From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mga01.intel.com (mga01.intel.com [192.55.52.88]) by dpdk.org (Postfix) with ESMTP id 9622EDE0 for ; Tue, 8 Dec 2015 07:05:24 +0100 (CET) Received: from orsmga002.jf.intel.com ([10.7.209.21]) by fmsmga101.fm.intel.com with ESMTP; 07 Dec 2015 22:05:23 -0800 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.20,398,1444719600"; d="scan'208";a="866927829" Received: from irvmail001.ir.intel.com ([163.33.26.43]) by orsmga002.jf.intel.com with ESMTP; 07 Dec 2015 22:05:21 -0800 Received: from sivswdev01.ir.intel.com (sivswdev01.ir.intel.com [10.237.217.45]) by irvmail001.ir.intel.com (8.14.3/8.13.6/MailSET/Hub) with ESMTP id tB865Jwc016474; Tue, 8 Dec 2015 06:05:19 GMT Received: from sivswdev01.ir.intel.com (localhost [127.0.0.1]) by sivswdev01.ir.intel.com with ESMTP id tB865JLF011167; Tue, 8 Dec 2015 06:05:19 GMT Received: (from ibetts@localhost) by sivswdev01.ir.intel.com with id tB865JYw011163; Tue, 8 Dec 2015 06:05:19 GMT From: Ian Betts To: dev@dpdk.org Date: Tue, 8 Dec 2015 06:05:14 +0000 Message-Id: <1449554717-11125-2-git-send-email-ian.betts@intel.com> X-Mailer: git-send-email 1.7.4.1 In-Reply-To: <1449554717-11125-1-git-send-email-ian.betts@intel.com> References: <1449511120-16899-git-send-email-ian.betts@intel.com> <1449554717-11125-1-git-send-email-ian.betts@intel.com> Cc: Ian Betts Subject: [dpdk-dev] [PATCH v10 1/4] doc: add sample application guide for performance-thread X-BeenThere: dev@dpdk.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: patches and discussions about DPDK List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Tue, 08 Dec 2015 06:05:26 -0000 This commit adds the sample application user guide for the performance thread sample application. Signed-off-by: Ian Betts --- .../sample_app_ug/img/performance_thread_1.svg | 799 +++++++++++++ .../sample_app_ug/img/performance_thread_2.svg | 865 ++++++++++++++ doc/guides/sample_app_ug/index.rst | 1 + doc/guides/sample_app_ug/performance_thread.rst | 1263 ++++++++++++++++++++ 4 files changed, 2928 insertions(+) create mode 100644 doc/guides/sample_app_ug/img/performance_thread_1.svg create mode 100644 doc/guides/sample_app_ug/img/performance_thread_2.svg create mode 100644 doc/guides/sample_app_ug/performance_thread.rst diff --git a/doc/guides/sample_app_ug/img/performance_thread_1.svg b/doc/guides/sample_app_ug/img/performance_thread_1.svg new file mode 100644 index 0000000..db01d7c --- /dev/null +++ b/doc/guides/sample_app_ug/img/performance_thread_1.svg @@ -0,0 +1,799 @@ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + image/svg+xml + + + + + + + + + + + Port 1 + + + Port 2 + + + rx-thread + + rings + + + + + + rx-thread + + + + Port 1 + + + Port 2 + + + tx-thread + + + + + + tx-thread + + + + + + + tx-thread + + + + + + + + diff --git a/doc/guides/sample_app_ug/img/performance_thread_2.svg b/doc/guides/sample_app_ug/img/performance_thread_2.svg new file mode 100644 index 0000000..48cf833 --- /dev/null +++ b/doc/guides/sample_app_ug/img/performance_thread_2.svg @@ -0,0 +1,865 @@ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + image/svg+xml + + + + + + + + + + + Port 1 + + + Port 2 + + + rx-thread + + rings + + + + + + rx-thread + + + + Port 1 + + + Port 2 + + + + + + + + tx-thread + + + + + + + tx-drain + + + + tx-thread + + + + tx-drain + + + + tx-thread + + + + tx-drain + + + + + diff --git a/doc/guides/sample_app_ug/index.rst b/doc/guides/sample_app_ug/index.rst index b8fca39..becd682 100644 --- a/doc/guides/sample_app_ug/index.rst +++ b/doc/guides/sample_app_ug/index.rst @@ -76,6 +76,7 @@ Sample Applications User Guide tep_termination proc_info ptpclient + performance_thread **Figures** diff --git a/doc/guides/sample_app_ug/performance_thread.rst b/doc/guides/sample_app_ug/performance_thread.rst new file mode 100644 index 0000000..d71bb84 --- /dev/null +++ b/doc/guides/sample_app_ug/performance_thread.rst @@ -0,0 +1,1263 @@ +.. BSD LICENSE + Copyright(c) 2015 Intel Corporation. All rights reserved. + All rights reserved. + + Redistribution and use in source and binary forms, with or without + modification, are permitted provided that the following conditions + are met: + + * Re-distributions of source code must retain the above copyright + notice, this list of conditions and the following disclaimer. + * Redistributions in binary form must reproduce the above copyright + notice, this list of conditions and the following disclaimer in + the documentation and/or other materials provided with the + distribution. + * Neither the name of Intel Corporation nor the names of its + contributors may be used to endorse or promote products derived + from this software without specific prior written permission. + + THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS + "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT + LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR + A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT + OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, + SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT + LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, + DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY + THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT + (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE + OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. + + +Performance Thread Sample Application +===================================== + +The performance thread sample application is a derivative of the standard L3 +forwarding application that demonstrates different threading models. + +Overview +-------- +For a general description of the L3 forwarding applications capabilities +please refer to the documentation of the standard application in +:doc:`l3_forward`. + +The performance thread sample application differs from the standard L3 +forwarding example in that it divides the TX and RX processing between +different threads, and makes it possible to assign individual threads to +different cores. + +Three threading models are considered: + +#. When there is one EAL thread per physical core. +#. When there are multiple EAL threads per physical core. +#. When there are multiple lightweight threads per EAL thread. + +Since DPDK release 2.0 it is possible to launch applications using the +``--lcores`` EAL parameter, specifying cpu-sets for a physical core. With the +performance thread sample application its is now also possible to assign +individual RX and TX functions to different cores. + +As an alternative to dividing the L3 forwarding work between different EAL +threads the performance thread sample introduces the possibility to run the +application threads as lightweight threads (L-threads) within one or +more EAL threads. + +In order to facilitate this threading model the example includes a primitive +cooperative scheduler (L-thread) subsystem. More details of the L-thread +subsystem can be found in :ref:`lthread_subsystem`. + +**Note:** Whilst theoretically possible it is not anticipated that multiple +L-thread schedulers would be run on the same physical core, this mode of +operation should not be expected to yield useful performance and is considered +invalid. + +Compiling the Application +------------------------- +The application is located in the sample application folder in the +``performance-thread`` folder. + +#. Go to the example applications folder + + .. code-block:: console + + export RTE_SDK=/path/to/rte_sdk + cd ${RTE_SDK}/examples/performance-thread/l3fwd-thread + +#. Set the target (a default target is used if not specified). For example: + + .. code-block:: console + + export RTE_TARGET=x86_64-native-linuxapp-gcc + + See the *DPDK Linux Getting Started Guide* for possible RTE_TARGET values. + +#. Build the application: + + make + + +Running the Application +----------------------- + +The application has a number of command line options:: + + ./build/l3fwd-thread [EAL options] -- + -p PORTMASK [-P] + --rx(port,queue,lcore,thread)[,(port,queue,lcore,thread)] + --tx(lcore,thread)[,(lcore,thread)] + [--enable-jumbo] [--max-pkt-len PKTLEN]] [--no-numa] + [--hash-entry-num] [--ipv6] [--no-lthreads] [--stat-lcore lcore] + +Where: + +* ``-p PORTMASK``: Hexadecimal bitmask of ports to configure. + +* ``-P``: optional, sets all ports to promiscuous mode so that packets are + accepted regardless of the packet's Ethernet MAC destination address. + Without this option, only packets with the Ethernet MAC destination address + set to the Ethernet address of the port are accepted. + +* ``--rx (port,queue,lcore,thread)[,(port,queue,lcore,thread)]``: the list of + NIC RX ports and queues handled by the RX lcores and threads. The parameters + are explained below. + +* ``--tx (lcore,thread)[,(lcore,thread)]``: the list of TX threads identifying + the lcore the thread runs on, and the id of RX thread with which it is + associated. The parameters are explained below. + +* ``--enable-jumbo``: optional, enables jumbo frames. + +* ``--max-pkt-len``: optional, maximum packet length in decimal (64-9600). + +* ``--no-numa``: optional, disables numa awareness. + +* ``--hash-entry-num``: optional, specifies the hash entry number in hex to be + setup. + +* ``--ipv6``: optional, set it if running ipv6 packets. + +* ``--no-lthreads``: optional, disables l-thread model and uses EAL threading + model. See below. + +* ``--stat-lcore``: optional, run CPU load stats collector on the specified + lcore. + +The parameters of the ``--rx`` and ``--tx`` options are: + +* ``--rx`` parameters + + .. _table_l3fwd_rx_parameters: + + +--------+------------------------------------------------------+ + | port | RX port | + +--------+------------------------------------------------------+ + | queue | RX queue that will be read on the specified RX port | + +--------+------------------------------------------------------+ + | lcore | Core to use for the thread | + +--------+------------------------------------------------------+ + | thread | Thread id (continuously from 0 to N) | + +--------+------------------------------------------------------+ + + +* ``--tx`` parameters + + .. _table_l3fwd_tx_parameters: + + +--------+------------------------------------------------------+ + | lcore | Core to use for L3 route match and transmit | + +--------+------------------------------------------------------+ + | thread | Id of RX thread to be associated with this TX thread | + +--------+------------------------------------------------------+ + +The ``l3fwd-thread`` application allows you to start packet processing in two +threading models: L-Threads (default) and EAL Threads (when the +``--no-lthreads`` parameter is used). For consistency all parameters are used +in the same way for both models. + + +Running with L-threads +~~~~~~~~~~~~~~~~~~~~~~ + +When the L-thread model is used (default option), lcore and thread parameters +in ``--rx/--tx`` are used to affinitize threads to the selected scheduler. + +For example, the following places every l-thread on different lcores:: + + l3fwd-thread -c ff -n 2 -- -P -p 3 \ + --rx="(0,0,0,0)(1,0,1,1)" \ + --tx="(2,0)(3,1)" + +The following places RX l-threads on lcore 0 and TX l-threads on lcore 1 and 2 +and so on:: + + l3fwd-thread -c ff -n 2 -- -P -p 3 \ + --rx="(0,0,0,0)(1,0,0,1)" \ + --tx="(1,0)(2,1)" + + +Running with EAL threads +~~~~~~~~~~~~~~~~~~~~~~~~ + +When the ``--no-lthreads`` parameter is used, the L-threading model is turned +off and EAL threads are used for all processing. EAL threads are enumerated in +the same way as L-threads, but the ``--lcores`` EAL parameter is used to +affinitize threads to the selected cpu-set (scheduler). Thus it is possible to +place every RX and TX thread on different lcores. + +For example, the following places every EAL thread on different lcores:: + + l3fwd-thread -c ff -n 2 -- -P -p 3 \ + --rx="(0,0,0,0)(1,0,1,1)" \ + --tx="(2,0)(3,1)" \ + --no-lthreads + + +To affinitize two or more EAL threads to one cpu-set, the EAL ``--lcores`` +parameter is used. + +The following places RX EAL threads on lcore 0 and TX EAL threads on lcore 1 +and 2 and so on:: + + l3fwd-thread -c ff -n 2 --lcores="(0,1)@0,(2,3)@1" -- -P -p 3 \ + --rx="(0,0,0,0)(1,0,1,1)" \ + --tx="(2,0)(3,1)" \ + --no-lthreads + + +Examples +~~~~~~~~ + +For selected scenarios the command line configuration of the application for L-threads +and its corresponding EAL threads command line can be realized as follows: + +a) Start every thread on different scheduler (1:1):: + + l3fwd-thread -c ff -n 2 -- -P -p 3 \ + --rx="(0,0,0,0)(1,0,1,1)" \ + --tx="(2,0)(3,1)" + + EAL thread equivalent:: + + l3fwd-thread -c ff -n 2 -- -P -p 3 \ + --rx="(0,0,0,0)(1,0,1,1)" \ + --tx="(2,0)(3,1)" \ + --no-lthreads + +b) Start all threads on one core (N:1). + + Start 4 L-threads on lcore 0:: + + l3fwd-thread -c ff -n 2 -- -P -p 3 \ + --rx="(0,0,0,0)(1,0,0,1)" \ + --tx="(0,0)(0,1)" + + Start 4 EAL threads on cpu-set 0:: + + l3fwd-thread -c ff -n 2 --lcores="(0-3)@0" -- -P -p 3 \ + --rx="(0,0,0,0)(1,0,0,1)" \ + --tx="(2,0)(3,1)" \ + --no-lthreads + +c) Start threads on different cores (N:M). + + Start 2 L-threads for RX on lcore 0, and 2 L-threads for TX on lcore 1:: + + l3fwd-thread -c ff -n 2 -- -P -p 3 \ + --rx="(0,0,0,0)(1,0,0,1)" \ + --tx="(1,0)(1,1)" + + Start 2 EAL threads for RX on cpu-set 0, and 2 EAL threads for TX on + cpu-set 1:: + + l3fwd-thread -c ff -n 2 --lcores="(0-1)@0,(2-3)@1" -- -P -p 3 \ + --rx="(0,0,0,0)(1,0,1,1)" \ + --tx="(2,0)(3,1)" \ + --no-lthreads + +Explanation +----------- + +To a great extent the sample application differs little from the standard L3 +forwarding application, and readers are advised to familiarize themselves with +the material covered in the :doc:`l3_forward` documentation before proceeding. + +The following explanation is focused on the way threading is handled in the +performance thread example. + + +Mode of operation with EAL threads +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +The performance thread sample application has split the RX and TX functionality +into two different threads, and the RX and TX threads are +interconnected via software rings. With respect to these rings the RX threads +are producers and the TX threads are consumers. + +On initialization the TX and RX threads are started according to the command +line parameters. + +The RX threads poll the network interface queues and post received packets to a +TX thread via a corresponding software ring. + +The TX threads poll software rings, perform the L3 forwarding hash/LPM match, +and assemble packet bursts before performing burst transmit on the network +interface. + +As with the standard L3 forward application, burst draining of residual packets +is performed periodically with the period calculated from elapsed time using +the timestamps counter. + +The diagram below illustrates a case with two RX threads and three TX threads. + +.. _figure_performance_thread_1: + +.. figure:: img/performance_thread_1.* + + +Mode of operation with L-threads +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Like the EAL thread configuration the application has split the RX and TX +functionality into different threads, and the pairs of RX and TX threads are +interconnected via software rings. + +On initialization an L-thread scheduler is started on every EAL thread. On all +but the master EAL thread only a a dummy L-thread is initially started. +The L-thread started on the master EAL thread then spawns other L-threads on +different L-thread schedulers according the the command line parameters. + +The RX threads poll the network interface queues and post received packets +to a TX thread via the corresponding software ring. + +The ring interface is augmented by means of an L-thread condition variable that +enables the TX thread to be suspended when the TX ring is empty. The RX thread +signals the condition whenever it posts to the TX ring, causing the TX thread +to be resumed. + +Additionally the TX L-thread spawns a worker L-thread to take care of +polling the software rings, whilst it handles burst draining of the transmit +buffer. + +The worker threads poll the software rings, perform L3 route lookup and +assemble packet bursts. If the TX ring is empty the worker thread suspends +itself by waiting on the condition variable associated with the ring. + +Burst draining of residual packets, less than the burst size, is performed by +the TX thread which sleeps (using an L-thread sleep function) and resumes +periodically to flush the TX buffer. + +This design means that L-threads that have no work, can yield the CPU to other +L-threads and avoid having to constantly poll the software rings. + +The diagram below illustrates a case with two RX threads and three TX functions +(each comprising a thread that processes forwarding and a thread that +periodically drains the output buffer of residual packets). + +.. _figure_performance_thread_2: + +.. figure:: img/performance_thread_2.* + + +CPU load statistics +~~~~~~~~~~~~~~~~~~~ + +It is possible to display statistics showing estimated CPU load on each core. +The statistics indicate the percentage of CPU time spent: processing +received packets (forwarding), polling queues/rings (waiting for work), +and doing any other processing (context switch and other overhead). + +When enabled statistics are gathered by having the application threads set and +clear flags when they enter and exit pertinent code sections. The flags are +then sampled in real time by a statistics collector thread running on another +core. This thread displays the data in real time on the console. + +This feature is enabled by designating a statistics collector core, using the +``--stat-lcore`` parameter. + + +.. _lthread_subsystem: + +The L-thread subsystem +---------------------- + +The L-thread subsystem resides in the examples/performance-thread/common +directory and is built and linked automatically when building the +``l3fwd-thread`` example. + +The subsystem provides a simple cooperative scheduler to enable arbitrary +functions to run as cooperative threads within a single EAL thread. +The subsystem provides a pthread like API that is intended to assist in +reuse of legacy code written for POSIX pthreads. + +The following sections provide some detail on the features, constraints, +performance and porting considerations when using L-threads. + + +.. _comparison_between_lthreads_and_pthreads: + +Comparison between L-threads and POSIX pthreads +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +The fundamental difference between the L-thread and pthread models is the +way in which threads are scheduled. The simplest way to think about this is to +consider the case of a processor with a single CPU. To run multiple threads +on a single CPU, the scheduler must frequently switch between the threads, +in order that each thread is able to make timely progress. +This is the basis of any multitasking operating system. + +This section explores the differences between the pthread model and the +L-thread model as implemented in the provided L-thread subsystem. If needed a +theoretical discussion of preemptive vs cooperative multi-threading can be +found in any good text on operating system design. + + +Scheduling and context switching +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +The POSIX pthread library provides an application programming interface to +create and synchronize threads. Scheduling policy is determined by the host OS, +and may be configurable. The OS may use sophisticated rules to determine which +thread should be run next, threads may suspend themselves or make other threads +ready, and the scheduler may employ a time slice giving each thread a maximum +time quantum after which it will be preempted in favor of another thread that +is ready to run. To complicate matters further threads may be assigned +different scheduling priorities. + +By contrast the L-thread subsystem is considerably simpler. Logically the +L-thread scheduler performs the same multiplexing function for L-threads +within a single pthread as the OS scheduler does for pthreads within an +application process. The L-thread scheduler is simply the main loop of a +pthread, and in so far as the host OS is concerned it is a regular pthread +just like any other. The host OS is oblivious about the existence of and +not at all involved in the scheduling of L-threads. + +The other and most significant difference between the two models is that +L-threads are scheduled cooperatively. L-threads cannot not preempt each +other, nor can the L-thread scheduler preempt a running L-thread (i.e. +there is no time slicing). The consequence is that programs implemented with +L-threads must possess frequent rescheduling points, meaning that they must +explicitly and of their own volition return to the scheduler at frequent +intervals, in order to allow other L-threads an opportunity to proceed. + +In both models switching between threads requires that the current CPU +context is saved and a new context (belonging to the next thread ready to run) +is restored. With pthreads this context switching is handled transparently +and the set of CPU registers that must be preserved between context switches +is as per an interrupt handler. + +An L-thread context switch is achieved by the thread itself making a function +call to the L-thread scheduler. Thus it is only necessary to preserve the +callee registers. The caller is responsible to save and restore any other +registers it is using before a function call, and restore them on return, +and this is handled by the compiler. For ``X86_64`` on both Linux and BSD the +System V calling convention is used, this defines registers RSP, RBP, and +R12-R15 as callee-save registers (for more detailed discussion a good reference +is `X86 Calling Conventions `_). + +Taking advantage of this, and due to the absence of preemption, an L-thread +context switch is achieved with less than 20 load/store instructions. + +The scheduling policy for L-threads is fixed, there is no prioritization of +L-threads, all L-threads are equal and scheduling is based on a FIFO +ready queue. + +An L-thread is a struct containing the CPU context of the thread +(saved on context switch) and other useful items. The ready queue contains +pointers to threads that are ready to run. The L-thread scheduler is a simple +loop that polls the ready queue, reads from it the next thread ready to run, +which it resumes by saving the current context (the current position in the +scheduler loop) and restoring the context of the next thread from its thread +struct. Thus an L-thread is always resumed at the last place it yielded. + +A well behaved L-thread will call the context switch regularly (at least once +in its main loop) thus returning to the scheduler's own main loop. Yielding +inserts the current thread at the back of the ready queue, and the process of +servicing the ready queue is repeated, thus the system runs by flipping back +and forth the between L-threads and scheduler loop. + +In the case of pthreads, the preemptive scheduling, time slicing, and support +for thread prioritization means that progress is normally possible for any +thread that is ready to run. This comes at the price of a relatively heavier +context switch and scheduling overhead. + +With L-threads the progress of any particular thread is determined by the +frequency of rescheduling opportunities in the other L-threads. This means that +an errant L-thread monopolizing the CPU might cause scheduling of other threads +to be stalled. Due to the lower cost of context switching, however, voluntary +rescheduling to ensure progress of other threads, if managed sensibly, is not +a prohibitive overhead, and overall performance can exceed that of an +application using pthreads. + + +Mutual exclusion +^^^^^^^^^^^^^^^^ + +With pthreads preemption means that threads that share data must observe +some form of mutual exclusion protocol. + +The fact that L-threads cannot preempt each other means that in many cases +mutual exclusion devices can be completely avoided. + +Locking to protect shared data can be a significant bottleneck in +multi-threaded applications so a carefully designed cooperatively scheduled +program can enjoy significant performance advantages. + +So far we have considered only the simplistic case of a single core CPU, +when multiple CPUs are considered things are somewhat more complex. + +First of all it is inevitable that there must be multiple L-thread schedulers, +one running on each EAL thread. So long as these schedulers remain isolated +from each other the above assertions about the potential advantages of +cooperative scheduling hold true. + +A configuration with isolated cooperative schedulers is less flexible than the +pthread model where threads can be affinitized to run on any CPU. With isolated +schedulers scaling of applications to utilize fewer or more CPUs according to +system demand is very difficult to achieve. + +The L-thread subsystem makes it possible for L-threads to migrate between +schedulers running on different CPUs. Needless to say if the migration means +that threads that share data end up running on different CPUs then this will +introduce the need for some kind of mutual exclusion system. + +Of course ``rte_ring`` software rings can always be used to interconnect +threads running on different cores, however to protect other kinds of shared +data structures, lock free constructs or else explicit locking will be +required. This is a consideration for the application design. + +In support of this extended functionality, the L-thread subsystem implements +thread safe mutexes and condition variables. + +The cost of affinitizing and of condition variable signaling is significantly +lower than the equivalent pthread operations, and so applications using these +features will see a performance benefit. + + +Thread local storage +^^^^^^^^^^^^^^^^^^^^ + +As with applications written for pthreads an application written for L-threads +can take advantage of thread local storage, in this case local to an L-thread. +An application may save and retrieve a single pointer to application data in +the L-thread struct. + +For legacy and backward compatibility reasons two alternative methods are also +offered, the first is modelled directly on the pthread get/set specific APIs, +the second approach is modelled on the ``RTE_PER_LCORE`` macros, whereby +``PER_LTHREAD`` macros are introduced, in both cases the storage is local to +the L-thread. + + +.. _constraints_and_performance_implications: + +Constraints and performance implications when using L-threads +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + + +.. _API_compatibility: + +API compatibility +^^^^^^^^^^^^^^^^^ + +The L-thread subsystem provides a set of functions that are logically equivalent +to the corresponding functions offered by the POSIX pthread library, however not +all pthread functions have a corresponding L-thread equivalent, and not all +features available to pthreads are implemented for L-threads. + +The pthread library offers considerable flexibility via programmable attributes +that can be associated with threads, mutexes, and condition variables. + +By contrast the L-thread subsystem has fixed functionality, the scheduler policy +cannot be varied, and L-threads cannot be prioritized. There are no variable +attributes associated with any L-thread objects. L-threads, mutexes and +conditional variables, all have fixed functionality. (Note: reserved parameters +are included in the APIs to facilitate possible future support for attributes). + +The table below lists the pthread and equivalent L-thread APIs with notes on +differences and/or constraints. Where there is no L-thread entry in the table, +then the L-thread subsystem provides no equivalent function. + +.. _table_lthread_pthread: + +.. table:: Pthread and equivalent L-thread APIs. + + +----------------------------+------------------------+-------------------+ + | **Pthread function** | **L-thread function** | **Notes** | + +============================+========================+===================+ + | pthread_barrier_destroy | | | + +----------------------------+------------------------+-------------------+ + | pthread_barrier_init | | | + +----------------------------+------------------------+-------------------+ + | pthread_barrier_wait | | | + +----------------------------+------------------------+-------------------+ + | pthread_cond_broadcast | lthread_cond_broadcast | See note 1 | + +----------------------------+------------------------+-------------------+ + | pthread_cond_destroy | lthread_cond_destroy | | + +----------------------------+------------------------+-------------------+ + | pthread_cond_init | lthread_cond_init | | + +----------------------------+------------------------+-------------------+ + | pthread_cond_signal | lthread_cond_signal | See note 1 | + +----------------------------+------------------------+-------------------+ + | pthread_cond_timedwait | | | + +----------------------------+------------------------+-------------------+ + | pthread_cond_wait | lthread_cond_wait | See note 5 | + +----------------------------+------------------------+-------------------+ + | pthread_create | lthread_create | See notes 2, 3 | + +----------------------------+------------------------+-------------------+ + | pthread_detach | lthread_detach | See note 4 | + +----------------------------+------------------------+-------------------+ + | pthread_equal | | | + +----------------------------+------------------------+-------------------+ + | pthread_exit | lthread_exit | | + +----------------------------+------------------------+-------------------+ + | pthread_getspecific | lthread_getspecific | | + +----------------------------+------------------------+-------------------+ + | pthread_getcpuclockid | | | + +----------------------------+------------------------+-------------------+ + | pthread_join | lthread_join | | + +----------------------------+------------------------+-------------------+ + | pthread_key_create | lthread_key_create | | + +----------------------------+------------------------+-------------------+ + | pthread_key_delete | lthread_key_delete | | + +----------------------------+------------------------+-------------------+ + | pthread_mutex_destroy | lthread_mutex_destroy | | + +----------------------------+------------------------+-------------------+ + | pthread_mutex_init | lthread_mutex_init | | + +----------------------------+------------------------+-------------------+ + | pthread_mutex_lock | lthread_mutex_lock | See note 6 | + +----------------------------+------------------------+-------------------+ + | pthread_mutex_trylock | lthread_mutex_trylock | See note 6 | + +----------------------------+------------------------+-------------------+ + | pthread_mutex_timedlock | | | + +----------------------------+------------------------+-------------------+ + | pthread_mutex_unlock | lthread_mutex_unlock | | + +----------------------------+------------------------+-------------------+ + | pthread_once | | | + +----------------------------+------------------------+-------------------+ + | pthread_rwlock_destroy | | | + +----------------------------+------------------------+-------------------+ + | pthread_rwlock_init | | | + +----------------------------+------------------------+-------------------+ + | pthread_rwlock_rdlock | | | + +----------------------------+------------------------+-------------------+ + | pthread_rwlock_timedrdlock | | | + +----------------------------+------------------------+-------------------+ + | pthread_rwlock_timedwrlock | | | + +----------------------------+------------------------+-------------------+ + | pthread_rwlock_tryrdlock | | | + +----------------------------+------------------------+-------------------+ + | pthread_rwlock_trywrlock | | | + +----------------------------+------------------------+-------------------+ + | pthread_rwlock_unlock | | | + +----------------------------+------------------------+-------------------+ + | pthread_rwlock_wrlock | | | + +----------------------------+------------------------+-------------------+ + | pthread_self | lthread_current | | + +----------------------------+------------------------+-------------------+ + | pthread_setspecific | lthread_setspecific | | + +----------------------------+------------------------+-------------------+ + | pthread_spin_init | | See note 10 | + +----------------------------+------------------------+-------------------+ + | pthread_spin_destroy | | See note 10 | + +----------------------------+------------------------+-------------------+ + | pthread_spin_lock | | See note 10 | + +----------------------------+------------------------+-------------------+ + | pthread_spin_trylock | | See note 10 | + +----------------------------+------------------------+-------------------+ + | pthread_spin_unlock | | See note 10 | + +----------------------------+------------------------+-------------------+ + | pthread_cancel | lthread_cancel | | + +----------------------------+------------------------+-------------------+ + | pthread_setcancelstate | | | + +----------------------------+------------------------+-------------------+ + | pthread_setcanceltype | | | + +----------------------------+------------------------+-------------------+ + | pthread_testcancel | | | + +----------------------------+------------------------+-------------------+ + | pthread_getschedparam | | | + +----------------------------+------------------------+-------------------+ + | pthread_setschedparam | | | + +----------------------------+------------------------+-------------------+ + | pthread_yield | lthread_yield | See note 7 | + +----------------------------+------------------------+-------------------+ + | pthread_setaffinity_np | lthread_set_affinity | See notes 2, 3, 8 | + +----------------------------+------------------------+-------------------+ + | | lthread_sleep | See note 9 | + +----------------------------+------------------------+-------------------+ + | | lthread_sleep_clks | See note 9 | + +----------------------------+------------------------+-------------------+ + + +**Note 1**: + +Neither lthread signal nor broadcast may be called concurrently by L-threads +running on different schedulers, although multiple L-threads running in the +same scheduler may freely perform signal or broadcast operations. L-threads +running on the same or different schedulers may always safely wait on a +condition variable. + + +**Note 2**: + +Pthread attributes may be used to affinitize a pthread with a cpu-set. The +L-thread subsystem does not support a cpu-set. An L-thread may be affinitized +only with a single CPU at any time. + + +**Note 3**: + +If an L-thread is intended to run on a different NUMA node than the node that +creates the thread then, when calling ``lthread_create()`` it is advantageous +to specify the destination core as a parameter of ``lthread_create()``. See +:ref:`memory_allocation_and_NUMA_awareness` for details. + + +**Note 4**: + +An L-thread can only detach itself, and cannot detach other L-threads. + + +**Note 5**: + +A wait operation on a pthread condition variable is always associated with and +protected by a mutex which must be owned by the thread at the time it invokes +``pthread_wait()``. By contrast L-thread condition variables are thread safe +(for waiters) and do not use an associated mutex. Multiple L-threads (including +L-threads running on other schedulers) can safely wait on a L-thread condition +variable. As a consequence the performance of an L-thread condition variables +is typically an order of magnitude faster than its pthread counterpart. + + +**Note 6**: + +Recursive locking is not supported with L-threads, attempts to take a lock +recursively will be detected and rejected. + + +**Note 7**: + +``lthread_yield()`` will save the current context, insert the current thread +to the back of the ready queue, and resume the next ready thread. Yielding +increases ready queue backlog, see :ref:`ready_queue_backlog` for more details +about the implications of this. + + +N.B. The context switch time as measured from immediately before the call to +``lthread_yield()`` to the point at which the next ready thread is resumed, +can be an order of magnitude faster that the same measurement for +pthread_yield. + + +**Note 8**: + +``lthread_set_affinity()`` is similar to a yield apart from the fact that the +yielding thread is inserted into a peer ready queue of another scheduler. +The peer ready queue is actually a separate thread safe queue, which means that +threads appearing in the peer ready queue can jump any backlog in the local +ready queue on the destination scheduler. + +The context switch time as measured from the time just before the call to +``lthread_set_affinity()`` to just after the same thread is resumed on the new +scheduler can be orders of magnitude faster than the same measurement for +``pthread_setaffinity_np()``. + + +**Note 9**: + +Although there is no ``pthread_sleep()`` function, ``lthread_sleep()`` and +``lthread_sleep_clks()`` can be used wherever ``sleep()``, ``usleep()`` or +``nanosleep()`` might ordinarily be used. The L-thread sleep functions suspend +the current thread, start an ``rte_timer`` and resume the thread when the +timer matures. The ``rte_timer_manage()`` entry point is called on every pass +of the scheduler loop. This means that the worst case jitter on timer expiry +is determined by the longest period between context switches of any running +L-threads. + +In a synthetic test with many threads sleeping and resuming then the measured +jitter is typically orders of magnitude lower than the same measurement made +for ``nanosleep()``. + + +**Note 10**: + +Spin locks are not provided because they are problematical in a cooperative +environment, see :ref:`porting_locks_and_spinlocks` for a more detailed +discussion on how to avoid spin locks. + + +.. _Thread_local_storage_performance: + +Thread local storage +^^^^^^^^^^^^^^^^^^^^ + +Of the three L-thread local storage options the simplest and most efficient is +storing a single application data pointer in the L-thread struct. + +The ``PER_LTHREAD`` macros involve a run time computation to obtain the address +of the variable being saved/retrieved and also require that the accesses are +de-referenced via a pointer. This means that code that has used +``RTE_PER_LCORE`` macros being ported to L-threads might need some slight +adjustment (see :ref:`porting_thread_local_storage` for hints about porting +code that makes use of thread local storage). + +The get/set specific APIs are consistent with their pthread counterparts both +in use and in performance. + + +.. _memory_allocation_and_NUMA_awareness: + +Memory allocation and NUMA awareness +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +All memory allocation is from DPDK huge pages, and is NUMA aware. Each +scheduler maintains its own caches of objects: lthreads, their stacks, TLS, +mutexes and condition variables. These caches are implemented as unbounded lock +free MPSC queues. When objects are created they are always allocated from the +caches on the local core (current EAL thread). + +If an L-thread has been affinitized to a different scheduler, then it can +always safely free resources to the caches from which they originated (because +the caches are MPSC queues). + +If the L-thread has been affinitized to a different NUMA node then the memory +resources associated with it may incur longer access latency. + +The commonly used pattern of setting affinity on entry to a thread after it has +started, means that memory allocation for both the stack and TLS will have been +made from caches on the NUMA node on which the threads creator is running. +This has the side effect that access latency will be sub-optimal after +affinitizing. + +This side effect can be mitigated to some extent (although not completely) by +specifying the destination CPU as a parameter of ``lthread_create()`` this +causes the L-thread's stack and TLS to be allocated when it is first scheduled +on the destination scheduler, if the destination is a on another NUMA node it +results in a more optimal memory allocation. + +Note that the lthread struct itself remains allocated from memory on the +creating node, this is unavoidable because an L-thread is known everywhere by +the address of this struct. + + +.. _object_cache_sizing: + +Object cache sizing +^^^^^^^^^^^^^^^^^^^ + +The per lcore object caches pre-allocate objects in bulk whenever a request to +allocate an object finds a cache empty. By default 100 objects are +pre-allocated, this is defined by ``LTHREAD_PREALLOC`` in the public API +header file lthread_api.h. This means that the caches constantly grow to meet +system demand. + +In the present implementation there is no mechanism to reduce the cache sizes +if system demand reduces. Thus the caches will remain at their maximum extent +indefinitely. + +A consequence of the bulk pre-allocation of objects is that every 100 (default +value) additional new object create operations results in a call to +``rte_malloc()``. For creation of objects such as L-threads, which trigger the +allocation of even more objects (i.e. their stacks and TLS) then this can +cause outliers in scheduling performance. + +If this is a problem the simplest mitigation strategy is to dimension the +system, by setting the bulk object pre-allocation size to some large number +that you do not expect to be exceeded. This means the caches will be populated +once only, the very first time a thread is created. + + +.. _Ready_queue_backlog: + +Ready queue backlog +^^^^^^^^^^^^^^^^^^^ + +One of the more subtle performance considerations is managing the ready queue +backlog. The fewer threads that are waiting in the ready queue then the faster +any particular thread will get serviced. + +In a naive L-thread application with N L-threads simply looping and yielding, +this backlog will always be equal to the number of L-threads, thus the cost of +a yield to a particular L-thread will be N times the context switch time. + +This side effect can be mitigated by arranging for threads to be suspended and +wait to be resumed, rather than polling for work by constantly yielding. +Blocking on a mutex or condition variable or even more obviously having a +thread sleep if it has a low frequency workload are all mechanisms by which a +thread can be excluded from the ready queue until it really does need to be +run. This can have a significant positive impact on performance. + + +.. _Initialization_and_shutdown_dependencies: + +Initialization, shutdown and dependencies +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +The L-thread subsystem depends on DPDK for huge page allocation and depends on +the ``rte_timer subsystem``. The DPDK EAL initialization and +``rte_timer_subsystem_init()`` **MUST** be completed before the L-thread sub +system can be used. + +Thereafter initialization of the L-thread subsystem is largely transparent to +the application. Constructor functions ensure that global variables are properly +initialized. Other than global variables each scheduler is initialized +independently the first time that an L-thread is created by a particular EAL +thread. + +If the schedulers are to be run as isolated and independent schedulers, with +no intention that L-threads running on different schedulers will migrate between +schedulers or synchronize with L-threads running on other schedulers, then +initialization consists simply of creating an L-thread, and then running the +L-thread scheduler. + +If there will be interaction between L-threads running on different schedulers, +then it is important that the starting of schedulers on different EAL threads +is synchronized. + +To achieve this an additional initialization step is necessary, this is simply +to set the number of schedulers by calling the API function +``lthread_num_schedulers_set(n)``, where ``n`` is the number of EAL threads +that will run L-thread schedulers. Setting the number of schedulers to a +number greater than 0 will cause all schedulers to wait until the others have +started before beginning to schedule L-threads. + +The L-thread scheduler is started by calling the function ``lthread_run()`` +and should be called from the EAL thread and thus become the main loop of the +EAL thread. + +The function ``lthread_run()``, will not return until all threads running on +the scheduler have exited, and the scheduler has been explicitly stopped by +calling ``lthread_scheduler_shutdown(lcore)`` or +``lthread_scheduler_shutdown_all()``. + +All these function do is tell the scheduler that it can exit when there are no +longer any running L-threads, neither function forces any running L-thread to +terminate. Any desired application shutdown behavior must be designed and +built into the application to ensure that L-threads complete in a timely +manner. + +**Important Note:** It is assumed when the scheduler exits that the application +is terminating for good, the scheduler does not free resources before exiting +and running the scheduler a subsequent time will result in undefined behavior. + + +.. _porting_legacy_code_to_run_on_lthreads: + +Porting legacy code to run on L-threads +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Legacy code originally written for a pthread environment may be ported to +L-threads if the considerations about differences in scheduling policy, and +constraints discussed in the previous sections can be accommodated. + +This section looks in more detail at some of the issues that may have to be +resolved when porting code. + + +.. _pthread_API_compatibility: + +pthread API compatibility +^^^^^^^^^^^^^^^^^^^^^^^^^ + +The first step is to establish exactly which pthread APIs the legacy +application uses, and to understand the requirements of those APIs. If there +are corresponding L-lthread APIs, and where the default pthread functionality +is used by the application then, notwithstanding the other issues discussed +here, it should be feasible to run the application with L-threads. If the +legacy code modifies the default behavior using attributes then if may be +necessary to make some adjustments to eliminate those requirements. + + +.. _blocking_system_calls: + +Blocking system API calls +^^^^^^^^^^^^^^^^^^^^^^^^^ + +It is important to understand what other system services the application may be +using, bearing in mind that in a cooperatively scheduled environment a thread +cannot block without stalling the scheduler and with it all other cooperative +threads. Any kind of blocking system call, for example file or socket IO, is a +potential problem, a good tool to analyze the application for this purpose is +the ``strace`` utility. + +There are many strategies to resolve these kind of issues, each with it +merits. Possible solutions include: + +* Adopting a polled mode of the system API concerned (if available). + +* Arranging for another core to perform the function and synchronizing with + that core via constructs that will not block the L-thread. + +* Affinitizing the thread to another scheduler devoted (as a matter of policy) + to handling threads wishing to make blocking calls, and then back again when + finished. + + +.. _porting_locks_and_spinlocks: + +Locks and spinlocks +^^^^^^^^^^^^^^^^^^^ + +Locks and spinlocks are another source of blocking behavior that for the same +reasons as system calls will need to be addressed. + +If the application design ensures that the contending L-threads will always +run on the same scheduler then it its probably safe to remove locks and spin +locks completely. + +The only exception to the above rule is if for some reason the +code performs any kind of context switch whilst holding the lock +(e.g. yield, sleep, or block on a different lock, or on a condition variable). +This will need to determined before deciding to eliminate a lock. + +If a lock cannot be eliminated then an L-thread mutex can be substituted for +either kind of lock. + +An L-thread blocking on an L-thread mutex will be suspended and will cause +another ready L-thread to be resumed, thus not blocking the scheduler. When +default behavior is required, it can be used as a direct replacement for a +pthread mutex lock. + +Spin locks are typically used when lock contention is likely to be rare and +where the period during which the lock may be held is relatively short. +When the contending L-threads are running on the same scheduler then an +L-thread blocking on a spin lock will enter an infinite loop stopping the +scheduler completely (see :ref:`porting_infinite_loops` below). + +If the application design ensures that contending L-threads will always run +on different schedulers then it might be reasonable to leave a short spin lock +that rarely experiences contention in place. + +If after all considerations it appears that a spin lock can neither be +eliminated completely, replaced with an L-thread mutex, or left in place as +is, then an alternative is to loop on a flag, with a call to +``lthread_yield()`` inside the loop (n.b. if the contending L-threads might +ever run on different schedulers the flag will need to be manipulated +atomically). + +Spinning and yielding is the least preferred solution since it introduces +ready queue backlog (see also :ref:`ready_queue_backlog`). + + +.. _porting_sleeps_and_delays: + +Sleeps and delays +^^^^^^^^^^^^^^^^^ + +Yet another kind of blocking behavior (albeit momentary) are delay functions +like ``sleep()``, ``usleep()``, ``nanosleep()`` etc. All will have the +consequence of stalling the L-thread scheduler and unless the delay is very +short (e.g. a very short nanosleep) calls to these functions will need to be +eliminated. + +The simplest mitigation strategy is to use the L-thread sleep API functions, +of which two variants exist, ``lthread_sleep()`` and ``lthread_sleep_clks()``. +These functions start an rte_timer against the L-thread, suspend the L-thread +and cause another ready L-thread to be resumed. The suspended L-thread is +resumed when the rte_timer matures. + + +.. _porting_infinite_loops: + +Infinite loops +^^^^^^^^^^^^^^ + +Some applications have threads with loops that contain no inherent +rescheduling opportunity, and rely solely on the OS time slicing to share +the CPU. In a cooperative environment this will stop everything dead. These +kind of loops are not hard to identify, in a debug session you will find the +debugger is always stopping in the same loop. + +The simplest solution to this kind of problem is to insert an explicit +``lthread_yield()`` or ``lthread_sleep()`` into the loop. Another solution +might be to include the function performed by the loop into the execution path +of some other loop that does in fact yield, if this is possible. + + +.. _porting_thread_local_storage: + +Thread local storage +^^^^^^^^^^^^^^^^^^^^ + +If the application uses thread local storage, the use case should be +studied carefully. + +In a legacy pthread application either or both the ``__thread`` prefix, or the +pthread set/get specific APIs may have been used to define storage local to a +pthread. + +In some applications it may be a reasonable assumption that the data could +or in fact most likely should be placed in L-thread local storage. + +If the application (like many DPDK applications) has assumed a certain +relationship between a pthread and the CPU to which it is affinitized, there +is a risk that thread local storage may have been used to save some data items +that are correctly logically associated with the CPU, and others items which +relate to application context for the thread. Only a good understanding of the +application will reveal such cases. + +If the application requires an that an L-thread is to be able to move between +schedulers then care should be taken to separate these kinds of data, into per +lcore, and per L-thread storage. In this way a migrating thread will bring with +it the local data it needs, and pick up the new logical core specific values +from pthread local storage at its new home. + + +.. _pthread_shim: + +Pthread shim +~~~~~~~~~~~~ + +A convenient way to get something working with legacy code can be to use a +shim that adapts pthread API calls to the corresponding L-thread ones. +This approach will not mitigate any of the porting considerations mentioned +in the previous sections, but it will reduce the amount of code churn that +would otherwise been involved. It is a reasonable approach to evaluate +L-threads, before investing effort in porting to the native L-thread APIs. + + +Overview +^^^^^^^^ +The L-thread subsystem includes an example pthread shim. This is a partial +implementation but does contain the API stubs needed to get basic applications +running. There is a simple "hello world" application that demonstrates the +use of the pthread shim. + +A subtlety of working with a shim is that the application will still need +to make use of the genuine pthread library functions, at the very least in +order to create the EAL threads in which the L-thread schedulers will run. +This is the case with DPDK initialization, and exit. + +To deal with the initialization and shutdown scenarios, the shim is capable of +switching on or off its adaptor functionality, an application can control this +behavior by the calling the function ``pt_override_set()``. The default state +is disabled. + +The pthread shim uses the dynamic linker loader and saves the loaded addresses +of the genuine pthread API functions in an internal table, when the shim +functionality is enabled it performs the adaptor function, when disabled it +invokes the genuine pthread function. + +The function ``pthread_exit()`` has additional special handling. The standard +system header file pthread.h declares ``pthread_exit()`` with +``__attribute__((noreturn))`` this is an optimization that is possible because +the pthread is terminating and this enables the compiler to omit the normal +handling of stack and protection of registers since the function is not +expected to return, and in fact the thread is being destroyed. These +optimizations are applied in both the callee and the caller of the +``pthread_exit()`` function. + +In our cooperative scheduling environment this behavior is inadmissible. The +pthread is the L-thread scheduler thread, and, although an L-thread is +terminating, there must be a return to the scheduler in order that the system +can continue to run. Further, returning from a function with attribute +``noreturn`` is invalid and may result in undefined behavior. + +The solution is to redefine the ``pthread_exit`` function with a macro, +causing it to be mapped to a stub function in the shim that does not have the +``noreturn`` attribute. This macro is defined in the file +``pthread_shim.h``. The stub function is otherwise no different than any of +the other stub functions in the shim, and will switch between the real +``pthread_exit()`` function or the ``lthread_exit()`` function as +required. The only difference is that the mapping to the stub by macro +substitution. + +A consequence of this is that the file ``pthread_shim.h`` must be included in +legacy code wishing to make use of the shim. It also means that dynamic +linkage of a pre-compiled binary that did not include pthread_shim.h is not be +supported. + +Given the requirements for porting legacy code outlined in +:ref:`porting_legacy_code_to_run_on_lthreads` most applications will require at +least some minimal adjustment and recompilation to run on L-threads so +pre-compiled binaries are unlikely to be met in practice. + +In summary the shim approach adds some overhead but can be a useful tool to help +establish the feasibility of a code reuse project. It is also a fairly +straightforward task to extend the shim if necessary. + +**Note:** Bearing in mind the preceding discussions about the impact of making +blocking calls then switching the shim in and out on the fly to invoke any +pthread API this might block is something that should typically be avoided. + + +Building and running the pthread shim +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +The shim example application is located in the sample application +in the performance-thread folder + +To build and run the pthread shim example + +#. Go to the example applications folder + + .. code-block:: console + + export RTE_SDK=/path/to/rte_sdk + cd ${RTE_SDK}/examples/performance-thread/pthread_shim + + +#. Set the target (a default target is used if not specified). For example: + + .. code-block:: console + + export RTE_TARGET=x86_64-native-linuxapp-gcc + + See the DPDK Getting Started Guide for possible RTE_TARGET values. + +#. Build the application: + + .. code-block:: console + + make + +#. To run the pthread_shim example + + .. code-block:: console + + lthread-pthread-shim -c core_mask -n number_of_channels + +.. _lthread_diagnostics: + +L-thread Diagnostics +~~~~~~~~~~~~~~~~~~~~ + +When debugging you must take account of the fact that the L-threads are run in +a single pthread. The current scheduler is defined by +``RTE_PER_LCORE(this_sched)``, and the current lthread is stored at +``RTE_PER_LCORE(this_sched)->current_lthread``. Thus on a breakpoint in a GDB +session the current lthread can be obtained by displaying the pthread local +variable ``per_lcore_this_sched->current_lthread``. + +Another useful diagnostic feature is the possibility to trace significant +events in the life of an L-thread, this feature is enabled by changing the +value of LTHREAD_DIAG from 0 to 1 in the file ``lthread_diag_api.h``. + +Tracing of events can be individually masked, and the mask may be programmed +at run time. An unmasked event results in a callback that provides information +about the event. The default callback simply prints trace information. The +default mask is 0 (all events off) the mask can be modified by calling the +function ``lthread_diagniostic_set_mask()``. + +It is possible register a user callback function to implement more +sophisticated diagnostic functions. +Object creation events (lthread, mutex, and condition variable) accept, and +store in the created object, a user supplied reference value returned by the +callback function. + +The lthread reference value is passed back in all subsequent event callbacks, +the mutex and APIs are provided to retrieve the reference value from +mutexes and condition variables. This enables a user to monitor, count, or +filter for specific events, on specific objects, for example to monitor for a +specific thread signalling a specific condition variable, or to monitor +on all timer events, the possibilities and combinations are endless. + +The callback function can be set by calling the function +``lthread_diagnostic_enable()`` supplying a callback function pointer and an +event mask. + +Setting ``LTHREAD_DIAG`` also enables counting of statistics about cache and +queue usage, and these statistics can be displayed by calling the function +``lthread_diag_stats_display()``. This function also performs a consistency +check on the caches and queues. The function should only be called from the +master EAL thread after all slave threads have stopped and returned to the C +main program, otherwise the consistency check will fail. -- 2.1.4