Re: [dpdk-dev] [PATCH v3] doc: add stack mempool guide

[Olivier Matz <olivier.matz@6wind.com>]

Hi Gage,

On Mon, Sep 14, 2020 at 04:11:53PM -0500, Gage Eads wrote:
> This guide describes the two stack modes, their tradeoffs, and (via a
> reference to the mempool guide) how to enable them.
> 
> Signed-off-by: Gage Eads <gage.eads@intel.com>
> ---
> v3: Fixed "Title underline too short" warning
> 
> v2: Added commit description
> 
>  doc/guides/mempool/index.rst          |  1 +
>  doc/guides/mempool/stack.rst          | 38 +++++++++++++++++++++++++++++++++++
>  doc/guides/prog_guide/mempool_lib.rst |  2 ++
>  doc/guides/prog_guide/stack_lib.rst   |  4 ++++
>  4 files changed, 45 insertions(+)
>  create mode 100644 doc/guides/mempool/stack.rst
> 
> diff --git a/doc/guides/mempool/index.rst b/doc/guides/mempool/index.rst
> index bbd02fd81..a0e55467e 100644
> --- a/doc/guides/mempool/index.rst
> +++ b/doc/guides/mempool/index.rst
> @@ -14,3 +14,4 @@ application through the mempool API.
>      octeontx
>      octeontx2
>      ring
> +    stack
> diff --git a/doc/guides/mempool/stack.rst b/doc/guides/mempool/stack.rst
> new file mode 100644
> index 000000000..bdf19cf04
> --- /dev/null
> +++ b/doc/guides/mempool/stack.rst
> @@ -0,0 +1,38 @@
> +..  SPDX-License-Identifier: BSD-3-Clause
> +    Copyright(c) 2020 Intel Corporation.
> +
> +Stack Mempool Driver
> +====================
> +
> +**rte_mempool_stack** is a pure software mempool driver based on the
> +``rte_stack`` DPDK library. A stack-based mempool is often better suited to
> +packet-processing workloads than a ring-based mempool, since its LIFO behavior
> +results in better temporal locality and a minimal memory footprint even if the
> +mempool is over-provisioned.

Would it make sense to give an example of a use-case where the stack
driver should be used in place of the standard ring-based one?

In most run-to-completion applications, the mbufs stay in per-core
caches, so changing the mempool driver won't have a big impact. However,
I suspect that for applications using a pipeline model (ex: rx on core0,
tx on core1), the stack model would be more efficient. Is it something
that you measured? If yes, it would be useful to explain this in the
documentation.


> +
> +The following modes of operation are available for the stack mempool driver and
> +can be selected as described in :ref:`Mempool_Handlers`:
> +
> +- ``stack``
> +
> +  The underlying **rte_stack** operates in standard (lock-based) mode.
> +  For more information please refer to :ref:`Stack_Library_Std_Stack`.
> +
> +- ``lf_stack``
> +
> +  The underlying **rte_stack** operates in lock-free mode. For more
> +  information please refer to :ref:`Stack_Library_LF_Stack`.
> +
> +The standard stack outperforms the lock-free stack on average, however the
> +standard stack is non-preemptive: if a mempool user is preempted while holding
> +the stack lock, that thread will block all other mempool accesses until it
> +returns and releases the lock. As a result, an application using the standard
> +stack whose threads can be preempted can suffer from brief, infrequent
> +performance hiccups.
> +
> +The lock-free stack, by design, is not susceptible to this problem; one thread can
> +be preempted at any point during a push or pop operation and will not impede
> +the progress of any other thread.
> +
> +For a more detailed description of the stack implementations, please refer to
> +:doc:`../prog_guide/stack_lib`.
> diff --git a/doc/guides/prog_guide/mempool_lib.rst b/doc/guides/prog_guide/mempool_lib.rst
> index e3e1f940b..6f3c0067f 100644
> --- a/doc/guides/prog_guide/mempool_lib.rst
> +++ b/doc/guides/prog_guide/mempool_lib.rst
> @@ -105,6 +105,8 @@ These user-owned caches can be explicitly passed to ``rte_mempool_generic_put()`
>  The ``rte_mempool_default_cache()`` call returns the default internal cache if any.
>  In contrast to the default caches, user-owned caches can be used by unregistered non-EAL threads too.
>  
> +.. _Mempool_Handlers:
> +
>  Mempool Handlers
>  ------------------------
>  
> diff --git a/doc/guides/prog_guide/stack_lib.rst b/doc/guides/prog_guide/stack_lib.rst
> index 8fe8804e3..3097cab0c 100644
> --- a/doc/guides/prog_guide/stack_lib.rst
> +++ b/doc/guides/prog_guide/stack_lib.rst
> @@ -28,6 +28,8 @@ Implementation
>  The library supports two types of stacks: standard (lock-based) and lock-free.
>  Both types use the same set of interfaces, but their implementations differ.
>  
> +.. _Stack_Library_Std_Stack:
> +
>  Lock-based Stack
>  ----------------
>  
> @@ -35,6 +37,8 @@ The lock-based stack consists of a contiguous array of pointers, a current
>  index, and a spinlock. Accesses to the stack are made multi-thread safe by the
>  spinlock.
>  
> +.. _Stack_Library_LF_Stack:
> +
>  Lock-free Stack
>  ------------------
>  
> -- 
> 2.13.6
>