Re: [dpdk-dev] [RFC PATCH 0/7] support multi-phtread per lcore

["Walukiewicz, Miroslaw" <Miroslaw.Walukiewicz@intel.com>]

Thank you Cunming for explanation. 

What about DPDK timers? They also depend on rte_lcore_id() to avoid spinlocks. 

Mirek

> -----Original Message-----
> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Cunming Liang
> Sent: Thursday, December 11, 2014 3:05 AM
> To: dev@dpdk.org
> Subject: [dpdk-dev] [RFC PATCH 0/7] support multi-phtread per lcore
> 
> 
> Scope & Usage Scenario
> ========================
> 
> DPDK usually pin pthread per core to avoid task switch overhead. It gains
> performance a lot, but it's not efficient in all cases. In some cases, it may
> too expensive to use the whole core for a lightweight workload. It's a
> reasonable demand to have multiple threads per core and each threads
> share CPU
> in an assigned weight.
> 
> In fact, nothing avoid user to create normal pthread and using cgroup to
> control the CPU share. One of the purpose for the patchset is to clean the
> gaps of using more DPDK libraries in the normal pthread. In addition, it
> demonstrates performance gain by proactive 'yield' when doing idle loop
> in packet IO. It also provides several 'rte_pthread_*' APIs to easy life.
> 
> 
> Changes to DPDK libraries
> ==========================
> 
> Some of DPDK libraries must run in DPDK environment.
> 
> # rte_mempool
> 
> In rte_mempool doc, it mentions a thread not created by EAL must not use
> mempools. The root cause is it uses a per-lcore cache inside mempool.
> And 'rte_lcore_id()' will not return a correct value.
> 
> The patchset changes this a little. The index of mempool cache won't be a
> lcore_id. Instead of it, using a linear number generated by the allocator.
> For those legacy EAL per-lcore thread, it apply for an unique linear id
> during creation. For those normal pthread expecting to use rte_mempool, it
> requires to apply for a linear id explicitly. Now the mempool cache looks like
> a per-thread base. The linear ID actually identify for the linear thread id.
> 
> However, there's another problem. The rte_mempool is not preemptable.
> The
> problem comes from rte_ring, so talk together in next section.
> 
> # rte_ring
> 
> rte_ring supports multi-producer enqueue and multi-consumer dequeue.
> But it's
> not preemptable. There's conversation talking about this before.
> http://dpdk.org/ml/archives/dev/2013-November/000714.html
> 
> Let's say there's two pthreads running on the same core doing enqueue on
> the
> same rte_ring. If the 1st pthread is preempted by the 2nd pthread while it
> has
> already modified the prod.head, the 2nd pthread will spin until the 1st one
> scheduled agian. It causes time wasting. In addition, if the 2nd pthread has
> absolutely higer priority, it's more terrible.
> 
> But it doesn't means we can't use. Just need to narrow down the situation
> when
> it's used by multi-pthread on the same core.
> - It CAN be used for any single-producer or single-consumer situation.
> - It MAY be used by multi-producer/consumer pthread whose scheduling
> policy
> are all SCHED_OTHER(cfs). User SHOULD aware of the performance penalty
> befor
> using it.
> - It MUST not be used by multi-producer/consumer pthread, while some of
> their
> scheduling policies is SCHED_FIFO or SCHED_RR.
> 
> 
> Performance
> ==============
> 
> It loses performance by introducing task switching. On packet IO perspective,
> we can gain some back by improving IO effective rate. When the pthread do
> idle
> loop on an empty rx queue, it should proactively yield. We can also slow
> down
> rx for a bit while to take more advantage of the bulk receiving in the next
> loop. In practice, increase the rx ring size also helps to improve the overrall
> throughput.
> 
> 
> Cgroup Control
> ================
> 
> Here's a simple example, there's four pthread doing packet IO on the same
> core.
> We expect the CPU share rate is 1:1:2:4.
> > mkdir /sys/fs/cgroup/cpu/dpdk
> > mkdir /sys/fs/cgroup/cpu/dpdk/thread0
> > mkdir /sys/fs/cgroup/cpu/dpdk/thread1
> > mkdir /sys/fs/cgroup/cpu/dpdk/thread2
> > mkdir /sys/fs/cgroup/cpu/dpdk/thread3
> > cd /sys/fs/cgroup/cpu/dpdk
> > echo 256 > thread0/cpu.shares
> > echo 256 > thread1/cpu.shares
> > echo 512 > thread2/cpu.shares
> > echo 1024 > thread3/cpu.shares
> 
> 
> -END-
> 
> Any comments are welcome.
> 
> Thanks
> 
> *** BLURB HERE ***
> 
> Cunming Liang (7):
>   eal: add linear thread id as pthread-local variable
>   mempool: use linear-tid as mempool cache index
>   ring: use linear-tid as ring debug stats index
>   eal: add simple API for multi-pthread
>   testpmd: support multi-pthread mode
>   sample: add new sample for multi-pthread
>   eal: macro for cpuset w/ or w/o CPU_ALLOC
> 
>  app/test-pmd/cmdline.c                    |  41 +++++
>  app/test-pmd/testpmd.c                    |  84 ++++++++-
>  app/test-pmd/testpmd.h                    |   1 +
>  config/common_linuxapp                    |   1 +
>  examples/multi-pthread/Makefile           |  57 ++++++
>  examples/multi-pthread/main.c             | 232 ++++++++++++++++++++++++
>  examples/multi-pthread/main.h             |  46 +++++
>  lib/librte_eal/common/include/rte_eal.h   |  15 ++
>  lib/librte_eal/common/include/rte_lcore.h |  12 ++
>  lib/librte_eal/linuxapp/eal/eal_thread.c  | 282
> +++++++++++++++++++++++++++---
>  lib/librte_mempool/rte_mempool.h          |  22 +--
>  lib/librte_ring/rte_ring.h                |   6 +-
>  12 files changed, 755 insertions(+), 44 deletions(-)
>  create mode 100644 examples/multi-pthread/Makefile
>  create mode 100644 examples/multi-pthread/main.c
>  create mode 100644 examples/multi-pthread/main.h
> 
> --
> 1.8.1.4