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

["Liang, Cunming" <cunming.liang@intel.com>]

Thanks Mirek. That's a good point which wasn't mentioned in cover letter.
For 'rte_timer', I only expect it be used within the 'legacy per-lcore' pthread.
I'm appreciate if you can give me some cases which can't use it to fit.
In case have to use 'rte_timer' in multi-pthread, there are some prerequisites and limitations.
1. Make sure thread local variable 'lcore_id' is set correctly (e.g. do pthread init by rte_pthread_prepare)
2. As 'rte_timer' is not preemptable, when using rte_timer_manager/reset in multi-pthread, make sure they're not on the same core.

-Cunming

> -----Original Message-----
> From: Walukiewicz, Miroslaw
> Sent: Thursday, December 11, 2014 5:57 PM
> To: Liang, Cunming; dev@dpdk.org
> Subject: RE: [dpdk-dev] [RFC PATCH 0/7] support multi-phtread per lcore
> 
> 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