Re: [dpdk-dev] [PATCH v8 1/3] eal/x86: run-time dispatch over memcpy

[Thomas Monjalon <thomas@monjalon.net>]

19/10/2017 10:50, Li, Xiaoyun:
> 
> > -----Original Message-----
> > From: Thomas Monjalon [mailto:thomas@monjalon.net]
> > Sent: Thursday, October 19, 2017 16:34
> > To: Li, Xiaoyun <xiaoyun.li@intel.com>
> > Cc: Ananyev, Konstantin <konstantin.ananyev@intel.com>; Richardson,
> > Bruce <bruce.richardson@intel.com>; dev@dpdk.org; Lu, Wenzhuo
> > <wenzhuo.lu@intel.com>; Zhang, Helin <helin.zhang@intel.com>;
> > ophirmu@mellanox.com
> > Subject: Re: [dpdk-dev] [PATCH v8 1/3] eal/x86: run-time dispatch over
> > memcpy
> > 
> > 19/10/2017 09:51, Li, Xiaoyun:
> > > From: Thomas Monjalon [mailto:thomas@monjalon.net]
> > > > 19/10/2017 04:45, Li, Xiaoyun:
> > > > > Hi
> > > > > > > >
> > > > > > > > The significant change of this patch is to call a function
> > > > > > > > pointer for packet size > 128 (RTE_X86_MEMCPY_THRESH).
> > > > > > > The perf drop is due to function call replacing inline.
> > > > > > >
> > > > > > > > Please could you provide some benchmark numbers?
> > > > > > > I ran memcpy_perf_test which would show the time cost of
> > > > > > > memcpy. I ran it on broadwell with sse and avx2.
> > > > > > > But I just draw pictures and looked at the trend not computed
> > > > > > > the exact percentage. Sorry about that.
> > > > > > > The picture shows results of copy size of 2, 4, 6, 8, 9, 12,
> > > > > > > 16, 32, 64, 128, 192, 256, 320, 384, 448, 512, 768, 1024,
> > > > > > > 1518, 1522, 1536, 1600, 2048, 2560, 3072, 3584, 4096, 4608,
> > > > > > > 5120, 5632, 6144, 6656, 7168,
> > > > > > 7680, 8192.
> > > > > > > In my test, the size grows, the drop degrades. (Using copy
> > > > > > > time indicates the
> > > > > > > perf.) From the trend picture, when the size is smaller than
> > > > > > > 128 bytes, the perf drops a lot, almost 50%. And above 128
> > > > > > > bytes, it approaches the original dpdk.
> > > > > > > I computed it right now, it shows that when greater than 128
> > > > > > > bytes and smaller than 1024 bytes, the perf drops about 15%.
> > > > > > > When above
> > > > > > > 1024 bytes, the perf drops about 4%.
> > > > > > >
> > > > > > > > From a test done at Mellanox, there might be a performance
> > > > > > > > degradation of about 15% in testpmd txonly with AVX2.
> > > > > >
> > > > >
> > > > > I did tests on X710, XXV710, X540 and MT27710 but didn't see
> > > > performance degradation.
> > > > >
> > > > > I used command "./x86_64-native-linuxapp-gcc/app/testpmd -c 0xf -n
> > > > > 4 -- -
> > > > I" and set fwd txonly.
> > > > > I tested it on v17.11-rc1, then revert my patch and tested it again.
> > > > > Show port stats all and see the throughput pps. But the results
> > > > > are similar
> > > > and no drop.
> > > > >
> > > > > Did I miss something?
> > > >
> > > > I do not understand. Yesterday you confirmed a 15% drop with buffers
> > > > between
> > > > 128 and 1024 bytes.
> > > > But you do not see this drop in your txonly tests, right?
> > > >
> > > Yes. The drop is using test.
> > > Using command "make test -j" and then " ./build/app/test -c f -n 4 "
> > > Then run "memcpy_perf_autotest"
> > > The results are the cycles that memory copy costs.
> > > But I just use it to show the trend because I heard that it's not
> > recommended to use micro benchmarks like test_memcpy_perf for memcpy
> > performance report as they aren't likely able to reflect performance of real
> > world applications.
> > 
> > Yes real applications can hide the memcpy cost.
> > Sometimes, the cost appear for real :)
> > 
> > > Details can be seen at
> > > https://software.intel.com/en-us/articles/performance-optimization-of-
> > > memcpy-in-dpdk
> > >
> > > And I didn't see drop in testpmd txonly test. Maybe it's because not a lot
> > memcpy calls.
> > 
> > It has been seen in a mlx4 use-case using more memcpy.
> > I think 15% in micro-benchmark is too much.
> > What can we do? Raise the threshold?
> > 
> I think so. If there is big drop, can try raise the threshold. Maybe 1024? but not sure.
> But I didn't reproduce the 15% drop on mellanox and not sure how to verify it.

I think we should focus on micro-benchmark and find a reasonnable threshold
for a reasonnable drop tradeoff.