Re: [dpdk-dev] [PATCH 0/4] DPDK memcpy optimization

[Stephen Hemminger <stephen@networkplumber.org>]

On Wed, 21 Jan 2015 13:26:20 +0000
Bruce Richardson <bruce.richardson@intel.com> wrote:

> On Wed, Jan 21, 2015 at 02:21:25PM +0100, Marc Sune wrote:
> > 
> > On 21/01/15 14:02, Bruce Richardson wrote:
> > >On Wed, Jan 21, 2015 at 01:36:41PM +0100, Marc Sune wrote:
> > >>On 21/01/15 04:44, Wang, Zhihong wrote:
> > >>>>-----Original Message-----
> > >>>>From: Richardson, Bruce
> > >>>>Sent: Wednesday, January 21, 2015 12:15 AM
> > >>>>To: Neil Horman
> > >>>>Cc: Wang, Zhihong; dev@dpdk.org
> > >>>>Subject: Re: [dpdk-dev] [PATCH 0/4] DPDK memcpy optimization
> > >>>>
> > >>>>On Tue, Jan 20, 2015 at 10:11:18AM -0500, Neil Horman wrote:
> > >>>>>On Tue, Jan 20, 2015 at 03:01:44AM +0000, Wang, Zhihong wrote:
> > >>>>>>>-----Original Message-----
> > >>>>>>>From: Neil Horman [mailto:nhorman@tuxdriver.com]
> > >>>>>>>Sent: Monday, January 19, 2015 9:02 PM
> > >>>>>>>To: Wang, Zhihong
> > >>>>>>>Cc: dev@dpdk.org
> > >>>>>>>Subject: Re: [dpdk-dev] [PATCH 0/4] DPDK memcpy optimization
> > >>>>>>>
> > >>>>>>>On Mon, Jan 19, 2015 at 09:53:30AM +0800, zhihong.wang@intel.com
> > >>>>wrote:
> > >>>>>>>>This patch set optimizes memcpy for DPDK for both SSE and AVX
> > >>>>platforms.
> > >>>>>>>>It also extends memcpy test coverage with unaligned cases and
> > >>>>>>>>more test
> > >>>>>>>points.
> > >>>>>>>>Optimization techniques are summarized below:
> > >>>>>>>>
> > >>>>>>>>1. Utilize full cache bandwidth
> > >>>>>>>>
> > >>>>>>>>2. Enforce aligned stores
> > >>>>>>>>
> > >>>>>>>>3. Apply load address alignment based on architecture features
> > >>>>>>>>
> > >>>>>>>>4. Make load/store address available as early as possible
> > >>>>>>>>
> > >>>>>>>>5. General optimization techniques like inlining, branch
> > >>>>>>>>reducing, prefetch pattern access
> > >>>>>>>>
> > >>>>>>>>Zhihong Wang (4):
> > >>>>>>>>   Disabled VTA for memcpy test in app/test/Makefile
> > >>>>>>>>   Removed unnecessary test cases in test_memcpy.c
> > >>>>>>>>   Extended test coverage in test_memcpy_perf.c
> > >>>>>>>>   Optimized memcpy in arch/x86/rte_memcpy.h for both SSE and AVX
> > >>>>>>>>     platforms
> > >>>>>>>>
> > >>>>>>>>  app/test/Makefile                                  |   6 +
> > >>>>>>>>  app/test/test_memcpy.c                             |  52 +-
> > >>>>>>>>  app/test/test_memcpy_perf.c                        | 238 +++++---
> > >>>>>>>>  .../common/include/arch/x86/rte_memcpy.h           | 664
> > >>>>>>>+++++++++++++++------
> > >>>>>>>>  4 files changed, 656 insertions(+), 304 deletions(-)
> > >>>>>>>>
> > >>>>>>>>--
> > >>>>>>>>1.9.3
> > >>>>>>>>
> > >>>>>>>>
> > >>>>>>>Are you able to compile this with gcc 4.9.2?  The compilation of
> > >>>>>>>test_memcpy_perf is taking forever for me.  It appears hung.
> > >>>>>>>Neil
> > >>>>>>Neil,
> > >>>>>>
> > >>>>>>Thanks for reporting this!
> > >>>>>>It should compile but will take quite some time if the CPU doesn't support
> > >>>>AVX2, the reason is that:
> > >>>>>>1. The SSE & AVX memcpy implementation is more complicated than
> > >>>>AVX2
> > >>>>>>version thus the compiler takes more time to compile and optimize 2.
> > >>>>>>The new test_memcpy_perf.c contains 126 constants memcpy calls for
> > >>>>>>better test case coverage, that's quite a lot
> > >>>>>>
> > >>>>>>I've just tested this patch on an Ivy Bridge machine with GCC 4.9.2:
> > >>>>>>1. The whole compile process takes 9'41" with the original
> > >>>>>>test_memcpy_perf.c (63 + 63 = 126 constant memcpy calls) 2. It takes
> > >>>>>>only 2'41" after I reduce the constant memcpy call number to 12 + 12
> > >>>>>>= 24
> > >>>>>>
> > >>>>>>I'll reduce memcpy call in the next version of patch.
> > >>>>>>
> > >>>>>ok, thank you.  I'm all for optimzation, but I think a compile that
> > >>>>>takes almost
> > >>>>>10 minutes for a single file is going to generate some raised eyebrows
> > >>>>>when end users start tinkering with it
> > >>>>>
> > >>>>>Neil
> > >>>>>
> > >>>>>>Zhihong (John)
> > >>>>>>
> > >>>>Even two minutes is a very long time to compile, IMHO. The whole of DPDK
> > >>>>doesn't take that long to compile right now, and that's with a couple of huge
> > >>>>header files with routing tables in it. Any chance you could cut compile time
> > >>>>down to a few seconds while still having reasonable tests?
> > >>>>Also, when there is AVX2 present on the system, what is the compile time
> > >>>>like for that code?
> > >>>>
> > >>>>	/Bruce
> > >>>Neil, Bruce,
> > >>>
> > >>>Some data first.
> > >>>
> > >>>Sandy Bridge without AVX2:
> > >>>1. original w/ 10 constant memcpy: 2'25"
> > >>>2. patch w/ 12 constant memcpy: 2'41"
> > >>>3. patch w/ 63 constant memcpy: 9'41"
> > >>>
> > >>>Haswell with AVX2:
> > >>>1. original w/ 10 constant memcpy: 1'57"
> > >>>2. patch w/ 12 constant memcpy: 1'56"
> > >>>3. patch w/ 63 constant memcpy: 3'16"
> > >>>
> > >>>Also, to address Bruce's question, we have to reduce test case to cut down compile time. Because we use:
> > >>>1. intrinsics instead of assembly for better flexibility and can utilize more compiler optimization
> > >>>2. complex function body for better performance
> > >>>3. inlining
> > >>>This increases compile time.
> > >>>But I think it'd be okay to do that as long as we can select a fair set of test points.
> > >>>
> > >>>It'd be great if you could give some suggestion, say, 12 points.
> > >>>
> > >>>Zhihong (John)
> > >>>
> > >>>
> > >>While I agree in the general case these long compilation times is painful
> > >>for the users, having a factor of 2-8x in memcpy operations is quite an
> > >>improvement, specially in DPDK applications which need to deal
> > >>(unfortunately) heavily on them -- e.g. IP fragmentation and reassembly.
> > >>
> > >>Why not having a fast compilation by default, and a tunable config flag to
> > >>enable a highly optimized version of rte_memcpy (e.g. RTE_EAL_OPT_MEMCPY)?
> > >>
> > >>Marc
> > >>
> > >Out of interest, are these 2-8x improvements something you have benchmarked
> > >in these app scenarios? [i.e. not just in micro-benchmarks].
> > 
> > How much that micro-speedup will end up affecting the performance of the
> > entire application is something I cannot say, so I agree that we should
> > probably have some additional benchmarks before deciding that pays off
> > maintaining 2 versions of rte_memcpy.
> > 
> > There are however a bunch of possible DPDK applications that could
> > potentially benefit; IP fragmentation, tunneling and specialized DPI
> > applications, among others, since they involve a reasonable amount of
> > memcpys per pkt. My point was, *if* it proves that is enough beneficial, why
> > not having it optionally?
> > 
> > Marc
> 
> I agree, if it provides the speedups then we need to have it in - and quite possibly
> on by default, even.
> 
> /Bruce

One issue I have is that as a vendor we need to ship on binary, not different distributions
for each Intel chip variant. There is some support for multi-chip version functions
but only in latest Gcc which isn't in Debian stable. And the multi-chip version
of functions is going to be more expensive than inlining. For some cases, I have
seen that the overhead of fancy instructions looks good but have nasty side effects
like CPU stall and/or increased power consumption which turns of turbo boost.


Distro's in general have the same problem with special case optimizations.