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

["EDMISON, Kelvin (Kelvin)" <kelvin.edmison@alcatel-lucent.com>]

On 2015-01-21, 3:54 PM, "Neil Horman" <nhorman@tuxdriver.com> wrote:

>On Wed, Jan 21, 2015 at 11:49:47AM -0800, Stephen Hemminger wrote:
>> 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.
>> 
>What we really need is to do something like borrow the alternatives
>mechanism
>from the kernel so that we can dynamically replace instructions at run
>time
>based on cpu flags.  That way we could make the choice at run time, and
>wouldn't
>have to do alot of special case jumping about.
>Neil

+1.  

I think it should be an anti-requirement that the build machine be the
exact same chip as the deployment platform.

I like the cpu flag inspection approach.  It would help in the case where
DPDK is in a VM and an odd set of CPU flags have been exposed.

If that approach doesn't work though, then perhaps DPDK memcpy could go
through a benchmarking at app startup time and select the most performant
option out of a set, like mdraid's raid6 implementation does.  To give an
example, this is what my systems print out at boot time re: raid6
algorithm selection.
raid6: sse2x1    3171 MB/s
raid6: sse2x2    3925 MB/s
raid6: sse2x4    4523 MB/s
raid6: using algorithm sse2x4 (4523 MB/s)

Regards,
   Kelvin