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

[Jim Thompson <jim@netgate.com>]

I’m not as concerned with compile times given the potential performance boost.

A long time ago (mid-80s) I was at Convex, and wanted to do a vector bcopy(), because it would make the I/O system (mostly disk then (*)) go faster.
The architect explained to me that the vector registers were for applications, not the kernel (as well as re-explaining the expense of vector context
switches, should the kernel be using the vector unit(s) and some application also wanted to use them.  

The same is true today of AVX/AVX2, SSE, and even the AES-NI instructions.  Normally we don’t use these in kernel code (which is traditionally where
the networking stack has lived).   

The differences with DPDK are that a) entire cores (including the AVX/SSE units and even AES-NI (FPU) are dedicated to DPDK, and b) DPDK is a library,
and the resulting networking applications are exactly that, applications.  The "operating system” is now a control plane.

Jim

(* Back then it was commonly thought that TCP would never be able to fill a 10Gbps Ethernet.)

> On Jan 21, 2015, at 2:54 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
>