Re: [PATCH 1/1] Add AVX512 support for xbzrle_encode_buffer function

[Dr. David Alan Gilbert]

* Zhao, Zhou (zhou.zhao@intel.com) wrote:
> Hi dainel:
>   Cause our code depend on intel intrinsics lib implement. And this lib 
> depend on macro like  " AVX512BW ". This macro need compile time check to 
> enable some machine options . if you only use that utility to do runtime 
> check ,you will met compile issue. And also if we want to save cpu time , 
> we'd better check it in compile time.

You need to do *both*:

  a) You need to check at compile time to see if you have the
intrinsics.
  b) You need to check at runtime to see if you're running on a suitable
CPU.

Other things to note (I've not checked the algorithm yet):
  c) The patch needs splitting up into compile checks, the algorithm,
the tests as at least 3 patches.
  d) The test includes a benchmark, we don't need to include a benchmark
program in the code, just something to check it works.
  e) The benchmark is a microbenchmark on the routine; what's it's
effect on the whole migration - is it significant?
  f) xbzrle isn't actually used that much these days, so I'm not sure
generally it's worth it.

Dave

> -----Original Message-----
> From: Daniel P. Berrangé <berrange@redhat.com> 
> Sent: Thursday, July 21, 2022 11:11 PM
> To: Xu, Ling1 <ling1.xu@intel.com>
> Cc: qemu-devel@nongnu.org; quintela@redhat.com; dgilbert@redhat.com; Zhao, 
> Zhou <zhou.zhao@intel.com>; Jin, Jun I <jun.i.jin@intel.com>
> Subject: Re: [PATCH 1/1] Add AVX512 support for xbzrle_encode_buffer function
> 
> On Thu, Jul 21, 2022 at 06:31:47PM +0800, ling xu wrote:
> > This commit adds AVX512 implementation of xbzrle_encode_buffer 
> > function to accelerate xbzrle encoding speed. Compared with C version 
> > of xbzrle_encode_buffer function,
> > AVX512 version can achieve almost 60%-70% performance improvement on unit 
> > test provided by qemu.
> > In addition, we provide one more unit test called 
> > "test_encode_decode_random", in which dirty data are randomly located in 4K 
> > page, and this case can achieve almost 140% performance gain.
> > 
> > Signed-off-by: ling xu <ling1.xu@intel.com>
> > Co-authored-by: Zhou Zhao <zhou.zhao@intel.com>
> > Co-authored-by: Jun Jin <jun.i.jin@intel.com>
> > ---
> >  configure                | 434 ++++++++++++++++++++++++++++++++++++++-
> >  migration/ram.c          |   6 +
> >  migration/xbzrle.c       | 177 ++++++++++++++++
> >  migration/xbzrle.h       |   4 +
> >  tests/unit/test-xbzrle.c | 307 +++++++++++++++++++++++++--
> >  5 files changed, 908 insertions(+), 20 deletions(-)
> 
> > diff --git a/migration/ram.c b/migration/ram.c index 
> > 01f9cc1d72..3b931c325f 100644
> > --- a/migration/ram.c
> > +++ b/migration/ram.c
> > @@ -747,9 +747,15 @@ static int save_xbzrle_page(RAMState *rs, uint8_t 
> > **current_data,
> >      memcpy(XBZRLE.current_buf, *current_data, TARGET_PAGE_SIZE);
> >  
> >      /* XBZRLE encoding (if there is no overflow) */
> > +    #if defined(__x86_64__) && defined(__AVX512BW__)
> > +    encoded_len = xbzrle_encode_buffer_512(prev_cached_page, 
> > XBZRLE.current_buf,
> > +                                       TARGET_PAGE_SIZE, 
> > XBZRLE.encoded_buf,
> > +                                       TARGET_PAGE_SIZE);
> > +    #else
> >      encoded_len = xbzrle_encode_buffer(prev_cached_page, 
> > XBZRLE.current_buf,
> >                                         TARGET_PAGE_SIZE, 
> > XBZRLE.encoded_buf,
> >                                         TARGET_PAGE_SIZE);
> > +    #endif
> 
> Shouldn't we be deciding which impl using a runtime check of the current 
> CPUID, rather than a compile time check ? I'm thinking along the lines of 
> what util/bufferiszero.c does to select different optimized versions based on 
> CPUID. The build host CPU features can't be expected to match the runtime 
> host CPU features.
> 
> 
> With regards,
> Daniel
> -- 
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-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK