Re: [Qemu-devel] [v2 0/2] add avx2 instruction optimization

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Michael S. Tsirkin (mst@redhat.com) wrote:
> On Thu, Apr 07, 2016 at 12:09:52PM +0100, Dr. David Alan Gilbert wrote:
> > * Eric Blake (eblake@redhat.com) wrote:
> > > On 11/12/2015 12:56 PM, Dr. David Alan Gilbert wrote:
> > > 
> > > >> One thing I still can't understand, why the unit test in host environment shows
> > > >> 'memcmp()' have better performance?
> > > 
> > > Have you tried running under a profiler, to see if there are hotspots or
> > > at least get an idea of where the time is being spent?
> > > 
> > > > 
> > > > Are you aware of any program other than QEMU that also wants to do something
> > > > similar?  Finding whether a block of memory is zero, sounds like something
> > > > that would be useful in lots of places, I just can't think which ones.
> > > 
> > > At least dd, cp, and probably several other utilities.  It would be nice
> > > to post an RFE to glibc to see if they can come up with a dedicated
> > > interface that is faster than memcmp(), although that still only helps
> > > us when targetting a system new enough to have that interface.
> > 
> > I've just posted that RFE:
> > https://sourceware.org/bugzilla/show_bug.cgi?id=19920
> > 
> > Dave
> 
> Have you guys seen the discussion in
> http://rusty.ozlabs.org/?p=560#respond
> 
> In particular it claims this is close to optimal:
> 
> 
> char check_zero(char *p, int len)
> {
>     char res = 0;
>     int i;
> 
>     for (i = 0; i < len; i++) {
>         res = res | p[i];
>     }
> 
>     return res;
> }
> 
> 
> If you compile this function with --tree-vectorize and --unroll-loops.
> 
> Now, this version always scans all of the buffer, so
> it will be slower when buffer is *not* all-zeroes.
> 
> Which might indicate that you need to know what your
> workload is to implement compare to zero efficiently,
> and if that is the case, it's not clear this is appropriate for libc.

On the contrary; anything that needs a couple of carefully chosen
compiler switches and assumes a particular workload is much
better optimised in a library for the general workload.

Dave

> 
> > > -- 
> > > Eric Blake   eblake redhat com    +1-919-301-3266
> > > Libvirt virtualization library http://libvirt.org
> > > 
> > 
> > 
> > --
> > Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK