On Thu, Jul 22, 2021 at 3:14 PM Dr. David Alan Gilbert <dgilbert@redhat.com>
wrote:

> * Richard Henderson (richard.henderson@linaro.org) wrote:
> > On 7/22/21 12:02 AM, Dr. David Alan Gilbert wrote:
> > > Hi Richard,
> > >    I think you were the last person to fiddle with the prefetching
> > > in buffer_zero_avx2 and friends; Joe (cc'd) wondered if explicit
> > > prefetching still made sense on modern CPUs, and that their hardware
> > > generally figures stuff out better on simple increments.
> > >
> > >    What was your thinking on this, and did you actually measure
> > > any improvement?
> >
> > Ah, well, that was 5 years ago so I have no particular memory of this.
> It
> > wouldn't surprise me if you can't measure any improvement on modern
> > hardware.
> >
> > Do you now measure an improvement with the prefetches gone?
>
> Not tried, it just came from Joe's suggestion that it was generally a
> bad idea these days; I do remember that the behaviour of those functions
> is quite tricky because there performance is VERY data dependent - many
> VMs actually have pages that are quite dirty so you never iterate the
> loop, but then you hit others with big zero pages and you spend your
> entire life in the loop.
>
>
Dave, Richard:
My curiosity got the best of me.  So I created a small test program that
used the buffer_zero_avx2() routine from qemu's bufferiszero.c.

When I run it on an Intel Cascade Lake processor, the cost of calling
"__builtin_prefetch(p)" is in the noise range .  It's always "just
slightly" slower.  I doubt it could ever be measured in qemu.

Ironically, when I disabled the hardware prefetchers, the program slowed
down over 33%.  And the call to "__builtin_prefetch(p)" actually hurt
performance by over 3%.

My results are below, (only with the hardware prefetchers enabled).  The
program is attached.
Joe

# gcc -mavx buffer_zero_avx.c -O -DDO_PREFETCH ; for i in {1..5}; do
./a.out; done
TSC 356144 Kcycles.
TSC 356714 Kcycles.
TSC 356707 Kcycles.
TSC 356565 Kcycles.
TSC 356853 Kcycles.
# gcc -mavx buffer_zero_avx.c -O ; for i in {1..5}; do ./a.out; done
TSC 355520 Kcycles.
TSC 355961 Kcycles.
TSC 355872 Kcycles.
TSC 355948 Kcycles.
TSC 355918 Kcycles.

Dave
> >
> > r~
> >
> --
> Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
>
>