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Re: Prefetches in buffer_zero_*

From: Dr. David Alan Gilbert
Subject: Re: Prefetches in buffer_zero_*
Date: Thu, 22 Jul 2021 20:13:57 +0100
User-agent: Mutt/2.0.7 (2021-05-04)

* 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.

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

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