linux-kernel - Re: [PATCH] mm: clear 1G pages with streaming stores on x86

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Message-ID: <20200309183704.GA1573@bombadil.infradead.org>
Date:   Mon, 9 Mar 2020 11:37:04 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     Michal Hocko <mhocko@...nel.org>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Cannon Matthews <cannonmatthews@...gle.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Rientjes <rientjes@...gle.com>,
        Greg Thelen <gthelen@...gle.com>,
        Salman Qazi <sqazi@...gle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH] mm: clear 1G pages with streaming stores on x86

On Mon, Mar 09, 2020 at 08:38:31AM -0700, Andi Kleen wrote:
> > Gigantic huge pages are a bit different. They are much less dynamic from
> > the usage POV in my experience. Micro-optimizations for the first access
> > tends to not matter at all as it is usually pre-allocation scenario. On
> > the other hand, speeding up the initialization sounds like a good thing
> > in general. It will be a single time benefit but if the additional code
> > is not hard to maintain then I would be inclined to take it even with
> > "artificial" numbers state above. There really shouldn't be other downsides
> > except for the code maintenance, right?
> 
> There's a cautious tale of the old crappy RAID5 XOR assembler functions which
> were optimized a long time ago for the Pentium1, and stayed around,
> even though the compiler could actually do a better job.
> 
> String instructions are constantly improving in performance (Broadwell is
> very old at this point) Most likely over time (and maybe even today
> on newer CPUs) you would need much more sophisticated unrolled MOVNTI variants
> (or maybe even AVX-*) to be competitive.

Presumably you have access to current and maybe even some unreleased
CPUs ... I mean, he's posted the patches, so you can test this hypothesis.