linux-kernel - Re: [PATCH 4/8] x86/asm: add clear_page

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Message-ID: <15b8e9b1-cc72-cbc3-5cc0-64b02b617a5b@oracle.com>
Date:   Wed, 14 Oct 2020 14:11:20 -0700
From:   Ankur Arora <ankur.a.arora@...cle.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        kirill@...temov.name, mhocko@...nel.org,
        boris.ostrovsky@...cle.com, konrad.wilk@...cle.com,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, Jiri Slaby <jslaby@...e.cz>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
Subject: Re: [PATCH 4/8] x86/asm: add clear_page_nt()

On 2020-10-14 12:56 p.m., Borislav Petkov wrote:
> On Wed, Oct 14, 2020 at 01:32:55AM -0700, Ankur Arora wrote:
>> This can potentially improve page-clearing bandwidth (see below for
>> performance numbers for two microarchitectures where it helps and one
>> where it doesn't) and can help indirectly by consuming less cache
>> resources.
>>
>> Any performance benefits are expected for extents larger than LLC-sized
>> or more -- when we are DRAM-BW constrained rather than cache-BW
>> constrained.
> 
> "potentially", "expected", I don't like those formulations.
That's fair. The reason for those weasel words is mostly because it
is microarchitecture specific.
For example on Intel where I did compare across generations: I see good
performance on Broadwellx, not good on Skylakex and then good again on
some pre-production CPUs.

> Do you have
> some actual benchmark data where this shows any improvement and not
> microbenchmarks only, to warrant the additional complexity?
Yes, guest creation under QEMU (pinned guests) shows similar improvements.
I've posted performance numbers in patches 7, 8 with a simple page-fault
test derived from that.

I can add numbers from QEMU as well.

Thanks,
Ankur

>