linux-kernel - Re: [PATCH v3 4/4] x86/folio_zero

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Message-ID: <87sem0qa9e.fsf@oracle.com>
Date: Tue, 22 Apr 2025 12:14:21 -0700
From: Ankur Arora <ankur.a.arora@...cle.com>
To: Raghavendra K T <raghavendra.kt@....com>
Cc: Ingo Molnar <mingo@...nel.org>, Ankur Arora <ankur.a.arora@...cle.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org, x86@...nel.org,
        torvalds@...ux-foundation.org, akpm@...ux-foundation.org, bp@...en8.de,
        dave.hansen@...ux.intel.com, hpa@...or.com, mingo@...hat.com,
        luto@...nel.org, peterz@...radead.org, paulmck@...nel.org,
        rostedt@...dmis.org, tglx@...utronix.de, willy@...radead.org,
        jon.grimm@....com, bharata@....com, boris.ostrovsky@...cle.com,
        konrad.wilk@...cle.com
Subject: Re: [PATCH v3 4/4] x86/folio_zero_user: multi-page clearing


Raghavendra K T <raghavendra.kt@....com> writes:

> On 4/14/2025 12:35 PM, Ingo Molnar wrote:
>> * Ankur Arora <ankur.a.arora@...cle.com> wrote:
>
> [...]
>
>>> Performance for preempt=none|voluntary remains unchanged.
>> CONFIG_PREEMPT_VOLUNTARY=y is the default on a number of major
>> distributions, such as Ubuntu, and a lot of enterprise distro kernels -
>> and this patch does nothing for them, for no good reason.
>> So could you please provide a sensible size granularity cutoff of 16MB
>> or so on non-preemptive kernels, instead of this weird build-time
>> all-or-nothing binary cutoff based on preemption modes?
>>
>
> Agree with Ingo here. We are too harsh on none and voluntary with this
> limit. At least 512KB size?

512KB seems much too small. I was thinking of something larger: 8MB. With
a zeroing BW of 10GBps (typical value I've seen with relatively new
Intel/AMD hardware), that would mean a preemption delay of 0.8ms
for preempt=none/voluntary.

Even with relatively old hardware -- say skylake -- we should see ~5
GBps/1.6ms delay.

> I will try to see if I can get numbers for different sizes that does not
> interfere much with other workloads' perf. (in none/voluntary mode).

Thanks that'll be useful to make the case.

--
ankur