Re: [PATCH] proc.5: Document inaccurate RSS due to SPLIT_RSS_COUNTING

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On 10/27/20 11:35 AM, Jann Horn wrote:
> On Tue, Oct 27, 2020 at 8:05 AM Michael Kerrisk (man-pages)
> <mtk.manpages@xxxxxxxxx> wrote:
>> On 10/12/20 4:52 PM, Jann Horn wrote:
>>> On Mon, Oct 12, 2020 at 1:49 PM Jann Horn <jannh@xxxxxxxxxx> wrote:
>>>> Since 34e55232e59f7b19050267a05ff1226e5cd122a5 (introduced back in
>>>> v2.6.34), Linux uses per-thread RSS counters to reduce cache contention on
>>>> the per-mm counters. With a 4K page size, that means that you can end up
>>>> with the counters off by up to 252KiB per thread.
>>>
>>> Actually, as Mark Mossberg pointed out to me off-thread, the counters
>>> can actually be off by many times more...
>>
>> So, does your patch to proc.5 need tweaking, or can I just apply as is?
> 
> The "(up to 63 pages per thread)" would have to go, the rest should be correct.
> 
> But as Michal said, if someone volunteers to get rid of this
> optimization, maybe we don't need the documentation after all? But
> that would probably require actually doing some careful
> heavily-multithreaded benchmarking on a big machine with a few dozen
> cores, or something like that, so that we know whether this
> optimization actually is unimportant enough that we can just get rid
> of it...

Okay -- in the meantime, I've applied your patch, with your 
suggested edit, to remove "(up to 63 pages per thread)".

Thanks!

Cheers,

Michael



-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/




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