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Subject: Re: [PATCH] fs/proc: introduce /proc/stat2 file
To:     Davidlohr Bueso <dave@stgolabs.net>,
        Andrew Morton <akpm@linux-foundation.org>
Cc:     Daniel Colascione <dancol@google.com>,
        linux-fsdevel@vger.kernel.org,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Davidlohr Bueso <dbueso@suse.de>
References: <20181029192521.23059-1-dave@stgolabs.net>
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 <20181106154840.3b448356214afa63dc8cb28c@linux-foundation.org>
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From:   Waiman Long <longman@redhat.com>
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Date:   Wed, 7 Nov 2018 11:31:33 -0500
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On 11/06/2018 10:32 PM, Davidlohr Bueso wrote:
> On Tue, 06 Nov 2018, Andrew Morton wrote:
>
>> It would be interesting to know precisely which stat fields the
>> database-which-shall-not-be-named is looking for.  Then we could cook
>> up a very whizzy way of getting at the info.
>
> The ctxt field, afaik. In any case they have been able to work around
> the bottleneck. I'm not sure if that is the case for Waiman, however.
>

In my case, the customers just complain about the slowdown in reading
/proc/stat on some platforms vs. the others because some had many more
interrupt lines than the others. They didn't specifically call out what
they were looking at.

>>
>> A downside of the stat2 approach is that applications will need to be
>> rebuilt.  And hopefully when people do this, they'll open
>> "/etc/my-app-name/symlink-to-proc-stat" (or use per-application config)
>> so they won't need a rebuild when we add /proc/stat3!
>>
>> A /proc/change-how-stat-works tunable would avoid the need to rebuild
>> applications.  But if a system still has some applications which want
>> the irq info then that doesn't work.
>>
>> It's all very sad, really.
>>
>> btw,
>>
>>> +The stat2 file acts as a performance alternative to /proc/stat for
>>> workloads
>>> +and systems that care and are under heavy irq load. In order to to
>>> be completely
>>> +compatible, /proc/stat and /proc/stat2 are identical with the
>>> exception that the
>>> +later will show 0 for any (hard)irq-related fields. This refers
>>> particularly
>>
>> "latter"
>>
>>> +to the "intr" line and 'irq' column for that aggregate in the cpu
>>> line.
>>
>> btw2, please quantify "poor performance".  That helps us determine how
>> much we care about all of this!
>
> Up to a quarter of a second is what was reported as being spent every
> time
> /proc/stat is used. This is with 1k cores and 4k interrupts.
>
> Thanks,
> Davidlohr

Yes, the time spent will scale more or less linearly with the # of cores
and # of interrupts.

Thanks,
Longman