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Subject: Re: [PATCH v5 0/3] mm, proc: Implement /proc/<pid>/totmaps
To: Michal Hocko <mhocko@kernel.org>
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On Mon, Sep 12, 2016 at 5:02 AM, Michal Hocko <mhocko@kernel.org> wrote:
> On Mon 05-09-16 16:14:06, robert.foss@collabora.com wrote:
>> From: Robert Foss <robert.foss@collabora.com>
>>
>> This series provides the /proc/PID/totmaps feature, which
>> summarizes the information provided by /proc/PID/smaps for
>> improved performance and usability reasons.
>>
>> A use case is to speed up monitoring of memory consumption in
>> environments where RSS isn't precise.
>>
>> For example Chrome tends to many processes which have hundreds of VMAs
>> with a substantial amount of shared memory, and the error of using
>> RSS rather than PSS tends to be very large when looking at overall
>> memory consumption.  PSS isn't kept as a single number that's exported
>> like RSS, so to calculate PSS means having to parse a very large smaps
>> file.
>>
>> This process is slow and has to be repeated for many processes, and we
>> found that the just act of doing the parsing was taking up a
>> significant amount of CPU time, so this patch is an attempt to make
>> that process cheaper.
>
> I still maintain my concerns about a single pss value. It might work in
> a very specific situations where the consumer knows what is shared but
> other than that the value can be more misleading than helpful. So a NACK
> from me until I am shown that this is usable in general and still
> helpful.

I know you think Pss isn't useful in general (though I'll point out
two other independent people said they found it useful) but how about
the other fields like Swap, Private_Dirty and Private_Shared?

If we removed Pss would you still NACK it?

>
> --
> Michal Hocko
> SUSE Labs