linux-kernel - Re: [PATCH v5 0/3] mm, proc: Implement /proc/<pid>/totmaps

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Date:   Mon, 12 Sep 2016 14:02:48 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     robert.foss@...labora.com
Cc:     corbet@....net, akpm@...ux-foundation.org, vbabka@...e.cz,
        hughd@...gle.com, koct9i@...il.com, n-horiguchi@...jp.nec.com,
        kirill.shutemov@...ux.intel.com, john.stultz@...aro.org,
        minchan@...nel.org, ross.zwisler@...ux.intel.com,
        jmarchan@...hat.com, hannes@...xchg.org, keescook@...omium.org,
        oleg@...hat.com, viro@...iv.linux.org.uk, mguzik@...hat.com,
        jdanis@...gle.com, calvinowens@...com, adobriyan@...il.com,
        ebiederm@...ssion.com, sonnyrao@...omium.org,
        seth.forshee@...onical.com, tixxdz@...il.com,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        Ben Zhang <benzh@...omium.org>,
        Bryan Freed <bfreed@...omium.org>,
        Filipe Brandenburger <filbranden@...omium.org>,
        Jann Horn <jann@...jh.net>, linux-api@...r.kernel.org,
        Jacek Anaszewski <j.anaszewski@...sung.com>
Subject: Re: [PATCH v5 0/3] mm, proc: Implement /proc/<pid>/totmaps

On Mon 05-09-16 16:14:06, robert.foss@...labora.com wrote:
> From: Robert Foss <robert.foss@...labora.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.

-- 
Michal Hocko
SUSE Labs