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

On Mon, Sep 12, 2016 at 5:02 AM, Michal Hocko [off-list ref] wrote:

On Mon 05-09-16 16:14:06, robert.foss-ZGY8ohtN/8qB+jHODAdFcQ@public.gmane.org wrote:

quoted

From: Robert Foss <robert.foss-ZGY8ohtN/8qB+jHODAdFcQ@public.gmane.org>

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