linux-kernel - Re: Detecting page cache trashing state

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Date:   Wed, 25 Oct 2017 13:54:24 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     "Ruslan Ruslichenko -X (rruslich - GLOBALLOGIC INC at Cisco)" 
        <rruslich@...co.com>
Cc:     Taras Kondratiuk <takondra@...co.com>,
        Michal Hocko <mhocko@...nel.org>, linux-mm@...ck.org,
        xe-linux-external@...co.com, linux-kernel@...r.kernel.org
Subject: Re: Detecting page cache trashing state

Hi Ruslan,

sorry about the delayed response, I missed the new activity in this
older thread.

On Thu, Sep 28, 2017 at 06:49:07PM +0300, Ruslan Ruslichenko -X (rruslich - GLOBALLOGIC INC at Cisco) wrote:
> Hi Johannes,
> 
> Hopefully I was able to rebase the patch on top v4.9.26 (latest supported
> version by us right now)
> and test a bit.
> The overall idea definitely looks promising, although I have one question on
> usage.
> Will it be able to account the time which processes spend on handling major
> page faults
> (including fs and iowait time) of refaulting page?

That's the main thing it should measure! :)

The lock_page() and wait_on_page_locked() calls are where iowaits
happen on a cache miss. If those are refaults, they'll be counted.

> As we have one big application which code space occupies big amount of place
> in page cache,
> when the system under heavy memory usage will reclaim some of it, the
> application will
> start constantly thrashing. Since it code is placed on squashfs it spends
> whole CPU time
> decompressing the pages and seem memdelay counters are not detecting this
> situation.
> Here are some counters to indicate this:
> 
> 19:02:44        CPU     %user     %nice   %system   %iowait %steal     %idle
> 19:02:45        all      0.00      0.00    100.00      0.00 0.00      0.00
> 
> 19:02:44     pgpgin/s pgpgout/s   fault/s  majflt/s  pgfree/s pgscank/s
> pgscand/s pgsteal/s    %vmeff
> 19:02:45     15284.00      0.00    428.00    352.00  19990.00 0.00      0.00
> 15802.00      0.00
> 
> And as nobody actively allocating memory anymore looks like memdelay
> counters are not
> actively incremented:
> 
> [:~]$ cat /proc/memdelay
> 268035776
> 6.13 5.43 3.58
> 1.90 1.89 1.26

How does it correlate with /proc/vmstat::workingset_activate during
that time? It only counts thrashing time of refaults it can actively
detect.

Btw, how many CPUs does this system have? There is a bug in this
version on how idle time is aggregated across multiple CPUs. The error
compounds with the number of CPUs in the system.

I'm attaching 3 bugfixes that go on top of what you have. There might
be some conflicts, but they should be minor variable naming issues.


View attachment "0001-mm-memdelay-fix-task-flags-race-condition.patch" of type "text/x-diff" (2548 bytes)

View attachment "0002-mm-memdelay-idle-time-is-not-productive-time.patch" of type "text/x-diff" (1796 bytes)

View attachment "0003-mm-memdelay-drop-IO-as-productive-time.patch" of type "text/x-diff" (1452 bytes)