linux-kernel - Re: Detecting page cache trashing state

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Date:   Fri, 27 Oct 2017 23:19:02 +0300
From:   "Ruslan Ruslichenko -X (rruslich - GLOBALLOGIC INC at Cisco)" 
        <rruslich@...co.com>
To:     Johannes Weiner <hannes@...xchg.org>
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 Johannes,

On 10/25/2017 08:54 PM, Johannes Weiner wrote:
> 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.
The workingset counters are growing quite actively too. Here are
some numbers per second:

workingset_refault   8201
workingset_activate   389
workingset_restore   187
workingset_nodereclaim   313

> 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.
The system has 2 CPU cores.
> 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.
>
I will test with your patches and get back to you.

Thanks,
Ruslan