Re: Detecting page cache trashing state

[Ruslan Ruslichenko -X (rruslich - GLOBALLOGIC INC at Cisco)]

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