Re: Detecting page cache trashing state

[Zdenek Kabelac <zkabelac@redhat.com>]

Dne 15.9.2017 v 02:16 Taras Kondratiuk napsal(a):
> Hi
> 
> In our devices under low memory conditions we often get into a trashing
> state when system spends most of the time re-reading pages of .text
> sections from a file system (squashfs in our case). Working set doesn't
> fit into available page cache, so it is expected. The issue is that
> OOM killer doesn't get triggered because there is still memory for
> reclaiming. System may stuck in this state for a quite some time and
> usually dies because of watchdogs.
> 
> We are trying to detect such trashing state early to take some
> preventive actions. It should be a pretty common issue, but for now we
> haven't find any existing VM/IO statistics that can reliably detect such
> state.
> 
> Most of metrics provide absolute values: number/rate of page faults,
> rate of IO operations, number of stolen pages, etc. For a specific
> device configuration we can determine threshold values for those
> parameters that will detect trashing state, but it is not feasible for
> hundreds of device configurations.
> 
> We are looking for some relative metric like "percent of CPU time spent
> handling major page faults". With such relative metric we could use a
> common threshold across all devices. For now we have added such metric
> to /proc/stat in our kernel, but we would like to find some mechanism
> available in upstream kernel.
> 
> Has somebody faced similar issue? How are you solving it?
> 
Hi

Well I witness this when running Firefox & Thunderbird on my desktop for a 
while on just 4G RAM machine till these 2app eat all free RAM...

It gets to the position (when I open new tab) that mouse hardly moves - 
kswapd eats  CPU  (I've no swap in fact - so likely just page-caching).

The only 'quick' solution for me as desktop user is to manually invoke OOM
with SYSRQ+F key -  and I'm also wondering why the system is not reacting 
better.  In most cases it kills one of those 2 - but sometime it kills whole 
Xsession...


Regards

Zdenek

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