Re: Caching/buffers become useless after some time

[Vlastimil Babka <vbabka@suse.cz>]

On 08/22/2018 10:02 PM, Marinko Catovic wrote:
>> It might be also interesting to do in the problematic state, instead of
>> dropping caches:
>>
>> - save snapshot of /proc/vmstat and /proc/pagetypeinfo
>> - echo 1 > /proc/sys/vm/compact_memory
>> - save new snapshot of /proc/vmstat and /proc/pagetypeinfo
> 
> There was just a worstcase in progress, about 100MB/10GB were used,
> super-low perfomance, but could not see any improvement there after echo 1,
> I watches this for about 3 minutes, the cache usage did not change.
> 
> pagetypeinfo before echo https://pastebin.com/MjSgiMRL
> pagetypeinfo 3min after echo https://pastebin.com/uWM6xGDd
> 
> vmstat before echo https://pastebin.com/TjYSKNdE
> vmstat 3min after echo https://pastebin.com/MqTibEKi

OK, that confirms compaction is useless here. Thanks.

It also shows that all orders except order-9 are in fact plentiful.
Michal's earlier summary of the trace shows that most allocations are up
to order-3 and should be fine, the exception is THP:

    277 9 GFP_TRANSHUGE|__GFP_THISNODE

Hmm it's actually interesting to see GFP_TRANSHUGE there and not
GFP_TRANSHUGE_LIGHT. What's your thp defrag setting? (cat
/sys/kernel/mm/transparent_hugepage/enabled). Maybe it's set to
"always", or there's a heavily faulting process that's using
madvise(MADV_HUGEPAGE). If that's the case, setting it to "defer" or
even "never" could be a workaround.