Re: lot of MemAvailable but falling cache and raising PSI

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Hi,

the current status is, that everything works well / fine since i
switched from CONFIG_TRANSPARENT_HUGEPAGE_ALWAYS to
CONFIG_TRANSPARENT_HUGEPAGE_MADVISE
Am 27.09.19 um 14:45 schrieb Vlastimil Babka:
> On 9/19/19 12:21 PM, Stefan Priebe - Profihost AG wrote:
>> Kernel 5.2.14 is now running since exactly 7 days and now we can easaly
>> view a trend i', not sure if i should post graphs.
>>
>> Cache size is continuously shrinking while memfree is rising.
>>
>> While there were 4,5GB free in avg in the beginnen we now have an avg of
>> 8GB free memory.
>>
>> Cache has shrinked from avg 24G to avg 18G.
>>
>> Memory pressure has rised from avg 0% to avg 0.1% - not much but if you
>> look at the graphs it's continuously rising while cache is shrinking and
>> memfree is rising.
> 
> Hi, could you try the patch below? I suspect you're hitting a corner
> case where compaction_suitable() returns COMPACT_SKIPPED for the
> ZONE_DMA, triggering reclaim even if other zones have plenty of free
> memory. And should_continue_reclaim() then returns true until twice the
> requested page size is reclaimed (compact_gap()). That means 4MB
> reclaimed for each THP allocation attempt, which roughly matches the
> trace data you preovided previously.
> 
> The amplification to 4MB should be removed in patches merged for 5.4, so
> it would be only 32 pages reclaimed per THP allocation. The patch below
> tries to remove this corner case completely, and it should be more
> visible on your 5.2.x, so please apply it there.

so i switched back to 4.19 LTS Kernel - as this is the kernel we run on
all our infrastructures. THP is now only in use von kvm host machines.
Your patch applies to 4.19 as well - but not sure if it is a good idea
to apply it to those machines.

Greets,
Stefan




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