Re: lot of MemAvailable but falling cache and raising PSI

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On 9/30/19 8:56 AM, Stefan Priebe - Profihost AG wrote:
> Hi,
> 
> the current status is, that everything works well / fine since i
> switched from CONFIG_TRANSPARENT_HUGEPAGE_ALWAYS to
> CONFIG_TRANSPARENT_HUGEPAGE_MADVISE

Thanks, that indeed confirms the problem is related to THP's.

> Am 27.09.19 um 14:45 schrieb Vlastimil Babka:
>> On 9/19/19 12:21 PM, Stefan Priebe - Profihost AG wrote:
>>
>> 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.

If you could try that, it would be great (and switch back hugepages to
always after applying). The problem is older than 4.19.

> Greets,
> Stefan
> 





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