Re: lot of MemAvailable but falling cache and raising PSI

[Stefan Priebe - Profihost AG <s.priebe@profihost.ag>]

Am 10.09.19 um 11:02 schrieb Michal Hocko:
> On Tue 10-09-19 10:38:25, Stefan Priebe - Profihost AG wrote:
>> Am 10.09.19 um 10:29 schrieb Michal Hocko:
>>> On Tue 10-09-19 07:56:36, Stefan Priebe - Profihost AG wrote:
>>>>
>>>> Am 09.09.19 um 14:56 schrieb Stefan Priebe - Profihost AG:
>>>>> Am 09.09.19 um 14:49 schrieb Michal Hocko:
>>>>>> On Mon 09-09-19 14:37:52, Stefan Priebe - Profihost AG wrote:
>>>>>>>
>>>>>>> Am 09.09.19 um 14:28 schrieb Michal Hocko:
>>>>>>>> On Mon 09-09-19 14:10:02, Stefan Priebe - Profihost AG wrote:
>>>>>>>>>
>>>>>>>>> Am 09.09.19 um 14:08 schrieb Michal Hocko:
>>>>>>>>>> On Mon 09-09-19 13:01:36, Michal Hocko wrote:
>>>>>>>>>>> and that matches moments when we reclaimed memory. There seems to be a
>>>>>>>>>>> steady THP allocations flow so maybe this is a source of the direct
>>>>>>>>>>> reclaim?
>>>>>>>>>>
>>>>>>>>>> I was thinking about this some more and THP being a source of reclaim
>>>>>>>>>> sounds quite unlikely. At least in a default configuration because we
>>>>>>>>>> shouldn't do anything expensinve in the #PF path. But there might be a
>>>>>>>>>> difference source of high order (!costly) allocations. Could you check
>>>>>>>>>> how many allocation requests like that you have on your system?
>>>>>>>>>>
>>>>>>>>>> mount -t debugfs none /debug
>>>>>>>>>> echo "order > 0" > /debug/tracing/events/kmem/mm_page_alloc/filter
>>>>>>>>>> echo 1 > /debug/tracing/events/kmem/mm_page_alloc/enable
>>>>>>>>>> cat /debug/tracing/trace_pipe > $file
>>>>>>>>
>>>>>>>> echo 1 > /debug/tracing/events/vmscan/mm_vmscan_direct_reclaim_begin/enable
>>>>>>>> echo 1 > /debug/tracing/events/vmscan/mm_vmscan_direct_reclaim_end/enable
>>>>>>>>  
>>>>>>>> might tell us something as well but it might turn out that it just still
>>>>>>>> doesn't give us the full picture and we might need
>>>>>>>> echo stacktrace > /debug/tracing/trace_options
>>>>>>>>
>>>>>>>> It will generate much more output though.
>>>>>>>>
>>>>>>>>> Just now or when PSI raises?
>>>>>>>>
>>>>>>>> When the excessive reclaim is happening ideally.
>>>>>>>
>>>>>>> This one is from a server with 28G memfree but memory pressure is still
>>>>>>> jumping between 0 and 10%.
>>>>>>>
>>>>>>> I did:
>>>>>>> echo "order > 0" >
>>>>>>> /sys/kernel/debug/tracing/events/kmem/mm_page_alloc/filter
>>>>>>>
>>>>>>> echo 1 > /sys/kernel/debug/tracing/events/kmem/mm_page_alloc/enable
>>>>>>>
>>>>>>> echo 1 >
>>>>>>> /sys/kernel/debug/tracing/events/vmscan/mm_vmscan_direct_reclaim_begin/enable
>>>>>>>
>>>>>>> echo 1 >
>>>>>>> /sys/kernel/debug/tracing/events/vmscan/mm_vmscan_direct_reclaim_end/enable
>>>>>>>
>>>>>>> timeout 120 cat /sys/kernel/debug/tracing/trace_pipe > /trace
>>>>>>>
>>>>>>> File attached.
>>>>>>
>>>>>> There is no reclaim captured in this trace dump.
>>>>>> $ zcat trace1.gz | sed 's@.*\(order=[0-9]\).*\(gfp_flags=.*\)@\1 \2@' | sort | uniq -c
>>>>>>     777 order=1 gfp_flags=__GFP_IO|__GFP_FS|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP|__GFP_NOMEMALLOC
>>>>>>     663 order=1 gfp_flags=__GFP_IO|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP|__GFP_NOMEMALLOC
>>>>>>     153 order=1 gfp_flags=__GFP_IO|__GFP_NOWARN|__GFP_RETRY_MAYFAIL|__GFP_NORETRY|__GFP_COMP|__GFP_NOMEMALLOC
>>>>>>     911 order=1 gfp_flags=GFP_KERNEL_ACCOUNT|__GFP_ZERO
>>>>>>    4872 order=1 gfp_flags=GFP_NOWAIT|__GFP_IO|__GFP_FS|__GFP_NOWARN|__GFP_COMP|__GFP_ACCOUNT
>>>>>>      62 order=1 gfp_flags=GFP_NOWAIT|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP|__GFP_NOMEMALLOC
>>>>>>      14 order=2 gfp_flags=GFP_ATOMIC|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP
>>>>>>      11 order=2 gfp_flags=GFP_ATOMIC|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP|__GFP_RECLAIMABLE
>>>>>>    1263 order=2 gfp_flags=__GFP_IO|__GFP_FS|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP|__GFP_NOMEMALLOC
>>>>>>      45 order=2 gfp_flags=__GFP_IO|__GFP_FS|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP|__GFP_NOMEMALLOC|__GFP_RECLAIMABLE
>>>>>>       1 order=2 gfp_flags=GFP_KERNEL|__GFP_COMP|__GFP_ZERO
>>>>>>    7853 order=2 gfp_flags=GFP_NOWAIT|__GFP_IO|__GFP_FS|__GFP_NOWARN|__GFP_COMP|__GFP_ACCOUNT
>>>>>>      73 order=3 gfp_flags=__GFP_IO|__GFP_FS|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP|__GFP_NOMEMALLOC
>>>>>>     729 order=3 gfp_flags=__GFP_IO|__GFP_FS|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP|__GFP_NOMEMALLOC|__GFP_RECLAIMABLE
>>>>>>     528 order=3 gfp_flags=__GFP_IO|__GFP_NOWARN|__GFP_RETRY_MAYFAIL|__GFP_NORETRY|__GFP_COMP|__GFP_NOMEMALLOC
>>>>>>    1203 order=3 gfp_flags=GFP_NOWAIT|__GFP_IO|__GFP_FS|__GFP_NOWARN|__GFP_COMP|__GFP_ACCOUNT
>>>>>>    5295 order=3 gfp_flags=GFP_NOWAIT|__GFP_IO|__GFP_FS|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP
>>>>>>       1 order=3 gfp_flags=GFP_NOWAIT|__GFP_IO|__GFP_FS|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP|__GFP_NOMEMALLOC
>>>>>>     132 order=3 gfp_flags=GFP_NOWAIT|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP|__GFP_NOMEMALLOC
>>>>>>      13 order=5 gfp_flags=GFP_KERNEL|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP|__GFP_ZERO
>>>>>>       1 order=6 gfp_flags=GFP_KERNEL|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP|__GFP_ZERO
>>>>>>    1232 order=9 gfp_flags=GFP_TRANSHUGE
>>>>>>     108 order=9 gfp_flags=GFP_TRANSHUGE|__GFP_THISNODE
>>>>>>     362 order=9 gfp_flags=GFP_TRANSHUGE_LIGHT|__GFP_THISNODE
>>>>>>
>>>>>> Nothing really stands out because except for the THP ones none of others
>>>>>> are going to even be using movable zone.
>>>>> It might be that this is not an ideal example is was just the fastest i
>>>>> could find. May be we really need one with much higher pressure.
>>>>
>>>> here another trace log where a system has 30GB free memory but is under
>>>> constant pressure and does not build up any file cache caused by memory
>>>> pressure.
>>>
>>> So the reclaim is clearly induced by THP allocations
>>> $ zgrep vmscan trace2.gz | grep gfp_flags | sed 's@.*\(gfp_flags=.*\) .*@\1@' | sort | uniq -c
>>>    1580 gfp_flags=GFP_TRANSHUGE
>>>      15 gfp_flags=GFP_TRANSHUGE|__GFP_THISNODE
>>>
>>> $ zgrep vmscan trace2.gz | grep nr_reclaimed | sed 's@nr_reclaimed=@@' |  awk '{nr+=$6+0}END{print nr}'
>>> 1541726
>>>
>>> 6GB of memory reclaimed in 1776s. That is a lot! But the THP allocation
>>> rate is really high as well
>>> $ zgrep "page_alloc.*GFP_TRANSHUGE" trace2.gz | wc -l
>>> 15340
>>>
>>> this is 30GB worth of THPs (some of them might get released of course).
>>> Also only 10% of requests ends up reclaiming.
>>>
>>> One additional interesting point
>>> $ zgrep vmscan trace2.gz | grep nr_reclaimed | sed 's@.*nr_reclaimed=\([[0-9]*\)@\1@' | calc_min_max.awk
>>> min: 1.00 max: 2792.00 avg: 965.99 std: 331.12 nr: 1596
>>>
>>> Even though the std is high there are quite some outliers when a lot of
>>> memory is reclaimed.
>>>
>>> Which kernel version is this. And again, what is the THP configuration.
>>
>> This is 4.19.66 regarding THP you mean this:
> 
> Do you see the same behavior with 5.3?

I rebootet with 5.3.0-rc8 - let's see what happens it might take some
    hours or even days.

>> /sys/kernel/mm/transparent_hugepage/defrag:always defer [defer+madvise]
>> madvise never
>>
>> /sys/kernel/mm/transparent_hugepage/enabled:[always] madvise never
>>
>> /sys/kernel/mm/transparent_hugepage/hpage_pmd_size:2097152
>>
>> /sys/kernel/mm/transparent_hugepage/shmem_enabled:always within_size
>> advise [never] deny force
>>
>> /sys/kernel/mm/transparent_hugepage/use_zero_page:1
>>
>> /sys/kernel/mm/transparent_hugepage/enabled was madvise until yesterday
>> where i tried to switch to defer+madvise - which didn't help.
> 
> Many processes hitting the reclaim are php5 others I cannot say because
> their cmd is not reflected in the trace. I suspect those are using
> madvise. I haven't really seen kcompactd interfering much. That would
> suggest using defer.

You mean i should set transparent_hugepage to defer?

Stefan