Re: lot of MemAvailable but falling cache and raising PSI - Stefan Priebe

From: Stefan Priebe - Profihost AG <s.priebe@profihost.ag>
To: Michal Hocko <mhocko@kernel.org>
Cc: "linux-mm@kvack.org" <linux-mm@kvack.org>,
	l.roehrs@profihost.ag, cgroups@vger.kernel.org,
	Johannes Weiner <hannes@cmpxchg.org>,
	Vlastimil Babka <vbabka@suse.cz>
Subject: Re: lot of MemAvailable but falling cache and raising PSI
Date: Tue, 10 Sep 2019 10:38:25 +0200	[thread overview]
Message-ID: <132e1fd0-c392-c158-8f3a-20e340e542f0@profihost.ag> (raw)
In-Reply-To: <20190910082919.GL2063@dhcp22.suse.cz>

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:
/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.

Greets,
Stefan