Re: lot of MemAvailable but falling cache and raising PSI

From: Michal Hocko <mhocko@kernel.org>
To: Stefan Priebe - Profihost AG <s.priebe@profihost.ag>
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 10:29:19 +0200	[thread overview]
Message-ID: <20190910082919.GL2063@dhcp22.suse.cz> (raw)
In-Reply-To: <52235eda-ffe2-721c-7ad7-575048e2d29d@profihost.ag>

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.
-- 
Michal Hocko
SUSE Labs