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
