Re: [PATCH 1/7] mm: remove ZONE_RECLAIM_LOCKED

[Andrea Arcangeli <aarcange@redhat.com>]

Hi!

On Mon, Jun 17, 2013 at 02:12:44PM -0400, Rik van Riel wrote:
> On 06/17/2013 05:30 AM, Mel Gorman wrote:
> > On Fri, Jun 14, 2013 at 12:16:47PM -0400, Rik van Riel wrote:
> >> On 06/06/2013 01:37 PM, Rik van Riel wrote:
> >>> On 06/06/2013 05:04 AM, Mel Gorman wrote:
> >>>> On Wed, Jun 05, 2013 at 05:10:31PM +0200, Andrea Arcangeli wrote:
> >>>>> Zone reclaim locked breaks zone_reclaim_mode=1. If more than one
> >>>>> thread allocates memory at the same time, it forces a premature
> >>>>> allocation into remote NUMA nodes even when there's plenty of clean
> >>>>> cache to reclaim in the local nodes.
> >>>>>
> >>>>> Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
> >>>>
> >>>> Be aware that after this patch is applied that it is possible to have a
> >>>> situation like this
> >>>>
> >>>> 1. 4 processes running on node 1
> >>>> 2. Each process tries to allocate 30% of memory
> >>>> 3. Each process reads the full buffer in a loop (stupid, just an example)
> >>>>
> >>>> In this situation the processes will continually interfere with each
> >>>> other until one of them gets migrated to another zone by the scheduler.
> >>>
> >>> This is a very good point.
> >>>
> >>> Andrea, I suspect we will need some kind of safeguard against
> >>> this problem.
> >>
> >> Never mind me.
> >>
> >> In __zone_reclaim we set the flags in swap_control so
> >> we never unmap pages or swap pages out at all by
> >> default, so this should not be an issue at all.
> >>
> >> In order to get the problem illustrated above, the
> >> user will have to enable RECLAIM_SWAP through sysfs
> >> manually.
> >>
> >
> > For the mapped case and the default tuning for zone_reclaim_mode then
> > yes. If instead of allocating 30% of memory the processes are using using
> > buffered reads/writes then they'll reach each others page cache pages and
> > it's a very similar problem.
> 
> Could we fix that problem by simply allowing page cache
> allocations (__GFP_WRITE) to fall back to other zones,
> regardless of the zone_reclaim setting?
> 
> The ZONE_RECLAIM_LOCKED function seems to break as many
> things as it fixes, so replacing it with something else
> seems like a worthwhile pursuit...

I actually don't see a connection between the various scenarios
described above with ZONE_RECLAIM_LOCKED. I mean whatever problem you
are having with swapping or excessive reclaim in a single zone/node
despite the other zones/nodes are completely free, could materialize
the same way with the current ZONE_RECLAIM_LOCKED code if you just use
a mutex in userland to serialize the memory allocations. Or if they
just happen to run serially for other reasons.

If it was a problem to keep insisting calling zone_reclaim in any
given zone, the problem would eventually materialize anyway, by just
running a single thread in the whole system pinned to a single node.

ZONE_RECLAIM_LOCKED isn't about swapping or memory pressure, it is
only about preventing running more than one zone_reclaim function at
once in any given zone. But that shall be ok. If all zone_reclaim()
running in parallel are doing a .nr_to_reclaim = SWAP_CLUSTER_MAX
shrinkage attempt with may_unmap/may_writepage unset
(zone_reclaim_mode is <=1), there shall be no problem. And
zone_reclaim won't be called anymore as soon as the watermark is above
"low".

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