Re: [patch] mm: vmscan: do not swap anon pages just because free+file is low

[Hugh Dickins <hughd@google.com>]

On Fri, 14 Mar 2014, Mel Gorman wrote:
> On Fri, Mar 14, 2014 at 12:06:25PM -0400, Rik van Riel wrote:
> > On 03/14/2014 11:35 AM, Johannes Weiner wrote:
> > > Page reclaim force-scans / swaps anonymous pages when file cache drops
> > > below the high watermark of a zone in order to prevent what little
> > > cache remains from thrashing.
> > > 
> > > However, on bigger machines the high watermark value can be quite
> > > large and when the workload is dominated by a static anonymous/shmem
> > > set, the file set might just be a small window of used-once cache.  In
> > > such situations, the VM starts swapping heavily when instead it should
> > > be recycling the no longer used cache.
> > > 
> > > This is a longer-standing problem, but it's more likely to trigger
> > > after 81c0a2bb515f ("mm: page_alloc: fair zone allocator policy")
> > > because file pages can no longer accumulate in a single zone and are
> > > dispersed into smaller fractions among the available zones.
> > > 
> > > To resolve this, do not force scan anon when file pages are low but
> > > instead rely on the scan/rotation ratios to make the right prediction.
> > 
> > I am not entirely sure that the scan/rotation ratio will be
> > meaningful when the page cache has been essentially depleted,
> > but on larger systems the distance between the low and high
> > watermark is gigantic, and I have no better idea on how to
> > fix the bug you encountered, so ...
> > 
> 
> I still agree with the direction in general even though I've not put
> thought into this specific patch yet. We've observed a problem whereby force
> reclaim was causing one or other LRU list to be trashed.  In one specific
> case, the inactive file is low logic was causing problems because while
> the relative size of inactive/active was taken into account, the absolute
> size vs anon was not. It was not a mainline kernel and we do not have a
> test configuration that properly illustrates the problem on mainline it's
> on our radar that it's a potential problem. The scan/rotation ratio at the
> moment does not take absolute sizes into account but we almost certainly
> want to go in that direction at some stage. Hugh's patch on altering how
> proportional shrinking works is also relevant.

That https://lkml.org/lkml/2014/3/13/217
is relevant, yes, but I think rather more so is
Suleiman's in https://lkml.org/lkml/2014/3/15/168

Hannes, your patch looks reasonable to me, and as I read it would
be well complemented by Suleiman's and mine; but I do worry that
the "scan_balance = SCAN_ANON" block you're removing was inserted
for good reason, and its removal bring complaint from some direction.

By the way, I notice you marked yours for stable [3.12+]:
if it's for stable at all, shouldn't it be for 3.9+?
(well, maybe nobody's doing a 3.9.N.M but 3.10.N is still alive).

Hugh

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