On Mon, Apr 18, 2011 at 10:32:51PM +1000, NeilBrown wrote:

On Thu, 14 Apr 2011 11:41:38 +0100 Mel Gorman [off-list ref] wrote:

quoted

If swap is backed by network storage such as NBD, there is a risk that a
large number of reclaimers can hang the system by consuming all
PF_MEMALLOC reserves. To avoid these hangs, the administrator must tune
min_free_kbytes in advance. This patch will throttle direct reclaimers
if half the PF_MEMALLOC reserves are in use as the system is at risk of
hanging. A message will be displayed so the administrator knows that
min_free_kbytes should be tuned to a higher value to avoid the
throttling in the future.

(I knew there was something else).

I understand that there are suggestions that direct reclaim should always be
serialised as this reduces lock contention and improve data patterns (or
something like that).

AFAIK, this suggestion never got much beyond the "hand-waving" stage
of development. It tended to trip up on the fact that such a feature
could also throttle processes on machines with plenty of free clean
unmapped pagecache which would be undesirable.

Would that make this patch redundant? 

Depends on how it was being serialised but ....

Or does this provide some extra
guarantee that the other proposal would not?

This patch could be extended to serialise direct reclaims in situations
other than PFMEMALLOC is low if someone demonstrated the benefit.

-- 
Mel Gorman
SUSE Labs

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