Re: [PATCH] mm: vmscan: check mem cgroup over reclaimed

On Tue, Jan 24, 2012 at 5:47 PM, Hillf Danton [off-list ref] wrote:

On Wed, Jan 25, 2012 at 7:22 AM, Ying Han [off-list ref] wrote:

quoted

On Mon, Jan 23, 2012 at 7:45 PM, Hillf Danton [off-list ref] wrote:

quoted

With soft limit available, what if nr_to_reclaim set to be the number of
pages exceeding soft limit? With over reclaim abused, what are the targets
of soft limit?

The nr_to_reclaim is set to SWAP_CLUSTER_MAX (32) for direct reclaim
and ULONG_MAX for background reclaim. Not sure we can set it, but it
is possible the res_counter_soft_limit_excess equal to that target
value. The current soft limit mechanism provides a clue of WHERE to
reclaim pages when there is memory pressure, it doesn't change the
reclaim target as it was before.

Decrement in sc->nr_to_reclaim was tried in another patch, you already saw it.

quoted

Overreclaim a cgroup under its softlimit is bad, but we should be
careful not introducing side effect before providing the guarantee.

Yes 8-)

quoted

Here, the should_continue_reclaim() has logic of freeing a bit more
order-0 pages for compaction. The logic got changed after this.

Compaction is to increase the successful rate of THP allocation, and in turn
to back up higher performance. In soft limit, performance guarantee is not
extra request but treated with less care.

Which one you prefer, compaction or guarantee?

The compaction is something we already supporting, while the softlimit
implementation is a new design. I would say that we need to guarantee
no regression introduced by any new code.

--Ying

Thanks
Hillf

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