Re: [PATCH] memcg: replace ss->id_lock with a rwlock

On Wed, 10 Aug 2011 11:20:33 -0700
Andrew Bresticker [off-list ref] wrote:

While back-porting Johannes Weiner's patch "mm: memcg-aware global reclaim"
for an internal effort, we noticed a significant performance regression
during page-reclaim heavy workloads due to high contention of the ss->id_lock.
This lock protects idr map, and serializes calls to idr_get_next() in
css_get_next() (which is used during the memcg hierarchy walk).  Since
idr_get_next() is just doing a look up, we need only serialize it with
respect to idr_remove()/idr_get_new().  By making the ss->id_lock a
rwlock, contention is greatly reduced and performance improves.

Tested: cat a 256m file from a ramdisk in a 128m container 50 times
on each core (one file + container per core) in parallel on a NUMA
machine.  Result is the time for the test to complete in 1 of the
containers.  Both kernels included Johannes' memcg-aware global
reclaim patches.
Before rwlock patch: 1710.778s
After rwlock patch: 152.227s

Signed-off-by: Andrew Bresticker <redacted>

Hopefully, the changelog should be based on the latest Linus's git tree
or mmotm. Even now, if a system has multiple hierarchies of memcg, I think
the contention will happen.

Acked-by: KAMEZAWA Hiroyuki <redacted>

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