Re: [PATCH 1/4] Add kswapd descriptor.

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

> On Tue, Dec 7, 2010 at 4:33 AM, Mel Gorman <mel@csn.ul.ie> wrote:
> > On Mon, Nov 29, 2010 at 10:49:42PM -0800, Ying Han wrote:
> >> There is a kswapd kernel thread for each memory node. We add a different kswapd
> >> for each cgroup.
> >
> > What is considered a normal number of cgroups in production? 10, 50, 10000?
> Normally it is less than 100. I assume there is a cap of number of
> cgroups can be created
> per system.
> 
> If it's a really large number and all the cgroups kswapds wake at the same time,
> > the zone LRU lock will be very heavily contended.
> 
> Thanks for reviewing the patch~
> 
> Agree. The zone->lru_lock is another thing we are looking at.
> Eventually, we need to break the lock to
> per-zone per-memcg lru.

This may make following bad scenario. That's the reason why now we are using zone->lru_lock.

1) start memcg reclaim
2) found the lru tail page has pte access bit
3) memcg reclaim decided that the page move to active list of memcg-lru.
    Also, pte access bit was cleaned. But, the page still remain inactive list of global-lru.
4) Sadly, global reclaim discard the page quickly because it has been lost accessed bit by memcg.


But, if we have to modify both memcg and global LRU, we can't avoid zone->lru_lock anyway.
Then, we don't use memcg special lock.

Thanks.


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