Messages in this thread | | | Date | Thu, 3 Jan 2013 12:24:04 -0800 | From | Andrew Morton <> | Subject | Re: [PATCH -repost] memcg,vmscan: do not break out targeted reclaim without reclaimed pages |
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On Thu, 3 Jan 2013 19:09:01 +0100 Michal Hocko <mhocko@suse.cz> wrote:
> Hi, > I have posted this quite some time ago > (https://lkml.org/lkml/2012/12/14/102) but it probably slipped through > --- > >From 28b4e10bc3c18b82bee695b76f4bf25c03baa5f8 Mon Sep 17 00:00:00 2001 > From: Michal Hocko <mhocko@suse.cz> > Date: Fri, 14 Dec 2012 11:12:43 +0100 > Subject: [PATCH] memcg,vmscan: do not break out targeted reclaim without > reclaimed pages > > Targeted (hard resp. soft) reclaim has traditionally tried to scan one > group with decreasing priority until nr_to_reclaim (SWAP_CLUSTER_MAX > pages) is reclaimed or all priorities are exhausted. The reclaim is > then retried until the limit is met. > > This approach, however, doesn't work well with deeper hierarchies where > groups higher in the hierarchy do not have any or only very few pages > (this usually happens if those groups do not have any tasks and they > have only re-parented pages after some of their children is removed). > Those groups are reclaimed with decreasing priority pointlessly as there > is nothing to reclaim from them. > > An easiest fix is to break out of the memcg iteration loop in shrink_zone > only if the whole hierarchy has been visited or sufficient pages have > been reclaimed. This is also more natural because the reclaimer expects > that the hierarchy under the given root is reclaimed. As a result we can > simplify the soft limit reclaim which does its own iteration. > > Reported-by: Ying Han <yinghan@google.com>
But what was in that report?
My guess would be "excessive CPU consumption", and perhaps "excessive reclaim in the higher-level memcgs".
IOW, what are the user-visible effects of this change?
(And congrats - you're the first person I've sent that sentence to this year! But not, I fear, the last)
I don't really understand what prevents limit reclaim from stealing lots of pages from the top-level groups. How do we ensure balancing/fairness in this case?
> --- a/mm/vmscan.c > +++ b/mm/vmscan.c > @@ -1973,18 +1973,17 @@ static void shrink_zone(struct zone *zone, struct scan_control *sc)
shrink_zone() might be getting a bit bloaty for CONFIG_MEMCG=n kernels.
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