Messages in this thread | | | Date | Wed, 18 Jan 2012 19:25:27 +0800 | From | Sha <> | Subject | Re: [patch 2/2] mm: memcg: hierarchical soft limit reclaim |
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On 01/18/2012 05:25 PM, Johannes Weiner wrote: > On Wed, Jan 18, 2012 at 03:17:25PM +0800, Sha wrote: >>>> I don't think it solve the root of the problem, example: >>>> root >>>> -> A (hard limit 20G, soft limit 12G, usage 20G) >>>> -> A1 ( soft limit 2G, usage 1G) >>>> -> A2 ( soft limit 10G, usage 19G) >>>> ->B1 (soft limit 5G, usage 4G) >>>> ->B2 (soft limit 5G, usage 15G) >>>> >>>> Now A is hitting its hard limit and start hierarchical reclaim under A. >>>> If we choose B1 to go through mem_cgroup_over_soft_limit, it will >>>> return true because its parent A2 has a large usage and will lead to >>>> priority=0 reclaiming. But in fact it should be B2 to be punished. >>> Because A2 is over its soft limit, the whole hierarchy below it should >>> be preferred over A1, so both B1 and B2 should be soft limit reclaimed >>> to be consistent with behaviour at the root level. >> Well it is just the behavior that I'm expecting actually. But with my >> humble comprehension, I can't catch the soft-limit-based hierarchical >> reclaiming under the target cgroup (A2) in the current implementation >> or after the patch. Both the current mem_cgroup_soft_reclaim or >> shrink_zone select victim sub-cgroup by mem_cgroup_iter, but it >> doesn't take soft limit into consideration, do I left anything ? > No, currently soft limits are ignored if pressure originates from > below root_mem_cgroup. > > But iff soft limits are applied right now, they are applied > hierarchically, see mem_cgroup_soft_limit_reclaim(). Er... I'm even more confused: mem_cgroup_soft_limit_reclaim indeed choses the biggest soft-limit excessor first, but in the succeeding reclaim mem_cgroup_hierarchical_reclaim just selects a child cgroup by css_id which has nothing to do with soft limit (see mem_cgroup_select_victim). IMHO, it's not a genuine hierarchical reclaim. I check this from the latest memcg-devel git tree (branch since-3.1)...
> In my opinion, the fact that soft limits are ignored when pressure is > triggered sub-root_mem_cgroup is an artifact of the per-zone tree, so > I allowed soft limits to be taken into account below root_mem_cgroup. > > But IMO, this is something different from how soft limit reclaim is > applied once triggered: currently, soft limit reclaim applies to a > whole hierarchy, including all children. And this I left unchanged.
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