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SubjectRe: [RFC 0/5] Memory controller soft limit introduction (v3)
On Mon, 30 Jun 2008 10:20:54 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:
> > > 2. *please* handle NUMA
> > > There is a fundamental difference between global VMM and memcg.
> > > global VMM - reclaim memory at memory shortage.
> > > memcg - for reclaim memory at memory limit
> > > Then, memcg wasn't required to handle place-of-memory at hitting limit.
> > > *just reducing the usage* was enough.
> > > In this set, you try to handle memory shortage handling.
> > > So, please handle NUMA, i.e. "what node do you want to reclaim memory from ?"
> > > If not,
> > > - memory placement of Apps can be terrible.
> > > - cannot work well with cpuset. (I think)
> > >
> >
> > try_to_free_mem_cgroup_pages() handles NUMA right? We start with the
> > node_zonelists of the current node on which we are executing. I can pass on the
> > zonelist from __alloc_pages_internal() to try_to_free_mem_cgroup_pages(). Is
> > there anything else you had in mind?
> >
> Assume following case of a host with 2 nodes. and following mount style.
>
> mount -t cgroup -o memory,cpuset none /opt/cgroup/
>
>
> /Group1: cpu 0-1, mem=0 limit=1G, soft-limit=700M
> /Group2: cpu 2-3, mem=1 limit=1G soft-limit=700M
> ....
> /Groupxxxx
>
> Assume a environ after some workload,
>
> /Group1: cpu 0-1, mem=0 limit=1G, soft-limit=700M usage=990M
> /Group2: cpu 2-3, mem=1 limit=1G soft-limit=700M usage=400M
>
> *And* memory of node"1" is in shortage and the kernel has to reclaim
> memory from node "1".
>
> Your routine tries to relclaim memory from a group, which exceeds soft-limit
> ....Group1. But it's no help because Group1 doesn't contains any memory in Node1.
> And make it worse, your routine doen't tries to call try_to_free_pages() in global
> LRU when your soft-limit reclaim some memory. So, if a task in Group 1 continues
> to allocate memory at some speed, memory shortage in Group2 will not be recovered,
> easily.
>
> This includes 2 aspects of trouble.
> - Group1's memory is reclaimed but it's wrong.
> - Group2's try_to_free_pages() may took very long time.
>
A bit more inforamtion, to be honest, I don't understand this perfectly.

But I convice there is some difference between limit and shortage.

in 2.6.26-rc5-mm3's shrink_zones() supprots cpuset by this.

==
if (scan_global_lru(sc)) {
if (!cpuset_zone_allowed_hardwall(zone, GFP_KERNEL))
continue;
note_zone_scanning_priority(zone, priority);
if (zone_is_all_unreclaimable(zone) &&
priority != DEF_PRIORITY)
continue; /* Let kswapd poll it */
sc->all_unreclaimable = 0;
} else {
/*
* Ignore cpuset limitation here. We just want to reduce
* # of used pages by us regardless of memory shortage.
*/
sc->all_unreclaimable = 0;
mem_cgroup_note_reclaim_priority(sc->mem_cgroup,
priority);
}
==
First point is (maybe) my mistake. We have to add cpuset hardwall check to memcg
part. (I will write a patch soon.)

Second point is when memory shortage is caused by some routine which is not in
cpuset. In this case, Group1's memory can be reclaimed w/o benefits.
not big trouble ?


Thanks,
-Kame





























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