linux-kernel - Re: [RFC 0/5] Memory controller soft limit introduction (v3)

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Date:	Mon, 30 Jun 2008 12:57:37 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	balbir@...ux.vnet.ibm.com
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	YAMAMOTO Takashi <yamamoto@...inux.co.jp>,
	Paul Menage <menage@...gle.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [RFC 0/5] Memory controller soft limit introduction (v3)

On Mon, 30 Jun 2008 09:11:19 +0530
Balbir Singh <balbir@...ux.vnet.ibm.com> wrote:

> KAMEZAWA Hiroyuki wrote:
> > On Sun, 29 Jun 2008 10:32:03 +0530
> > Balbir Singh <balbir@...ux.vnet.ibm.com> wrote:
> >>> I have a couple of comments.
> >>>
> >>> 1. Why you add soft_limit to res_coutner ?
> >>>    Is there any other controller which uses soft-limit ?
> >>>    I'll move watermark handling to memcg from res_counter becasue it's
> >>>    required only by memcg.
> >>>
> >> I expect soft_limits to be controller independent. The same thing can be applied
> >> to an io-controller for example, right?
> >>
> > 
> > I can't imagine how soft-limit works on i/o controller. could you explain ?
> > 
> 
> An io-controller could have the same concept. A hard-limit on the bandwidth and
> a soft-limit to allow a group to exceed the soft-limit provided there is no i/o
> bandwidth congestion.
> 
Hmm, that is the case where "share" works well. Why soft-limit ?
i/o conroller doesn't support share ? (I don' know sorry.)



> > 
> >>> 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.
> > 
> > (Current page shrinking under cpuset seems to scan all nodes,
> >  his seems not to be quick, but it works  because it scans all.
> >  This will be another problem, anyway ;).
> > 
> > 
> > BTW, currently mem_cgroup_try_to_free_pages() assumes GFP_HIGHUSER_MOVABLE
> > always.
> > ==
> > unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *mem_cont,
> >                                                 gfp_t gfp_mask)
> > {
> >         struct scan_control sc = {
> >                 .may_writepage = !laptop_mode,
> >                 .may_swap = 1,
> >                 .swap_cluster_max = SWAP_CLUSTER_MAX,
> >                 .swappiness = vm_swappiness,
> >                 .order = 0,
> >                 .mem_cgroup = mem_cont,
> >                 .isolate_pages = mem_cgroup_isolate_pages,
> >         };
> >         struct zonelist *zonelist;
> > 
> >         sc.gfp_mask = (gfp_mask & GFP_RECLAIM_MASK) |
> >                         (GFP_HIGHUSER_MOVABLE & ~GFP_RECLAIM_MASK);
> >         zonelist = NODE_DATA(numa_node_id())->node_zonelists;
> >         return do_try_to_free_pages(zonelist, &sc);
> > }
> > ==
> > please select appropriate zonelist here.
> > 
> 
> We do have zonelist information in __alloc_pages_internal(), it should be easy
> to pass the zonelist or come up with a good default (current one) if no zonelist
> is provided to the routine.
> 
yes. what I want to say is you should take care of this.

Anyway, I think you should revisit the whole memory reclaim and fixes small bugs?
which doesn't meet soft-limit.

Thanks,
-Kame

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