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SubjectRe: [RFC 0/5] Memory controller soft limit introduction (v3)
KAMEZAWA Hiroyuki wrote:
> On Sun, 29 Jun 2008 10:32:03 +0530
> Balbir Singh <balbir@linux.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.

>
>>> 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.


>
>>> 3. I think when "mem_cgroup_reclaim_on_contention" exits is unclear.
>>> plz add explanation of algorithm. It returns when some pages are reclaimed ?
>>>
>> Sure, I will do that.
>>
>>> 4. When swap-full cgroup is on the top of heap, which tends to contain
>>> tons of memory, much amount of cpu-time will be wasted.
>>> Can we add "ignore me" flag ?
>>>
>> Could you elaborate on swap-full cgroup please? Are you referring to changes
>> introduced by the memcg-handle-swap-cache patch? I don't mind adding a ignore me
>> flag, but I guess we need to figure out when a cgroup is swap full.
>>
> No. no-available-swap, or all-swap-are-used situation.
>
> This situation will happen very easily if swap-controller comes.

We'll definitely deal with it when the swap-controller comes in.

--
Warm Regards,
Balbir Singh
Linux Technology Center
IBM, ISTL


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