Date:	Wed, 20 Jan 2010 13:48:49 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	Daisuke Nishimura <nishimura@....nes.nec.co.jp>
CC:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] Shared page accounting for memory cgroup

On Wednesday 20 January 2010 12:45 PM, Daisuke Nishimura wrote:
> On Wed, 20 Jan 2010 13:09:02 +0900, Daisuke Nishimura <nishimura@....nes.nec.co.jp> wrote:
>> On Tue, 19 Jan 2010 09:22:41 +0530, Balbir Singh <balbir@...ux.vnet.ibm.com> wrote:
>>> On Tuesday 19 January 2010 08:04 AM, Daisuke Nishimura wrote:
>>>> On Tue, 19 Jan 2010 07:19:42 +0530, Balbir Singh <balbir@...ux.vnet.ibm.com> wrote:
>>>>> On Tue, Jan 19, 2010 at 6:52 AM, Daisuke Nishimura
>>>>> <nishimura@....nes.nec.co.jp> wrote:
>>>>> [snip]
>>>>>>> Correct, file cache is almost always considered shared, so it has
>>>>>>>
>>>>>>> 1. non-private or shared usage of 10MB
>>>>>>> 2. 10 MB of file cache
>>>>>>>
>>>>>>>> I don't think "non private usage" is appropriate to this value.
>>>>>>>> Why don't you just show "sum_of_each_process_rss" ? I think it would be easier
>>>>>>>> to understand for users.
>>>>>>>
>>>>>>> Here is my concern
>>>>>>>
>>>>>>> 1. The gap between looking at memcg stat and sum of all RSS is way
>>>>>>> higher in user space
>>>>>>> 2. Summing up all rss without walking the tasks atomically can and
>>>>>>> will lead to consistency issues. Data can be stale as long as it
>>>>>>> represents a consistent snapshot of data
>>>>>>>
>>>>>>> We need to differentiate between
>>>>>>>
>>>>>>> 1. Data snapshot (taken at a time, but valid at that point)
>>>>>>> 2. Data taken from different sources that does not form a uniform
>>>>>>> snapshot, because the timestamping of the each of the collected data
>>>>>>> items is different
>>>>>>>
>>>>>> Hmm, I'm sorry I can't understand why you need "difference".
>>>>>> IOW, what can users or middlewares know by the value in the above case
>>>>>> (0MB in 01 and 10MB in 02)? I've read this thread, but I can't understande about
>>>>>> this point... Why can this value mean some of the groups are "heavy" ?
>>>>>>
>>>>>
>>>>> Consider a default cgroup that is not root and assume all applications
>>>>> move there initially. Now with a lot of shared memory,
>>>>> the default cgroup will be the first one to page in a lot of the
>>>>> memory and its usage will be very high. Without the concept of
>>>>> showing how much is non-private, how does one decide if the default
>>>>> cgroup is using a lot of memory or sharing it? How
>>>>> do we decide on limits of a cgroup without knowing its actual usage -
>>>>> PSS equivalent for a region of memory for a task.
>>>>>
>>>> As for limit, I think we should decide it based on the actual usage because
>>>> we account and limit the accual usage. Why we should take account of the sum of rss ?
>>>
>>> I am talking of non-private pages or potentially shared pages - which is
>>> derived as follows
>>>
>>> sum_of_all_rss - (rss + file_mapped) (from .stat file)
>>>
>>> file cache is considered to be shared always
>>>
>>>
>>>> I agree that we'd better not to ignore the sum of rss completely, but could you show me
>>>> how the value 0MB/10MB can be used to caluculate the limit in 01/02 in detail ?
>>>
>>> In your example, usage shows that the real usage of the cgroup is 20 MB
>>> for 01 and 10 MB for 02.
>> right.
>>
>>> Today we show that we are using 40MB instead of
>>> 30MB (when summed).
>> Sorry, I can't understand here.
>> If we sum usage_in_bytes in both groups, it would be 30MB.
>> If we sum "actual rss(rss_file, rss_anon) via stat file" in both groups, it would be 30M.
>> If we sum "total rss(rss_file, rss_anon) of all process via mm_counter" in both groups,
>> it would be 40MB.
>>
>>> If an administrator has to make a decision to say
>>> add more resources, the one with 20MB would be the right place w.r.t.
>>> memory.
>>>
>> You mean he would add the additional resource to 00, right? Then, 
>> the smaller "shared_usage_in_bytes" is, the more likely an administrator should
>> add additional resources to the group ?
>>
>> But when both /cgroup/memory/aa and /cgroup/memory/bb has 20MB as acutual usage,
>> and aa has 10MB "shared"(used by multiple processes *in aa*) usage while bb has none,
>> "shared_usage_in_bytes" is 10MB in aa and 0MB in bb(please consider there is
>> no "shared" usage between aa and bb).
>> Should an administrator consider bb is heavier than aa ? I don't think so.
>>
>> IOW, "shared_usage_in_bytes" doesn't have any consistent meaning about which
>> group is unfairly "heavy".
>>
>> The problem here is, "shared_usage_in_bytes" doesn't show neither one of nor the sum
>> of the following value(*IFF* we have only one cgroup, "shared_usage_in_bytes" would
>> mean a), but it has no use in real case).
>>
>>   a) memory usage used by multiple processes inside this group.
>>   b) memory usage used by both processes inside this and another group.
>>   c) memory usage not used by any processes inside this group, but used by
>>      that of in another group.
>>
>> IMHO, we should take account of all the above values to determine which group
>> is unfairly "heavy". I agree that the bigger the size of a) is, the bigger
>> "shared_usage_in_bytes" of the group would be, but we cannot know any information about
>> the size of b) by it, becase those usages are included in both actual usage(rss via stat)
>> and sum of rss(via mm_counter). To make matters warse, "shared_usage_in_bytes" has
>> the opposite meaning about b), i.e., the more a processe in some group(foo) has actual
>> charges in *another* group(baa), the bigger "shared_usage_in_bytes" in "foo" would be
>> (as 00 and 01 in my example).
>>
>> I would agree with you if you add interfaces to show some hints to users about above values,
>> but "shared_usage_in_bytes" doesn't meet it at all.
>>
> This is just an idea(At least, we need interfaces to read and reset them).
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c> index 385e29b..bf601f2 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -83,6 +83,8 @@ enum mem_cgroup_stat_index {
>  					used by soft limit implementation */
>  	MEM_CGROUP_STAT_THRESHOLDS, /* decrements on each page in/out.
>  					used by threshold implementation */
> +	MEM_CGROUP_STAT_SHARED_IN_GROUP,
> +	MEM_CGROUP_STAT_SHARED_FROM_OTHERS,
> 
>  	MEM_CGROUP_STAT_NSTATS,
>  };
> @@ -1707,8 +1709,25 @@ static void __mem_cgroup_commit_charge(struct mem_cgroup *mem,
> 
>  	lock_page_cgroup(pc);
>  	if (unlikely(PageCgroupUsed(pc))) {
> +		struct mem_cgroup *charged = pc->mem_cgroup;
> +		struct mem_cgroup_stat *stat;
> +		struct mem_cgroup_stat_cpu *cpustat;
> +		int cpu;
> +		int shared_type;
> +
>  		unlock_page_cgroup(pc);
>  		mem_cgroup_cancel_charge(mem);
> +
> +		stat = &charged->stat;
> +		cpu = get_cpu();
> +		cpustat = &stat->cpustat[cpu];
> +		if (charged == mem)
> +			shared_type = MEM_CGROUP_STAT_SHARED_IN_GROUP;
> +		else
> +			shared_type = MEM_CGROUP_STAT_SHARED_FROM_OTHERS;
> +		__mem_cgroup_stat_add_safe(cpustat, shared_type, 1);
> +		put_cpu();
> +
>  		return;

How will this work during uncharge? If the original cgroup that owns the
pages has unmapped them?

Balbir Singh.
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