Re: [PATCH v2 2/5] memcg: provide root figures from system totals

[Sha Zhengju <handai.szj@gmail.com>]

On Wed, Mar 6, 2013 at 6:59 PM, Kamezawa Hiroyuki
<kamezawa.hiroyu@jp.fujitsu.com> wrote:
> (2013/03/06 19:52), Glauber Costa wrote:
>> On 03/06/2013 02:45 PM, Kamezawa Hiroyuki wrote:
>>> (2013/03/06 17:30), Glauber Costa wrote:
>>>> On 03/06/2013 04:27 AM, Kamezawa Hiroyuki wrote:
>>>>> (2013/03/05 22:10), Glauber Costa wrote:
>>>>>> + case _MEMSWAP: {
>>>>>> +         struct sysinfo i;
>>>>>> +         si_swapinfo(&i);
>>>>>> +
>>>>>> +         return ((memcg_read_root_rss() +
>>>>>> +         atomic_long_read(&vm_stat[NR_FILE_PAGES])) << PAGE_SHIFT) +
>>>>>> +         i.totalswap - i.freeswap;
>>>>>
>>>>> How swapcache is handled ? ...and How kmem works with this calc ?
>>>>>
>>>> I am ignoring kmem, because we don't account kmem for the root cgroup
>>>> anyway.
>>>>
>>>> Setting the limit is invalid, and we don't account until the limit is
>>>> set. Then it will be 0, always.
>>>>
>>>> For swapcache, I am hoping that totalswap - freeswap will cover
>>>> everything swap related. If you think I am wrong, please enlighten me.
>>>>
>>>
>>> i.totalswap - i.freeswap = # of used swap entries.
>>>
>>> SwapCache can be rss and used swap entry at the same time.
>>>
>>
>> Well, yes, but the rss entries would be accounted for in get_mm_rss(),
>> won't they ?
>>
>> What am I missing ?
>
>
> I think the correct caluculation is
>
>   Sum of all RSS + All file caches + (i.total_swap - i.freeswap - # of mapped SwapCache)
>
>
> In the patch, mapped SwapCache is counted as both of rss and swap.
>

After a quick look, swapcache is counted as file pages and meanwhile
use a swap entry at the same time(__add_to{delete_from}_swap_cache()).
Even though, I think we still do not need to exclude swapcache out,
because it indeed uses two copy of resource: one is swap entry, one is
cache, so the usage should count both of them in.

What I think it matters is that swapcache may be counted as both file
pages and rss(if it's a process's anonymous page), which we need to
subtract # of swapcache to avoid double-counting. But it isn't always
so: a shmem/tmpfs page may use swapcache and be counted as file pages
but not a rss, then we can not subtract swapcache... Is there anything
I lost?

> BTW, how about
>
>   Sum of all LRU + (i.total_swap - i.freeswap - # of all SwapCache)
> ?
>
> Thanks,
> -Kame
>
>
>
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>



-- 
Thanks,
Sha

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