linux-kernel - Re: [PATCH 1/7] mm: memcontrol: charge swap to cgroup2

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Date:	Thu, 17 Dec 2015 13:29:58 +0900
From:	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Vladimir Davydov <vdavydov@...tuozzo.com>,
	Michal Hocko <mhocko@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/7] mm: memcontrol: charge swap to cgroup2

On 2015/12/17 12:32, Johannes Weiner wrote:
> On Thu, Dec 17, 2015 at 11:46:27AM +0900, Kamezawa Hiroyuki wrote:
>> On 2015/12/16 20:09, Johannes Weiner wrote:
>>> On Wed, Dec 16, 2015 at 12:18:30PM +0900, Kamezawa Hiroyuki wrote:
>>>>   - swap-full notification via vmpressure or something mechanism.
>>>
>>> Why?
>>>
>>
>> I think it's a sign of unhealthy condition, starting file cache drop rate to rise.
>> But I forgot that there are resource threshold notifier already. Does the notifier work
>> for swap.usage ?
>
> That will be reflected in vmpressure or other distress mechanisms. I'm
> not convinced "ran out of swap space" needs special casing in any way.
>
Most users checks swap space shortage as "system alarm" in enterprise systems.
At least, our customers checks swap-full.

>>>>   - force swap-in at reducing swap.limit
>>>
>>> Why?
>>>
>> If full, swap.limit cannot be reduced even if there are available memory in a cgroup.
>> Another cgroup cannot make use of the swap resource while it's occupied by other cgroup.
>> The job scheduler should have a chance to fix the situation.
>
> I don't see why swap space allowance would need to be as dynamically
> adjustable as the memory allowance. There is usually no need to be as
> tight with swap space as with memory, and the performance penalty of
> swapping, even with flash drives, is high enough that swap space acts
> as an overflow vessel rather than be part of the regularly backing of
> the anonymous/shmem working set. It really is NOT obvious that swap
> space would need to be adjusted on the fly, and that it's important
> that reducing the limit will be reflected in consumption right away.
>

With my OS support experience, some customers consider swap-space as a resource.


> We shouldn't be adding hundreds of lines of likely terrible heuristics
> code* on speculation that somebody MIGHT find this useful in real life.
> We should wait until we are presented with a real usecase that applies
> to a whole class of users, and then see what the true requirements are.
>
ok, we should wait.  I'm just guessing (japanese) HPC people will want the
feature for their job control. I hear many programs relies on swap.

> * If a group has 200M swapped out and the swap limit is reduced by 10M
> below the current consumption, which pages would you swap in? There is
> no LRU list for swap space.
>
If a rotation can happen when a swap-in-by-real-pagefault, random swap-in
at reducing swap.limit will work enough.

Thanks,
-Kame



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