linux-kernel - Re: [RFC v3 0/3] vmpressure_fd: Linux VM pressure notifications

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Date:	Mon, 19 Nov 2012 17:57:19 +0400
From:	Glauber Costa <glommer@...allels.com>
To:	David Rientjes <rientjes@...gle.com>
CC:	Anton Vorontsov <anton.vorontsov@...aro.org>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Pekka Enberg <penberg@...nel.org>,
	Mel Gorman <mgorman@...e.de>,
	Leonid Moiseichuk <leonid.moiseichuk@...ia.com>,
	"KOSAKI Motohiro" <kosaki.motohiro@...il.com>,
	Minchan Kim <minchan@...nel.org>,
	Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
	John Stultz <john.stultz@...aro.org>, <linux-mm@...ck.org>,
	<linux-kernel@...r.kernel.org>, <linaro-kernel@...ts.linaro.org>,
	<patches@...aro.org>, <kernel-team@...roid.com>,
	<linux-man@...r.kernel.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Michal Hocko <mhocko@...e.cz>,
	Johannes Weiner <hannes@...xchg.org>, Tejun Heo <tj@...nel.org>
Subject: Re: [RFC v3 0/3] vmpressure_fd: Linux VM pressure notifications

On 11/17/2012 01:57 AM, David Rientjes wrote:
> On Sat, 17 Nov 2012, Glauber Costa wrote:
> 
>>> I'm wondering if we should have more than three different levels.
>>>
>>
>> In the case I outlined below, for backwards compatibility. What I
>> actually mean is that memcg *currently* allows arbitrary notifications.
>> One way to merge those, while moving to a saner 3-point notification, is
>> to still allow the old writes and fit them in the closest bucket.
>>
> 
> Yeah, but I'm wondering why three is the right answer.
> 

This is unrelated to what I am talking about.
I am talking about pre-defined values with a specific event meaning (in
his patchset, 3) vs arbitrary numbers valued in bytes.

>>> Umm, why do users of cpusets not want to be able to trigger memory 
>>> pressure notifications?
>>>
>> Because cpusets only deal with memory placement, not memory usage.
> 
> The set of nodes that a thread is allowed to allocate from may face memory 
> pressure up to and including oom while the rest of the system may have a 
> ton of free memory.  Your solution is to compile and mount memcg if you 
> want notifications of memory pressure on those nodes.  Others in this 
> thread have already said they don't want to rely on memcg for any of this 
> and, as Anton showed, this can be tied directly into the VM without any 
> help from memcg as it sits today.  So why implement a simple and clean 
> mempressure cgroup that can be used alone or co-existing with either memcg 
> or cpusets?
> 
>> And it is not that moving a task to cpuset disallows you to do any of
>> this: you could, as long as the same set of tasks are mounted in a
>> corresponding memcg.
>>
> 
> Same thing with a separate mempressure cgroup.  The point is that there 
> will be users of this cgroup that do not want the overhead imposed by 
> memcg (which is why it's disabled in defconfig) and there's no direct 
> dependency that causes it to be a part of memcg.
> 
I think we should shoot the duck where it is going, not where it is. A
good interface is more important than overhead, since this overhead is
by no means fundamental - memcg is fixable, and we would all benefit
from it.

Now, whether or not memcg is the right interface is a different
discussion - let's have it!

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