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

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Date:	Wed, 7 Nov 2012 14:11:10 +0200
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Anton Vorontsov <anton.vorontsov@...aro.org>
Cc:	Mel Gorman <mgorman@...e.de>, Pekka Enberg <penberg@...nel.org>,
	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, Glauber Costa <glommer@...allels.com>
Subject: Re: [RFC v3 0/3] vmpressure_fd: Linux VM pressure notifications

On Wed, Nov 07, 2012 at 03:43:46AM -0800, Anton Vorontsov wrote:
> On Wed, Nov 07, 2012 at 01:21:36PM +0200, Kirill A. Shutemov wrote:
> [...]
> > Sorry, I didn't follow previous discussion on this, but could you
> > explain what's wrong with memory notifications from memcg?
> > As I can see you can get pretty similar functionality using memory
> > thresholds on the root cgroup. What's the point?
> 
> There are a few reasons we don't use cgroup notifications:
> 
> 1. We're not interested in the absolute number of pages/KB of available
>    memory, as provided by cgroup memory controller. What we're interested
>    in is the amount of easily reclaimable memory and new memory
>    allocations' cost.
> 
>    We can have plenty of "free" memory, of which say 90% will be caches,
>    and say 10% idle. But we do want to differentiate these types of memory
>    (although not going into details about it), i.e. we want to get
>    notified when kernel is reclaiming. And we also want to know when the
>    memory comes from swapping others' pages out (well, actually we don't
>    call it swap, it's "new allocations cost becomes high" -- it might be a
>    result of many factors (swapping, fragmentation, etc.) -- and userland
>    might analyze the situation when this happens).
> 
>    Exposing all the VM details to userland is not an option

IIUC, you want MemFree + Buffers + Cached + SwapCached, right?
It's already exposed to userspace.

> -- it is not
>    possible to build a stable ABI on this. Plus, it makes it really hard
>    for userland to deal with all the low level details of Linux VM
>    internals.
> 
>    So, no, raw numbers of "free/used KBs" are not interesting at all.
> 
> 1.5. But it is important to understand that vmpressure_fd() is not
>      orthogonal to cgroups (like it was with vmevent_fd()). We want it to
>      be "cgroup'able" too. :) But optionally.
> 
> 2. The last time I checked, cgroups memory controller did not (and I guess
>    still does not) not account kernel-owned slabs. I asked several times
>    why so, but nobody answered.

Almost there. Glauber works on it.

-- 
 Kirill A. Shutemov
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