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Re: How to handle TIF_MEMDIE stalls?

To: Johannes Weiner <hannes@xxxxxxxxxxx>
Subject: Re: How to handle TIF_MEMDIE stalls?
From: Michal Hocko <mhocko@xxxxxxx>
Date: Mon, 2 Mar 2015 18:10:58 +0100
Cc: Dave Chinner <david@xxxxxxxxxxxxx>, Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>, dchinner@xxxxxxxxxx, linux-mm@xxxxxxxxx, rientjes@xxxxxxxxxx, oleg@xxxxxxxxxx, akpm@xxxxxxxxxxxxxxxxxxxx, mgorman@xxxxxxx, torvalds@xxxxxxxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx
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User-agent: Mutt/1.5.23 (2014-03-12)
On Mon 02-03-15 11:05:37, Johannes Weiner wrote:
> On Mon, Mar 02, 2015 at 04:18:32PM +0100, Michal Hocko wrote:
> > Typical busy system won't be very far away from the high watermark
> > so there would be a reclaim performed during increased watermaks
> > (aka reservation) and that might lead to visible performance
> > degradation. This might be acceptable but it also adds a certain level
> > of unpredictability when performance characteristics might change
> > suddenly.
> There is usually a good deal of clean cache.  As Dave pointed out
> before, clean cache can be considered re-allocatable from NOFS
> contexts, and so we'd only have to maintain this invariant:
>       min_wmark + private_reserves < free_pages + clean_cache

Do I understand you correctly that we do not have to reclaim clean pages
as per the above invariant?

If yes, how do you reflect overcommit on the clean_cache from multiple
requestor (who are doing reservations)?
My point was that if we keep clean pages on the LRU rather than forcing
to reclaim them via increased watermarks then it might happen that
different callers with access to reserves wouldn't get promissed amount
of reserved memory because clean_cache is basically a shared resource.

Michal Hocko

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