linux-kernel - Re: [patch] mm, vmpressure: add high level

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Message-ID: <alpine.DEB.2.02.1310162042050.30329@chino.kir.corp.google.com>
Date:	Wed, 16 Oct 2013 20:44:12 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Anton Vorontsov <anton@...msg.org>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Michal Hocko <mhocko@...e.cz>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [patch] mm, vmpressure: add high level

On Wed, 16 Oct 2013, Anton Vorontsov wrote:

> > Vmpressure has two important levels: medium and critical.  Medium is 
> > defined at 60% and critical is defined at 95%.
> > 
> > We have a customer who needs a notification at a higher level than medium, 
> > which is slight to moderate reclaim activity, and before critical to start 
> > throttling incoming requests to save memory and avoid oom.
> > 
> > This patch adds the missing link: a high level defined at 80%.
> > 
> > In the future, it would probably be better to allow the user to specify an 
> > integer ratio for the notification rather than relying on arbitrarily 
> > specified levels.
> 
> Does the customer need to differentiate the two levels (medium and high),
> or the customer only interested in this (80%) specific level?
> 

Only high.

> In the latter case, instead of adding a new level I would vote for adding
> a [sysfs] knob for modifying medium level's threshold.
> 

Hmm, doesn't seem like such a good idea.  If one process depends on this 
being 60% and another depends on it being 80%, we're stuck.  I think it's 
legitimate to have things like low, medium, high, and critical as rough 
approximations (and to keep backwards compatibility), but as mentioned in 
the changelog I want to extend the interface to allow integer writes to 
specify their own ratio.
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