linux-kernel - Re: min_free_kbytes documentation, /proc/sys/vm docs

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Message-ID: <20070813141231.GA9834@atrey.karlin.mff.cuni.cz>
Date:	Mon, 13 Aug 2007 16:12:31 +0200
From:	Pavel Machek <pavel@....cz>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Andrew Morton <akpm@...l.org>,
	kernel list <linux-kernel@...r.kernel.org>, andrea@...e.de,
	npiggin@...e.de
Subject: Re: min_free_kbytes documentation, /proc/sys/vm docs

Hi!

> > min_free_kbytes documentation says:
> > 
> > min_free_kbytes:
> > 
> > This is used to force the Linux VM to keep a minimum number 
> > of kilobytes free.  The VM uses this number to compute a pages_min
> > value for each lowmem zone in the system.  Each lowmem zone gets 
> > a number of reserved free pages based proportionally on its size.
> > 
> > ....which is not too helpful :-(. In particular, should kernel survive
> > setting this to 0 and still work reliably? IOW is this
> > performance-only tweak, or does it correctness implications?
> 
> correctness
> 
> sort-of, since PF_MEMALLOC usage is currently un-bounded.

/me is scared. If this is for correctness... why is it different on
different machines? Should we specify some minimum value in the
documentation?

> > I guess it should also say something like "do not set this to more
> > than half of your physical RAM (or half of lowmem?) or you get an
> > instant OOM".
> 
> yeah, lowmem, not sure on the oom part, it should work with quite high
> limits, although I've never tried it with ridiculous numbers.

I did, and it indeed OOMs instantly.

Perhaps something like this should be applied?

Signed-off-by: Pavel Machek <pavel@...e.cz>

diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt
index a0ccc5b..e460c19 100644
--- a/Documentation/sysctl/vm.txt
+++ b/Documentation/sysctl/vm.txt
@@ -111,6 +111,12 @@ of kilobytes free.  The VM uses this num
 value for each lowmem zone in the system.  Each lowmem zone gets 
 a number of reserved free pages based proportionally on its size.
 
+Some minimal ammount of memory is needed to satisfy PF_MEMALLOC
+allocations; if you set this to lower than 1024KB, your system will
+become subtly broken, and prone to deadlock under high loads.
+
+Setting this too high will OOM your machine instantly.
+
 ==============================================================
 
 percpu_pagelist_fraction

								Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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