linux-kernel - Re: Should calculation of vm.overcommit

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Message-ID: <20100420183252.0c61208f@lxorguk.ukuu.org.uk>
Date:	Tue, 20 Apr 2010 18:32:52 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Dave Wright <wrightd@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Should calculation of vm.overcommit_ratio be changed?

> might have 4GB RAM and 1GB swap. I don't think you would expect
> Desktop users to understand or tweak overcommit_ratio, but I also
> don't think having the distro simply change the default from 50 (to
> 100 or something else) would cover all the cases well.

Sounds like the distribution should be tuning the value according to the
> 
> Would it make more sense to have the overcommit formula be calculated as:
> 
> max commit = min(swap, ram) * overcommit_ratio + max(swap, ram) ?
> 
> When swap>=ram, the formula works exactly the same as it does now, but
> when ram>>swap, you are guaranteed to always be able to your full RAM
> (even when swap=0).

Which is wromg - some of your RAM ends up eaten by the kernel, by
pagetables and buffers etc. 50% is probably very conservative but the
point of VM overcommit is exactly that - and you end up deploying swap
as a precaution against disaster rather than because you need it.

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