Re: Overcommitable memory??

From: Alan Curry (pacman-kernel@cqc.com)
Date: Thu Mar 16 2000 - 14:48:40 EST


James Sutherland writes the following:
>
>On 15 Mar 2000, Rask Ingemann Lambertsen wrote:
>> Not at all. COW is a performance optimisation which does not depend on
>> overcommitment of memory in any way. Why would you want to turn it off?
>
>Because it *IS* overcommitment of memory. You can have two processes, each
>with their 200Mb of data, in a machine with 256Mb RAM+swap, quite happily
>- until they start writing to it, at which point you discover you have
>overcommitted your memory, and things go wrong.

Just because you can describe an example scenario in which COW and
overcommit are both used, does not mean that they are inseparable. You can do
COW by simply *reserving* RAM or swap space at fork() time and copying data
into it later. That is COW without overcommit.

Unfortunately nobody with the necessary skills seems interested in
implementing it that way.

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