Re: Overcommitable memory??

From: James Sutherland (
Date: Thu Mar 16 2000 - 14:57:03 EST

On Thu, 16 Mar 2000, Alan Curry wrote:

> 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.

This would entail reserving (in some cases extremely large amounts of)
memory which is never used. This just isn't an itch it is worth scratching
- reserving memory in this way would achieve very little. Particularly in
cases such as WWW servers under Apache, you could end up reserving 1 Gb or
more of swap which you never use - is this desirable at all, let alone
"worth it"??


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