On Fri, 17 Mar 2000, Peter Svensson wrote:
> On Thu, 16 Mar 2000, James Sutherland wrote:
>
> > The problem is, that fork() allocates memory - the same amount as the
> > parent process has allocated. If you want to honour this without
> > overcommit, you must disable COW - otherwise you've overcommitted your
> > memory.
>
> Of course not. COW is simply a speed optimization. You can still allocate
> the pages at fork time but delay the actual copying until write time. COW
> is orthogonal to the allocation (or at least the accounting) of pages.
Yes, you COULD have COW without overcommitting - but you still lose one of
the major benefits of COW, namely huge savings on VM usage. If I fork()
100 Apache processes of 20Mb each, I need perhaps 30Mb of VM total.
WITHOUT overcommitted COW, I end up needing 2Gb of swap space - 1.98Gb of
which I will never use! This is certainly not an efficient use of swap
space, IMO...
James.
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