On 15 Mar 2000, Rask Ingemann Lambertsen wrote:
> Den 15-Mar-00 11:49:45 skrev James Sutherland følgende om "Re: Overcommitable memory??":
> > On Wed, 15 Mar 2000, David Whysong wrote:
>
> >> The only possibilities are to (a) enforce hard memory limits with no
> >> overcommit, thereby wasting large amounts of swap space that will never
> >> get used while not really solving the problem,
>
> > As well as *destroying* performance (you'd effectively eliminate COW
> > capability on fork - if your 500Mb simulation wants to fork a 100k mailer
> > process to send you an update, the kernel has to allocate and copy 500Mb
> > of RAM/swap first, then discard it all again.)
>
> 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.
James.
-
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