Messages in this thread | | | Date | Fri, 17 Mar 2000 10:58:51 +0000 (GMT) | From | James Sutherland <> | Subject | Re: Overcommitable memory?? |
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On Fri, 17 Mar 2000, Andreas Bombe wrote:
> On Thu, Mar 16, 2000 at 11:04:23AM +0000, James Sutherland wrote: > > 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. > > He means avoiding overcommit by counting vm requirements but without > actually copying COW pages (denying a COW allocation if it could not > be faulted in at a later time). Resulting in vast areas of unused > RAM.
Yes, I know. This does avoid the performance hit - but it still wastes obscene amounts of swap space unnecessarily, and makes the original problem worse by reducing the available amount of memory.
On a WWW server with 100 Apache processes of 20Mb, for example, I would need 30Mb or so normally - or 2Gb with this strategy, even though 1.97Gb of this is never used. This means I will run out of memory a LOT sooner - I have 1.97Gb less VM than I otherwise would! This certainly doesn't help the original problem...
James.
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