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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This archive was generated by hypermail 2b29 : Thu Mar 23 2000 - 21:00:23 EST