Re: why swap at all?

[Nick Piggin]

Matthias Schniedermeyer wrote:
> On Wed, May 26, 2004 at 07:48:10PM +1000, Nick Piggin wrote:
>
> > John Bradford wrote:
> >
> > > Quote from Nick Piggin <nickpiggin@xxxxxxxxxxxx>:
> > >
> > >
> > > > Even for systems that don't *need* the extra memory space, swap can
> > > > actually provide performance improvements by allowing unused memory
> > > > to be replaced with often-used memory.
> > >
> > >
> > > That's true, but it's not a magical property of swap space - extra physical
> > > RAM would do more or less the same thing.
> >
> > Well it is a magical property of swap space, because extra RAM
> > doesn't allow you to replace unused memory with often used memory.
> >
> > The theory holds true no matter how much RAM you have. Swap can
> > improve performance. It can be trivially demonstrated.
>
>
> The other way around can be "demonstrated" equally trivially.
>
> In my personal machine i have 3GB of RAM and i regularly create
> DVD-ISO-Images (about 2 per day). After creating an image (reading up to
> 4,4GB and writing up to 4,4GB) the cache is 100% trashed(1). With swap
> it would be even more trashed then it is without swap(1).

I don't disagree that you could find a situation where swap
is worse than no swap. I don't understand what you mean by
trashed and more trashed though :)

Creating your ISOs makes your system swap a lot when swap
is enabled?

> 1: This has "always(tm)" been so since i began burning DVDs 3 years ago.
> Beginning from kernel 2.4.4-2.4.25 and 2.6.4-2.6.6. Currently i use 2.6.5. (This is no typo!)
>
> I have only tested the "with swap"-case with 2.4.4 as i didn't use swap
> after 2.4.4 trashed so badly with swap enabled. But i don't think that
> things have changed so fundamentaly that the "with swap"-case is
> better(FOR ME!) than the "without swap"-case.

The 2.6 VM has changed pretty fundamentally. It would be good
if you could retest.
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