Re: why swap at all?
From: Matthias Schniedermeyer (ms_at_citd.de)
Date: 05/26/04
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Date: Wed, 26 May 2004 12:58:37 +0200 To: Nick Piggin <nickpiggin@yahoo.com.au>
On Wed, May 26, 2004 at 08:33:28PM +1000, Nick Piggin wrote:
> Matthias Schniedermeyer wrote:
> >On Wed, May 26, 2004 at 07:48:10PM +1000, Nick Piggin wrote:
> >
> >>John Bradford wrote:
> >>
> >>>Quote from Nick Piggin <nickpiggin@yahoo.com.au>:
> >>>
> >>>
> >>>>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 :)
trashed means "everything i need(tm)" is paged out (mozilla/konsole/xine
...)
with swap the data-part of running programs was swapped out, without
swap only the program-part is thrown out of memory as the data-part
can't be moved anywhere else.
I have a 10KPRM SCSI-HDD, i can here what my system is doing. :-)
> Creating your ISOs makes your system swap a lot when swap
> is enabled?
Transfering up to 8,8GB tends to trash the cache.
> >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.
Bis denn
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