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SubjectRe: why swap at all?
Anthony DiSante wrote:

> As a general question about ram/swap and relating to some of the
> issues in this thread:
>
> ~500 megs cached yet 2.6.5 goes into swap hell
>
> Consider this: I have a desktop system with 256MB ram, so I make a
> 256MB swap partition. So I have 512MB "memory" and if some process
> wants more, too bad, there is no more.
>
> Now I buy another 256MB of ram, so I have 512MB of real memory. Why
> not just disable my swap completely now? I won't have increased my
> memory's size at all, but won't I have increased its performance lots?

This is correct. You now have 512M of fast memory instead of
256M fast memory and 256M "slow" memory. You don't _need_ to have additional
swap, but it is usually a good idea. If you keep your 256M of swap,
then you now
have 512M fast memory + 256M slow memory for a total of 768M. This is
even better.

Please note that your machine _will_ do one kind of swapping even if you
don't configure any swap: Executable files are a kind of swap-files,
if memory pressure happens then (part of) your programs will be evicted
from memory _because_ they can be reloaded from their executables.

This cause the same sort of performance degradations as swapping to
a swap partition. Actually, it is worse because swapping to a swap
partition
allows swapping out little-used writeable memory before discarding
program code that might see more use. So if swapping happens, then
you're better off with a swap partition because then it is the least used
stuff that goes first. Without a swap partition, the least used program code
goes, but it may or may not be the least used memory overall.

Helge Hafting



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