Re: why swap at all?

[Helge Hafting]

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



-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/