Re: why swap at all?

[Catalin BOIE]

Hello!

> But swap behaviour kills performance even when memory is more than adequate. 
> Consider building a DVD image in a 4GB system. The i/o forces all of the 
> unused programs out, in spite of the fact that an extra 100MB doesn't make a 
> measurable difference in performance. But when I click Mozilla paging most of 
> it in from disk make a big difference in performance to the user.

I think that kernel cannot know that you need some data once or more.
This is fadvise for.
With my wrapper (http://kernel.umbrella.ro) for fadvise you can do this:
NOCA_SIZE=128 NOCA_READ=1 NOCA_WRITE=1 NOCA_RA=1 \
	noca mkisofs -R -o /tmp/1.iso /tmp/data

This means:
NOCA_SIZE: Call fadvise only after 128KiB was read/wrote.
NOCA_RA: call fadvise with POSIX_FADV_SEQUENTIAL
NOCA_READ: use fadvise(POSIX_FADV_DONTNEED) for reads (because you don't 
need anymore the source files)
NOCA_WRITE: use fadvise(POSIX_FADV_DONTNEED) for writes (because it's 
useless to cache the end of the ISO)

Do this program resolve your problem?

> The problems with small memory are different in kind, when not even the 
> programs will fit in memory at the same time, or will leave next to nothing 
> for i/o, swap is required for performance. But on a large memory system I 
> believe the gain to pain ratio is way too low with the current VM. The 
> solution at the moment is to turn off swap, which as you note has other 
> problems (can't move between zones without swap?) which in theory could 
> really hang a system.
>
> --
>   -bill davidsen (davidsen@xxxxxxx)
> "The secret to procrastination is to put things off until the
> last possible moment - but no longer"  -me
> -
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---
Catalin(ux aka Dino) BOIE
catab at deuroconsult.ro
http://kernel.umbrella.ro/
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