Re: Swap

[Tim Connors]

On Sun, 18 Nov 2001, [ISO-8859-15] François Cami wrote:

> Dan Maas wrote:
> 
> 
> > Still, it puzzles me why a system with no swap space would appear to be more
> > responsive than one with swap (assuming their working sets are quite a bit
> > smaller than total amount of RAM)... Can you do a controlled test somehow,
> > to rule out any sort of placebo effect?
> 
> It's pretty simple... Try putting as much progs as you can into RAM
> (but less than total RAM size) when you have RAM+swap.
> Switching from one prog to another now takes time, because if you need
> to go e.g. from mozilla to openoffice for example, if openoffice has
> been swapped, it'll take ages.
> 
> Another good example is launching X and a few heavy X apps, going back
> to console, doing a few things, like compiling different kernel trees.
> If you have swap, the X + X apps will be swapped. going back to X will
> take ages, because all that data + code has to be moved out to RAM to
> cache the data in the two kernel trees.
> If you don't have swap, maybe one, or both of the two kernel trees
> will end up being not cached into main memory, depending on how much
> RAM left you have. but going back to X will take 1 second instead of 20,
> and thus the system will be more responsive.
> 
> It depends clearly on the situation you're in. I believe running with
> swap is beneficial when your memory load is more than 75% of total
> RAM, and less so when you have a few hundred megs of RAM left with all
> useful apps loaded into RAM (which is not too unlikely these days,
> due to the low price of SD/DDR RAM).

A perfect example of why a system _needs_ tuning knobs - this view of
Linus's that we need a self tuning system is idiotic, because some of us
don't care how long a kernel compile takes (or even how long it takes to
serve a couple of web pages per hour), but _do_ care about the general
system responsiveness. The system cannot predict what *I* the user wants
out of it. Hence we need /proc interfaces to the the VM that say this is a
compiling machine, or this is a desktop machine.....

-- 
TimC -- http://www.physics.usyd.edu.au/~tcon/

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