Re: why swap at all?

[Nick Piggin]

Martin Olsson wrote:
> Hi Linux-gurus,
>
> I agree with Anthony Disante, maybe not all users want swapping. I have 
> myself felt very annoying with swapping lately but I've not yet tried to 
> disable it.
>
> In school I've studied the swapping concept from a theoretical point
> of view, and I fully understand the fact that swapping, if used 
> properly, can both increase performance and provide a safe way to get 
> out of a bad situation when the box runs out of memory. The problem is 
> that in reality this does not work, not on Linux nor on Windows 2000 
> which I use at home. Unfortunately I cannot provide a specific reason 
> why it does not work, I'm very much a end-user/desktop-user, I'm not a 
> kernel hacker (yet). But I see two things that needs improvement atm:

You don't need to provide a specific reason, a report would be
valuable too.

> A) when I do large data processing operations the computer is always 
> very very slow afterwards

Time how long the large data processing operations take, then turn
swap off and time them again.

> B) if I have X Mb of RAM then there should not be imho a single swap 
> read/write until the whole of my X Mb RAM is completely stuffed, is this 
> so today?

Yes, Linux doesn't start swapping or reclaiming at all until your
RAM is full.

> ---
>
> Also, imagine that I disable swap today and start a large data 
> processing operation. During this operation I try to start a new 
> process, here ideally the program should not OOM but instead the memory 
> allocated for the data processing operation should be decreased. Is this 
> possible using today's technology? Can be divide memory into two sorts, 
> one for processes (here to stay memory) and another sort for batch 
> operations (where the amount of memory does not really matter but less 
> memory means less performance). I see the problem with "taking memory 
> back" though, I guess its impossible.

File backed data will be able to be reclaimed, yes.
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