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SubjectRe: Very aggressive swapping after 2 hours rest
Byron Stanoszek wrote:
>
> I've finally had a chance to test out the new VM patch on my 32mb system.
>
> It runs much, much better than the previous test8, and the pages->swap change
> is actually much smoother than I had expected it to be considering the recent
> talk about making it more gradual. I'm against having the swap more gradual
> because of the low amount of available memory and the high amount of memory
> actually taken up by processes required for normal operation.
>
> At the moment, there's only room for about 5-6 meg of cache. If a gradual swap
> goes into effect, then I'm afraid that the processes that actually 'need' to
> stay in memory will start swapping out and thrashing, even when there's 6 meg
> still available for use. This was precisely the problem with the old VM on my
> machine, only the system wanted to keep 16 meg free for cache (*gag*).

I hope we can have a swap mechanism that gradually writes stuff
into swap early on, but don't actually unmap the memory.
So we get clean pages that are still available but may be dropped
quickly if needed. This will smooth things a bit without
degrading performance.

Some memory should be kept around for cache (if the processes do
any io at all) or you'll get io trashing: the same stuff read
over and over from files instead of from cache. And a read
before every write that isn't a complete block.

Almost all processes have some pages they don't use much, such
as startup initialization. Swapping out that doesn't hurt.

Helge Hafting
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