Re: Background scanning change on 2.4.6-pre1

[Marcelo Tosatti <marcelo@conectiva.com.br>]

On Fri, 8 Jun 2001, Linus Torvalds wrote:

> 
> 
> On Fri, 8 Jun 2001, Marcelo Tosatti wrote:
> >
> > I've tried that in the past, and the behaviour I got was pages being
> > swapped out with little (or not any) VM pressure.
> 
> What is "past"?
> 
> Remember: these days we don't do any IO at all inside "swap_out()", the
> _only_ thing we do is to age the VM and possibly move pages to the swap
> cache.

Yes, in the days where we did actual swapouts at page_launder(). 

> Which is really what you wanted - it's just that we delay moving anonymous
> pages to the swap-cache until we have some reason to (ie we delay it until
> we want to re-fill the inactive list).
> 
> Think of it as a simple issue of
>  - when we age pages, we should also check whether they've been dirtied by
>    being mapped, and whether the mappings have accessed them.
> 
> Put that way, I doubt you'll disagree.

I agree. 

> (Now, whether it gets the balancing _right_ is another matter altogether.
> We may have to tune the amount of pages that get looked at, both on the VM
> mapping side and on the active/inactive list sides).

> 
> We actually always used to do this, it was just that we delayed it until
> the active list scan started failing. Which may have been delaying it too
> much, causing "spikes" of activity.
> 
> Or maybe not. I'd like people to explore the balancing space more, instead
> of trying to tune specific parts of the existing balance.
> 
> > Yes, we want fair aging. No, we dont want more pages being swapped out.
> 
> Absolutely. "swap_out()" does not really swap pages out. The name is
> purely due to historical reasons. It should really be called
> "scan_process_mappings()" or similar.
> 
> The actual swap-out obviously happens in page_launder().

Yes. Now the problem is having swap space allocated with _NO_ pressure
may sound a bit weird to people. _I_ know that we're just allocating the
swap space, but not everybody does. 

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