Re: Inclusion of zoned inactive/free shortage patch

[Marcelo Tosatti]

On Wed, 18 Jul 2001, Linus Torvalds wrote:

> 
> On Wed, 18 Jul 2001, Marcelo Tosatti wrote:
> >
> > I ended up using the "zone_t *zone" as a boolean and forgot to change it
> > before sending the patch.
> 
> Ok.
> 
> My main worry really is that I absolutely detest special cases. Especially
> special cases that just make the code uglier.
> 
> If it is right and necessary to _sometimes_ take zone inactive shortage
> into account, why not do it always?
> 
> I realize that this makes a difference for how you'd do the test. If you
> do it sometimes, you have something like
> 
> 	if (shortage_only && !inactive_shortage(page->zone))
> 		return;
> 
> while if you decide that it is actually ok to always have this heuristic
> you'd probably write it
> 
> 	if (inactive_plenty(page->zone))
> 		return;
> 
> instead. See the difference? The first one says "this time we're only
> interested in zones that have shortage". The second one says "in general,
> if we have plenty of inactive pages in this zone, we don't want to bother
> with it".
> 
> The reason I'd much prefer the latter is:
>  - code that doesn't have special cases is more likely to be correct and
>    have good behaviour over a wide variety of loads - simply because it
>    gets tested under _all_ loads, not just the loads that trigger the
>    special cases
>  - code like the above means that we can more gradually approach the state
>    of some zone shortage. We can do background shortage scanning, and
>    nicely handle the case where we're not actually _short_ on any zone,
>    but some zones are getting close to being short. Which should make the
>    load smoother.
> 
> > Because I tried to avoid strict perzone shortage handling, keeping the
> > global scanning to have _some_ "fair" aging between the zones.
> 
> Sure. But at the same time, if some zone has tons of memory and another
> zone doesn't, then it is ok to say "we can ignore the zone with lots of
> memory for now".
> 
> Yes, it's "unfair". Yes, it will cause the tight zone to be aged out
> quicker. But yes, that's actually what we want.
> 
> Think something more NUMA-like for example - imagine walking a VM tree
> where the process has pages mapped from multiple nodes. At the same time,
> because of node affinity, some nodes would end up being under higher
> memory pressure because of the processes they are running. Do we want to
> age the pages on those nodes faster? Sure.
> 
> And we do NOT want to get into the situation that one zone/node ends up
> being close to the shortage line all the time, and then when it crosses
> over we have a clear "behaviour change".
> 
> Changing behaviour like that is bad.

Ok, I understand and I agree with doing _unconditional_
"zone_inactive_plenty()" instead of conditional
"zone_inactive_shortage()".

This way we do not get _strict_ zoned behaviour (with strict I mean only
doing scanning for zones which have a shortage), making the shortage
handling smoother and doing "fair" aging in cases where there are not
specific zones under pressure.