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SubjectRe: 2.4.8/2.4.9 VM problems


On Mon, 20 Aug 2001, Daniel Phillips wrote:

> On August 20, 2001 09:14 pm, Mike Galbraith wrote:
> > On Mon, 20 Aug 2001, Daniel Phillips wrote:
> > > On August 20, 2001 05:40 pm, Mike Galbraith wrote:
> > > > I'll give your patch a shot. In the meantime, below is what I did
> > > > to it here. I might have busted use_once all to pieces ;-) but it
> > > > cured my problem, so I'll show it anyway.
> > >
> > > No, this doesn't break it at all, what it does is require the IO page
> > > to be touched more times before it's considered truly active. This
> > > partly takes care of the theory that an intial burst of activity on
> > > the page should be considered as only one use.
> >
> > (it turns it into a ~sortof used twiceish in my specific case I think..
>
> Actually, used-thriceish.
>
> > the aging must happen to make it work right though.. very very tricky.
>
> I doubt the aging has much to do with it, what's more important is the length
> of the inactive_dirty queue. Of course, aging affects that and so does
> scanning policy, both a little "uncalibrated" at the moment.
>
> > Nope, I don't have anything other than a 'rough visual' to work with..
> > might be totally out there ;-)
>
> What made you think of trying the higher activation threshold? ;-)
>
> > > We can expose this activation threshold through proc so you can adjust it
> > > without recompiling. I'll prepare a patch for that.
> > >
> > > Another thing you might try is just reversing the unlazy activation patch
> > > I posted previously (and Linus put into 2.4.9) because that will achieve
> > > the effect of treating all touches of the page while it's on the inactive
> > > list as a single reference. But that has the disadvantage of making the
> > > system think it has more inactive pages than it really does, and since the
> > > scanning logic is a little fragile it doesn't sound like such a good idea
> > > right now.
> >
> > I don't think this is a big issue. I do inactive listscanning to improve
> > the informational content of the lists, but it only has a _minor_ effect.
> > For maximum performance, it matters, but really we are not to the point
> > that it matters in the general case at all.
>
> OK, but people were seeing the inactive_dirty list getting longer than normal
> and getting worried about it. Before the fixes to zone scanning it likely
> would have been a problem, now most probably not.
>
> > > I intend to try a separate queue for newly activated pages so that the
> > > time spent on the queue can be decoupled from the number of aged-to-zero
> > > inactive pages, and we can get finer control over the period during which
> > > all touches on the page are grouped together into a single reference.
> > > This is 2.5 material.
> >
> > We need to get the pages 'actioned' (the only thing that really matters)
> > off of the dirty list so that they are out of the equation.. that I'm
> > sure of.
>
> Well, except when the page is only going to be used once, or not at all (in
> the case of an unused readahead page). Otherwise, no, we don't want to have
> frequently used pages or pages we know nothing about dropping of the inactive
> queue into the bit-bucket. There's more work to do to make that come true.

Find riel's message with topic "VM tuning" to linux-mm, then take a look
at the 4th aging option.

That one _should_ be able to make us remove all kinds of "hacks" to do
drop behind, and also it should keep hot/warm active memory _in cache_
for more time.

>
> > How is the right way, I don't have a clue ;-) One thing I
> > feel strongly about: the only thing that matters is getting the right
> > number of pages moving in the right direction. (since we are not able
> > to predict the future accurately.. we approximate, and we don't _ever_
> > want to tie that to real time [sync IO is utterly evil] because that
> > then impacts our ability to react to new input to correct our fsckups:)
>
> True, true and true. Personally, I'm training myself to think of everything
> that happens inside the mm on a timescale of allocation events (one page
> alloced = one tick) not real time. Sometimes this happens to correspond
> linearly to real time, but more often not.


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