Re: [PATCH] remove throttle_vm_writeout()

[Peter Zijlstra]

On Thu, 2007-10-04 at 15:00 +0200, Miklos Szeredi wrote:
> > > 1) File backed pages -> file
> > > 
> > >   dirty + writeback count remains constant
> > > 
> > > 2) Anonymous pages -> swap
> > > 
> > >   writeback count increases, dirty balancing will hold back file
> > >   writeback in favor of swap
> > > 
> > > So the real question is: does case 2 need rate limiting, or is it OK
> > > to let the device queue fill with swap pages as fast as possible?
> > 
> > > Because balance_dirty_pages() maintains:
> > 
> >  nr_dirty + nr_unstable + nr_writeback < 
> > 	total_dirty + nr_cpus * ratelimit_pages
> > 
> > throttle_vm_writeout() _should_ not deadlock on that, unless you're
> > caught in the error term: nr_cpus * ratelimit_pages. 
> 
> And it does get caught on that in small memory machines.  This
> deadlock is easily reproducable on a 32MB UML instance.  

Ah, yes, for those that is indeed easily doable.

> I haven't yet
> tested with the per-bdi patches, but I don't think they make a
> difference in this case.

Correct, they would not.

> > Which can only happen when it is larger than 10% of dirty_thresh.
> > 
> > Which is even more unlikely since it doesn't account nr_dirty (as I
> > think it should).
> 
> I think nr_dirty is totally irrelevant.  Since we don't care about
> case 1), and in case 2) nr_dirty doesn't play any role.

Ah, but its correct to have since we compare against dirty_thresh, which
is defined to be a unit of nr_dirty + nr_unstable + nr_writeback. if we
take one of these out, then we get an undefined amount of space extra.

> > As for 2), yes I think having a limit on the total number of pages in
> > flight is a good thing.
> 
> Why?

for my swapping over network thingies I need to put a bound on the
amount of outgoing traffic in flight because that bounds the amount of
memory consumed by the sending side.

> > But that said, there might be better ways to do that.
> 
> Sure, if we do need to globally limit the number of under-writeback
> pages, then I think we need to do it independently of the dirty
> accounting.

It need not be global, it could be per BDI as well, but yes.

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