Re: [PATCH] remove throttle_vm_writeout()

[Miklos Szeredi]

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

> 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.

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

Why?

> 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.

Miklos
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