Re: [RFC PATCH 0/5] IO-less balance dirty pages

[Jan Kara <jack@suse.cz>]

On Sun 06-02-11 19:54:37, Boaz Harrosh wrote:
> On 02/04/2011 03:38 AM, Jan Kara wrote:
> > The basic idea (implemented in the third patch) is that processes throttled
> > in balance_dirty_pages() wait for enough IO to complete. The waiting is
> > implemented as follows: Whenever we decide to throttle a task in
> > balance_dirty_pages(), task adds itself to a list of tasks that are throttled
> > against that bdi and goes to sleep waiting to receive specified amount of page
> > IO completions. Once in a while (currently HZ/10, in patch 5 the interval is
> > autotuned based on observed IO rate), accumulated page IO completions are
> > distributed equally among waiting tasks.
> > 
> > This waiting scheme has been chosen so that waiting time in
> > balance_dirty_pages() is proportional to
> >   number_waited_pages * number_of_waiters.
> > In particular it does not depend on the total number of pages being waited for,
> > thus providing possibly a fairer results.
> > 
> > I gave the patches some basic testing (multiple parallel dd's to a single
> > drive) and they seem to work OK. The dd's get equal share of the disk
> > throughput (about 10.5 MB/s, which is nice result given the disk can do
> > about 87 MB/s when writing single-threaded), and dirty limit does not get
> > exceeded. Of course much more testing needs to be done but I hope it's fine
> > for the first posting :).
> 
> So what is the disposition of Wu's patches in light of these ones?
> * Do they replace Wu's, or Wu's just get rebased ontop of these at a
>   later stage?
  They are meant as a replacement.

> * Did you find any hard problems with Wu's patches that delay them for
>   a long time?
  Wu himself wrote that the current patchset probably won't fly because it
fluctuates too much. So he decided to try to rewrite patches from per-bdi
limits to global limits when he has time...

> * Some of the complicated stuff in Wu's patches are the statistics and
>   rate control mechanics. Are these the troubled area? Because some of
>   these are actually some things that I'm interested in, and that appeal
>   to me the most.
  Basically yes, this logic seems to be the problematic one.

								Honza
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR