Re: per bdi dirty balancing (was Re: kupdate weirdness)

[Peter Zijlstra <peterz@infradead.org>]

On Fri, 2007-08-03 at 08:43 +0200, Miklos Szeredi wrote:
> (cc restored)
> 
> > > > There were heaps of problems in there and it is surprising how few people
> > > > were hitting them.  Ordered-mode journalling filesystems will fix it all up
> > > > behind the scenes, of course.
> > > > 
> > > > I just have a bad feeling about that code - list_heads are the wrong data
> > > > structure and it all needs to be ripped and redone using some indexable
> > > > data structure.  There has been desultory discussion, but nothing's
> > > > happening and nothing will happen in the medium term, so we need to keep
> > > > on whapping bandainds on it.
> > > 
> > > The reason why I'm looking at that code is because of those
> > > balance_dirty_pages() deadlocks.  I'm not perfectly happy with the
> > > per-pdi-per-cpu counters Peter's patch is introducing.
> > 
> > What is your biggest concern regarding them?
> 
> Complexity.  I've started to review the patches, and they are just too
> damn complex.
> 
> For example introducing the backing_dev_info initializer and
> destructor adds potential bugs if we miss to add them somewhere.

yeah, that was/is a pain.

> Now maybe this is unavoidable.  I'm just trying to look for a solution
> involving less uncertanties and complexities.
> 
> My plan is to extract the minimal set of features from your patchset,
> that solves the dirty balancing deadlocks and submit them as quickly
> as possible.

I had hoped to post a new version yesterday, but lets hope for today.

> After that we can look at trying to solve the more ambitious problem
> of the slow vs. fast devices in a way that not only you can understand ;)

Drad, and here I thought all that documentation in the proportions lib
would have solved that :-(