Re: + mm-balance_dirty_pages-reduce-calls-to-global_page_state-to-reduce-c ache-references.patch added to -mm tree

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Sun, 2009-08-23 at 09:32 +0800, Wu Fengguang wrote:
> > > I'd propose to remove the above 'if' and liberate the following three 'if's.
> > 
> > That might work, but it looses the total dirty_thresh constraint. The
> > sum of per-bdi dirties _should_ not be larger than that, but I'm not
> > sure it won't ever be.
> > 
> > The clip code Richard removed ensured that, and I think I wrote that out
> > of more than sheer paranoia, but I'm not sure anymore :/
> 
> Oh I assumed that your per-bdi throttling is not too permissive to
> exceed the global dirty_thresh. In theory the per-bdi throttling
> should be able to quickly stop the growing of (nr_reclaimable +
> nr_writeback).  Once dirty_thresh is reached we already los

Right, so:

 bdi_thresh_n = dirty_thresh * p(n) + eps.

and

 \Sum_n p(n) = 1

So:

 \Sum_n bdi_thresh_n = dirty_thresh + n*eps

Which yields an O(n) error bound.

I'm just not sure how large the thing is in reality, and paranoia won
out.