Re: [PATCH RFC] mm: Implement balance_dirty_pages() through waiting for flusher thread

[Dave Chinner <david@fromorbit.com>]

On Fri, Jun 18, 2010 at 11:11:00AM +0200, Peter Zijlstra wrote:
> On Fri, 2010-06-18 at 16:09 +1000, Dave Chinner wrote:
> > > +             bdi->wb_written_head = bdi_stat(bdi, BDI_WRITTEN) + wc->written;
> > 
> > The resolution of the percpu counters is an issue here, I think.
> > percpu counters update in batches of 32 counts per CPU. wc->written
> > is going to have a value of roughly 8 or 32 depending on whether
> > bdi->dirty_exceeded is set or not. I note that you take this into
> > account when checking dirty threshold limits, but it doesn't appear
> > to be taken in to here. 
> 
> The BDI stuff uses a custom batch-size, see bdi_stat_error() and
> related. The total error is in the order of O(n log n) where n is the
> number of CPUs.

#define BDI_STAT_BATCH (8*(1+ilog2(nr_cpu_ids)))

A dual socket server I have here:

[    0.000000] NR_CPUS:512 nr_cpumask_bits:512 nr_cpu_ids:32 nr_node_ids:2

Which means that the bdi per-cpu counter batch size on it would be 48
and the inaccuracy would be even bigger than I described. ;)

> But yeah, the whole dirty_exceeded thing makes life more
> interesting.

I suspect the 8 vs 32 pages could go away without too much impact
with the Jan's mechanism...

Another thing to consider is the impact of mulipage writes on the
incoming value to balance_dirty_pages - if we get 1024 page writes
running, then that may impact on the way throttling behaves, too.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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