Re: [PATCH 3/6] cpuset write throttle

[Ethan Solomita <solo@google.com>]

Christoph Lameter wrote:
> On Tue, 2 Oct 2007, Ethan Solomita wrote:
> 
>> 	Unfortunately this eliminates one of the main reasons for the
>> per-cpuset throttling. If one cpuset is responsible for pushing one
>> disk/BDI to its dirty limit, someone in another cpuset can get throttled.
> 
> I think that is acceptable. All processes that write to one disk/BDI must 
> be affected by congestion on that device. We may have to deal with 
> fairness issues later if it indeed becomes a problem.

	We do see a fairness issue. We've seen delays on the order of 100
seconds for just a few writes to disk, and latency is important to us.
Perhaps we can detect that the bdi already has a long queue of pending
writes and not force more writes at this time so long as the per-cpuset
dirty threshold is not too high.

	On a side note, get_dirty_limits() now returns two dirty counts, both
the dirty and bdi_dirty, yet its callers only ever want one of those
results. Could we change get_dirty_limits to only calculate one dirty
value based upon whether bdi is non-NULL? This would save calculation of
regular dirty when a bdi is passed.
	-- Ethan

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