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[dm-devel] Re: IO scheduler based IO controller V10



On Sat, 2009-10-03 at 09:24 +0200, Jens Axboe wrote:

> After shutting down the computer yesterday, I was thinking a bit about
> this issue and how to solve it without incurring too much delay. If we
> add a stricter control of the depth, that may help. So instead of
> allowing up to max_quantum (or larger) depths, only allow gradual build
> up of that the farther we get away from a dispatch from the sync IO
> queues. For example, when switching to an async or seeky sync queue,
> initially allow just 1 in flight. For the next round, if there still
> hasn't been sync activity, allow 2, then 4, etc. If we see sync IO queue
> again, immediately drop to 1.
> 
> It could tie in with (or partly replace) the overload feature. The key
> to good latency and decent throughput is knowing when to allow queue
> build up and when not to.

Hm.  Starting at 1 sounds a bit thin (like IDLE), multiple iterations to
build/unleash any sizable IO, but that's just my gut talking.

	-Mike


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