Re: CFQ iosched praise: good perfomance and better latency

From: Nick Piggin
Date: Mon Apr 19 2004 - 01:14:15 EST

Andrew Morton wrote:
Nick Piggin <nickpiggin@xxxxxxxxxxxx> wrote:

Pedro Larroy wrote:


I've been trying CFQ ioscheduler in my software raid5 with nice results,
I've observed that a latency pattern still exists, just as in the
anticipatory ioscheduler, but those spikes are now much lower (from
6ms with AS to 2ms with CFQ as seen in the bottom of,
plus apps seems to get a fair amount of io so they don't get starved.

Seems a good choice for io loaded boxes. Thanks Jens Axboe.

Although AS isn't at its best when behind raid devices (it should
probably be in front of them), you could be seeing some problem
with the raid code.

I'd be interested to see what the graph looks like with elevator=noop

This isn't a very surprising result, is it? AS throws away latency to gain
throughput. Pedro is measuring latency...

Well I think Pedro actually means *seconds*, not ms. The URL
shows AS peaks at nearly 10 seconds latency, and CFQ over 2s.

It really seems like a raid problem though, because latency
measured at the individual devices is under 250ms for AS.
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