[ltt-dev] [RFC PATCH] block: Fix bio merge induced high I/O latency

[Mathieu Desnoyers]

* Ben Gamari (bgamari at gmail.com) wrote:
> On Tue, Jan 20, 2009 at 7:25 PM, Mathieu Desnoyers
> <mathieu.desnoyers at polymtl.ca> wrote:
> > * Mathieu Desnoyers (mathieu.desnoyers at polymtl.ca) wrote:
> >
> > As a side-note : I'd like to have my results confirmed by others.
> 
> Well, I think the (fixed) patch did help to some degree (I haven't
> done fio benchmarks to compare against yet). Unfortunately, the I/O
> wait time problem still remains. I have been waiting 3 minutes now for
> evolution to start with 88% I/O wait time yet no visible signs of
> progress. I've confirmed I'm using the CFQ scheduler, so that's not
> the problem.
> 

Did you also 

echo 1 > /sys/block/sd{a,b}/device/queue_depth
echo 1 > /sys/block/sd{a,b}/queue/iosched/slice_async_rq
echo 1 > /sys/block/sd{a,b}/queue/iosched/quantum

(replacing sd{a,b} with your actual drives) ?

It seems to have been part of the factors that helped (along with the
patch).

And hopefully you don't have a recent Seagate hard drive like me ? :-)

So you test case is :
- start a large dd with 1M block size
- time evolution

?

Mathieu

> Also, Jens, I'd just like to point out that the problem is
> reproducible across all schedulers. Does your patch seek to tackle a
> problem specific to the CFQ scheduler, leaving the I/O wait issue for
> later? Just wondering.
> 
> I'll post some benchmarks numbers once I have them. Thanks,
> 
> - Ben
> 
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-- 
Mathieu Desnoyers
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