Re: Weirdness in block device queues.

From: Andrea Arcangeli (
Date: Thu Sep 07 2000 - 20:21:06 EST

Eric, sorry for the late reply.

On Thu, 7 Sep 2000, Eric Youngdale wrote:

> The oddness is this. We were observing stalls in the processing of
>commands that was traced to the fact that the queue had remained plugged
>for an excessive amount of time. The stalls last for about 5 seconds or

If during the stalls the disk is writing to disk at full speed (not idle),
then I agree the right fix is to unplug the queue before putting kflushd
to sleep (after all it makes sense to do the I/O while the disk is
otherwise idle).

However I'm also worried by the size of the queues, maybe the high latency
is a sign that the queue is too big (5 seconds at 20mbyte/sec are
100mbytes of stuff into the I/O queue, are you really waiting 5 seconds
while the disk is writing?).


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