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SubjectWeirdness in block device queues.
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<DIV><FONT face=Arial size=2></FONT>&nbsp;</DIV>
<DIV><FONT face=Arial size=2>&nbsp;&nbsp;&nbsp; Doug Gilbert and I ran across
some weirdness in the way the block device queues are plugged/unplugged.&nbsp;
It turned up with some benchmarks of the SCSI generics driver - with the new
queueing code, the generics driver is inserting requests into the same queue
that block device requests are inserted.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT>&nbsp;</DIV>
<DIV><FONT face=Arial size=2>&nbsp;&nbsp;&nbsp; The oddness is this.&nbsp; 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.&nbsp; The
stalls last for about 5 seconds or so.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT>&nbsp;</DIV>
<DIV><FONT face=Arial size=2>&nbsp;&nbsp;&nbsp; Some investigation revealed that
part of the answer is that the bdflush daemon essentially forces a bunch of
dirty pages to be written to disk, but never bothers to unplug the queue when it
is done.&nbsp; The result is that the queue remains plugged until someone else
comes along and unplugs it.&nbsp; As it turns out, kupdate() does unplug the
queue, and kupdate runs every 5 seconds or so.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT>&nbsp;</DIV>
<DIV><FONT face=Arial size=2>&nbsp;&nbsp;&nbsp; Patching bdflush to run tq_disk
after flushing buffers (i.e. before the schedule()) fixed *most* of the problem,
but evidently not all of it (Doug was still observing stalls, but a lot less
frequently).&nbsp; In other words, there is someone else out there queueing
requests in such a way that the queue can remain plugged for some amount of
time.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT>&nbsp;</DIV>
<DIV><FONT face=Arial size=2>&nbsp;&nbsp;&nbsp; My gut tells me that it is wrong
for bdflush to not unplug the queue when it is done queueing I/O requests.&nbsp;
My gut also tells me that the generics driver probably wants to be unplugging
the one specific queue that it is using to ensure that I/O gets queued right
away (it doesn't make sense to unplug all queues in this instance).</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT>&nbsp;</DIV>
<DIV><FONT face=Arial size=2>&nbsp;&nbsp;&nbsp; Comments?</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT>&nbsp;</DIV>
<DIV><FONT face=Arial size=2>-Eric</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT>&nbsp;</DIV></BODY></HTML>
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