[Regression] High latency when doing large I/O

[Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>]

Hi,

A long standing I/O regression (since 2.6.18, still there today) has hit
Slashdot recently :
http://bugzilla.kernel.org/show_bug.cgi?id=12309
http://it.slashdot.org/article.pl?sid=09/01/15/049201

I've taken a trace reproducing the wrong behavior on my machine and I
think it's getting us somewhere.

LTTng 0.83, kernel 2.6.28
Machine : Intel Xeon E5405 dual quad-core, 16GB ram
(just created a new block-trace.c LTTng probe which is not released yet.
It basically replaces blktrace)


echo 3 > /proc/sys/vm/drop_caches

lttctl -C -w /tmp/trace -o channel.mm.bufnum=8 -o channel.block.bufnum=64 trace

dd if=/dev/zero of=/tmp/newfile bs=1M count=1M
cp -ax music /tmp   (copying 1.1GB of mp3)

ls  (takes 15 seconds to get the directory listing !)

lttctl -D trace

I looked at the trace (especially at the ls surroundings), and bash is
waiting for a few seconds for I/O in the exec system call (to exec ls).

While this happens, we have dd doing lots and lots of bio_queue. There
is a bio_backmerge after each bio_queue event. This is reasonable,
because dd is writing to a contiguous file.

However, I wonder if this is not the actual problem. We have dd which
has the head request in the elevator request queue. It is progressing
steadily by plugging/unplugging the device periodically and gets its
work done. However, because requests are being dequeued at the same
rate others are being merged, I suspect it stays at the top of the queue
and does not let the other unrelated requests run.

There is a test in the blk-merge.c which makes sure that merged requests
do not get bigger than a certain size. However, if the request is
steadily dequeued, I think this test is not doing anything.

If you are interested in looking at the trace I've taken, I could
provide it.

Does that make sense ?

Mathieu

-- 
Mathieu Desnoyers
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