[Regression] High latency when doing large I/O

From: Mathieu Desnoyers
Date: Fri Jan 16 2009 - 19:44:54 EST


A long standing I/O regression (since 2.6.18, still there today) has hit
Slashdot recently :

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 Desnoyers
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