linux-kernel - io_submit delay problems revisited

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Message-ID: <Pine.LNX.4.63.0612221816590.9502@miro.cs.duke.edu>
Date:	Fri, 22 Dec 2006 22:16:06 -0500 (EST)
From:	Patrick Reynolds <reynolds@...duke.edu>
To:	linux-kernel@...r.kernel.org
Subject: io_submit delay problems revisited

I believe that io_submit() blocks when it shouldn't.  I have read the two
recent LKML threads on the subject.

My computer is a Thinkpad X60: Core Duo 1.83 with a SATA hard disk and
1.5GB of RAM.  I'm running a plain-vanilla 2.6.19.1 kernel, though the
error is also present under 2.6.18.1.  I have Debian's libaio-dev
0.3.106-3 installed.

I've written a handful of test programs that call the kernel's AIO system
calls (io_submit and io_getevents) directly.  They all exhibit the same
behavior: io_submit() calls take long enough to perform the actual I/O
operation, and io_getevents() returns instantly.  This behavior happens
whether the submitted requests are files on disk or network sockets.

I don't believe it's related to block-layer queueing, per the recent LKML
thread.  The delays happen even when I'm performing 32 parallel 512-byte
I/Os -- probably not enough to overflow any queues at the block layer.
And it happens from the very first io_submit() call, before the
block-layer queues would be full.  Changing
/sys/block/sda/queue/nr_requests has no effect.

I've attached my test program (compilation instructions included) and the
output of strace -T.  Note that nearly all io_submit calls take 10+
milliseconds, which is right where you'd expect the seek time on a laptop
hard drive to be.  Also note that the io_getevents calls return all 32
outstanding I/Os at once -- why would 32 disk seeks all finish at the same
time?

Any ideas?

--Patrick
View attachment "aiotest.c" of type "TEXT/PLAIN" (2570 bytes)

View attachment "aio.trace" of type "TEXT/PLAIN" (56629 bytes)