[PATCH] self-unplugging request queues

The patch teaches a queue to unplug itself:

a) if is has four requests OR
b) if it has had plugged requests for 3 milliseconds.

These numbers may need to be tuned, although doing so doesn't seem to
make much difference.  10 msecs works OK, so HZ=100 machines will be
fine.

Instrumentation shows that about 5-10% of requests were started due to
the three millisecond timeout (during a kernel compile).  That's
somewhat significant.  It means that the kernel is leaving stuff in the
queue, plugged, for too long.  This testing was with a uniprocessor
preemptible kernel, which is particularly vulnerable to unplug latency
(submit some IO, get preempted before the unplug).

This patch permits the removal of a lot of rather lame unplugging in
page reclaim and in the writeback code, which kicks the queues
(globally!) every four megabytes to get writeback underway.

This patch doesn't use blk_run_queues().  It is able to kick just the
particular queue.

The patch is not expected to make much difference really, except for
AIO.  AIO needs a blk_run_queues() in its io_submit() call.  For each
request.  This means that AIO has to disable plugging altogether,
unless something like this patch does it for it.  It means that AIO
will unplug *all* queues in the machine for every io_submit().  Even
against a socket!

This patch was tested by disabling blk_run_queues() completely.  The
system ran OK.

The 3 milliseconds may be too long.  It's OK for the heavy writeback
code, but AIO may want less.  Or maybe AIO really wants zero (ie:
disable plugging).  If that is so, we need new code paths by which AIO
can communicate the "immediate unplug" information - a global unplug is
not good.


To minimise unplug latency due to user CPU load, this patch gives keventd
`nice -10'.  This is of course completely arbitrary.  Really, I think keventd
should be SCHED_RR/MAX_RT_PRIO-1, as it has been in -aa kernels for ages.

1 files changed, 10 insertions, 0 deletions

diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index c599ea36233b..82766b7e60b0 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -4,6 +4,8 @@
 #include <linux/major.h>
 #include <linux/genhd.h>
 #include <linux/list.h>
+#include <linux/timer.h>
+#include <linux/workqueue.h>
 #include <linux/pagemap.h>
 #include <linux/backing-dev.h>
 #include <linux/wait.h>
@@ -188,6 +190,14 @@ struct request_queue
 	unplug_fn		*unplug_fn;
 	merge_bvec_fn		*merge_bvec_fn;
 
+	/*
+	 * Auto-unplugging state
+	 */
+	struct timer_list	unplug_timer;
+	int			unplug_thresh;	/* After this many requests */
+	unsigned long		unplug_delay;	/* After this many jiffies */
+	struct work_struct	unplug_work;
+
 	struct backing_dev_info	backing_dev_info;
 
 	/*