kcopyd performance

[Kevin Jamieson <kjamieson@bycast.com>]

Hi,

I've been investigating some performance issues with writes to a LVM2
snapshot origin volume being much slower than one would expect (i.e.,
10-50x slower for a logical volume with a single snapshot volume).

Using a simple dd test (I get similar numbers with other tools like
bonnie) on a P4 3GHz w/SATA disks (no RAID):

dd if=/dev/zero of=/vol/file bs=1M count=1000

Logical volume w/o snapshot: 45.4 MB/s
LV w/snapshot on same disk, chunksize=8K: 1.1 MB/s
LV w/snapshot on same disk, chunksize=512K: 4.0 MB/s
LV w/snapshot on separate disk, chunksize=8K: 1.2 MB/s
LV w/snapshot on separate disk, chunksize=512K: 5.6 MB/s

I tracked the main cause of the poor performance to the behaviour of
process_jobs() in kcopyd. If kcopyd's page pool has no free pages, it
moves the next job in the _pages_jobs list to the tail of the list.
New jobs are also added to the tail of the _pages_jobs list (in
dispatch_job()). This re-ordering of the I/Os results in a lot of extra
seek activity for workloads consisting of sequential writes.

The below patch modifies process_jobs() to push the job back to the head
of the list instead of the tail when there are no free pages. I see
significantly improved performance with this change:

LV w/snapshot on same disk, chunksize=8K: 9.6 MB/s
LV w/snapshot on same disk, chunksize=512K: 8.4 MB/s
LV w/snapshot on separate disk, chunksize=8K: 17.2 MB/s
LV w/snapshot on separate disk, chunksize=512K: 14.5 MB/s

Thanks,
Kevin


--- linux/drivers/md/kcopyd.c.bak	2007-05-30 14:18:36.000000000 -0700
+++ linux/drivers/md/kcopyd.c	2007-05-30 15:54:33.000000000 -0700
@@ -280,6 +280,15 @@
 	spin_unlock_irqrestore(&_job_lock, flags);
 }
 
+static inline void push_head(struct list_head *jobs, struct kcopyd_job *job)
+{
+	unsigned long flags;
+
+	spin_lock_irqsave(&_job_lock, flags);
+	list_add(&job->list, jobs);
+	spin_unlock_irqrestore(&_job_lock, flags);
+}
+
 /*
  * These three functions process 1 item from the corresponding
  * job list.
@@ -404,7 +413,7 @@
 			 * We couldn't service this job ATM, so
 			 * push this job back onto the list.
 			 */
-			push(jobs, job);
+			push_head(jobs, job);
 			break;
 		}