From b673c3a8192e28f13e2050a4b82c1986be92cc15 Mon Sep 17 00:00:00 2001 From: Kazuo Ito Date: Tue, 21 Oct 2008 17:44:50 +0100 Subject: [PATCH] dm kcopyd: avoid queue shuffle Write throughput to LVM snapshot origin volume is an order of magnitude slower than those to LV without snapshots or snapshot target volumes, especially in the case of sequential writes with O_SYNC on. The following patch originally written by Kevin Jamieson and Jan Blunck and slightly modified for the current RCs by myself tries to improve the performance by modifying the behaviour of kcopyd, so that it pushes back an I/O job to the head of the job queue instead of the tail as process_jobs() currently does when it has to wait for free pages. This way, write requests aren't shuffled to cause extra seeks. I tested the patch against 2.6.27-rc5 and got the following results. The test is a dd command writing to snapshot origin followed by fsync to the file just created/updated. A couple of filesystem benchmarks gave me similar results in case of sequential writes, while random writes didn't suffer much. dd if=/dev/zero of= bs=4096 count=... [conv=notrunc when updating] 1) linux 2.6.27-rc5 without the patch, write to snapshot origin, average throughput (MB/s) 10M 100M 1000M create,dd 511.46 610.72 11.81 create,dd+fsync 7.10 6.77 8.13 update,dd 431.63 917.41 12.75 update,dd+fsync 7.79 7.43 8.12 compared with write throughput to LV without any snapshots, all dd+fsync and 1000 MiB writes perform very poorly. 10M 100M 1000M create,dd 555.03 608.98 123.29 create,dd+fsync 114.27 72.78 76.65 update,dd 152.34 1267.27 124.04 update,dd+fsync 130.56 77.81 77.84 2) linux 2.6.27-rc5 with the patch, write to snapshot origin, average throughput (MB/s) 10M 100M 1000M create,dd 537.06 589.44 46.21 create,dd+fsync 31.63 29.19 29.23 update,dd 487.59 897.65 37.76 update,dd+fsync 34.12 30.07 26.85 Although still not on par with plain LV performance - cannot be avoided because it's copy on write anyway - this simple patch successfully improves throughtput of dd+fsync while not affecting the rest. Signed-off-by: Jan Blunck Signed-off-by: Kazuo Ito Signed-off-by: Alasdair G Kergon Cc: stable@kernel.org --- drivers/md/dm-kcopyd.c | 13 ++++++++++++- 1 file changed, 12 insertions(+), 1 deletion(-) diff --git a/drivers/md/dm-kcopyd.c b/drivers/md/dm-kcopyd.c index 996802b8a452..8f153530d6df 100644 --- a/drivers/md/dm-kcopyd.c +++ b/drivers/md/dm-kcopyd.c @@ -268,6 +268,17 @@ static void push(struct list_head *jobs, struct kcopyd_job *job) spin_unlock_irqrestore(&kc->job_lock, flags); } + +static void push_head(struct list_head *jobs, struct kcopyd_job *job) +{ + unsigned long flags; + struct dm_kcopyd_client *kc = job->kc; + + spin_lock_irqsave(&kc->job_lock, flags); + list_add(&job->list, jobs); + spin_unlock_irqrestore(&kc->job_lock, flags); +} + /* * These three functions process 1 item from the corresponding * job list. @@ -398,7 +409,7 @@ static int process_jobs(struct list_head *jobs, struct dm_kcopyd_client *kc, * We couldn't service this job ATM, so * push this job back onto the list. */ - push(jobs, job); + push_head(jobs, job); break; } -- 2.20.1