Hi Mikulas,
On Sun, Oct 24 2010 at 9:02pm -0400,
Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote:
> Remove BIO_RW_SYNCIO flag from kcopyd
>
> This improves write throughput twice when writing to the origin with snapshot
> on the same device.
Can you please be more specific on the performance improvement you
observed? You're saying something like: baseline=10MB/s patched=20MB/s?
Did you use a larger chunk_size for the snapshot device (e.g. 512K)?
I can't reproduce such significant performance improvements but my testbed
may be overly constrained (I'm passing an entire disk from host into KVM
guest using virtio). I can look to test on bare metal early next week
(unless you have more insight that would obviate the need to do so).
> This helps for CFQ I/O scheduler that has separate queues for sync and async
> I/O, async is optimized for throughput, sync for latency.
> Reallocations are triggered by writes that are usually async, so mark it
> as async as well.
This change is at odds with a previous patch you made to reduce latency
of SYNC IO, see linux-2.6 commit:
7ff14a36159d947872870 "dm: unplug queues in threads"
So we're now trading latency for throughput? Given the concerns about
snapshot-origin throughput (on lvm-general) I think that is reasonable.
But I just want to make sure we're consciously favoring throughput over
latency (I'll likely update the patch header to explicitly say as much).
> Index: linux-2.6.36-rc7-fast/drivers/md/dm-kcopyd.c
> ===================================================================
> --- linux-2.6.36-rc7-fast.orig/drivers/md/dm-kcopyd.c 2010-10-07 16:09:13.000000000 +0200
> +++ linux-2.6.36-rc7-fast/drivers/md/dm-kcopyd.c 2010-10-15 03:18:52.000000000 +0200
> @@ -345,7 +345,7 @@ static int run_io_job(struct kcopyd_job
> {
> int r;
> struct dm_io_request io_req = {
> - .bi_rw = job->rw | REQ_SYNC | REQ_UNPLUG,
> + .bi_rw = job->rw | REQ_UNPLUG,
> .mem.type = DM_IO_PAGE_LIST,
> .mem.ptr.pl = job->pages,
> .mem.offset = job->offset,
>
> --
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