Re: Remove BIO_RW_SYNCIO flag from kcopyd

[Mike Snitzer <snitzer@redhat.com>]

Hi Mikulas,

On Sun, Oct 24 2010 at  9:02pm -0400,
Mikulas Patocka <mpatocka@redhat.com> 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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