On 08/02/17 18:29, Benoit Depail wrote:

On 08/01/2017 11:48 AM, Roger Pau Monné wrote:

quoted

On Fri, Jul 28, 2017 at 04:50:27PM +0200, Benoit Depail wrote:

quoted

On 07/26/17 00:25, Keith Busch wrote:

quoted

On Fri, Jul 21, 2017 at 07:07:06PM +0200, Benoit Depail wrote:

quoted

On 07/21/17 18:07, Roger Pau Monné wrote:

quoted

Hm, I'm not sure I follow either. AFAIK this problem came from
changing the Linux version in the Dom0 (where the backend, blkback is
running), rather than in the DomU right?

Regarding the queue/sectors stuff, blkfront uses several blk_queue
functions to set those parameters, maybe there's something wrong
there, but I cannot really spot what it is:

http://elixir.free-electrons.com/linux/latest/source/drivers/block/xen-blkfront.c#L929

In the past the number of pages that could fit in a single ring
request was limited to 11, but some time ago indirect descriptors
where introduced in order to lift this limit, and now requests can
have a much bigger number of pages.

Could you check the max_sectors_kb of the underlying storage you are
using in Dom0?

Roger.

I checked the value for the loop device as well

With 4.4.77 (bad write performance)
$ cat /sys/block/sda/queue/max_sectors_kb
1280
$ cat /sys/block/loop1/queue/max_sectors_kb
127


With 4.1.42 (normal write performance)
$ cat /sys/block/sda/queue/max_sectors_kb
4096
$ cat /sys/block/loop1/queue/max_sectors_kb
127

Thank you for the confirmations so far. Could you confirm performance dom0
running 4.4.77 with domU running 4.1.42, and the other way around? Would
like to verify if this is just isolated to blkfront.

Hi,

I've ran the tests, and I can tell that the domU kernel version have no
influence on the performance.

Dom0 with 4.4.77 always shows bad performance, wether the domU runs
4.1.42 or 4.4.77.

Dom0 with 4.1.42 always shows good performance, wether the domU runs
4.1.42 or 4.4.77.

Hello,

I haven't yet got time to look into this sadly. Can you please try to
use fio [0] in order to run the tests against the loop device in Dom0?

If possible, could you test several combinations of block sizes, I/O
sizes and I/O depths?

Thanks, Roger.

[0] http://git.kernel.dk/?p=fio.git;a=summary

Ok I'll give a try later when I have more time. Probably next week.

Thanks,

Hi,

I had some time to play around with fio. I am not really sure what I was
supposed to do with it so I made a wild guess.

Using the setup showing bad write performances (dom0 with v4.4.77), I
ran a fio job with the following parameters :

[global]
size=5g
filename=<device>
direct=1
readwrite=write

[job1]
runtime=1m


I then used a loop to change the blocksize from 512 to 8192, steps 512,
and the io depth from 1 to 32, steps 4.

On the loop device (from the dom0), write speed was about 34MB/s most of
the time, reaching 110MB/s with a blocksize multiple of 4096 (4096 and
8192).

On the domU, write speed was the same, but reaching only 52MB/s when the
blocksize is a multiple of 4096.

Io depth did not show any significant change.

Feel free to suggest any improvment on my bench suit.

Thansk,

-- 
Benoit Depail
Senior Infrastructures Architect
NBS System