Re: [PATCHSET v3] block: IO polling improvements

["Stephen Bates" <sbates@raithlin.com>]

On Fri, November 11, 2016 11:11 pm, Jens Axboe wrote:
> Respun on top of for-4.10/block, dropping patches that have been
> merged. This patchset adds Christoph's simplified sync O_DIRECT bdev access
> mode, and implements a more efficient IO polling on top of it. For more
> details, see the v2 posting here:
>
> http://www.mail-archive.com/linux-block@vger.kernel.org/msg02079.html
>
>
> Changes since v2:
>
>
> - Adapt to blk stat changes
> - Make it explicit that only blk-mq supports polling
> - Fix a bug in the hrtimer code, switching to absolute mode if we
> have to loop around (thanks Omar).
>

Hi Jens

I applied this series cleanly on top of 2868f13c303e147 in your
for-4.10/block branch. I tested using a simple fio wrapper script [1] on a
low-latency NVMe SSD [2]. Here are some results:

io_poll  io_poll_delay  threads  latency  CPU/threads

0        N/A            1        17.0     47%
1        -1             1        13.8     100%
1        0              1        14.6     73%
1        10             1        17.0     56%

0        N/A            8        18.7     47%
1        -1             8        14.4     100%
1        0              8        15.7     73%
1        10             8        19.0     56%

The io_poll_delay option 0 is definitely a nice compromise between no
polling and 100% polling. The result for io_poll_delay=10us is interesting
as it implies there might be some mismatch between that value and the
completion time. However I think that is something we can improve on over
time and I don't see it as a reason to not get this series upstream.

For the series:

Tested-By: Stephen Bates <sbates@raithlin.com>
Reviewed-By: Stephen Bates <sbates@raithlin.com>

Cheers

Stephen

[1] https://github.com/sbates130272/fio-stuff/blob/master/misc/iopoll-test.sh
[2]
http://www.microsemi.com/products/storage/flashtec-nvram-drives/flashtec-nvram-drives