Re: [PATCH 1/3] block: add flag for single-threaded submission

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Hannes,

> On Jul 21, 2016, at 15:37, Hannes Reinecke <hare@xxxxxxx> wrote:
> 
> On 07/21/2016 08:01 AM, Hannes Reinecke wrote:
>> On 07/21/2016 07:54 AM, Christoph Hellwig wrote:
>>> On Tue, Jul 19, 2016 at 04:02:56PM +0200, Hannes Reinecke wrote:
>>>> Some devices (most notably SMR drives) support only
>>>> one I/O stream eg for ensuring ordered I/O submission.
>>>> This patch adds a new block queue flag
>>>> 'BLK_QUEUE_SINGLE' to support these devices.
>>> 
>>> We'll need a blk-mq implementation of this flag as well before it can
>>> be merged.
>>> 
>> Yes, indeed. Will be fixed for the next round of submissions.
>> 
> Hmm.
> Looking closer do I _really_ need that for blk-mq?
> From my understanding any hctx can only run on one dedicated cpu, to
> which the hctx is bound.
> So if we only have one CPU serving that hctx how can we have several
> concurrent calls to queue_rq() here?

Ex: an application with 2 threads running on different CPUs, with both
threads writing to the same sequential zone with a proper mutual exclusion
to ensure sequential write order at the application level.

For this example, each thread requests will end up in a different hw queue
(hctx), and a single CPU serving each hctx may not result in an overall
in-order delivery to the disk.

So we need to ensure that all requests for a sequential zone end up in the
same hctx, which I think means simply that we need to allow only a single
queue for a ZBC device. If one day we get a fast SSD supporting ZBC/ZAC
commands, this may impact performance though...

I may be completely wrong about this though.

Cheers.

------------------------
Damien Le Moal, Ph.D.
Sr. Manager, System Software Group, HGST Research,
HGST, a Western Digital brand
Damien.LeMoal@xxxxxxxx
(+81) 0466-98-3593 (ext. 513593)
1 kirihara-cho, Fujisawa, 
Kanagawa, 252-0888 Japan
www.hgst.com 
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