linux-kernel - Re: [PATCH 1/9] move blk_iopoll to limit and make it generally available

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Date:	Sun, 15 Nov 2015 10:48:41 +0200
From:	Sagi Grimberg <sagig@....mellanox.co.il>
To:	Christoph Hellwig <hch@....de>, Or Gerlitz <gerlitz.or@...il.com>
Cc:	"linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
	Bart Van Assche <bart.vanassche@...disk.com>, axboe@...com,
	"linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/9] move blk_iopoll to limit and make it generally
 available


>> On Fri, Nov 13, 2015 at 3:46 PM, Christoph Hellwig <hch@....de> wrote:
>>> The new name is irq_poll as iopoll is already taken.  Better suggestions
>>> welcome.
>>
>> Sagi (or Christoph if you can address that),
>>
>> @ some pointer over the last 18 months there was a port done at
>> mellanox for iser to use blk-iopoll and AFAIR it didn't work well or
>> didn't work at all. Can you tell now what was the problem and how did
>> you address it at your generalization?
>
> Hi Or,
>
> Sagi mentioned last time he tried a similar approach in iSER he saw
> some large latency sparks.  We've seen nothing worse than the original
> approach.  The Flash memory summit slide set has some numbers:
>
> http://www.flashmemorysummit.com/English/Collaterals/Proceedings/2015/20150811_FA11_Bandic.pdf
>
> they aren't quite up to date, but the latency distribution hasn't
> really changed.

Or is correct,

I have attempted to convert iser to use blk_iopoll in the past, however
I've seen inconsistent performance and latency skews (comparing to
tasklets iser is using today). This was manifested in IOPs test cases
where I ran multiple threads with higher queue-depth and not in
sanitized pure latency (QD=1) test cases. Unfortunately I didn't have
the time to pick it up since.

I do have every intention of testing it again with this. If it still
exist we will need to find the root-cause of it before converting
drivers to use it.

Sagi.
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