linux-kernel - Re: QUEUE_FLAG_NO_SG

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Date:	Fri, 27 Nov 2015 15:29:59 +0100
From:	Hannes Reinecke <hare@...e.de>
To:	Ming Lei <ming.lei@...onical.com>
CC:	Jens Axboe <axboe@...com>, Christoph Hellwig <hch@....de>,
	"Martin K. Petersen" <martin.petersen@...cle.com>,
	Johannes Thumshirn <jthumshirn@...e.de>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	linux-nvme@...ts.infradead.org,
	SCSI Mailing List <linux-scsi@...r.kernel.org>
Subject: Re: QUEUE_FLAG_NO_SG_MERGE and non-block-mq

On 11/26/2015 10:21 AM, Ming Lei wrote:
> On Thu, Nov 26, 2015 at 4:13 PM, Hannes Reinecke <hare@...e.de> wrote:
>> Hi all,
>>
>> while investigating the crash in scsi_lib.c I found a rather curious
>> behaviour for QUEUE_FLAG_NO_SG_MERGE.
>>
>> While the flag is evaluated in blk_recalc_rq_segments and
>> blk_recount_segments (resulting in nr_phys_segments being
>> computed based on that flag) it is completely ignored
>> during blk_rq_map_sg() or the actual merging itself.
> 
> Yes, I guess Jens introduced the flag for decreasing CPU
> consumption when comuputing segments, but it is still
> ignored by blk_rq_map_sg(), but it may not be used
> by some drivers.
> 
> After bio splitting is introduced, the flag is also ignored
> when computing segments.
> 
>>
>> This typically shouldn't be an issue, seeing that with
>> QUEUE_FLAG_NO_SG_MERGE nr_phys_segments will always be
>> larger than the actual segment count.
>>
>> However, it still makes me wonder:
>> What is the point of having a QUEUE_FLAG_NO_SG_MERGE
>> which doesn't work as advertised?
>> Or, to be precise, which only works for blk-mq?
>> Should we make it work for non-block-mq, too?
> 
> Thanks bio splitting, this flag has little effect on performance now,
> so I think it can be removed if Jens has no objection.
> 
As per your suggestion we've made some performance measurements,
and 4k fio showed little if no impact:

NO_SG_MERGE:
  IOPS R/W: 148097.7+-125.7 / 148124.1+-123.1
  BW   R/W: 592392.4+-502.7 / 592498.3+-492.3
SG_MERGE:
  IOPS R/W: 148054.4+-123.3 / 148082.6+-120.0
  BW   R/W: 592219.2+-493.5 / 592332.3+-479.7

So the performance benefit lies squarely within the
error margin, making me wonder if it's worth bothering
with having the NO_SG_MERGE flag at all.

Thanks to Johannes for doing the measurements :-)

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		               zSeries & Storage
hare@...e.de			               +49 911 74053 688
SUSE LINUX GmbH, Maxfeldstr. 5, 90409 Nürnberg
GF: F. Imendörffer, J. Smithard, J. Guild, D. Upmanyu, G. Norton
HRB 21284 (AG Nürnberg)
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