Re: QUEUE_FLAG_NO_SG_MERGE and non-block-mq

[Jens Axboe]

On 11/27/2015 07:29 AM, Hannes Reinecke wrote:
> On 11/26/2015 10:21 AM, Ming Lei wrote:
> > On Thu, Nov 26, 2015 at 4:13 PM, Hannes Reinecke <hare@xxxxxxx> 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 :-)

150K iops is on the slow side, though. It's pointless to iterate the sg 
list if we don't have to. I can try and run a few tests next week.

--
Jens Axboe

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