Messages in this thread | | | Subject | Re: QUEUE_FLAG_NO_SG_MERGE and non-block-mq | From | Jens Axboe <> | Date | Fri, 27 Nov 2015 09:14:08 -0700 |
| |
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@suse.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 :-)
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
| |