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 :-) Cheers, Hannes -- Dr. Hannes Reinecke zSeries & Storage hare@xxxxxxx +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) -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
