QUEUE_FLAG_NO_SG_MERGE and non-block-mq

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.

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?

Cheers,

Hannes
--
Dr. Hannes Reinecke zSeries & Storage
[email protected] +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 [email protected]
More majordomo info at http://vger.kernel.org/majordomo-info.html

Quoted 9 lines · Hannes Reinecke

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.

Quoted 5 lines

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.

Quoted 5 lines

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.

Quoted 24 lines · Ming Lei

Quoted 9 lines · Hannes Reinecke

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.

Quoted 5 lines

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.

Quoted 9 lines

Quoted 5 lines

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 :-)