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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