RE: [RFC] add sysfs to dynamically control blk request tag maintenance

[Chen, Kenneth W]

Jens Axboe wrote on Friday, October 07, 2005 12:42 AM
> > It starts out with scsi_end_request being a fairly hot function in
the
> > execution profile, then I noticed blk_queue_start/end_tag() are
being
> > called but no actual consumer of using the tag.  I'm trying to find
a
> > way to avoid making these blk_queue_start/end_tag calls.  I got the
> > answer
> > now. The proper way is to fix it in the scsi LLDD.  Scratch this
patch,
> > new patch to follow :-)
> 
> Ok that makes more sense! But it's a little worrying that
> blk_queue_end_tag() would show up as hot in the profile, it is
actually
> quite lean.

It's probably a very small number that I'm chasing with avoiding blk
layer tagging.  Nevertheless, any number no matter how small, is a gold
mine to me :-)

Latest execution profile taken with 2.6.14-rc2 kernel with "industry
standard transaction processing database workload".  First column is
clock ticks (a direct measure of time), 2nd column is instruction
retired,
and 3rd column is number of L3 misses occurred inside the function.

Symbol			Clockticks	Inst. Retired	L3 Misses
scsi_request_fn		8.12%	9.27%	11.18%
Schedule			6.52%	4.93%	7.26%
scsi_end_request		4.44%	3.59%	6.76%
__blockdev_direct_IO	4.28%	4.38%	3.98%
__make_request		3.59%	4.16%	3.47%
__wake_up			2.46%	1.56%	3.33%
dio_bio_end_io		2.14%	1.67%	3.18%
aio_complete		2.05%	1.27%	3.56%
kmem_cache_free		1.95%	1.70%	0.71%
kmem_cache_alloc		1.45%	1.84%	0.45%
put_page			1.42%	0.60%	1.27%
follow_hugetlb_page	1.41%	0.75%	1.27%
__generic_file_aio_read	1.37%	0.36%	1.68%
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