RE: Direct io on block device has performance regression on 2.6.x kernel

[Chen, Kenneth W]

Chen, Kenneth W wrote on Wednesday, March 09, 2005 5:45 PM
> Andrew Morton wrote on Wednesday, March 09, 2005 5:34 PM
> > What are these percentages?  Total CPU time?  The direct-io stuff doesn't
> > look too bad.  It's surprising that tweaking the direct-io submission code
> > makes much difference.
>
> Percentage is relative to total kernel time.  There are three DIO functions
> showed up in the profile:
>
> __blockdev_direct_IO	4.97%
> dio_bio_end_io		2.70%
> dio_bio_complete	1.20%

For the sake of comparison, let's look at the effect of performance patch on
raw device, in place of the above three functions, we now have two:

raw_file_rw			1.59%
raw_file_aio_rw		1.19%

A total saving of 6.09% (4.97+2.70+1.20 -1.59-1.19).  That's only counting
the cpu cycles.  We have tons of other data showing significant kernel path
length reduction with the performance patch.  Cache misses reduced across
the entire 3 level cache hierarchy, that's a secondary effect can not be
ignored since kernel is also competing cache resource with application.

- Ken


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