low(er) disk performance with sched_4bsd then with sched_ule

Andrew Gallatin gallatin at cs.duke.edu
Sat Sep 17 14:33:47 PDT 2005


Oliver Lehmann writes:
 > Joseph Koshy wrote:
 > 
 > > ol> Wow, that update to BETA4 did the trick! While running 
 > > ol> SCHED_4BSD:
 > > 
 > > Fantastic!  What is the profile like with the new 4BSD kernel?
 > 
 > http://pofo.de/tmp/gprof.4bsd.3

I don't know the disk codepath very well, but the samples look a
little suspect.  We're copying a lot of data into and out of the
kernel, so I would expect the majority of non disk wait time would be
spent simply copying out the zero-filled pages, and copying them back
in (AFAIK, dd uses read/write).  Where is the time spent in read,
write, uiomove, bcopy?

What about ionode allocations, etc?  And why do things like
g_bsd_modify and g_bsd_ioctl rank so high?  Aren't those only used
when dealing with disklabels?

BTW, I *love* that we've got access to the hw counters, and an easy
way to do low-overhead profiling of the kernel.

Drew



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