VFS accounting performance benchmarks

[Francois Tigeot]

Most of the VFS accounting work is now in the tree (and disabled by default,
enable it with vfs.accounting_enabled="1" in /boot/loader.conf).
Only a potentially disruptive commit to enable it for / has not been pushed.

Since VFS accounting uses a per mount-point spinlock to protect its counters, 
there is some concern it will degrade write performance on SMP machines.

To measure if this was the case or not, I've run some blogbench benchmarks on
a small Xeon system; the details of the test and the results are in the
attached PDF.

VFS accounting kernel was from 2011-12-19
Kernel without VFS accounting was from 2011-11-19

For my part I think the results are inconclusive: either there's no measurable
performance difference or the choice of benchmark was bad.

-- 
Francois Tigeot
Attachment:
2011-12-20 VFS-Accounting Blogbench performance.pdf
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