witness performance improvements
Jeff Roberson
jroberson at jroberson.net
Sat Jul 19 03:07:08 UTC 2008
Hello,
I have a patch that improves witness performance available at:
http://people.freebsd.org/~jeff/witness.diff
This improvement comes at the cost of some significant space overhead. It
changes the witness graph from a linked tree to a matrix based approach.
Relationships can be quickly resolved with a table lookup. The table size
is WITNESS_COUNT^2, or 1MB with the current count of 1024.
This patch also makes struct witness objects persistent even after the
last lock using this name has been removed. This is helpful for short
lived objects which may be created frequently.
To reduce lock contention on SMP witness_checkorder() now runs without the
w_mtx when there are no lock violations. I also cache a lock_list_entry
in each thread as allocating these requires the w_mtx. The entry is
disposed of at thread_exit().
There is also a new sysctl that produces dot output which graphs lock
order relationships with the graphviz program.
Most of this work was done by Ilya Maykov while he was at Isilon systems.
The locking work and some cleanup/porting/refinement was done by me on
behalf of Nokia.
The performance improvement can be significant. It is only on the order
of 10-20% for buildkernel but on a packet forwarding test at nokia it sped
things up by 5x putting a witness enabled kernel within about 50% of the
performance of a kernel without. I believe buildworld isn't helped as
much because forking and exiting a lot would then contend on the witness
lock.
I'm mostly interested in hearing what people have to say about the space
bloat. I believe it is in a commit ready state.
Thanks,
Jeff
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