rw_semaphore performance, was: new metadata reader/writer locks inintegration-test

[Christoph Hellwig]

On Tue, Jul 19, 2011 at 01:30:22PM -0400, Chris Mason wrote:
> We've seen a number of benchmarks dominated by contention on the root
> node lock.  This changes our locks into a simple reader/writer lock.
> They are based on mutexes so that we still take advantage of the mutex
> adaptive spins for write locks (rwsemaphores were much slower).

Interesting.  Do you have set up some artifical benchmarks for this?

I wonder if the lack of adaptive spinning has something to do with the
slightly slower XFS performance on Joern's flash testing, given that
we extensively use the rw_semaphore as the primary I/O mutex, while
all others rely on plain mutexes as the primary synchronization
primitive.
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