Archive for July, 2011

Lock Performance

5. July 2011

I’m currently working on a low-level library where locked data access has to be optimized. Therefore I benchmarked the performance of the three lock types in Collage on Linux and Mac OS X. The test just runs a number of threads which just set and unset the lock without any other operation. Click on the image below to get a full-resolution image. Be aware the chart uses double-log scale.

The two benchmarks can not be directly compared since they did not run on the same hardware. There are nevertheless a few interesting observations:

(1) Spinlocks are faster than ‘real’ locks. I’ve blogged about this before. Since they consume CPU time while spinning they should only be hold for a very short time, i.e., to read a value. The Collage implement immediately backs off when encountering a set lock by yielding the thread. This avoids priority inversion, which can be observed by some pthread spin lock implementations.

(2) pthread locks are dead slow on Mac OS X. Be aware that the graph uses log scale – a spin lock is up to three orders of magnitude faster than a pthread lock!

(3) Timed locks are slower than un-timed. This meets my intuitive expectation, since the timed implementation is more complex. The timed lock in Collage is implemented using pthread_cond_timedwait.

(4) The Spinlock is faster on OS X on slower hardware than on Linux. Not sure why that is the case. The Collage spin lock uses an atomic variable and compare_and_set. Either these operations are faster on the Core i5, or the thread yield behaves ‘better’ on OS X.

(5) Single-threaded lock access in pthread libraries seems to be optimized.

(6) pthread conditions on Linux observe a steep performance drop once you have more threads than cores. Could be a scheduling issue again.

Next I’ll work on benchmarking and optimizing read/write locking in the Collage Spinlock. Stay tuned for updates!

EDIT: I discovered a bug in my micro-benchmark which wrongly multiplied the results with the number of threads – doh! The figure is fixed now with a new test run.