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Saturday, May 06, 2006

To Skew or Not To Skew - Part 2

It looks like my analysis of this lock-free priority queue was right on the money: the algorithm can be quite inefficient (about half as fast as the priority queue using a spin-lock). It seems that Herlihy (the guy who wrote the algorithm) did a fair bit of research on it back in 1993. He also managed to identify a solution to the problem: exponential back off.

While the article doesn't do benchmarks on the skew heap with exponential back off, the normal heap with exponential back off shows a remarkable increase in speed, performing superior to the spin lock version beginning at about 6 threads executing concurrently. At 16 processes contending for the same priority queue, the lock-free heap with exponential back off is about twice as fast as the spin lock version.

However, using exponential back off with the spinlock gets faster, still. The heap using a spinlock with exponential back off is about 65% faster than the lock-free heap with exponential back off.

Last, it should be noted that there's an inherent bias in these results. They all use artificial situations where memory access is minimized in the case of the spin lock version, so most of the access can be done from cache, in contrast to the lock-free algorithm. So it's possible that in real world scenarios the lock-free version will be faster, relative to the spin-lock version.

So, we got some nice information, but it really doesn't answer the question of whether I should implement this sucker :P

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