Nice talk. The blog he started just before the video was hosted at his employer who changed the URL to http://www.azulsystems.com/blog/cliff, which also links to his new blog, http://www.cliffc.org/blog/. I also found a link to one version of the code.

The design actually comes quite close to RCU in many ways. He uses atomic Check-And-Set as a form of optimistic locking for updaters. (And there is significant work beyond just RCU in order to make that all work.) This means updaters don't have to wait for a lock but they may have to retry their updates (but the number of retries is bounded by the number of threads and, in practice, is almost always very small).

Sounds like it never got folded in to java.util, though. I'm a bit curious why but haven't yet spent the time to try to find any discussions on that.

The discussion about how the size of the hash is tracked indicates that having a single generation number could be a bottleneck when having a huge number of simultaneous updates. He tracks the number of entries in the hash by having a striped counter where each thread will only update one (partial) count and to get the count you have to add up all of the partial counts. So the multiple (cyclic) generation counts in a single 64-bit word that you mentioned would likely be a bottleneck for huge numbers of threads. There are probably tons of use cases where that ends up mattering very little, though.

- tye        

In reply to Re^3: Design thoughts: iterator invalidation (links) by tye
in thread Design thoughts: iterator invalidation by BrowserUk

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