Poor performance / lockup with STM

Question

I\'m writing a program where a large number of agents listen for events and react on them. Since Control.Concurrent.Chan.dupChan is deprecated I decided to use TCha

Answer 1

This is a great test case! I think you've actually created a rare instance of genuine livelock/starvation. We can test this by compiling with -eventlog and running with -vst or by compiling with -debug and running with -Ds. We see that even as the program "hangs" the runtime still is working like crazy, jumping between blocked threads.

The high-level reason is that you have one (fast) writer and many (fast) readers. The readers and writer both need to access the same tvar representing the end of the queue. Let's say that nondeterministically one thread succeeds and all others fail when this happens. Now, as we increase the number of threads in contention to 100*100, then the probability of the reader making progress rapidly goes towards zero. In the meantime, the writer in fact takes longer in its access to that tvar than do the readers, so that makes things worse for it.

In this instance, putting a tiny throttle between each invocation of go for the writer (say, threadDelay 100) is enough to fix the problem. It gives the readers enough time to all block between successive writes, and so eliminates the livelock. However, I do think that it would be an interesting problem to improve the behavior of the runtime scheduler to deal with situations like this.