On 24. 05. 20 3:38, Matthew Flatt wrote: > At Sat, 23 May 2020 18:51:23 +0200, Dominik Pantůček wrote: >> But that is just where the issue is showing up. The real question is how >> the counter gets decremented twice (given that fsemaphores should be >> futures-safe). > > I found a configuration that triggered the bug often enough on my > machine. Yes, it was a dumb mistake in the implementation of > fsemaphores. In your example, it wasn't so much that the counter > fsemaphore was decremented twice, but that the lock fsemaphore could go > negative --- so it didn't actually lock. > > I've pushed a repair.
Thank you for the repair (and good idea with the test case). > > > FWIW, after looking at this example more, I see why it's still not > enough in principle to make a thread that waits on the result of > `do-start-workers` or to run `do-start-workers` after the enqueue > loops. The `start-workers` function can run a dequeue loop after > starting a future to do the same, and before touching that future. So, > the dependencies aren't as linear as I thought before. > > If your real code looks anything like this, consider using a CAS > operation, like `box-cas!`, instead of an fsemaphore as lock for the > queue. The queue's use of an fsemaphore for counting and signaling > seems fine, though. > Yes, the real code is a binary tree of futures. However each future performs a lot of fixnum/flonum operations. At first I was surprised that you are basically suggesting using spinlocks (busy-wait loops) instead of futex-backed (at least on Linux) fsemaphores. That is a waste of CPU time... But yes, CAS-based locking outperforms fsemaphores locking and distributes the work way more evenly. Now when I see it in action and look at the source it kind of makes sense as the spinlock does not need to wait long and does not cause any re-scheduling (which is the reason why I get so uneven counts with fsemaphore-based locking). > In any case, the example worked well to expose the fsemaphore bug. > Probably expect to see more in the future :) Dominik -- You received this message because you are subscribed to the Google Groups "Racket Users" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/racket-users/9fdcb07a-a596-fc84-5529-4517f6b8b1fd%40trustica.cz.
