Linked by Hadrien Grasland on Fri 27th May 2011 11:34 UTC
After having an interesting discussion with Brendan on the topic of deadlocks in threaded and asynchronous event handling systems (see the comments on this blog post), I just had something to ask to the developers on OSnews: could you live without blocking API calls? Could you work with APIs where lengthy tasks like writing to a file, sending a signal, doing network I/O, etc is done in a nonblocking fashion, with only callbacks as a mechanism to return results and notify your software when an operation is done?
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Alfman,
I kinda get what you mean. I was talking about non-blocking schemes, not blocking schemes, so I think we are a bit out of sync ourselves.
I am basing myself off wikipedia as of right now, and I do not see it saying that CAS blocks at the hardware level. In fact, it does say that CAS is unsafe -- it does not even try to block the content being overwritten twice! (ABA problem). As such, I doubt your atomics is exactly the same as my CAS solution.
If my reading is not failing me, the CMPXCHG instruction should be hardware-non-blocking and yet atomic, which is kinda like heavenly. The LL/SC is even better -- it does that and avoids ABA, at the slight cost of speed. Definitely nothing as long as the "lock add" you are referring to.
The (b) about interrupts is because atomics are usually done on single cores by simply disabling interrupts for the transaction. The CAS is not much faster than that case, but scales a lot better for the many-cores case.
Can you please clarify? I am still of the impression that this CAS runs in one instruction cycle, not at bus speed due to a tiny mutex. Or else it would be kind of defeating the point of such an invention.