On Fri, Feb 29, 2008 at 4:27 PM, Roberto Zunino
Josef Svenningsson wrote:
What I want to know boils down to this: what order are processes run which have been woken up from a call to retry?
IIUC, the order of wake up is irrelevant, since *all* the threads will re-run the transaction in parallel. So, even if thread 1 is the first to wake up, thread 2 might beat it in the race, and complete its transaction first.
That's not quite right since there is no true parallelism here. I'm running on a single core (which I suppose I could have mentioned) and so it is up the scheduler to make sure that processes get a fair chance at doing their business, i.e. achieving fairness. The point I was trying to make is that the scheduler isn't doing a very good job in this case.
I suggest you put some random delay in your fairness tests, maybe using unsafeIOtoSTM, so that you can improve starvation ;-)
I'd rather fix the scheduler.
Also, try running a very slow (much-delayed) transaction againts several fast ones. I expect the slow one will never reach completion.
Indeed. This is a well known problem with STM but afaict orthogonal to the problem I'm talking about.
AFAIK, achieving fairness in STM can be quite hard (not unlike other mainstream approaches to concurrency, sadly).
Yes. Still, in the particular situation I showed I think we can do a better job than what is currently being done. Cheers, Josef