[Python-Dev] Pythonic concurrency - cooperative MT (original) (raw)
Antoine Pitrou solipsis at pitrou.net
Fri Sep 30 11:45:17 CEST 2005
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(C) That scheduler is non-preemptive. A single greedy generator can starve all the others.
Instead of looking at this as a problem, you could look at it as a feature. Since generators can't be switched at arbitrary places, the programmer has to define his/her synchronization points explicitly. Let me contrast this:
- in preemptive MT, threads can switch at every moment by default, and you have to explicitly wrap some pieces of code in critical sections (or other primitives) to protect the semantics of your program; every time you forget a critical section, you introduce a bug
- in cooperative MT, threads can only switch where the programmer explicitly allows it; every time you forget a synchronization point, you give your program worse performance (latency), but you don't introduce a bug
So you have a scheme where you seek optimal performance (latency) but you take the risk of a huge number of difficult bugs, and you have a scheme where good performance needs more careful coding but the paradigm avoids difficult bugs /by construction/.
By the way, the cooperative MT is exactly the Twisted approach, except implemented in a different manner (event loop instead of explicit cooperative tasks).
Regards
Antoine.
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