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Datum: Mon, 21 Mar 2011 06:38:58 -0700 (PDT) Von: Artyom <artyomtnk@yahoo.com> An: boost@lists.boost.org Betreff: Re: [boost] [context review] Several Questions
From: Oliver Kowalke <oliver.kowalke@gmx.de>
sched_yield(): 2108 cycles ucontext_t: 1795 cycles fcontext_t: 156 cycles
Actually it is quite consistent with what you had show, the Boost.Context is better (but not significantly) then real OS (kernel) context switch.
factor 13x ?
I was talking about _default_ build which uses ucontext.
OK - ucontext does system calls which are time consuming. Anyway with sched_yield() you do not achive the same as with ucontext. The idea is not to block your current process/thread if some conditions are not met but to proceed with other work an return if the the conditions are met. void my_action_1() { // compute something ... // some condtions are not met // don not block thread; // jump out and do other stuff (for instance my_action-2() // in the meantime yield(); // conditions now met and we can proceed with our computation } void my_action_2() {...} the code has choosen to return execution control and let the thread do other things (like process my_action_2()). If, for instance an external event happend and the conditions are met for my_action_1() the execution control is given back to my_action_1() with all its local state (variables, stackframe, etc.). Using event-loops you can achive similiar things (not fully equivalent) but you have to be always aware of the event loop - and at least I don't feel this paradigm straigth forward. If you take a look into boos.tasklet you could see that this lib has some classes like mutex,condition- and event-variables etc. whic hhide the yield() call. You can programm like for boost.thread. boost.context and boost.tasklet are derivative work from boost.task. boost.task implements a thread-pool and my aim was that the tasks I push to the poll don't block the worker-thread if the current computation can not be fullfilled because some criteria are not met. For instance a task creates certain amount of subtasks and has to wait for the result of its sub-tasks. Until all sub-tasks are finished the parent-task blocks its worker-thread inside the pool. If you create more sub-tasks than worker-threads than your thead-pool is blocked. In order to solve this problem I've developed boost.tasklet (fromaly boost.fiber) and boost.context. As Phil Endecott requested I should move the cotnext switching code into a library (== boost.context) so other libs could probably benefit from it (boost.coroutine). several other languages provide such context switching facilities: Lua : http://lua-users.org/wiki/CoroutinesTutorial Go : http://sites.google.com/site/gopatterns/concurrency/coroutines Scheme : http://en.wikipedia.org/wiki/Scheme_%28programming_language%29#First-class_c... Stackless Python : http://www.stackless.com/ why shouldn't C++ not provide such a facility (at least this is my motivation) best regards, Oliver -- NEU: FreePhone - kostenlos mobil telefonieren und surfen! Jetzt informieren: http://www.gmx.net/de/go/freephone