Implementing Queues for Event-Driven Programs
1–10 of 28 posts
Re: Implementing Queues for Event-Driven Programs
#2From the first code example:
> Yep, notifying outside of lock is usually BETTER. Otherwise the other thread would be released but will immediately run into our own lock above our own lock above, causing unnecessary (and Very Expensive) context switch
Is this true and if so, on which platform(s)? Aren't most mutex/condition variable implementations optimized to avoid this case by deferring the condition signal to the mutex unlock?
Additionally, in the implementation of kill() in the latter examples, there should be notify_all() instead of notify_one().
Re: Implementing Queues for Event-Driven Programs
#3From the first code example: > Yep, notifying outside of lock is usually BETTER. Otherwise the other thread would be released but will immediately run into our own lock above our own lock above, causing unnecessary (and Very Expensive) context switch Is this true and if so, on which platform(s)? Aren't most mutex/condition variable implementations optimized to avoid this case by deferring the condition signal to the…
That would be relying on the OS to do the right thing. One problem is that on some platform or for some usage pattern, then mutex and notifying object will be two independent objects, so the OS would have no way of knowing that it should not immediately wake up the notified threads.
Furthermore, it is a good rule of thumb anyway: hold the mutex for the shortest time possible. The corresponding pattern is to hold a mutex, copy the data you need to a local copy, release the mutex and then do the processing on the local copy. Sometimes, the processing can have unknown dependency and so it avoids potential deadlocks caused by some function called during processing trying to take a lock that is held be another thread.
Basically, never hold a lock while calling functions. You never know what might be hidden behind them now and will be hidden in the future.
Re: Implementing Queues for Event-Driven Programs
#4From the first code example: > Yep, notifying outside of lock is usually BETTER. Otherwise the other thread would be released but will immediately run into our own lock above our own lock above, causing unnecessary (and Very Expensive) context switch Is this true and if so, on which platform(s)? Aren't most mutex/condition variable implementations optimized to avoid this case by deferring the condition signal to the…
> Aren't most mutex/condition variable implementations optimized to avoid this case by deferring the condition signal to the mutex unlock?
Yes, _some_ but not _all_ implementations are doing it; however, as it is not really guaranteed (and actually is a workaround for poorly written programs) - standing recommendation is still to notify after the lock (which can be better, can be the same, but won't be worse than doing it within), see for example http://en.cppreference.com/w/cpp/thread/condition_variable/n...:
"The notifying thread does not need to hold the lock on the same mutex as the one held by the waiting thread(s); in fact doing so is a pessimization, since the notified thread would immediately block again, waiting for the notifying thread to release the lock. However, some implementations (in particular many implementations of pthreads) recognize this situation and avoid this "hurry up and wait" scenario by transferring the waiting thread from the condition variable's queue directly to the queue of the mutex within the notify call, without waking it up."
> Additionally, in the implementation of kill() in the latter examples, there should be notify_all() instead of notify_one().
In MOST cases, it didn't matter (as there was only one blocking thread - the one reading), but yes, there was one case when it was indeed important. Fixed now, THANKS!
Re: Implementing Queues for Event-Driven Programs
#5On Linux, another way to signal arbitrary events instead of an anonymous pipe is via eventfd(2).
Re: Implementing Queues for Event-Driven Programs
#6From the first code example: > Yep, notifying outside of lock is usually BETTER. Otherwise the other thread would be released but will immediately run into our own lock above our own lock above, causing unnecessary (and Very Expensive) context switch Is this true and if so, on which platform(s)? Aren't most mutex/condition variable implementations optimized to avoid this case by deferring the condition signal to the…
That would be relying on the OS to do the right thing. One problem is that on some platform or for some usage pattern, then mutex and notifying object will be two independent objects, so the OS would have no way of knowing that it should not immediately wake up the notified threads. Furthermore, it is a good rule of thumb anyway: hold the mutex for the shortest time possible. The corresponding pattern is to hold a mu…
> One problem is that on some platform or for some usage pattern, then mutex and notifying object will be two independent objects, so the OS would have no way of knowing that it should not immediately wake up the notified threads.
On all significant platforms I know, condition wait works like this:
condition_wait(locked_mutex, condition_to_wait);
Did you have a platform in mind that works differently from this?
Additionally, when you unlock a mutex before signalling the condition, another thread may be scheduled in between that may invalidate the condition that was signalled. This may introduce a race condition.
I signal my condition variables with mutexes locked because it's easier to guarantee correctness that way.
Re: Implementing Queues for Event-Driven Programs
#7From the first code example: > Yep, notifying outside of lock is usually BETTER. Otherwise the other thread would be released but will immediately run into our own lock above our own lock above, causing unnecessary (and Very Expensive) context switch Is this true and if so, on which platform(s)? Aren't most mutex/condition variable implementations optimized to avoid this case by deferring the condition signal to the…
> Aren't most mutex/condition variable implementations optimized to avoid this case by deferring the condition signal to the mutex unlock? Yes, _some_ but not _all_ implementations are doing it; however, as it is not really guaranteed (and actually is a workaround for poorly written programs) - standing recommendation is still to notify after the lock (which can be better, can be the same, but won't be worse than doi…
Pthreads on Linux encourage holding the lock during a signal or broadcast (http://linux.die.net/man/3/pthread_cond_signal):
The pthread_cond_broadcast() or pthread_cond_signal()
functions may be called by a thread whether or not it
currently owns the mutex that threads calling
pthread_cond_wait() or pthread_cond_timedwait() have
associated with the condition variable during their waits;
however, if predictable scheduling behavior is required,
then that mutex shall be locked by the thread calling
pthread_cond_broadcast() or pthread_cond_signal().
Re: Implementing Queues for Event-Driven Programs
#8From the first code example: > Yep, notifying outside of lock is usually BETTER. Otherwise the other thread would be released but will immediately run into our own lock above our own lock above, causing unnecessary (and Very Expensive) context switch Is this true and if so, on which platform(s)? Aren't most mutex/condition variable implementations optimized to avoid this case by deferring the condition signal to the…
> Aren't most mutex/condition variable implementations optimized to avoid this case by deferring the condition signal to the mutex unlock? Yes, _some_ but not _all_ implementations are doing it; however, as it is not really guaranteed (and actually is a workaround for poorly written programs) - standing recommendation is still to notify after the lock (which can be better, can be the same, but won't be worse than doi…
Since most platforms defer condition signalling to mutex unlocks, the way the code is now written will cause more spurious wakeups and context switching than necessary.
The pathological case in scheduling goes like this:
1. Reader A enters pop_front on an empty queue, goes to wait on the condition variable
2. Writer W enters push_back, adds an element to list and releases mutex and get pre-empted (on line 25 first example)
3. Reader B enters pop_front, sees queue not empty, pops element and leaves
4. Writer W signals condition, wakes up Reader A
5. Reader A wakes up but the queue is empty, goes back to sleep causing unnecessary context switch and trashing the TLB
I would say it's usually better to signal your condition variables with mutexes locked unless you know you're running on a platform with flaky condition variables. Easier to guarantee correctness and avoid spurious wakeups that way.
related note: cppreference.com is a terrible resource. It was worse 10 years ago, but it's not improved much. I would not trust the weasel wording ("some implementations" etc) in the link you posted, what it says about pthreads contradicts the pthread man pages (that encourage signal while holding lock)
Re: Implementing Queues for Event-Driven Programs
#9Earlier quoted context omitted.
That would be relying on the OS to do the right thing. One problem is that on some platform or for some usage pattern, then mutex and notifying object will be two independent objects, so the OS would have no way of knowing that it should not immediately wake up the notified threads. Furthermore, it is a good rule of thumb anyway: hold the mutex for the shortest time possible. The corresponding pattern is to hold a mu…
> One problem is that on some platform or for some usage pattern, then mutex and notifying object will be two independent objects, so the OS would have no way of knowing that it should not immediately wake up the notified threads. On all significant platforms I know, condition wait works like this: condition_wait(locked_mutex, condition_to_wait); For each thread in the wait list, the condition variable "knows" which…
> Additionally, when you unlock a mutex before signalling the condition, another thread may be scheduled in between that may invalidate the condition that was signalled. This may introduce a race condition.
And if you unlock the mutex after signalling the condition, another thread can be still scheduled in between the call to notify() and whatever-notified-thread-which-starts-to-run (and can still invalidate whatever-conditions-it-can-invalidate). There is absolutely no guarantee whatsoever that notified-thread gets to your mutex first (not that it really should matter for correctness in any sane program BTW).
> I signal my condition variables with mutexes locked because it's easier to guarantee correctness that way.
There is no difference in guarantees in practice whether you notify within or after; I'm not even sure that there is any difference in theory (exactly because the thread-which-was-notified is not guaranteed to get mutex first).
Re: Implementing Queues for Event-Driven Programs
#10Earlier quoted context omitted.
> Aren't most mutex/condition variable implementations optimized to avoid this case by deferring the condition signal to the mutex unlock? Yes, _some_ but not _all_ implementations are doing it; however, as it is not really guaranteed (and actually is a workaround for poorly written programs) - standing recommendation is still to notify after the lock (which can be better, can be the same, but won't be worse than doi…
Since most platforms defer condition signalling to mutex unlocks, the way the code is now written will cause more spurious wakeups and context switching than necessary. The pathological case in scheduling goes like this: 1. Reader A enters pop_front on an empty queue, goes to wait on the condition variable 2. Writer W enters push_back, adds an element to list and releases mutex and get pre-empted (on line 25 first ex…
From what I've seen (YMMV), chances of it happening are MUCH smaller than chances of getting context switch under the lock (with lots of threads running into this lock and having their own context switches), because of spending more time under the lock than it is really necessary (and ANY call, especially kernel call at 300+ clocks, is a LOT of time). Strictly speaking, it needs to be measured, but until that point - I'm keeping my mutex locks as small as possible.