"vicente.botet" <vicente.botet@wanadoo.fr> writes:
From: "Anthony Williams" <anthony_w.geo@yahoo.com>
"vicente.botet" <vicente.botet@wanadoo.fr> writes:
In the Join library I have see the following idiom
mutex mtx; condition_variable cond; unsigned num_blocked;
void foo() { unique_lock l(mtx); ++num_blocked; cond.wait(lock); --num_blocked;
// ... }
void foo() { unique_lock l(mtx); // ... if (num_blocked > 0) cond.notify_one(); // ... }
Does this perform better in some contexts? if yes in which ones?
It really shouldn't do. The check is part of the notify.
On windows notify_one is defined as:
void notify_one() { if(detail::interlocked_read_acquire(&total_count)) { boost::mutex::scoped_lock internal_lock(internal_mutex); // ... } }
detail::interlocked_read_acquire do a system call.
Not with MSVC it doesn't: it's just a plain volatile read and a compiler barrier to prevent optimizations changing the ordering. See boost/thread/win32/interlocked_read.hpp
This should be more expensive than doing a test >0, isn't it?
It could be on non-MSVC compilers.
With pthreads notify_one is defined as
inline void condition_variable::notify_one() { BOOST_VERIFY(!pthread_cond_signal(&cond)); }
Does pthread_cond_signal needs to protect its own counter from multiple threads? Could some one point to the pthread_cond_signal source code, or how to get it?
pthread_cond_signal is the POSIX condition variable notify. You can get the source code from your favourite linux distro. The POSIX standard requires that it is thread-safe and only notifies threads currently waiting on the cond var: if there aren't any it does nothing.
Note that this idiom is only applicable if the application use the same lock to wait and to notify, which is quite current.
In general, you don't need to hold a mutex across the notify. If you take an extra lock in order to check the count, you're imposing unnecessary synchronization. Anthony -- Anthony Williams | Just Software Solutions Ltd Custom Software Development | http://www.justsoftwaresolutions.co.uk Registered in England, Company Number 5478976. Registered Office: 15 Carrallack Mews, St Just, Cornwall, TR19 7UL