Re: [boost] [thread] RFC standard proposed mutex, read-write mutex, condition

From: Peter Dimov (pdimov_at_[hidden])
Date: 2007-08-20 19:37:51

Howard Hinnant wrote:

> I realize that this is stricter than POSIX, however I'm not sure that
> the difference in strictness has a practical consequence.  We're
> talking about a condition which would be waited on with mutex1 by a
> group of threads, and then at some later date (synchronized properly
> so as not to overlap) waited on with mutex2.  Do you have motivating
> use cases needing this functionality?

No, but I also have no motivating use cases needing the check when the 
condition has been default-constructed.

> As far as overhead goes, I'm more concerned about space overhead than
> performance overhead.  On Mac, sizeof(pthread_cond_t) is 28, and
> sizeof(pthread_mutex_t) is 44.  I would really like for there to be be
> an option for sizeof(std::condition) to be as close to 44 as
> possible.  Consider:
>
> class MyClass
> {
>     std::tr2::upgrade_mutex mut_;
>     ...
> };
>
> MyClass already has two std::condition's buried under the
> upgrade_mutex (at least by my reference implementation).

I believe that the main cost of a mutex or a condition is measured in kernel 
objects, not bytes. One will typically run out of kernel memory much 
earlier, IMO.