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

From: Howard Hinnant (hinnant_at_[hidden])
Date: 2007-08-22 10:31:46

On Aug 22, 2007, at 10:11 AM, David Abrahams wrote:

>
> on Mon Aug 20 2007, Howard Hinnant <howard.hinnant-AT-gmail.com>  
> wrote:
>
>> Here is a link to a reference implementation and a FAQ for mutexes,
>> locks and condition variables I am currently anticipating proposing
>> for C++ standardization (or subsequent TR).
>>
>> http://home.twcny.rr.com/hinnant/cpp_extensions/concurrency_rationale.html
>
>  "However if the external mutex is (or is layout compatible with) a
>  pthread_mutex_t, then there is no need for the internal
>  pthread_mutex_t. When one wants to wait, one simply waits on the
>  external pthread_mutex_t with the internal pthread_cond_t."
>
> This smells a little fishy.  It seems to imply that pthreads can wait
> on any old data type that happens to be layout-compatible with
> pthread_mutex_t.  Is that really true?

Perhaps the wording isn't precise enough.

The intent is that on pthreads:

class mutex
{
     pthread_mutex_t mut_;
public:
     ...
};

template <>
class condition<mutex>
{
     pthread_cond_t cv_;
public:
     ...
};

When condition<mutex>::wait is called with a lock whose mutex_type is  
std::mutex (e.g. unique_lock<mutex>), then there is no need for an  
internal pthread_mutex_t in condition<mutex>.  Internally the wait  
function can simply:

    pthread_cond_wait(&cv_, lock.mutex()->native_handle());

where lock.mutex()->native_handle() is returning a pointer to the  
member mut_ of the std::mutex referenced by the passed in lock.

Imho this is a critical optimization.  It isn't that it makes the wait  
faster (it does, but who cares, it blocks anyway).  It is that it  
makes the std::condition<std::mutex> smaller in size.  Since  
condition<mutex> is anticipated to be a low level, foundation tool,  
possibly used in many places, its size becomes important just for  
reducing L1 cache misses (for the exact same reason that people care  
that sizeof(vector<int>) is 3 words, not 4 words).

-Howard