Re: [boost] [thread] countdown_latch

Subject: Re: [boost] [thread] countdown_latch
From: Vicente J. Botet Escriba (vicente.botet_at_[hidden])
Date: 2013-05-13 17:13:06

Le 13/05/13 21:05, Gaetano Mendola a écrit :
> On 12/05/2013 01.03, Vicente J. Botet Escriba wrote:
>> Le 11/05/13 23:25, Gaetano Mendola a écrit :
>>> On 27/04/2013 09.05, Vicente J. Botet Escriba wrote:
>>>> The change set is https://svn.boost.org/trac/boost/changeset/84055. To
>>>> see the sources, you would need to update the trunk or to take a 
>>>> look at
>>>> https://svn.boost.org/svn/boost/trunk/boost/thread/
>>>>
>>>> Please let me know what do you think.
>>>
>>> Is the explicit lk.unlock() inside the
>>>
>>> bool count_down(unique_lock<mutex> &lk);
>>>
>>> needed ?
>>>
>>
>>> I see the two places where it's called are:
>>>
>>> void count_down()
>>> {
>>>   boost::unique_lock<boost::mutex> lk(mutex_);
>>>   count_down(lk);
>>> }
>>>
>>> and
>>>
>>> void count_down_and_wait()
>>> {
>>>   boost::unique_lock<boost::mutex> lk(mutex_);
>>>   if (count_down(lk))
>>>   {
>>>     return;
>>>   }
>>>   count_.cond_.wait(lk, detail::counter_is_zero(count_));
>>> }
>>>
>>> in both cases the unique_lock does the job, or I'm missing
>>> something?
>> Not really. It is an optimization.
>
> Let me understand, instead of:
>
> - long jump
> - unlock inside boost::~unique_lock
>
> with your optimization the following will happen:optimization:
>
> - explicit unlock
> - long jump
> - boost::~unique_lock
>
> is that optimization realy worth ? I mean a mutex unlock is hundred
> of instructions, involving even a syscall, while a long jump is just
> a long jump.
>
> Do you have any evidence of the gain you have doing so?
>
> Also while we talk about optimization why not use lock_guard when
> possible ?
>
> > The mutex don't need to be lock when the condition is notified.
>
> Of course is not needed, but my point was another one.
>
I didn't expressed myself clearly. The optimization consist in unlocking 
as soon as possible not in executing less instructions.
>>>
>>> Also I would add a new method to boost::detail::counter
>>>
>>>   bool dec_and_notify_all_if_value(const std::size a_value)
>>>   {
>>>     if (--value_ == a_value)
>>>     {
>>>       cond_.notify_all();
>>>       return true;
>>>     }
>>>     return false;
>>>   }
>>>
>>>
>>> This way you can further simplify the count_down(unique_lock<mutex> 
>>> &lk)
>>> method:
>>>
>>>     //lk is here only to assure it's a thread safe call
>>>     bool count_down(unique_lock<mutex> &lk)
>>>     /// pre_condition (count_.value_ > 0)
>>>     {
>>>       BOOST_ASSERT(count_.value_ > 0);
>>>       return count_.dec_and_notify_all_if_value(0);
>>>     }
>> When this function would be used?
>
> Look at the new version of bool count_down(unique_lock<mutex> &lk);
> I have expressed above.
>
I meant in addition to the use in count_down.

Best,
Vicente