Re: [boost] [thread] countdown_latch

Subject: Re: [boost] [thread] countdown_latch
From: Gaetano Mendola (mendola_at_[hidden])
Date: 2013-05-13 15:05:03

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.

>>
>> 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.

Regards
Gaetano Mendola