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Subject: Re: [boost] [thread] countdown_latch
From: Michael Marcin (mike.marcin_at_[hidden])
Date: 2013-05-13 16:27:50

On 5/13/13 2:19 PM, Gaetano Mendola wrote:
> On 13/05/2013 02.46, Rob Stewart wrote:
>> On May 12, 2013, at 6:45 PM, Michael Marcin <mike.marcin_at_[hidden]>
>> wrote:
>>> On 5/12/2013 6:49 AM, Rob Stewart wrote:
>>>> On May 11, 2013, at 5:29 PM, Gaetano Mendola <mendola_at_[hidden]> wrote:
>>>>> On 21/04/2013 12.54, Rob Stewart wrote:
>>>>>> I'd prefer to create a barrier class and, in your example, it
>>>>>> would release waiting threads when two are blocked behind it. IOW,
>>>>>> you'd create a barrier for two threads and both thread_proc() and
>>>>>> the constructor would wait() on the barrier. Once both threads
>>>>>> have called wait(), they are both released. (I plan to present
>>>>>> that at C++ Now, this year.)
>>>>> As said that's create a not needed "wait" on the thread body, what
>>>>> the OP (and me for the matter) needs is that *only* thread creator
>>>>> is blocked waiting for the threads to arrive at certain point of
>>>>> the execution.
>>>> It seems to me that wait is inconsequential relative to the cost of
>>>> creating a thread. If this were part of the thread creation process,
>>>> an option to thread's constructor, say, there would be some
>>>> convenience, but the performance difference doesn't seem worthwhile.
>>>> Have I missed something?
>>> The created thread is already executing when it gets to the latch.
>>> Why would you want to introduce synchronization, and potentially
>>> block the thread, where none is needed?
>> Is the latch to just cause the creator to block until the created
>> thread begins, or is it more general purpose to cause a number of
>> threads to wait until they are all ready? I thought it was the latter
>> and, if so, how can it be done without synchronization? I must still
>> be missing something.
> The latch is to just cause the creator to block until the created
> thread begins.
> At the moment with boost off-the-shelf you can achieve it using a
> barrier but doing so you are blocking the thread as well, what me
> and the OP needs is that the thread creator will eventualy block
> waiting for the thread not the other way around. You can achieve it
> using a syncronization mechanims named "latch":
> boost::latch myLatch(1);
> myLatch.count_down(); /// This is not a blocking operation
> myLatch.wait(); /// This is blocking if a count_down() was not issued
> of course latch is implemented with condition.

More generally you can use the latch to block N threads until N events
occur. But the threads signaling the events don't have to block.

 From the Java docs:

A CountDownLatch is a versatile synchronization tool and can be used for
a number of purposes. A CountDownLatch initialized with a count of one
serves as a simple on/off latch, or gate: all threads invoking await
wait at the gate until it is opened by a thread invoking countDown(). A
CountDownLatch initialized to N can be used to make one thread wait
until N threads have completed some action, or some action has been
completed N times.

A useful property of a CountDownLatch is that it doesn't require that
threads calling countDown wait for the count to reach zero before
proceeding, it simply prevents any thread from proceeding past an await
until all threads could pass.

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