boost:asio thread pool implementation for occasionally synchronized tasks

Question

I have a \"main\" function that performs many small, independent tasks each once per time step. However, after each time step, I must wait for all of the tasks to complete befor

Answer 1

may be you can use boost::barrier as follow:

void thread_proc( boost::barrier& b ) {
    while( true ) {
        if( !ioservice.run_one() ) break; // io_service stopped
        b.wait();
    }
}

Answer 2

Rost's method essentially works, but the boost::make_shared can not compile as is. The following is a working version (in vs2012):

#include <boost/asio.hpp>
#include <boost/bind.hpp>
#include <boost/make_shared.hpp>
#include <boost/function_types/result_type.hpp>
#include <boost/shared_ptr.hpp>
#include <boost/function.hpp>
#include <boost/thread.hpp>

std::vector<boost::unique_future<void>> pending_data;
typedef boost::packaged_task<void> task_t;

boost::shared_ptr< boost::packaged_task<void> > pt(new boost::packaged_task<void> ([&,i](){...}));
boost::unique_future<void> result = pt->get_future();
pending_data.push_back(boost::move(result));
io_service.post(boost::bind(&task_t::operator(), pt));

boost::wait_for_all(pending_data.begin(), pending_data.end()); 
pending_data.clear();

It will not compile if use argument in the packaged_task typedef. This thread pool by asio and future method only saved 8% time compared with each loop create new thread methods.

Answer 3

You may use futures for data processing and synchronize with them using boost::wait_for_all(). This will allow you to operate in terms of parts of work done, not threads.

int process_data() {...}

// Pending futures
std::vector<boost::unique_future<int>> pending_data;

for(int i = 0; i < numSmallTasks; ++i)
{
   // Create task and corresponding future
   // Using shared ptr and binding operator() trick because
   // packaged_task is non-copyable, but asio::io_service::post requires argument to be copyable

   // Boost 1.51 syntax
   // For Boost 1.53+ or C++11 std::packaged_task shall be boost::packaged_task<int()>
   typedef boost::packaged_task<int> task_t;

   boost::shared_ptr<task_t> task = boost::make_shared<task_t>(
      boost::bind(&process_data, i, theTime));

   boost::unique_future<int> fut = task->get_future();

   pending_data.push_back(std::move(fut));
   io_service.post(boost::bind(&task_t::operator(), task));    
}

// After loop - wait until all futures are evaluated
boost::wait_for_all(pending_data.begin(), pending_data.end());