Summary

"cpptask" is an open source library which implements load balanced task scheduler for developing parallel C++ programs.

The main advantages are:

1. It was made to be similar to native C++ async interfaces as much as possible.
2. You can specify number of threads, library will not create new ones.
3. "Work stealing" technique is used for load balancing.
4. You can use special type of synchronization primitives and special lock type to guarantee workload of thread during lock waiting operation.
5. Library implemented as headers only library.

Features

1. Task management

• future class
• async function

These functionality copies standard C++ signatures, methods and behaviour except shared_future class.

Usage example:

#include <cpptask/cpptask.h>
#include <chrono>
#include <cassert>

int main()
{
   cpptask::initializer init(4); //initialize library with 4 threads
   auto f = cpptask::async(std::launch::async, []()
   {
       //do something
       std::this_thread::sleep_for(std::chrono::milliseconds(10));
       return 5;
   });

   auto x = f.get();
   assert(5 == x);
}

2. Synchronization

• mutex class - Special synchronization primitive which allows to workload thread during waiting operation. should be used only in pair with process_lock type, signatures identical to std::mutex.

  Usage example:

  ...
  cpptask::mutex guard;
  ...
  {
    std::unique_lock<cpptask::mutex> lock(guard); //totaly block thread
  }
  ...
  {
    cpptask::process_lock<cpptask::mutex> lock(guard); //will process other tasks during lock
  }
  ...

• event class - Event synchronization abstraction.

  Usage example:

  cpptask::event e;
  ...
  //somewhere in thread
  {
    e.wait();
    //do some work if event fired
  }
  ...
  //somewehere in another thread
  e.notify();
  ...
  

• lockable_event class - Same as cpptask::event but should be used in pair with process_lock type to workload thread during waiting operation.

  Usage example:

  ...
  cpptask::lockable_event e;
  ...
  //somewhere in thread
  {
    cpptask::process_lock<cpptask::lockable_event> lock(e); //will process other tasks during lock
    //do some work when event fired
  }
  ...
  //somewehere in another thread
  e.notify();
  ...

• semaphore class - Counting semaphore synchronization abstraction. Object of this type can be used for example to limit threads count accessing some resource. Object can be initialized with initial counter value and maximum value for counter. When lock operation is called, counter value is decreased if counter value is non zero. When unlock operation is called, counter value is increased, but only in case if new value will be less or equal to maximum allowed. Also maximum value is negative by default, this mean that maximum value is unbounded.

  Usage example:

  ...
  cpptask::semaphore s(3,3); //We limit access to resource only to 3 threads concurrently
  SharedResource res;
  ...
  //somewhere in threads
  {
    std::unique_lock<cpptask::semaphore> lock(s);
    //now we can access resource
    process(res);
  }
  

• lockable_semaphore class - Same as cpptask::semaphore but should be used in pair with process_lock type to workload thread during waiting operation.

  Usage example:

  ...
  cpptask::lockable_semaphore s(3,3); //We limit access to resource only to 3 threads concurrently
  SharedResource res;
  ...
  //somewhere in threads
  {
    std::process_lock<cpptask::lockable_semaphore> lock(s); //will process other tasks during lock
    //now we can access resource
    process(res);
  }
  

• process_lock class - type for instantiating RAII lock on special synchronization primitives to workload thread during waiting. See lockable_mutex, lockable_event and lockable_semaphore.

3. Algorithms

• range class - simple right opened range. Can be initialized with integer types or with iterators. Used internally for parallel for_each implementation.

• split_range function - split input iterator range to number of smaller ranges sequentially. Used internally for parallel for_each implementation.

  Usage example:

  template<class Iterator>
  void function(Iterator start, Iterator end)
  {
      typedef range<Iterator> RangeType;
      typedef std::vector<RangeType> Ranges;
      Ranges ranges = split_range(start, end, 4); //split input iterator range to 4 sub ranges sequentially
      ...
  }
  

• split_num_range function - split input integers range to number of smaller ranges sequentially. Used internally for parallel for_each implementation.

  Usage example:

  ...
  typedef range<Int> RangeType;
  typedef std::vector<RangeType> Ranges;
  Ranges ranges = split_range(1, 100, 4); //split input integer range[1,100) to 4 sub ranges sequentially
   ...
  

• for_each function - apply operation to each element of input right opened range. Function splits input range to number of sub ranges and proccess them in parallel. Number of sub ranges equals to number of threads used for library initialization. Input range can be specified with iterators or with integer numbers.

  Usage example:

  ...
  cpptask::for_each(1,100,[&](int x){ heavyFunction(x); });
  ...
  

• reduce function - perform reduction on input right opened range, specified with iterators. Function recursively split input range and process each sub range in parallel with others.

  Usage example:

  ...
  int process_func(const cpptask::range<std::vector<int>::iterator>& range)
  {
      return std::accumulate(range.start, range.end, 0);
  }
  
  int join_func(int a, int b)
  {
     return a + b;
  }
  ...
  std::vector<int> m(BIG_NUMBER_OF_ELEMENTS);
  ...
  cpptask::reduce<int>(m.begin(), m.end(), process_func, join_func); // calculate sum in parallel
  ...
  

4. Miscellaneous

• initializer class - should be used for library initialization, use number of threads as parameter.

  Usage example:

  ...
  cpptask::initializer init(4); //initialize library with 4 threads
  ...
  

• cpptask::exception class - this type of exception will be throwed in case of exceptional situation from library functions.

Supported Platforms (was tested on)

• Linux (Arch)
• gcc 5.1.0 (64bits)
• clang 3.6.2 (64bits)
• Windows
• Visual Studio 2015 (64 bit)
• MinGw-64 (gcc 5.1)

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