A parallel for using std::thread?

Question

I'm new with std::thread and I try to code a parallel_for. I coded the following thing:

// parallel_for.cpp
// compilation: g++ -O3 -std=c++0x parallel_for.cpp -o parallel_for -lpthread
// execution: time ./parallel_for 100 50000000 
// (100: number of threads, 50000000: vector size)
#include <iostream>
#include <iomanip>
#include <cstdlib>
#include <vector>
#include <thread>
#include <cmath>
#include <algorithm>
#include <numeric>
#include <utility>

// Parallel for
template<typename Iterator, class Function>
void parallel_for(const Iterator& first, const Iterator& last, Function&& f, const int nthreads = 1, const int threshold = 1000)
{
    const unsigned int group = std::max(std::max(1, std::abs(threshold)), (last-first)/std::abs(nthreads));
    std::vector<std::thread> threads;
    for (Iterator it = first; it < last; it += group) {
        threads.push_back(std::thread([=](){std::for_each(it, std::min(it+group, last), f);}));
    }
    std::for_each(threads.begin(), threads.end(), [=](std::thread& x){x.join();});
}

// Function to apply
template<typename Type>
void f1(Type& x)
{
    x = std::sin(x)+std::exp(std::cos(x))/std::exp(std::sin(x)); 
}

// Main
int main(int argc, char* argv[]) {

    const unsigned int nthreads = (argc > 1) ? std::atol(argv[1]) : (1);
    const unsigned int n = (argc > 2) ? std::atol(argv[2]) : (100000000);
    double x = 0;
    std::vector<double> v(n);
    std::iota(v.begin(), v.end(), 0);

    parallel_for(v.begin(), v.end(), f1<double>, nthreads);

    for (unsigned int i = 0; i < n; ++i) x += v[i];
    std::cout<<std::setprecision(15)<<x<<std::endl;
    return 0;
}

But this is not working: (error codes from g++ 4.6)

parallel_for.cpp: In instantiation of ‘parallel_for(const Iterator&, const Iterator&, Function&&, int, int) [with Iterator = __gnu_cxx::__normal_iterator<double*, std::vector<double> >, Function = void (&)(double&)]::<lambda()>’:
parallel_for.cpp:22:9:   instantiated from ‘void parallel_for(const Iterator&, const Iterator&, Function&&, int, int) [with Iterator = __gnu_cxx::__normal_iterator<double*, std::vector<double> >, Function = void (&)(double&)]’
parallel_for.cpp:43:58:   instantiated from here
parallel_for.cpp:22:89: erreur: field ‘parallel_for(const Iterator&, const Iterator&, Function&&, int, int) [with Iterator = __gnu_cxx::__normal_iterator<double*, std::vector<double> >, Function = void (&)(double&)]::<lambda()>::__f’ invalidly declared function type

How to solve this problem ?

EDIT: This new version compiles but does not give the good result:

// parallel_for.cpp
// compilation: g++ -O3 -std=c++0x parallel_for.cpp -o parallel_for -lpthread
// execution: time ./parallel_for 100 50000000 
// (100: number of threads, 50000000: vector size)
#include <iostream>
#include <iomanip>
#include <cstdlib>
#include <vector>
#include <thread>
#include <cmath>
#include <algorithm>
#include <numeric>
#include <utility>

// Parallel for
template<typename Iterator, class Function>
void parallel_for(const Iterator& first, const Iterator& last, Function&& f, const int nthreads = 1, const int threshold = 1000)
{
    const unsigned int group = std::max(std::max(1, std::abs(threshold)), (last-first)/std::abs(nthreads));
    std::vector<std::thread> threads;
    for (Iterator it = first; it < last; it += group) {
        threads.push_back(std::thread([=, &f](){std::for_each(it, std::min(it+group, last), f);}));
    }
    std::for_each(threads.begin(), threads.end(), [](std::thread& x){x.join();});
}

// Function to apply
template<typename Type>
void f(Type& x)
{
    x = std::sin(x)+std::exp(std::cos(x))/std::exp(std::sin(x)); 
}

// Main
int main(int argc, char* argv[]) {

    const unsigned int nthreads = (argc > 1) ? std::atol(argv[1]) : (1);
    const unsigned int n = (argc > 2) ? std::atol(argv[2]) : (100000000);
    double x = 0;
    double y = 0;
    std::vector<double> v(n);

    std::iota(v.begin(), v.end(), 0);
    std::for_each(v.begin(), v.end(), f<double>);
    for (unsigned int i = 0; i < n; ++i) x += v[i];

    std::iota(v.begin(), v.end(), 0);
    parallel_for(v.begin(), v.end(), f<double>, nthreads);
    for (unsigned int i = 0; i < n; ++i) y += v[i];

    std::cout<<std::setprecision(15)<<x<<" "<<y<<std::endl;
    return 0;
}

The result is :

./parallel_for 1 100
155.524339894552 4950

The parallel version returns 4950 whereas the sequential version returns 155..... Where is the problem ?

Johan Lundberg · Answer 1 · 2012-12-26T20:35:15.480

You need a cast or type conversion at (last-first). The reason is that type conversions are never done during template argument deduction.

This works just fine (also fixing the problem DeadMG and Ben Voigt found). Both versions give 156608294.151782 with n=100000000.

template<typename Iterator, class Function>
void parallel_for(const Iterator& first, const Iterator& last, Function&& f, const int nthreads = 1, const int threshold = 1000)
{
    const unsigned int group = std::max(std::max(ptrdiff_t(1), ptrdiff_t(std::abs(threshold))), ((last-first))/std::abs(nthreads));
    std::vector<std::thread> threads;
    threads.reserve(nthreads);
    Iterator it = first;
    for (; it < last-group; it += group) {
        threads.push_back(std::thread([=,&f](){std::for_each(it, std::min(it+group, last), f);}));
    }
    std::for_each(it, last, f); // last steps while we wait for other threads

    std::for_each(threads.begin(), threads.end(), [](std::thread& x){x.join();});
}

Since the step for_each(it, last, f) is smaller than the others, we may as well use the calling thread to finish that while waiting for the other results.

awesoon · Accepted Answer · 2012-12-27T01:16:59.997

1

You must capture functions by reference.

[=, &f] () { /* your code */ };
Look at the code.
```
#include <iostream>

template <class T>
void foo(const T& t)
{
    const int a = t;
    [&]
    {
        std::cout << a << std::endl;
    }();
}


int main()
{
    foo(42);
    return 0;
}
```
The clang gives output 42, but g++ throws warning: ‘a’ is used uninitialized in this function, and prints 0. Looks like a bug.

Workaround: use const auto(for variable group in your code).

UPD: I think, that's it. http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52026

edited Dec 27 '12 at 01:16

answered Dec 26 '12 at 18:42

awesoon

32,469
11
74
99

Thanks! But now there is a new problem. – Vincent Dec 26 '12 at 18:52

Ben Voigt · Answer 3 · 2012-12-26T20:32:13.483

One problem is that it += group can be last legally, but creating a value off the end is undefined behavior. Merely checking it < last is too late to fix that.

You need to instead test last - it while it is still valid. (Neither it + group nor last - group is certain to be safe, although the latter should be due to the way group is calculated.)

For example:

template<typename Iterator, class Function>
void parallel_for(const Iterator& first, const Iterator& last, Function f, const int nthreads = 1, const int threshold = 100)
{
    const unsigned int group = std::max(std::max(1, std::abs(threshold)), (last-first)/std::abs(nthreads));
    std::vector<std::thread> threads;
    threads.reserve(nthreads);
    Iterator it = first;
    for (; last - it > group; it += group) {
        threads.push_back(std::thread([=, &f](){std::for_each(it, it+group, last), f);}));
    }
    threads.push_back(std::thread([=, &f](){std::for_each(it, last, f);}));

    std::for_each(threads.begin(), threads.end(), [](std::thread& x){x.join();});
}

yes. OP cross-posted this, so there was already answers in the other tread. Minor thing: *it* is not in scope for use in the second for_each. — Johan Lundberg, Dec 26 '12 at 19:59

score 0 · Answer 4 · answered Dec 26 '12 at 18:57

0

You give std::min(it+group, last) to std::for_each, but always add group on to the end. This means that if last is not a multiple of group on from it, you will move it past last, which is UB.

answered Dec 26 '12 at 18:57

Puppy

144,682
38
256
465

Johan Lundberg · Answer 5 · 2012-12-26T20:33:32.077

0

You need to capture by reference, and you need a cast or type conversion at (last-first). The reason is that type conversions are never done during template argument deduction.

Also, fix the problem DeadMG found, and you end up with the code below.

It works just fine, both versions give 156608294.151782 with n=100000000.

template<typename Iterator, class Function>
void parallel_for(const Iterator& first, const Iterator& last, Function&& f, const int nthreads = 1, const int threshold = 1000)
{
    const unsigned int group = std::max(std::max(ptrdiff_t(1), ptrdiff_t(std::abs(threshold))), ((last-first))/std::abs(nthreads));
    std::vector<std::thread> threads;
    Iterator it = first;
    for (; it < last-group; it += group) {
        threads.push_back(std::thread([=,&f](){std::for_each(it, std::min(it+group, last), f);}));
    }
    std::for_each(it, last, f); // use calling thread while we wait for the others
    std::for_each(threads.begin(), threads.end(), [](std::thread& x){x.join();});
}

edited Dec 26 '12 at 20:33

answered Dec 26 '12 at 18:57

Johan Lundberg

26,184
12
71
97

Why on earth would the second lambda need a capture by reference? It doesn't refer to any local variables. – Puppy Dec 26 '12 at 18:58
Also, the cast is quite irrelevant. If anything, you're now breaking iterators which have a difference which is larger than `int`. – Puppy Dec 26 '12 at 19:00
@DeadMG, just try. You need a cast to get past template argument deduction, but I agree that int can be too small. – Johan Lundberg Dec 26 '12 at 19:04
Seriously, just... just stop. You haven't changed anything meaningful, and you don't know what the problem is. Go reproduce it somewhere, and fix it, and then post an answer. – Puppy Dec 26 '12 at 19:14
I did... With your fix this is fine. – Johan Lundberg Dec 26 '12 at 19:16

score 0 · Answer 6 · answered Jan 08 '15 at 15:56

vc11 solution, please let me know if it´s not work with gcc.

template<typename Iterator, class Function>
void parallel_for( const Iterator& first, const Iterator& last, Function&& f, const size_t nthreads = std::thread::hardware_concurrency(), const size_t threshold = 1 )
{
    const size_t portion = std::max( threshold, (last-first) / nthreads );
    std::vector<std::thread> threads;
    for ( Iterator it = first; it < last; it += portion )
    {
        Iterator begin = it;
        Iterator end = it + portion;
        if ( end > last )
            end = last;

        threads.push_back( std::thread( [=,&f]() {
            for ( Iterator i = begin; i != end; ++i )
                f(i);
        }));
    }
    std::for_each(threads.begin(), threads.end(), [](std::thread& x){x.join();});
}

A parallel for using std::thread?

6 Answers6

Linked

Related