c++ - threads - ¿Qué implementaciones std:: async usan grupos de hilos?

threads on c++

Question

Se supone que una de las ventajas de usar std::async lugar de crear manualmente objetos std::thread es que std::async puede usar pools de hilos bajo las cubiertas para evitar problemas de sobresuscripción. Pero, ¿qué implementaciones hacen esto? Según tengo entendido, la implementación de Microsoft sí lo hace, pero ¿qué hay de estas otras implementaciones async ?

• Libstdc de Gnu
• Gnu''s libc ++
• La biblioteca de Just Software
• Boost (para boost::thread::async , no std::async )

Gracias por cualquier información que pueda ofrecer.

Answer 1

Prueba Black-Box

Aunque la comprobación de "caja blanca" se puede realizar inspeccionando fuentes boost, libstdc ++ o libc ++, o comprobando documentación como just::thread o MSVC Concurrency Runtime , ¡pero no puedo negarme el placer de escribir el código C ++ 11!

He hecho una prueba de caja negra :

DEMO EN VIVO

#define BOOST_THREAD_PROVIDES_FUTURE
#define BOOST_RESULT_OF_USE_DECLTYPE
#include <boost/exception/exception.hpp>
#include <boost/range/algorithm.hpp>
#include <boost/move/iterator.hpp>
#include <boost/phoenix.hpp>
#include <boost/thread.hpp>
#include <boost/config.hpp>

#include <unordered_set>
#include <functional>
#include <algorithm>
#include <iterator>
#include <iostream>
#include <ostream>
#include <cstddef>
#include <string>
#include <vector>
#include <future>
#include <thread>
#include <chrono>
#include <mutex>

// _____________________[CONFIGURATION]________________________ //
namespace async_lib = std;
const bool work_is_sleep = false;
// ____________________________________________________________ //

using namespace std;
using boost::phoenix::arg_names::arg1;
using boost::thread_specific_ptr;
using boost::back_move_inserter;
using boost::type_name;
using boost::count_if;
using boost::copy;

template<typename Mutex>
unique_lock<Mutex> locker(Mutex &m)
{
    return unique_lock<Mutex>(m);
}

void do_work()
{
    if(work_is_sleep)
    {
        this_thread::sleep_for( chrono::milliseconds(20) );
    }
    else
    {
        volatile double result=0.0;
        for(size_t i=0; i!=1<<22; ++i)
            result+=0.1;
    }
}

int main()
{
    typedef thread::id TID;
    typedef async_lib::future<TID> FTID;

    unordered_set<TID> tids, live_tids;
    vector<FTID> ftids;
    vector<int> live_tids_count;
    async_lib::mutex m;
    generate_n
    (
        back_move_inserter(ftids), 64*thread::hardware_concurrency(),
        [&]()
        {
            return async_lib::async([&]() -> TID
            {
                static thread_specific_ptr<bool> fresh;
                if(fresh.get() == nullptr)
                    fresh.reset(new bool(true));
                TID tid = this_thread::get_id();
                locker(m),
                    live_tids.insert(tid),
                    live_tids_count.push_back(int(live_tids.size()) * (*fresh ? -1 : 1));
                do_work();
                locker(m),
                    live_tids.erase(tid);
                *fresh = false;
                return tid;
            });
        }
    );
    transform
    (
        begin(ftids), end(ftids),
        inserter(tids, tids.end()),
        [](FTID &x){return x.get();}
    );

    cout << "Compiler = " << BOOST_COMPILER                             << endl;
    cout << "Standard library = " << BOOST_STDLIB                       << endl;
    cout << "Boost = " << BOOST_LIB_VERSION                             << endl;
    cout << "future type = " << type_name<FTID>()                       << endl;
    cout << "Only sleep in do_work = " << boolalpha << work_is_sleep    << endl;
    cout << string(32,''_'')                                              << endl;
    cout << "hardware_concurrency = " << thread::hardware_concurrency() << endl;
    cout << "async count = " << ftids.size()                            << endl;
    cout << "unique thread id''s = " << tids.size()                      << endl;
    cout << "live threads count (negative means fresh thread):"                ;
    copy(live_tids_count, ostream_iterator<int>(cout," "));        cout << endl;
    cout << "fresh count = " << count_if(live_tids_count, arg1 < 0)     << endl;
}

Compiler = Microsoft Visual C++ version 11.0
Standard library = Dinkumware standard library version 540
Boost = 1_53
future type = class std::future<class std::thread::id>
Only sleep in do_work = false
________________________________
hardware_concurrency = 4
async count = 256
unique thread id''s = 4
live threads count (negative means fresh thread):-1 -2 2 2 -3 -4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4
 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 3 4 4 4 4 4 4 4 4 4
 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4
 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4
fresh count = 4

Compiler = Microsoft Visual C++ version 11.0
Standard library = Dinkumware standard library version 540
Boost = 1_53
future type = class std::future<class std::thread::id>
Only sleep in do_work = true
________________________________
hardware_concurrency = 4
async count = 256
unique thread id''s = 34
live threads count (negative means fresh thread):-1 -2 -3 -4 -5 -6 -7 -8 -9 -10 -11 -12 -13 -14 -15 -16 -17 -18 18 18 18 18 18 18 18 18 18 18 18 18 18
 18 18 18 18 -19 19 -20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 -21 21 21 -22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 -
23 23 23 23 -24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 -25 25 25 25 25 -26 26 26 26 26 26 26 26 26 26 26 26 26 26 26 26 26 26 26
26 26 26 -27 27 27 27 27 27 -28 28 28 28 28 28 28 28 28 28 28 28 28 28 28 28 28 28 28 28 28 28 28 -29 29 29 29 29 29 29 -30 30 30 30 30 30 30 30 30 30
 30 30 30 30 30 30 30 30 30 30 30 30 30 30 -31 31 31 31 31 31 31 31 -32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 -33 33
 33 33 33 33 33 33 33 -34 34 34 34 34 34 34 34 34 11 12 11 12 13 14 15 16 15 10 11 12 13 14
fresh count = 34

Compiler = Microsoft Visual C++ version 11.0
Standard library = Dinkumware standard library version 540
Boost = 1_53
future type = class boost::future<class std::thread::id>
Only sleep in do_work = false
________________________________
hardware_concurrency = 4
async count = 256
unique thread id''s = 256
live threads count (negative means fresh thread):-1 -2 -2 -3 -3 -3 -3 -3 -3 -3 -3 -3 -3 -3 -3 -3 -4 -4 -4 -4 -4 -4 -4 -4 -4 -4 -3 -4 -4 -4 -4 -4 -4 -4
 -4 -4 -4 -4 -4 -4 -4 -4 -4 -4 -4 -4 -4 -4 -4 -4 -4 -4 -4 -4 -4 -4 -4 -4 -4 -4 -4 -4 -4 -4 -4 -4 -4 -4 -4 -4 -2 -3 -3 -3 -3 -4 -4 -4 -4 -2 -2 -2 -2 -3
 -2 -2 -3 -1 -2 -2 -3 -3 -1 -2 -1 -2 -3 -2 -2 -2 -2 -1 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2 -3 -3 -2 -2 -2 -2 -3 -3 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2
 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2 -3 -1 -2 -1 -2 -1 -2 -1 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2 -3 -3 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2
 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2 -3 -3 -2 -2 -3 -1 -2 -1 -2 -2 -2 -3 -2 -3 -1 -2 -2 -2 -3 -2 -3 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2
 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2 -3 -3 -3 -2 -2 -2 -3
fresh count = 256

Compiler = Microsoft Visual C++ version 11.0
Standard library = Dinkumware standard library version 540
Boost = 1_53
future type = class boost::future<class std::thread::id>
Only sleep in do_work = true
________________________________
hardware_concurrency = 4
async count = 256
unique thread id''s = 256
live threads count (negative means fresh thread):-1 -2 -3 -4 -5 -6 -7 -8 -9 -10 -11 -12 -13 -14 -15 -16 -17 -18 -19 -20 -21 -22 -23 -24 -25 -26 -27 -2
8 -29 -30 -31 -32 -33 -34 -35 -36 -37 -38 -39 -40 -41 -42 -43 -44 -45 -46 -47 -48 -49 -50 -51 -52 -53 -54 -55 -56 -57 -58 -59 -60 -61 -62 -63 -64 -65
-66 -67 -68 -69 -70 -71 -72 -73 -74 -75 -76 -77 -78 -79 -80 -81 -82 -83 -84 -85 -86 -87 -88 -89 -90 -91 -92 -93 -94 -95 -96 -97 -98 -99 -100 -101 -102
 -103 -104 -105 -106 -107 -108 -109 -110 -111 -112 -113 -114 -115 -116 -117 -118 -119 -120 -121 -122 -123 -124 -125 -126 -127 -128 -129 -130 -131 -132
 -133 -134 -135 -136 -137 -138 -139 -140 -141 -142 -143 -144 -145 -146 -147 -148 -149 -150 -151 -152 -153 -154 -155 -156 -157 -158 -159 -160 -161 -162
 -163 -164 -165 -166 -167 -168 -169 -170 -171 -172 -173 -174 -175 -176 -177 -178 -179 -180 -181 -182 -183 -184 -185 -186 -187 -188 -189 -190 -191 -192
 -193 -194 -195 -196 -197 -198 -199 -200 -201 -202 -203 -204 -205 -206 -207 -208 -209 -210 -211 -212 -213 -214 -215 -216 -217 -218 -219 -220 -221 -222
 -223 -224 -225 -226 -227 -228 -229 -230 -231 -232 -233 -234 -235 -236 -237 -238 -239 -240 -241 -242 -243 -244 -245 -246 -247 -248 -249 -250 -251 -252
 -253 -254 -255 -256
fresh count = 256

Compiler = GNU C++ version 4.8.0-alpha20121216 20121216 (experimental)
Standard library = GNU libstdc++ version 20121216
Boost = 1_53
future type = std::future<std::thread::id>
Only sleep in do_work = false
________________________________
hardware_concurrency = 4
async count = 256
unique thread id''s = 1
live threads count (negative means fresh thread):-1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
fresh count = 1

Compiler = GNU C++ version 4.8.0-alpha20121216 20121216 (experimental)
Standard library = GNU libstdc++ version 20121216
Boost = 1_53
future type = boost::future<std::thread::id>
Only sleep in do_work = false
________________________________
hardware_concurrency = 4
async count = 256
unique thread id''s = 122
live threads count (negative means fresh thread):-1 -2 -3 -4 -5 -6 -7 -8 -9 -10 -11 -12 -13 -14 -15 -16 -17 -18 -19 -20 -21 -20 -21 -22 -23 -24 -25 -25 -26 -27 -28 -29 -29 -30 -31 -32 -33 -34 -35 -36 -37 -38 -39 -40 -41 -42 -43 -44 -45 -46 -47 -48 -49 -50 -51 -52 -53 -54 -55 -56 -57 -58 -59 -59 -60 -61 -62 -63 -64 -64 -64 -63 -61 -55 -48 -41 -33 -34 -27 -27 -28 -29 -29 -30 -31 -31 -30 -28 -29 -28 -28 -28 -29 -30 -30 -31 -30 -31 -31 -31 -32 -31 -26 -24 -25 -26 -25 -23 -23 -22 -20 -21 -19 -20 -20 -19 -20 -21 -21 -22 -21 -22 -23 -24 -24 -25 -26 -27 -25 -25 -25 -22 -23 -22 -23 -23 -24 -25 -26 -27 -27 -28 -29 -29 -30 -31 -32 -33 -34 -35 -36 -37 -38 -39 -40 -41 -42 -43 -44 -45 -46 -47 -48 -49 -50 -51 -51 -52 -53 -54 -55 -56 -57 -56 -54 -3 -3 -4 -4 -5 -6 -7 -8 -9 -10 -11 -12 -13 -14 -15 -16 -17 -18 -19 -20 -21 -22 -23 -24 -25 -26 -27 -28 -29 -30 -31 -32 -33 -34 -35 -36 -37 -38 -39 -40 -41 -42 -43 -44 -45 -46 -47 -48 -49 -50 -51 -52 -53 -54 -55 -55 -56 -57 -58 -59 -60 -61 -61 -61 -62 -63 -64 -65 -64 -62 -63 -63 -61 -62 -61 -59 -60 -59 -57 -55 -56
fresh count = 256

Compiler = Clang version 3.2 (tags/RELEASE_32/final)
Standard library = libc++ version 1101
Boost = 1_53
future type = NSt3__16futureINS_11__thread_idEEE
Only sleep in do_work = false
________________________________
hardware_concurrency = 4
async count = 256
unique thread id''s = 255
live threads count (negative means fresh thread):-1 -2 -3 -4 -5 -6 -7 -8 -9 -10 -11 -12 -12 -13 -14 -15 -16 -17 -18 -19 -20 -21 -22 -23 -24 -25 -26 -27 -27 -28 -29 -30 -31 -32 -33 -34 -35 -36 -37 -38 -39 -40 -41 -42 -43 -44 -45 -46 -47 -48 -49 -50 -51 -52 -53 -54 -55 -56 -57 -58 -59 -60 -61 -62 -63 -64 -65 -66 -67 -68 -69 -69 -70 -71 -72 -73 -74 -75 -76 -77 -78 -79 -80 -81 -82 -83 -84 -85 -86 -87 -88 -89 -90 -91 -92 -93 -94 -95 -96 -97 -98 -99 -100 -101 -102 -102 -103 -104 -105 -106 -107 -108 -109 -110 -111 -112 -113 -114 -115 -116 -117 -118 -119 -120 -121 -121 -122 -122 -123 -123 -124 -125 -126 -127 -126 -127 -128 -129 -129 -128 -129 -128 -129 -129 -128 -129 -129 -128 -126 -127 -128 -128 -129 -129 -130 -129 -129 -128 -129 -129 -130 -131 -132 -133 -134 -135 -136 -134 -132 -133 -134 -134 -133 -132 -133 -132 -133 -134 -133 -131 -129 -127 -124 -125 -121 -119 -120 -118 -119 -118 -117 -115 -111 -107 -105 -106 -103 -100 -97 -95 -96 -94 -90 -87 -81 -73 -74 -71 -72 -73 -74 -75 -70 -71 -66 -60 -59 -60 -61 -62 -63 -64 -61 -58 -55 -55 -52 -53 -54 -54 -55 -56 -56 -57 -54 -55 -56 -56 -57 -57 -58 -56 -54 -55 -56 -56 -57 -58 -58 -59 -58 -58 -58 -58 -59 -60
fresh count = 256

Como se puede ver, MSVC sí reutiliza los subprocesos, es decir, es muy probable que su esquema sea una especie de grupo de subprocesos.