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Stockfish, a UCI chess playing engine derived from Glaurung 2.1
Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
Copyright (C) 2008-2012 Marco Costalba, Joona Kiiski, Tord Romstad
Stockfish is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.
Stockfish is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program. If not, see <>.
#include <cassert>
#include <iostream>
#include "movegen.h"
#include "search.h"
#include "thread.h"
#include "ucioption.h"
using namespace Search;
ThreadPool Threads; // Global object
namespace { extern "C" {
// start_routine() is the C function which is called when a new thread
// is launched. It is a wrapper to member function pointed by start_fn.
long start_routine(Thread* th) { (th->*(th->start_fn))(); return 0; }
} }
// Thread c'tor starts a newly-created thread of execution that will call
// the idle loop function pointed by start_fn going immediately to sleep.
Thread::Thread(Fn fn) {
is_searching = do_exit = false;
maxPly = splitPointsCnt = 0;
curSplitPoint = NULL;
start_fn = fn;
idx = Threads.size();
do_sleep = (fn != &Thread::main_loop); // Avoid a race with start_searching()
if (!thread_create(handle, start_routine, this))
std::cerr << "Failed to create thread number " << idx << std::endl;
// Thread d'tor waits for thread termination before to return.
Thread::~Thread() {
do_exit = true; // Search must be already finished
thread_join(handle); // Wait for thread termination
// Thread::timer_loop() is where the timer thread waits maxPly milliseconds and
// then calls check_time(). If maxPly is 0 thread sleeps until is woken up.
extern void check_time();
void Thread::timer_loop() {
while (!do_exit)
sleepCondition.wait_for(mutex, maxPly ? maxPly : INT_MAX);
// Thread::main_loop() is where the main thread is parked waiting to be started
// when there is a new search. Main thread will launch all the slave threads.
void Thread::main_loop() {
while (true)
do_sleep = true; // Always return to sleep after a search
is_searching = false;
while (do_sleep && !do_exit)
Threads.sleepCondition.notify_one(); // Wake up UI thread if needed
if (do_exit)
is_searching = true;
// Thread::wake_up() wakes up the thread, normally at the beginning of the search
// or, if "sleeping threads" is used at split time.
void Thread::wake_up() {
// Thread::wait_for_stop_or_ponderhit() is called when the maximum depth is
// reached while the program is pondering. The point is to work around a wrinkle
// in the UCI protocol: When pondering, the engine is not allowed to give a
// "bestmove" before the GUI sends it a "stop" or "ponderhit" command. We simply
// wait here until one of these commands (that raise StopRequest) is sent and
// then return, after which the bestmove and pondermove will be printed.
void Thread::wait_for_stop_or_ponderhit() {
Signals.stopOnPonderhit = true;
while (!Signals.stop) sleepCondition.wait(mutex);;
// Thread::cutoff_occurred() checks whether a beta cutoff has occurred in the
// current active split point, or in some ancestor of the split point.
bool Thread::cutoff_occurred() const {
for (SplitPoint* sp = curSplitPoint; sp; sp = sp->parent)
if (sp->cutoff)
return true;
return false;
// Thread::is_available_to() checks whether the thread is available to help the
// thread 'master' at a split point. An obvious requirement is that thread must
// be idle. With more than two threads, this is not sufficient: If the thread is
// the master of some active split point, it is only available as a slave to the
// slaves which are busy searching the split point at the top of slaves split
// point stack (the "helpful master concept" in YBWC terminology).
bool Thread::is_available_to(Thread* master) const {
if (is_searching)
return false;
// Make a local copy to be sure doesn't become zero under our feet while
// testing next condition and so leading to an out of bound access.
int spCnt = splitPointsCnt;
// No active split points means that the thread is available as a slave for any
// other thread otherwise apply the "helpful master" concept if possible.
return !spCnt || (splitPoints[spCnt - 1].slavesMask & (1ULL << master->idx));
// init() is called at startup. Initializes lock and condition variable and
// launches requested threads sending them immediately to sleep. We cannot use
// a c'tor becuase Threads is a static object and we need a fully initialized
// engine at this point due to allocation of endgames in Thread c'tor.
void ThreadPool::init() {
timer = new Thread(&Thread::timer_loop);
threads.push_back(new Thread(&Thread::main_loop));
// exit() cleanly terminates the threads before the program exits.
void ThreadPool::exit() {
for (size_t i = 0; i < threads.size(); i++)
delete threads[i];
delete timer;
// read_uci_options() updates internal threads parameters from the corresponding
// UCI options and creates/destroys threads to match the requested number. Thread
// objects are dynamically allocated to avoid creating in advance all possible
// threads, with included pawns and material tables, if only few are used.
void ThreadPool::read_uci_options() {
maxThreadsPerSplitPoint = Options["Max Threads per Split Point"];
minimumSplitDepth = Options["Min Split Depth"] * ONE_PLY;
useSleepingThreads = Options["Use Sleeping Threads"];
size_t requested = Options["Threads"];
assert(requested > 0);
while (threads.size() < requested)
threads.push_back(new Thread(&Thread::idle_loop));
while (threads.size() > requested)
delete threads.back();
// wake_up() is called before a new search to start the threads that are waiting
// on the sleep condition and to reset maxPly. When useSleepingThreads is set
// threads will be woken up at split time.
void ThreadPool::wake_up() const {
for (size_t i = 0; i < threads.size(); i++)
threads[i]->maxPly = 0;
threads[i]->do_sleep = false;
if (!useSleepingThreads)
// sleep() is called after the search finishes to ask all the threads but the
// main one to go waiting on a sleep condition.
void ThreadPool::sleep() const {
// Main thread will go to sleep by itself to avoid a race with start_searching()
for (size_t i = 1; i < threads.size(); i++)
threads[i]->do_sleep = true;
// available_slave_exists() tries to find an idle thread which is available as
// a slave for the thread 'master'.
bool ThreadPool::available_slave_exists(Thread* master) const {
for (size_t i = 0; i < threads.size(); i++)
if (threads[i]->is_available_to(master))
return true;
return false;
// split() does the actual work of distributing the work at a node between
// several available threads. If it does not succeed in splitting the node
// (because no idle threads are available, or because we have no unused split
// point objects), the function immediately returns. If splitting is possible, a
// SplitPoint object is initialized with all the data that must be copied to the
// helper threads and then helper threads are told that they have been assigned
// work. This will cause them to instantly leave their idle loops and call
// search(). When all threads have returned from search() then split() returns.
template <bool Fake>
Value ThreadPool::split(Position& pos, Stack* ss, Value alpha, Value beta,
Value bestValue, Move* bestMove, Depth depth,
Move threatMove, int moveCount, MovePicker* mp, int nodeType) {
assert(bestValue > -VALUE_INFINITE);
assert(bestValue <= alpha);
assert(alpha < beta);
assert(beta <= VALUE_INFINITE);
assert(depth > DEPTH_ZERO);
Thread* master = pos.this_thread();
if (master->splitPointsCnt >= MAX_SPLITPOINTS_PER_THREAD)
return bestValue;
// Pick the next available split point from the split point stack
SplitPoint& sp = master->splitPoints[master->splitPointsCnt];
sp.parent = master->curSplitPoint;
sp.master = master;
sp.cutoff = false;
sp.slavesMask = 1ULL << master->idx;
sp.depth = depth;
sp.bestMove = *bestMove;
sp.threatMove = threatMove;
sp.alpha = alpha;
sp.beta = beta;
sp.nodeType = nodeType;
sp.bestValue = bestValue; = mp;
sp.moveCount = moveCount;
sp.pos = &pos;
sp.nodes = 0; = ss;
master->curSplitPoint = &sp;
int slavesCnt = 0;
// Try to allocate available threads and ask them to start searching setting
// is_searching flag. This must be done under lock protection to avoid concurrent
// allocation of the same slave by another master.
for (size_t i = 0; i < threads.size() && !Fake; ++i)
if (threads[i]->is_available_to(master))
sp.slavesMask |= 1ULL << i;
threads[i]->curSplitPoint = &sp;
threads[i]->is_searching = true; // Slave leaves idle_loop()
if (useSleepingThreads)
if (++slavesCnt + 1 >= maxThreadsPerSplitPoint) // Master is always included
// Everything is set up. The master thread enters the idle loop, from which
// it will instantly launch a search, because its is_searching flag is set.
// The thread will return from the idle loop when all slaves have finished
// their work at this split point.
if (slavesCnt || Fake)
// In helpful master concept a master can help only a sub-tree of its split
// point, and because here is all finished is not possible master is booked.
// We have returned from the idle loop, which means that all threads are
// finished. Note that setting is_searching and decreasing splitPointsCnt is
// done under lock protection to avoid a race with Thread::is_available_to().
sp.mutex.lock(); // To protect sp.nodes
master->is_searching = true;
master->curSplitPoint = sp.parent;
pos.set_nodes_searched(pos.nodes_searched() + sp.nodes);
*bestMove = sp.bestMove;
return sp.bestValue;
// Explicit template instantiations
template Value ThreadPool::split<false>(Position&, Stack*, Value, Value, Value, Move*, Depth, Move, int, MovePicker*, int);
template Value ThreadPool::split<true>(Position&, Stack*, Value, Value, Value, Move*, Depth, Move, int, MovePicker*, int);
// set_timer() is used to set the timer to trigger after msec milliseconds.
// If msec is 0 then timer is stopped.
void ThreadPool::set_timer(int msec) {
timer->maxPly = msec;
timer->sleepCondition.notify_one(); // Wake up and restart the timer
// wait_for_search_finished() waits for main thread to go to sleep, this means
// search is finished. Then returns.
void ThreadPool::wait_for_search_finished() {
Thread* t = main_thread();
t->sleepCondition.notify_one(); // In case is waiting for stop or ponderhit
while (!t->do_sleep) sleepCondition.wait(t->mutex);
// start_searching() wakes up the main thread sleeping in main_loop() so to start
// a new search, then returns immediately.
void ThreadPool::start_searching(const Position& pos, const LimitsType& limits,
const std::vector<Move>& searchMoves, StateStackPtr& states) {
SearchTime = Time::now(); // As early as possible
Signals.stopOnPonderhit = Signals.firstRootMove = false;
Signals.stop = Signals.failedLowAtRoot = false;
RootPosition = pos;
Limits = limits;
SetupStates = states; // Ownership transfer here
for (MoveList<LEGAL> ml(pos); !ml.end(); ++ml)
if (searchMoves.empty() || count(searchMoves.begin(), searchMoves.end(), ml.move()))
main_thread()->do_sleep = false;
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