Added an example of parallel pi.

This commit is contained in:
Stephen M. McQuay 2012-07-05 09:45:05 -06:00
parent 4a84a1fe84
commit 6448b41ae1
2 changed files with 60 additions and 2 deletions

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@ -12,7 +12,7 @@ CPPFLAGS=-Wall -g -std=c++0x -pthread
# CXX=clang++ # CXX=clang++
# CPPFLAGS=-Wall -g -std=c++0x -stdlib=libc++ -L/opt/clang/lib -pthread # CPPFLAGS=-Wall -g -std=c++0x -stdlib=libc++ -L/opt/clang/lib -pthread
all: auto constexpr ilists init init_new lambda loops regex1 regex2 thread thread2 tuple all: auto constexpr ilists init init_new lambda loops pi regex1 regex2 thread thread2 tuple
run: all run: all
./auto ./auto
@ -30,7 +30,7 @@ run: all
clean: clean:
@rm -vf auto constexpr core* ilists init init_new lambda loops regex1 regex2 thread thread2 tuple @rm -vf auto constexpr core* ilists init init_new lambda loops pi regex1 regex2 thread thread2 tuple
stress: thread2 stress: thread2
zsh stress.zsh zsh stress.zsh

58
pi.cc Normal file
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@ -0,0 +1,58 @@
#include <iostream>
#include <thread>
#include <atomic>
#include <sys/time.h>
#include <ctime>
#include <cmath>
#include <random>
static const int num_threads = 8;
static const int max_ = 100000;
void slice_of_pi(int tries, int my_id, std::atomic<int> & in_circle) {
struct timeval tv;
gettimeofday(&tv, NULL);
srand(tv.tv_usec + my_id);
std::uniform_int_distribution<int> distribution {0, max_};
std::mt19937 engine; // Mersenne twister MT19937
auto generator = std::bind(distribution, engine);
double x, y, d;
int _in_circle = 0;
for(int i = 0; i < tries; i++) {
x = (float)generator() / (float)max_;
y = (float)generator() / (float)max_;
d = sqrt(x * x + y * y);
if(d < 1.0)
_in_circle++;
}
in_circle += _in_circle;
}
int main(int argc, char * argv[]) {
if(argc != 2) {
std::cerr << "pi <number of attempts>" << std::endl;
return 1;
}
std::atomic<int> successes {0};
int attempts = atoi(argv[1]);
std::thread t[num_threads];
for (int i = 0; i < num_threads; ++i) {
t[i] = std::thread(slice_of_pi, attempts, i, std::ref(successes));
}
for (int i = 0; i < num_threads; ++i) {
t[i].join();
}
float pi = 4 * float(successes) / float(attempts * num_threads);
printf("%0.8f\n", pi);
return 0;
}