why introducing sleep causes producer consumer to wait? - c++

Question

I'm just learning to use the C++ threading library.
If anyone is curious - my code is just a modified version in a tutorial https://www.youtube.com/watch?v=13dFggo4t_I&t=6m45s
I wrote a simple producer/consumer code. I tried to introduce a sleep to make the producer and consumer in lockstep fashion, so item is consumed as soon as it is produced. But making the producer waits causes some deadlock. I couldn't figure out why.
Can you please help me point out what I'm missing in the code?
#include <iostream>
#include <thread>
#include <mutex>
#include <vector>
#include <chrono>
#define BUFFER_SIZE 10
std::mutex mu;
std::condition_variable full,empty;
std::vector<int> vec;
int i=0;
std::chrono::milliseconds slp(10);
void produce()
{
while (i < 2*BUFFER_SIZE) {
std::unique_lock<std::mutex> locker(mu);
full.wait(locker, [] {return vec.size() != BUFFER_SIZE;});
vec.push_back(i++);
empty.notify_one();
//std::this_thread::sleep_for(slp); <--- introducing this causes program to hang.
}
}
void consume()
{
while(!vec.empty()) {
std::unique_lock<std::mutex> locker(mu);
empty.wait(locker, [] {return !vec.empty();});
std::cout << "Consumed:" << vec.back() <<"\n";
vec.pop_back();
full.notify_one();
}
}
int main() {
vec.reserve(BUFFER_SIZE*2);
std::thread producer(produce), consumer(consume);
producer.join();
consumer.join();
return 0;
}
EDIT:
void produce()
{
while (i < 2*BUFFER_SIZE) {
std::unique_lock<std::mutex> locker(mu);
full.wait(locker, [] {return vec.size() != BUFFER_SIZE;});
vec.push_back(i++);
locker.unlock();
empty.notify_one();
std::this_thread::sleep_for(slp);
}
}
void consume()
{
while(!vec.empty()) {
std::unique_lock<std::mutex> locker(mu);
empty.wait(locker, [] {return !vec.empty();});
std::cout << "Consumed:" << vec.back() <<"\n";
vec.pop_back();
locker.unlock();
full.notify_one();
}
}

Answer 1

You have a couple of problems:
The consumer is accessing vec without the mutex held:
while(!vec.empty()) {
This is easily addressed by ensuring that all acceses to vec are "inside" the mutex.
If the consumer gets ahead of the producer and manages to empty vec, it will exit early. You can fix this by using some other mechanism to indicate completion.
You are performing some processing/io/sleep with the mutex held, reducing possible concurrency. ideally you should only hold the mutex while accessing shared state.
const unsigned BUFFER_SIZE = 10;
std::mutex mu;
std::condition_variable full,empty;
std::vector<int> vec;
const std::chrono::milliseconds slp(10);
auto done = false;
void produce()
{
for (auto i = 0u; i < 2 * BUFFER_SIZE; ++i) {
std::unique_lock<std::mutex> locker(mu);
full.wait(locker, [] {return vec.size() < BUFFER_SIZE;});
auto was_empty = vec.empty();
vec.push_back(i);
locker.unlock();
// Only notify if the buffer was empty before the push_back
if (was_empty) {
empty.notify_all();
}
std::this_thread::sleep_for(slp);
}
}
void consume()
{
for (;;) {
std::unique_lock<std::mutex> locker(mu);
while (vec.empty()) {
if (done) {
return;
}
empty.wait(locker);
}
auto was_full = vec.size() >= BUFFER_SIZE;
auto value = vec.back();
vec.pop_back();
locker.unlock();
if (was_full) {
full.notify_all();
}
std::cout << "Consumed: " << value << '\n';
}
}
int main() {
vec.reserve(BUFFER_SIZE*2);
std::thread producer(produce), consumer(consume);
producer.join();
// Produce some more
producer = std::thread(produce);
producer.join();
// Produce A LOT more
std::vector<std::thread> many_producers(8);
for (auto&& t : many_producers) {
t = std::thread(produce);
}
for (auto && t : many_producers) {
t.join();
}
// Tell consumer we are done producing
{
std::lock_guard<std::mutex> lock(mu);
done = true;
}
empty.notify_one();
consumer.join();
}
See it live at Coliru.