How to apply a concurrent solution to a Producer-Consumer like situation - c++

Question

I have a XML file with a sequence of nodes. Each node represents an element that I need to parse and add in a sorted list (the order must be the same of the nodes found in the file).
At the moment I am using a sequential solution:
struct Graphic
{
bool parse()
{
// parsing...
return parse_outcome;
}
};
vector<unique_ptr<Graphic>> graphics;
void producer()
{
for (size_t i = 0; i < N_GRAPHICS; i++)
{
auto g = new Graphic();
if (g->parse())
graphics.emplace_back(g);
else
delete g;
}
}
So, only if the graphic (that actually is an instance of a class derived from Graphic, a Line, a Rectangle and so on, that is why the new) can be properly parse, it will be added to my data structure.
Since I only care about the order in which thes graphics are added to my list, I though to call the parse method asynchronously, such that the producer has the task of read each node from the file and add this graphic to the data structure, while the consumer has the task of parse each graphic whenever a new graphic is ready to be parsed.
Now I have several consumer threads (created in the main) and my code looks like the following:
queue<pair<Graphic*, size_t>> q;
mutex m;
atomic<size_t> n_elements;
void producer()
{
for (size_t i = 0; i < N_GRAPHICS; i++)
{
auto g = new Graphic();
graphics.emplace_back(g);
q.emplace(make_pair(g, i));
}
n_elements = graphics.size();
}
void consumer()
{
pair<Graphic*, size_t> item;
while (true)
{
{
std::unique_lock<std::mutex> lk(m);
if (n_elements == 0)
return;
n_elements--;
item = q.front();
q.pop();
}
if (!item.first->parse())
{
// here I should remove the item from the vector
assert(graphics[item.second].get() == item.first);
delete item.first;
graphics[item.second] = nullptr;
}
}
}
I run the producer first of all in my main, so that when the first consumer starts the queue is already completely full.
int main()
{
producer();
vector<thread> threads;
for (auto i = 0; i < N_THREADS; i++)
threads.emplace_back(consumer);
for (auto& t : threads)
t.join();
return 0;
}
The concurrent version seems to be at least twice as faster as the original one.
The full code has been uploaded here.
Now I am wondering:
Are there any (synchronization) errors in my code?
Is there a way to achieve the same result faster (or better)?
Also, I noticed that on my computer I get the best result (in terms of elapsed time) if I set the number of thread equals to 8. More (or less) threads give me worst results. Why?

Answer 1

Blockquote
There isn't synchronization errors, but I think that the memory managing could be better, since your code leaked if parse() throws an exception.
There isn't synchronization errors, but I think that your memory managing could be better, since you will have leaks if parse() throw an exception.
Blockquote
Is there a way to achieve the same result faster (or better)?
Probably. You could use a simple implementation of a thread pool and a lambda that do the parse() for you.
The code below illustrate this approach. I use the threadpool implementation
here
#include <iostream>
#include <stdexcept>
#include <vector>
#include <memory>
#include <chrono>
#include <utility>
#include <cassert>
#include <ThreadPool.h>
using namespace std;
using namespace std::chrono;
#define N_GRAPHICS (1000*1000*1)
#define N_THREADS 8
struct Graphic;
using GPtr = std::unique_ptr<Graphic>;
static vector<GPtr> graphics;
struct Graphic
{
Graphic()
: status(false)
{
}
bool parse()
{
// waste time
try
{
throw runtime_error("");
}
catch (runtime_error)
{
}
status = true;
//return false;
return true;
}
bool status;
};
int main()
{
auto start = system_clock::now();
auto producer_unit = []()-> GPtr {
std::unique_ptr<Graphic> g(new Graphic);
if(!g->parse()){
g.reset(); // if g don't parse, return nullptr
}
return g;
};
using ResultPool = std::vector<std::future<GPtr>>;
ResultPool results;
// ThreadPool pool(thread::hardware_concurrency());
ThreadPool pool(N_THREADS);
for(int i = 0; i <N_GRAPHICS; ++i){
// Running async task
results.emplace_back(pool.enqueue(producer_unit));
}
for(auto &t : results){
auto value = t.get();
if(value){
graphics.emplace_back(std::move(value));
}
}
auto duration = duration_cast<milliseconds>(system_clock::now() - start);
cout << "Elapsed: " << duration.count() << endl;
for (size_t i = 0; i < graphics.size(); i++)
{
if (!graphics[i]->status)
{
cerr << "Assertion failed! (" << i << ")" << endl;
break;
}
}
cin.get();
return 0;
}
It is a bit faster (1s) on my machine, more readable, and removes the necessity of shared datas (synchronization is evil, avoid it or hide it in a reliable and efficient way).