More threads allocated than wanted - c++

Question

I have implemented a thread pool in C++, in which I create Nthread workers to which I assign some jobs from a queue I keep pushing task to. When The queue is empty and/or when I say so, the threads stop working. Everything runs on WSL (Ubuntu 20.04 Focal).
If I open Windows task manager when I launch the program, the number of threads actually working is more than the ones allocated. For example, if I run the program with 4 threads on a 12 cores machine, I can see at least 6 cores with above-average activity; if I use 10 threads, all 12 cores go to 100%. Is this behavior somehow expected or am I doing something wrong? I would expect to see one more thread than the ones allocated because I spawn Nthread threads from the main one (which by the way should stay quiet waiting for the others to finish...), but I cannot explain what I see.
I want to stress that I create all the Nthread threads before all operations, then I populate and process the queue and finally I destroy the threads, i.e. as far as I can see I do not create/destroy threads continouosly during the calculations.
EDIT
I forgot to mention I operate under C++11.
Here is the relevant C++ code.
In main.cc
ThreadPool *pool = new ThreadPool(fNThreads);
std::vector<std::function<void(int)>> *caller =
new std::vector<std::function<void(int)>>;
for (size_t iter = 0; iter < nIter; ++iter)
{
pool->ResetQueue();
for (size_t j = 0; nmax < 2; ++j)
{
caller->push_back(
[=](int iThr){function(iter, j, iThr);});
pool->PushTask((*caller)[j]);
}
pool->WaitForCompletion(1.e-4);
caller->clear();
}
delete caller;
delete pool;
SynchronizedQueue.hh
#ifndef SYNCQUEUE_H
#define SYNCQUEUE_H
#include <list>
#include <mutex>
#include <condition_variable>
template<typename T>
class SynchronizedQueue
{
public:
SynchronizedQueue();
~SynchronizedQueue();
void Put(T const & data);
void Put(T const && data);
T Get();
size_t Size();
SynchronizedQueue(SynchronizedQueue const &) = delete;
SynchronizedQueue & operator=(SynchronizedQueue const &) = delete;
SynchronizedQueue(SynchronizedQueue&&) = delete;
SynchronizedQueue & operator=(SynchronizedQueue&&) = delete;
private:
std::list<T> queue;
std::mutex mut;
std::condition_variable condvar;
};
template<typename T>
SynchronizedQueue<T>::SynchronizedQueue()
{}
template<typename T>
SynchronizedQueue<T>::~SynchronizedQueue()
{}
template<typename T>
void SynchronizedQueue<T>::Put(T const & data)
{
std::unique_lock<std::mutex> lck(mut);
queue.push_back(data);
condvar.notify_one();
}
template<typename T>
T SynchronizedQueue<T>::Get()
{
std::unique_lock<std::mutex> lck(mut);
while (queue.empty())
{
condvar.wait(lck);
}
T result = queue.front();
queue.pop_front();
return result;
}
template<typename T>
size_t SynchronizedQueue<T>::Size()
{
std::unique_lock<std::mutex> lck(mut);
return queue.size();
}
#endif
ThreadPool.hh
#ifndef THREADPOOL_H
#define THREADPOOL_H
#include "SynchronizedQueue.hh"
#include <atomic>
#include <functional>
#include <mutex>
#include <thread>
#include <vector>
class ThreadPool
{
public:
ThreadPool(unsigned int nThreads = 1);
virtual ~ThreadPool();
void PushTask(std::function<void(int)> func);
void WaitForCompletion();
void WaitForCompletion(int sec);
void ResetQueue();
void JoinThreads();
void Delay(int sec);
size_t GetWorkQueueLength();
private:
void WorkerThread(int i);
std::atomic<bool> done;
unsigned int threadCount;
SynchronizedQueue<std::function<void(int)>> workQueue;
std::vector<std::thread> threads;
};
#endif
ThreadPool.cc
#include "ThreadPool.hh"
#include "SynchronizedQueue.hh"
#include <chrono>
//#include <iostream>
void doNothing(int i)
{}
ThreadPool::ThreadPool(unsigned int nThreads)
: done(false)
{
if (nThreads <= 0)
{
threadCount = std::thread::hardware_concurrency();
}
else
{
threadCount = nThreads;
}
for (unsigned int i = 0; i < threadCount; ++i)
{
threads.push_back(std::thread(&ThreadPool::WorkerThread, this, i));
}
}
ThreadPool::~ThreadPool()
{
WaitForCompletion();
JoinThreads();
}
void ThreadPool::WaitForCompletion(int sec)
{
if (!done)
{
while (GetWorkQueueLength())
{
std::this_thread::sleep_for(std::chrono::seconds(sec));
}
done = true;
for (unsigned int i = 0; i < threadCount; ++i)
{
PushTask(&doNothing);
}
}
}
void ThreadPool::WaitForCompletion()
{
if (!done)
{
while (GetWorkQueueLength())
{}
done = true;
for (unsigned int i = 0; i < threadCount; ++i)
{
PushTask(&doNothing);
}
}
}
void ThreadPool::JoinThreads()
{
for (auto& th : threads)
{
if (th.joinable())
{
th.join();
}
}
}
void ThreadPool::Delay(int sec)
{
std::this_thread::sleep_for(std::chrono::seconds(sec));
}
void ThreadPool::PushTask(std::function<void(int)> func)
{
workQueue.Put(func);
}
void ThreadPool::ResetQueue()
{
done = false;
}
void ThreadPool::WorkerThread(int i)
{
while (!done)
{
workQueue.Get()(i);
}
}
size_t ThreadPool::GetWorkQueueLength()
{
return workQueue.Size();
}