Why is threading slowing down over time?
If I run the following sample code the calls getting slower and slower:
#include "opencv2\opencv.hpp"
#include <vector>
#include <chrono>
#include <thread>
using namespace cv;
using namespace std;
using namespace std::chrono;
void blurSlowdown(void*) {
Mat m1(360, 640, CV_8UC3);
Mat m2(360, 640, CV_8UC3);
medianBlur(m1, m2, 3);
}
int main()
{
for (;;) {
high_resolution_clock::time_point start = high_resolution_clock::now();
for (int k = 0; k < 100; k++) {
thread t(blurSlowdown, nullptr);
t.join();
}
high_resolution_clock::time_point end = high_resolution_clock::now();
cout << duration_cast<microseconds>(end - start).count() << endl;
}
}
Why does that happen? Is it a bug?
Some observations / remarks:
-The effect is amplified when using the debug Library, even if no debugger is used
-The processor boost clock is stable and is not causing the problem
-It does not matter if m1 and m2 are allocated on the heap instead of the stack
-The slowdown effect does not appear when medianBlur isn't called
My system:
-Windows 10 64bit
-MSVC compiler
-Newest offical OpenCV 3.4.2 binaries
I think you used the number of threads so much. Did you try with k=4,8 ?? (Depend on your CPU)
The are only 2 threads active at the same time.
thread tis getting destroyed after each loop.@Tycos, please avoid using your own multithreading with opencv. a lot of functions are explicitly not thread-safe.
rather rebuild the opencv libs with TBB or openmp support.
(and debug builds have most optimizations off, so, no wonder it's slow)
@berak this is not the problem. First only one thread is interfering with any cv context at the same time. Second the performance is good but it is getting worse and worse over time. Third opencv is mostly thread safe and wants you to use multithreading: http://answers.opencv.org/question/69...
that is about internal parallelism. again, imho, you're on the wrong path here.
"OpenCV philosophy here is that application should be multi-threaded, not OpenCV functions." - http://answers.opencv.org/question/69...
sorry dear, but you misread all of it.
it says the opposite.
I don't think so. From the context I clearly understand that internal parallelism (TBB) is only supported in a "may be a dozen" functions yet because OpenCV philosophy is that application should use multiple thread on their own to call the functions instead.
definitively not so.
So can you explain me what Iam getting wrong?