in a small program demonstrating C++ threads, I noticed that linking with openCV ruins the concurrency of the threads. NOTE: the demo program doesn't even use openCV!. I stumbled on this problem when I observed that my own application, which does use openCV, did not show the concurrency as expected. It turned out that linking with openCV was the problem.
In the sample code (file main.cpp given below) three threads are launched, each doing the same calculation. From the same source, I create two executables, called 'mttok' and 'mttno', as follows:
g++ -o mttok -O3 main.cpp -lpthread
g++ -o mttno -O3 main.cpp -lpthread -L/usr/local/opencv/lib64 -lopencv_core
When I run the first executable with the gnu 'time' command I get:
=> time ./mttok
All threads are running...
result1: 5e+19
result2: 5e+19
result3: 5e+19
27.005u 0.003s 0:09.00 300.0% 0+0k 0+0io 0pf+0w
The third field is the elapsed time (9 secs) whereas the fourth number (300%) is the cpu-time, clearly showing the three threads running concurrently. This is also seen from an applet on my desktop visualising the CPU-activity: three bars corresponding to 3 'CPUs' climb to ~100%.
The other executable (linked with opencv) gives
=> time ./mttno
All threads are running...
result1: 5e+19
result2: 5e+19
result3: 5e+19
26.690u 0.203s 0:26.45 101.6% 0+0k 43648+0io 8pf+0w
Note the ~3x larger elapsed time (now 26 secs) and the CPU-percentage (101%). The CPU activity shows only one bar climbing to 100%.
I have tried this both with and without TBB, and both with and without openmp. The results are the same. The source code is a recent clone of the git repository (4.1.2-dev) but I saw the same phenomenon with the precompiled version of openCV that comes with SUSE leap 15.1, i.e. opencv 3.3 Who can explain this behaviour and suggest what can be done to keep proper concurrent behaviour?
I have asked this question at link:stackoverflow, with more detail and the source code. But this did not lead to a solution. Perhaps somebody of hte openCV community can help?
Here follows the code:
=== main.cpp ===
#include <iostream>
#include <thread>
const unsigned long NMAX=10000000000;
class MTTest
{
public:
void foo( double& r )
{
double s = 0;
for (unsigned long u=0; u<NMAX; u++)
{
s += u;
}
r = s;
}
};
int main()
{
double s1, s2, s3;
std::unique_ptr<MTTest> ptr1( new MTTest );
std::unique_ptr<MTTest> ptr2( new MTTest );
std::unique_ptr<MTTest> ptr3( new MTTest );
std::thread t1( &MTTest::foo, ptr1.get(), std::ref(s1) );
std::thread t2( &MTTest::foo, ptr2.get(), std::ref(s2) );
std::thread t3( &MTTest::foo, ptr3.get(), std::ref(s3) );
std::cout << "All threads are running..." << std::endl;
// synchronize threads:
t1.join();
t2.join();
t3.join();
std::cout << "result1: " << s1 << std::endl;
std::cout << "result2: " << s2 << std::endl;
std::cout << "result3: " << s3 << std::endl;
return 0;
}
// ---- eof ---
