opencv use cuda memory anyway - c++

Question

I want to know how much my code is faster with CUDA, so I compile OpenCV with below switches (including cuda)
cmake CMAKE_BUILD_TYPE=RELEASE \
CMAKE_INSTALL_PREFIX=/usr/local \
WITH_CUDA=ON \
ENABLE_FAST_MATH=1 \
CUDA_FAST_MATH=1 \
WITH_CUBLAS=1 \
INSTALL_PYTHON_EXAMPLES=OFF \
OPENCV_EXTRA_MODULES_PATH=/home/saeed/opencv_contrib/modules \
BUILD_EXAMPLES=OFF ..
and I'm working on Linux based machine and with dedicated GPU on my system so how I can measure gpu usage for opencv code? I did that with using
$ nvidia-smi
command,which give me general information about gpu usage and etc (in bash) .
for trace this output I use
$ watch -n 0.1 nvidia-smi
command,which updates informations every 0.1 second(0.1 second is minimum)
So I have a code like below in opencv(You can see I don't use any cuda function in it)
#include "opencv2/core.hpp"
#include "opencv2/imgproc/imgproc.hpp"
using namespace cv;
int main()
{
cv::Mat src(1500,1500,CV_8UC3),res;
for (int var = 0; var < 70; ++var)
{
Canny(src,res,50,150,5);
}
}
So when i run above code I take a screen shot in my nvidia-smi page(to know how much gpu ram is in use)
As you can see there is 23 MiB is in use !!! so why this happen !!if it is normal how we could stop that !? How to stop GPU for this problem

Answer 1

You can make the CUDA runtime indicate that there are no available GPUs with the following environment variable:
CUDA_VISIBLE_DEVICES="" ./my_opencv_code_that_wont_use_gpu
If you want OpenCV to actually not do anything with the GPU, my best guess would be to compile it without CUDA support:
cmake CMAKE_BUILD_TYPE=RELEASE \
CMAKE_INSTALL_PREFIX=/usr/local \
WITH_CUDA=ON \ ********MODIFY
ENABLE_FAST_MATH=1 \
CUDA_FAST_MATH=1 \ ********MODIFY
WITH_CUBLAS=1 \ ********MODIFY
INSTALL_PYTHON_EXAMPLES=OFF \
OPENCV_EXTRA_MODULES_PATH=/home/saeed/opencv_contrib/modules \
BUILD_EXAMPLES=OFF ..