Qt + CUDA, loosing accuracy

Accelerated Computing CUDA CUDA Setup and Installation
1

Hello, i encountered weird issue, .cu code compiled with C++/Qt code has worse accuracy than the same code compiled in terminal with nvcc compiler. The margin of error is around 2%, but in my calculation it is unacceptable. Im running this code on ubuntu 18.04, GTX 1050 Ti mobile with installed cuda toolkit 9.2. I assume it must be something with .pro file configuration. Please let me know if I made any serious mistake.

.pro file:

# CUDA settings <-- may change depending on your system
CUDA_SOURCES += $$OTHER_FILES
CUDA_SDK = "/usr/local/cuda-9.2/"   # Path to cuda SDK install
CUDA_DIR = "/usr/local/cuda-9.2/"            # Path to cuda toolkit install

# DO NOT EDIT BEYOND THIS UNLESS YOU KNOW WHAT YOU ARE DOING....

SYSTEM_NAME = x64         # Depending on your system either 'Win32', 'x64', or 'Win64'
SYSTEM_TYPE = 64            # '32' or '64', depending on your system
CUDA_ARCH = sm_60           # Type of CUDA architecture, for example 'compute_10', 'compute_11', 'sm_10'
NVCC_OPTIONS = --use_fast_math -O2


# include paths
INCLUDEPATH += $$CUDA_DIR/include


CUDA_OBJECTS_DIR = ./


# Add the necessary libraries
CUDA_LIBS = -lcuda -lcudart

# The following makes sure all path names (which often include spaces) are put between quotation marks
CUDA_INC = $$join(INCLUDEPATH,'" -I"','-I"','"')
#LIBS += $$join(CUDA_LIBS,'.so ', '', '.so')
LIBS += -L$$CUDA_DIR/lib64 -lcuda -lcudart

# Configuration of the Cuda compiler
CONFIG(debug, debug|release) {
    # Debug mode
    cuda_d.input = CUDA_SOURCES
    cuda_d.output = $$CUDA_OBJECTS_DIR/${QMAKE_FILE_BASE}_cuda.o
    cuda_d.commands = $$CUDA_DIR/bin/nvcc -D_DEBUG $$NVCC_OPTIONS $$CUDA_INC $$NVCC_LIBS --machine $$SYSTEM_TYPE -arch=$$CUDA_ARCH -c -o ${QMAKE_FILE_OUT} ${QMAKE_FILE_NAME}
    cuda_d.dependency_type = TYPE_C
    QMAKE_EXTRA_COMPILERS += cuda_d
}
else {
    # Release mode
    cuda.input = CUDA_SOURCES
    cuda.output = $$CUDA_OBJECTS_DIR/${QMAKE_FILE_BASE}_cuda.o
    cuda.commands = $$CUDA_DIR/bin/nvcc $$NVCC_OPTIONS $$CUDA_INC $$NVCC_LIBS --machine $$SYSTEM_TYPE -arch=$$CUDA_ARCH -c -o ${QMAKE_FILE_OUT} ${QMAKE_FILE_NAME}
    cuda.dependency_type = TYPE_C
    QMAKE_EXTRA_COMPILERS += cuda
}
2

I would get rid of this:

–use_fast_math

if you are concerned about accuracy. It’s also not clear why you would specify -O2.

What command line do you use compile with nvcc “from the terminal” ?

3

I tried without those flags and result was the same.

By compiling from terminal i mean “nvcc -o program program.cu” and execute ./program.

4

Without code to look at and without access to the data you are comparing, we are reduced to guessing. I’ll throw out a few ideas:

This suggests your problem may be in the host portion of the code.

Use cuda-memcheck to make sure your code does not have race conditions, or out-of-bounds accesses.

Use valgrind to make sure your host code doesn’t operate on invalid data.

Check whether your code contains atomic operations on floating-point math, which results in an indeterminate order of operations, and floating-point math isn’t associative.

Check whether you are compiling for the same GPU architecture in both environments.

Check that you are using the same CUDA compiler version on both platforms.

Check whether you are JIT-compiling GPU code on at least one of the platforms.

Try – for debug purposes only! – the -fmad=false flag for GPU compilation.

It is not clear what you are comparing when establishing a 2% error. You may be using an unsuitable reference. Your 2% error bound may not be realistic: I have seen bigger differences than that simply by the use of different compiler flags, in host-only code.