Difference between global and device functions
I will explain it with an example:
main()
{
// Your main function. Executed by CPU
}
__global__ void calledFromCpuForGPU(...)
{
//This function is called by CPU and suppose to be executed on GPU
}
__device__ void calledFromGPUforGPU(...)
{
// This function is called by GPU and suppose to be executed on GPU
}
i.e. when we want a host(CPU) function to call a device(GPU) function, then 'global' is used. Read this: "https://code.google.com/p/stanford-cs193g-sp2010/wiki/TutorialGlobalFunctions"
And when we want a device(GPU) function (rather kernel) to call another kernel function we use 'device'. Read this "https://code.google.com/p/stanford-cs193g-sp2010/wiki/TutorialDeviceFunctions"
This should be enough to understand the difference.
__global__
- Runs on the GPU, called from the CPU or the GPU*. Executed with<<<dim3>>>
arguments.__device__
- Runs on the GPU, called from the GPU. Can be used with variabiles too.__host__
- Runs on the CPU, called from the CPU.
*) __global__
functions can be called from other __global__
functions starting
compute capability 3.5.
Differences between __device__
and __global__
functions are:
__device__
functions can be called only from the device, and it is executed only in the device.
__global__
functions can be called from the host, and it is executed in the device.
Therefore, you call __device__
functions from kernels functions, and you don't have to set the kernel settings. You can also "overload" a function, e.g : you can declare void foo(void)
and __device__ foo (void)
, then one is executed on the host and can only be called from a host function. The other is executed on the device and can only be called from a device or kernel function.
You can also visit the following link: http://code.google.com/p/stanford-cs193g-sp2010/wiki/TutorialDeviceFunctions, it was useful for me.
Global functions are also called "kernels". It's the functions that you may call from the host side using CUDA kernel call semantics (<<<...>>>
).
Device functions can only be called from other device or global functions. __device__
functions cannot be called from host code.