Detection of CUDA

CUDA detection at run-time by any script ( such as recon-all ) is possible by executing a binary file called cudadetect. If it exits with 0, CUDA is available. Otherwise, not.

The following pseudocode is how one can use the detection scheme in their own scripts:

..
cudadetect
if $status == 0
  setenv CUDAENABLED
endif
..
..
if CUDAENABLED
  <cuda_enabled_binary> <options>
else
  <normal_binary> <options>
endif

There are a few reasons why cudadetect might not detect your CUDA setup.

* The LD_LIBRARY_PATH or DYLD_LIBRARY_PATH doesn't have the PATH where CUDA libraries reside. Usually they reside in /usr/local/cuda/lib . So check to see whether these environment variables have the CUDA libraries path. Better to put it in .profile or .cshrc

* cudadetect works by calling dlopen() system call which calls a function from arbitrary shared library. In this case, cudadetect tries to call cudaGetDeviceCount from libcudart. If either the name of the library or the name of the function change, cudadetect might not work. Currently, cudadetect works for CUDA version 2.0 but effort will be taken to ensure that cudadetect calls the correct library and API is called whenever CUDA is upgraded.

Changes to enable CUDA during configuration ( configure.in )

This section gives an overview of how configure.in is changed and what stuff it exports. More often than not, one need not tamper with configure.in -- it is already changed to accommodate CUDA.

* CUDA support is enabled by using a --with-cuda=<path of cuda> switch to ./configure. Suppose, the CUDA installation is in /usr/local/cuda ( which is the default path, usually ), then the ./configure command must have the switch --with-cuda=/usr/local/cuda. The script checks to see if it can find nvcc, the CUDA compiler. This is a sanity check and if it finds nvcc, the ./configure assumes CUDA exists and exports certain things which are helpful in compiling CUDA programs ( and which are to be used in Makefile.am )

Flags which configure.in exports

These flags are typically used in the Makefile.am of any binary which has a CUDA replacement.

#ifdef CUDA
  #include "cudaproc.h"
  cuda_procedure(args);
#else
  cpu_procedure(args);
#endif

And then specific rules in Makefile.am can be used to compile the .cu file where the cuda_procedure() usually resides. cudaproc.h contains the definition of the cuda_procedure() function. More in the next section.

If there are a large number of functions which are to be replaced this way, a separate source file should be written with _cuda suffix. For instance, if mri_em_register is re-written with functions calling CUDA code, the file is named mri_em_register_cuda.c.

Tweaking Makefile.am of the binary

Listing of Makefile.am of mri_em_register to illustrate how Makefile.am changes:

## 
## Makefile.am 
##

AM_CFLAGS=-I$(top_srcdir)/include -I$(top_srcdir)/include/dicom 
AM_LDFLAGS=

bin_PROGRAMS = mri_em_register
mri_em_register_SOURCES=mri_em_register.c
mri_em_register_LDADD= $(addprefix $(top_builddir)/, $(LIBS_MGH))
mri_em_register_LDFLAGS=$(OS_LDFLAGS)

## ----
## CUDA
## ----

# BUILDCUDA is defined if configure.in finds CUDA
if BUILDCUDA
# rules for building cuda files
.cu.o:
        $(NVCC) -o $@ -c $< $(NVCCFLAGS) $(AM_CFLAGS) $(MNI_CFLAGS)
bin_PROGRAMS += mri_em_register_cuda
mri_em_register_cuda_SOURCES = mri_em_register_cuda.c \
computelogsampleprob.cu findoptimaltransform.cu findoptimaltranslation.cu
mri_em_register_cuda_CFLAGS = $(AM_CFLAGS)  $(CUDA_CFLAGS)
mri_em_register_cuda_CXXFLAGS = $(AM_CFLAGS) $(CUDA_CFLAGS)
mri_em_register_cuda_LDADD = $(addprefix $(top_builddir)/, $(LIBS_MGH)) $(CUDA_LIBS)
mri_em_register_cuda_LDFLAGS = $(OS_LDFLAGS) 
mri_em_register_cuda_LINK = $(LIBTOOL) --tag=CC $(AM_LIBTOOLFLAGS) \
        $(LIBTOOLFLAGS) --mode=link $(CCLD) $(mri_em_register_cuda_CFLAGS) \
        $(CFLAGS) $(mri_em_register_cuda_LDFLAGS) $(LDFLAGS) -o $@
endif

# Our release target. Include files to be excluded here. They will be
# found and removed after 'make install' is run during the 'make
# release' target.
EXCLUDE_FILES=""
include $(top_srcdir)/Makefile.extra

As one can notice, the Makefile.am of a CUDA enabled code differs from normal Makefile.am only in the if BUILDCUDA.. endif block.

Notes :

Steps to CUDA-ize a binary

Suppose you have a CUDA replacement for a binary ( like the mri_em_register case above). Following steps are like guidelines:

if BUILDCUDA
# rules for building cuda files
.cu.o:
        $(NVCC) -o $@ -c $< $(NVCCFLAGS) $(AM_CFLAGS) 
bin_PROGRAMS += dummy_cuda
dummy_SOURCES = dummy_cuda.c \
dummy.cu
dummy_CFLAGS = $(AM_CFLAGS)  $(CUDA_CFLAGS)
dummy_CXXFLAGS = $(AM_CFLAGS) $(CUDA_CFLAGS)
dummy_LDADD = $(addprefix $(top_builddir)/, $(LIBS_MGH)) $(CUDA_LIBS)
dummy_LDFLAGS = $(OS_LDFLAGS) 
dummy_LINK = $(LIBTOOL) --tag=CC $(AM_LIBTOOLFLAGS) \
        $(LIBTOOLFLAGS) --mode=link $(CCLD) $(dummy_CFLAGS) \
        $(CFLAGS) $(dummy_LDFLAGS) $(LDFLAGS) -o $@
endif