On 02/29/2012 05:35 PM, Number Cruncher wrote:
Do transitive dependencies reduce number of jobs that can be compiled in
parallel?
If I have two libraries A and B, with an executable C, whose
dependencies are described by:
add_library(A ${A_SRC})
add_library(B ${B_SRC})
target_link_libraries(B A)
add_executable(C ${C_SRC})
target_link_libraries(C B)
I understand that when *linking* C, the transitive dependency A will be
added. However, if I build C in parallel make -j N, will CMake build
libraries A and B simultaneously, or fully compile and link A before
starting compilation of B? I.e. just because the link steps are serial
dependencies, are the compilation steps? Would it be faster to do:
add_library(A ${A_SRC})
add_library(B ${B_SRC})
add_executable(C ${C_SRC})
target_link_libraries(C B A)
Thanks.
Look at the following exemplary project:
CMAKE_MINIMUM_REQUIRED(VERSION 2.8 FATAL_ERROR)
PROJECT(P C)
FILE(WRITE ${CMAKE_BINARY_DIR}/a.c void a(void){}\n)
FILE(WRITE ${CMAKE_BINARY_DIR}/b.c void b(void){a();}\n)
FILE(WRITE ${CMAKE_BINARY_DIR}/c.c int main(void){b(); return 0;}\n)
ADD_LIBRARY(A SHARED a.c)
ADD_LIBRARY(B SHARED b.c)
ADD_EXECUTABLE(C c.c)
IF(TRANSITIVE)
TARGET_LINK_LIBRARIES(B A)
TARGET_LINK_LIBRARIES(C B)
ELSE()
TARGET_LINK_LIBRARIES(C B A)
ENDIF()
Configure with -DTRANSITIVE=ON and inspect CMakeFiles/Makefile2:
CMakeFiles/A.dir/all:
CMakeFiles/B.dir/all: CMakeFiles/A.dir/all
CMakeFiles/C.dir/all: CMakeFiles/B.dir/all
With -DTRANSITIVE=OFF, these lines read:
CMakeFiles/A.dir/all:
CMakeFiles/B.dir/all:
CMakeFiles/C.dir/all: CMakeFiles/A.dir/all
CMakeFiles/C.dir/all: CMakeFiles/B.dir/all
The CMakeFiles/X.dir/all targets do:
$(MAKE) -f CMakeFiles/X.dir/build.make CMakeFiles/X.dir/build
Finally, CMakeFiles/X.dir/build in CMakeFiles/X.dir/build.make
does build target X completely, i.e. including the linking step.
Thus, the two-part transitive linking with -DTRANSITIVE=ON indeed
completes A before addressing B, so A and B can not be compiled in
parallel. In contrast, the one-part non-transitive linking with -D
TRANSITIVE=OFF allows for A and B to be compiled and even linked in
parallel since they haven't any interdependencies. So, with -j, the
latter is potentially faster than the former, but...
...reconsider what you're about to do: If B actually references A,
you might possibly not want to drop the TARGET_LINK_LIBRARIES(B A)
command. Run readelf -d libB.so on both results for -DTRANSITIVE
and you will see the difference. If A and B were static libraries,
CMake would lose the awareness that B must pull in A in the linker
command line.
In short, the answers to your questions are: N/Y, Y and Y.
Regards,
Michael
--
Powered by www.kitware.com
Visit other Kitware open-source projects at
http://www.kitware.com/opensource/opensource.html
Please keep messages on-topic and check the CMake FAQ at:
http://www.cmake.org/Wiki/CMake_FAQ
Follow this link to subscribe/unsubscribe:
http://www.cmake.org/mailman/listinfo/cmake
