cmake: sitl_gazebo build use memory information from the system to estimate the parallel jobs

Using cmake_host_system_information, grabs AVAILABLE_PHYSICAL_MEMORY and adds another job for every 1.5GB of available memory.

This is tested on a single system with 16 logical cores and 16GB RAM (~11.5GB available, reported correctly by cmake).

platforms/posix/cmake/sitl_target.cmake

@@ -21,15 +21,37 @@ px4_add_git_submodule(TARGET git_jmavsim PATH "${PX4_SOURCE_DIR}/Tools/jMAVSim")
 
 # Add support for external project building
 include(ExternalProject)
+
+# Estimate an appropriate number of parallel jobs
+cmake_host_system_information(RESULT AVAILABLE_PHYSICAL_MEMORY QUERY AVAILABLE_PHYSICAL_MEMORY)
+cmake_host_system_information(RESULT NUMBER_OF_LOGICAL_CORES QUERY NUMBER_OF_LOGICAL_CORES)
+
+set(parallel_jobs 1)
+
+if(NOT NUMBER_OF_LOGICAL_CORES)
 	include(ProcessorCount)
+	ProcessorCount(NUMBER_OF_LOGICAL_CORES)
+endif()
 
-set(build_cores 1)
+if(NOT AVAILABLE_PHYSICAL_MEMORY AND NUMBER_OF_LOGICAL_CORES GREATER_EQUAL 4)
+	# Memory estimate unavailable, use N-2 jobs
+	math(EXPR parallel_jobs "${NUMBER_OF_LOGICAL_CORES} - 2")
+endif()
 
-ProcessorCount(N)
-if(N GREATER_EQUAL 4)
-	math(EXPR build_cores "${N} - 2")
+if (AVAILABLE_PHYSICAL_MEMORY)
+	# Allow an additional job for every 1.5GB of available physical memory
+	math(EXPR parallel_jobs "${AVAILABLE_PHYSICAL_MEMORY}/(3*1024/2)")
+else()
+	set(AVAILABLE_PHYSICAL_MEMORY "?")
+endif()
+
+if(parallel_jobs GREATER NUMBER_OF_LOGICAL_CORES)
+	set(parallel_jobs ${NUMBER_OF_LOGICAL_CORES})
 endif()
 
+message(DEBUG  "${NUMBER_OF_LOGICAL_CORES} logical cores detected and ${AVAILABLE_PHYSICAL_MEMORY} megabytes of memory available.
+		Limiting sitl_gazebo and simulation-ignition concurrent jobs to ${parallel_jobs}")
+
 # project to build sitl_gazebo if necessary
 px4_add_git_submodule(TARGET git_gazebo PATH "${PX4_SOURCE_DIR}/Tools/sitl_gazebo")
 ExternalProject_Add(sitl_gazebo
@@ -45,7 +67,7 @@ ExternalProject_Add(sitl_gazebo
 	USES_TERMINAL_BUILD true
 	EXCLUDE_FROM_ALL true
 	BUILD_ALWAYS 1
-	BUILD_COMMAND ${CMAKE_COMMAND} --build <BINARY_DIR> -- -j ${build_cores}
+	BUILD_COMMAND ${CMAKE_COMMAND} --build <BINARY_DIR> -- -j ${parallel_jobs}
 )
 
 px4_add_git_submodule(TARGET git_ign_gazebo PATH "${PX4_SOURCE_DIR}/Tools/simulation-ignition")
@@ -59,7 +81,7 @@ ExternalProject_Add(simulation-ignition
 	USES_TERMINAL_BUILD true
 	EXCLUDE_FROM_ALL true
 	BUILD_ALWAYS 1
-	BUILD_COMMAND ${CMAKE_COMMAND} --build <BINARY_DIR> -- -j ${build_cores}
+	BUILD_COMMAND ${CMAKE_COMMAND} --build <BINARY_DIR> -- -j ${parallel_jobs}
 )
 
 ExternalProject_Add(mavsdk_tests