Restricting Parallelism

Sometimes file collisions cannot be avoided in a makefile. An example is xstr(1), which extracts strings from a C program to implement shared strings. The xstr command writes the modified C program to the fixed file x.c and appends the strings to the fixed file strings. Since xstr must be run over each C file, the following new .c.o rule is commonly defined:

.c.o:
	$(CC) $(CPPFLAGS) -E $*.c | xstr -c - 
	$(CC) $(CFLAGS) $(TARGET_ARCH) -c x.c
	mv x.o $*.o

The dmake utility cannot concurrently build targets using this rule since the build of each target writes to the same x.c and strings files. Nor is it possible to change the files used. You can use the special target .NO_PARALLEL: to tell dmake not to build these targets concurrently. For example, if the objects being built using the .c.o rule were defined by the OBJECTS macro, the following entry would force dmake to build those targets serially:

.NO_PARALLEL: $(OBJECTS)

If most of the objects must be built serially, it is easier and safer to force all objects to default to serial processing by including the .NO_PARALLEL: target without any dependents. Any targets that can be built in parallel can be listed as dependencies of the .PARALLEL: target:

.NO_PARALLEL:
.PARALLEL: $(LIB_OBJECT)

Nested Invocations of Distributed Make

When dmake encounters a target that invokes another dmake command, it builds that target serially, rather than concurrently. This prevents problems where two different dmake invocations attempt to build the same targets in the same directory. Such a problem might occur when two different programs are built concurrently, and each must access the same library. The only way for each dmake invocation to be sure that the library is up-to-date is for each to invoke dmake recursively to build that library. The dmake utility recognizes a nested invocation only when the $(MAKE) macro is used in the command line.

If you nest commands that you know will not collide, you can force them to be done in parallel by using the .PARALLEL: construct.

When a makefile contains many nested commands that run concurrently, the load-balancing algorithm may force too many builds to be assigned to the local machine. This may cause high loads and possibly other problems, such as running out of swap space. If such problems occur, allow the nested commands to run serially.