TopolearnParallel builds cut build times by the number of cores, but only if the dependency graph is honest.
make -j8 runs up to eight recipes at once; make -j$(nproc) uses every core. make only starts a target once all its prerequisites are done, so a correct dependency graph is automatically parallel-safe.
Parallel builds expose missing dependencies. If app needs gen/config.h but the rule doesn't say so, a serial build may work by luck of ordering while -j builds fail randomly. Fix the graph rather than adding .NOTPARALLEL.
Tools for a precise graph:
- Order-only prerequisites, after a
|:build/%.o: src/%.c | buildmeans "build must exist first, but its timestamp doesn't matter". Perfect for output directories, whose timestamps change whenever a file is added to them. - A grouped target
a.c a.h &: spec.y(GNU make 4.3+) says one recipe produces several files together. A plaina.c a.h: spec.ymeans two independent rules, and under-jthe generator may run twice at once. --output-sync=target(-O) buffers each target's output so logs from parallel jobs don't interleave.
When make calls itself recursively, use $(MAKE), so the jobserver shares the job slots instead of each sub-make starting its own eight. MAKEFLAGS += -j8 inside the Makefile is possible but surprises users; usually leave -j to the caller.
Example
SRC := $(wildcard src/*.c)
OBJ := $(SRC:src/%.c=build/%.o)
build/app: $(OBJ)
cc -o $@ $^
# order-only: the directory must exist, its timestamp is ignored
build/%.o: src/%.c gen/version.h | build
cc -Igen -c $< -o $@
build:
mkdir -p $@
# grouped target: one run of the generator produces both files
gen/version.h gen/version.c &: version.txt
@mkdir -p gen
./mkversion.sh version.txt gen/version.h gen/version.c
# $ make -j"$(nproc)" -ORemember
- make -jN runs independent recipes in parallel
- Random -j failures mean the dependency graph is missing an edge
- Use | for order-only prerequisites (directories) and &: for grouped targets
Watch out for
- Making object files depend normally on their output directory, so every new file in it triggers rebuilds.