On Wed, Nov 17 2021, Mike Hommey wrote: > On Tue, Nov 16, 2021 at 01:00:18PM +0100, Ævar Arnfjörð Bjarmason wrote: >> Add a template to do the "mkdir -p" of $(@D) (the parent dir of $@) >> for us, and use it for the "make lint-docs" targets I added in >> 8650c6298c1 (doc lint: make "lint-docs" non-.PHONY, 2021-10-15). >> >> As seen in 4c64fb5aad9 (Documentation/Makefile: fix lint-docs mkdir >> dependency, 2021-10-26) maintaining these manual lists of parent >> directory dependencies is fragile, in addition to being obviously >> verbose. >> >> I used this pattern at the time because I couldn't find another method >> than "order-only" prerequisites to avoid doing a "mkdir -p $(@D)" for >> every file being created, which as noted in [1] would be significantly >> slower. >> >> But as it turns out we can use this neat trick of only doing a "mkdir >> -p" if the $(wildcard) macro tells us the path doesn't exist. A re-run >> of a performance test similar to thatnoted downthread of [1] in [2] >> shows that this is faster, in addition to being less verbose and more >> reliable (this uses my "git-hyperfine" thin wrapper for "hyperfine"[3]): >> >> $ git hyperfine -L rev HEAD~0,HEAD~1 -b 'make -C Documentation lint-docs' -p 'rm -rf Documentation/.build' 'make -C Documentation lint-docs' >> Benchmark 1: make -C Documentation lint-docs' in 'HEAD~0 >> Time (mean ± σ): 2.129 s ± 0.011 s [User: 1.840 s, System: 0.321 s] >> Range (min … max): 2.121 s … 2.158 s 10 runs >> >> Benchmark 2: make -C Documentation lint-docs' in 'HEAD~1 >> Time (mean ± σ): 2.659 s ± 0.002 s [User: 2.306 s, System: 0.397 s] >> Range (min … max): 2.657 s … 2.662 s 10 runs >> >> Summary >> 'make -C Documentation lint-docs' in 'HEAD~0' ran >> 1.25 ± 0.01 times faster than 'make -C Documentation lint-docs' in 'HEAD~1' >> >> So let's use that pattern both for the "lint-docs" target, and a few >> miscellaneous other targets. >> >> This method of creating parent directories is explicitly racy in that >> we don't know if we're going to say always create a "foo" followed by >> a "foo/bar" under parallelism, or skip the "foo" because we created >> "foo/bar" first. In this case it doesn't matter for anything except >> that we aren't guaranteed to get the same number of rules firing when >> running make in parallel. > > Something else that is racy is that $(wildcard) might be saying the > directory doesn't exist while there's another make subprocess that has > already started spawning `mkdir -p` for that directory. > That doesn't make a huge difference, but you can probably still end up > with multiple `mkdir -p` runs for the same directory. > > I think something like the following could work while avoiding those > races: > > define create_parent_dir_RULE > $(1): | $(dir $(1)). > ALL_DIRS += $(dir $(1)) > endef > > define create_parent_dir_TARGET > $(1)/.: $(dir $(1)). > echo mkdir $$(@D) > endef > > $(eval $(call create_parent_dir_RULE, first/path/file)) > $(eval $(call create_parent_dir_RULE, second/path/file)) > # ... > > $(foreach dir,$(sort $(ALL_DIRS)),$(eval $(call create_parent_dir_TARGET,$(dir:%/=%)))) I think the "race" just isn't a problem, and makes managing this much simpler. I.e. we already rely on "mkdir -p" not failing on an existing directory, so the case where we redundantly try to create a directory that just got created by a concurrent process is OK, and as the quoted benchmark shows is much faster than a similar (but not quite the same as) a dependency-based implementaiton. I haven't implemented your solution, but it seems to be inherently more complex. I.e. with the one I've got you just stick the "mkdir if needed" one-liner in each rule, with yours you'll need to accumulate things in ALL_DIRS, and have some foreach somewhere or dependency relationship to create those beforehand if they're nested, no?
