Thoughts on limiting the parallel build load, suggestion for a new -j auto option
Hi all: I recently came across a build test script that launched make -j with no limits, which consumed all available RAM and ground my Linux system to a halt. I had to power the computer off, there was nothing else I could do. After this experience, I believe that the default limitless behaviour of -j is too dangerous. Even if your PC does not end up dying, GNU Make may inadvertently consume too many resources. An explicit -j infinite would be a better option, if anybody really needs something like that. I recently came across option -l , which limits the amount of parallel tasks based on the system's average load. However, this flag does not seem safe enough, as, according to Wikipedia, not all Unix systems include in the average load those processes currently waiting for disk I/O. Besides, the maximum average load one would almost certainly want to use depends on the number of CPUs, so the calling script (or user) has to find out how many CPUs there are. How you find out may also depend on the operating system underneath, so everybody gets a little extra work every time. I am writing a build test framework myself, and I have been trying to coordinate all sub-makes from a main makefile. The top-level script decides how many parallel processes are allowed for the entire build and relies on MAKEFLAGS in order to let all sub-makes talk to each other so as to limit the overall load. Because different makefiles may require different GNU Make options, I am filtering out all others and leaving just the parallel build flags in place, like this: export MAKEFLAGS=$(filter --jobserver-fds=%,$(MAKEFLAGS)) $(filter -j,$(MAKEFLAGS)) $(MAKE) ...etc... By the way, option --jobserver-fds is not documented, but I think it should be. As you can see, the user may need to filter it out manually after all. The trouble with this MAKEFLAGS technique is that I often come across some third-party script which insists on calculating and setting its own -j value, rendering my coordination efforts useless. When this happens, I get warnings like this: warning: -jN forced in submake: disabling jobserver mode Needless to say, most heuristics to calculate the -j value are as lame as mine (see below). When writing build scripts, nobody seems to have much time left for finding out how to retrieve the relevant system information in bash/perl/whatever in a portable way and then calculate a good -j value out of it. I have been thinking about the best way to overcome such parallel woes, and I wanted to share this thought with you all. How about adding to GNU Make a new -j parameter like this: make -j auto The behaviour of -j auto could be as follows: 1) If MAKEFLAGS specifies -j and --jobserver-fds , then use those settings (no warning required). 2) Otherwise, calculate the maximum number of parallel tasks with some trivial heuristic based on the number of CPUs and/or the system load. I'm using CPU count + 1 at the moment, but I'm sure there are better guesses. I could think of several alternative heuristics: make -j auto-system-load # Use -l CPU count + 0.5 make -j auto-processor-count # Use -j CPU count + 1 I guess most people would then end up using some -j auto variant, in order to avoid overloading or underloading the system without having to implement their own heuristics. That way, a top-level script will be much more likely to succeed at setting a global limit when launching third-party sub-makes in parallel. Please copy me on the answers, as I'm not on this list. Thanks, R. Diez ___ Bug-make mailing list Bug-make@gnu.org https://lists.gnu.org/mailman/listinfo/bug-make
Re: Thoughts on limiting the parallel build load, suggestion for a new -j auto option
R. Diez wrote: Hi all: I recently came across a build test script that launched make -j with no limits, which consumed all available RAM and ground my Linux system to a halt. I had to power the computer off, there was nothing else I could do. Go and shoot the moron who wrote that script. After this experience, I believe that the default limitless behaviour of -j is too dangerous. Even if your PC does not end up dying, GNU Make may inadvertently consume too many resources. An explicit -j infinite would be a better option, if anybody really needs something like that. I recently came across option -l , which limits the amount of parallel tasks based on the system's average load. However, this flag does not seem safe enough, as, according to Wikipedia, not all Unix systems include in the average load those processes currently waiting for disk I/O. Besides, the maximum average load one would almost certainly want to use depends on the number of CPUs, so the calling script (or user) has to find out how many CPUs there are. How you find out may also depend on the operating system underneath, so everybody gets a little extra work every time. -l is utterly useless. Load average is computed too slowly; by the time it passes any useful threshold the actual make load will have spiralled out of control. I am writing a build test framework myself, and I have been trying to coordinate all sub-makes from a main makefile. The top-level script decides how many parallel processes are allowed for the entire build and relies on MAKEFLAGS in order to let all sub-makes talk to each other so as to limit the overall load. Because different makefiles may require different GNU Make options, I am filtering out all others and leaving just the parallel build flags in place, like this: export MAKEFLAGS=$(filter --jobserver-fds=%,$(MAKEFLAGS)) $(filter -j,$(MAKEFLAGS)) $(MAKE) ...etc... By the way, option --jobserver-fds is not documented, but I think it should be. As you can see, the user may need to filter it out manually after all. The trouble with this MAKEFLAGS technique is that I often come across some third-party script which insists on calculating and setting its own -j value, rendering my coordination efforts useless. When this happens, I get warnings like this: Go and shoot the morons who wrote those scripts. The make -j value should never be encoded in any file; it should only ever be set by the user invoking the actual top level make command. warning: -jN forced in submake: disabling jobserver mode Needless to say, most heuristics to calculate the -j value are as lame as mine (see below). When writing build scripts, nobody seems to have much time left for finding out how to retrieve the relevant system information in bash/perl/whatever in a portable way and then calculate a good -j value out of it. Nobody writing scripts should ever bother with such a thing. It's for the end-user to control; any other effort will be wrong 99.9% of the time. I have been thinking about the best way to overcome such parallel woes, and I wanted to share this thought with you all. How about adding to GNU Make a new -j parameter like this: make -j auto The behaviour of -j auto could be as follows: 1) If MAKEFLAGS specifies -j and --jobserver-fds , then use those settings (no warning required). 2) Otherwise, calculate the maximum number of parallel tasks with some trivial heuristic based on the number of CPUs and/or the system load. I'm usingCPU count + 1 at the moment, but I'm sure there are better guesses. I could think of several alternative heuristics: make -j auto-system-load # Use -lCPU count + 0.5 make -j auto-processor-count # Use -jCPU count + 1 I guess most people would then end up using some -j auto variant, in order to avoid overloading or underloading the system without having to implement their own heuristics. That way, a top-level script will be much more likely to succeed at setting a global limit when launching third-party sub-makes in parallel. A good value depends entirely on the actual workload. For most compiles I find that CPU count * 1.5 works well. Some people use make for things other than invoking compilers though, and some compilers have different resource requirements than a typical gcc run, so even that isn't a widely applicable estimate. Only the end user can determine the best value. Please copy me on the answers, as I'm not on this list. -- -- Howard Chu CTO, Symas Corp. http://www.symas.com Director, Highland Sun http://highlandsun.com/hyc/ Chief Architect, OpenLDAP http://www.openldap.org/project/ ___ Bug-make mailing list Bug-make@gnu.org https://lists.gnu.org/mailman/listinfo/bug-make
Re: Thoughts on limiting the parallel build load, suggestion for a new -j auto option
-l is utterly useless. Load average is computed too slowly; by the time it passes any useful threshold the actual make load will have spiralled out of control. If that's the case, exactly that reasoning should be written in the documentation. Go and shoot the morons who wrote those scripts. The make -j value should never be encoded in any file; it should only ever be set by the user invoking the actual top level make command. Again, such a recommendation should be in the manual. However, for practical reasons, I still think GNU Make should offer some simple heuristic for people without the necessary time or inclination to calculate their own optimum value (that would be most humans). That simple heuristic need not be totally fixed. The user could provide their own factor for a simple CPU * factor formula, something like -jcpufactor 1.5 could do the trick. I just want to spare the user the effort to retrieve the CPU count, which is always a small, system-dependant pain. Please copy me on the answers, as I'm not on this list. Regards, R. Diez ___ Bug-make mailing list Bug-make@gnu.org https://lists.gnu.org/mailman/listinfo/bug-make
