File: make.info,  Node: Parallel,  Next: Errors,  Prev: Execution,  Up: Recipes

5.4 Parallel Execution
======================

GNU 'make' knows how to execute several recipes at once.  Normally,
'make' will execute only one recipe at a time, waiting for it to finish
before executing the next.  However, the '-j' or '--jobs' option tells
'make' to execute many recipes simultaneously.  You can inhibit
parallelism for some or all targets from within the makefile (*note
Disabling Parallel Execution: Parallel Disable.).

   On MS-DOS, the '-j' option has no effect, since that system doesn't
support multi-processing.

   If the '-j' option is followed by an integer, this is the number of
recipes to execute at once; this is called the number of "job slots".
If there is nothing looking like an integer after the '-j' option, there
is no limit on the number of job slots.  The default number of job slots
is one, which means serial execution (one thing at a time).

   Handling recursive 'make' invocations raises issues for parallel
execution.  For more information on this, see *note Communicating
Options to a Sub-'make': Options/Recursion.

   If a recipe fails (is killed by a signal or exits with a nonzero
status), and errors are not ignored for that recipe (*note Errors in
Recipes: Errors.), the remaining recipe lines to remake the same target
will not be run.  If a recipe fails and the '-k' or '--keep-going'
option was not given (*note Summary of Options: Options Summary.),
'make' aborts execution.  If make terminates for any reason (including a
signal) with child processes running, it waits for them to finish before
actually exiting.

   When the system is heavily loaded, you will probably want to run
fewer jobs than when it is lightly loaded.  You can use the '-l' option
to tell 'make' to limit the number of jobs to run at once, based on the
load average.  The '-l' or '--max-load' option is followed by a
floating-point number.  For example,

     -l 2.5

will not let 'make' start more than one job if the load average is above
2.5.  The '-l' option with no following number removes the load limit,
if one was given with a previous '-l' option.

   More precisely, when 'make' goes to start up a job, and it already
has at least one job running, it checks the current load average; if it
is not lower than the limit given with '-l', 'make' waits until the load
average goes below that limit, or until all the other jobs finish.

   By default, there is no load limit.

* Menu:

* Parallel Disable::            Disabling parallel execution
* Parallel Output::             Handling output during parallel execution
* Parallel Input::              Handling input during parallel execution