GNU make knows how to execute several commands at once.
Normally, make will execute only one command at a time, waiting
for it to finish before executing the next. However, the ‘-j’ or
‘--jobs’ option tells make to execute many commands
simultaneously.
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 commands 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).
One unpleasant consequence of running several commands simultaneously is that output generated by the commands appears whenever each command sends it, so messages from different commands may be interspersed.
Another problem is that two processes cannot both take input from the
same device; so to make sure that only one command tries to take input
from the terminal at once, make will invalidate the standard
input streams of all but one running command. This means that
attempting to read from standard input will usually be a fatal error (a
‘Broken pipe’ signal) for most child processes if there are
several.
It is unpredictable which command will have a valid standard input stream
(which will come from the terminal, or wherever you redirect the standard
input of make). The first command run will always get it first, and
the first command started after that one finishes will get it next, and so
on.
We will change how this aspect of make works if we find a better
alternative. In the mean time, you should not rely on any command using
standard input at all if you are using the parallel execution feature; but
if you are not using this feature, then standard input works normally in
all commands.
Finally, handling recursive make invocations raises issues. For
more information on this, see
Communicating Options to a Sub-make.
If a command fails (is killed by a signal or exits with a nonzero
status), and errors are not ignored for that command
(see Errors in Commands),
the remaining command lines to remake the same target will not be run.
If a command fails and the ‘-k’ or ‘--keep-going’
option was not given
(see Summary of Options),
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.