If you allow multiple builds to update the derived-file cache directory simultaneously, two builds that occur at the same time can sometimes start "racing" with one another to build the same files in the same order. If, for example, you are linking multiple files into an executable program:
Program('prog', ['f1.c', 'f2.c', 'f3.c', 'f4.c', 'f5.c'])
SCons will normally build the input object files on which the program depends in their normal, sorted order:
% scons -Q
cc -o f1.o -c f1.c
cc -o f4.o -c f4.c
cc -o f5.o -c f5.c
cc -o f2.o -c f2.c
cc -o f3.o -c f3.c
cc -o prog f1.o f2.o f3.o f4.o f5.o
But if two such builds take place simultaneously,
they may each look in the cache at nearly the same
time and both decide that f1.o
must be rebuilt and pushed into the derived-file cache directory,
then both decide that f2.o
must be rebuilt (and pushed into the derived-file cache directory),
then both decide that f3.o
must be rebuilt...
This won't cause any actual build problems--both
builds will succeed,
generate correct output files,
and populate the cache--but
it does represent wasted effort.
To alleviate such contention for the cache,
you can use the --random
command-line option
to tell SCons to build dependencies
in a random order:
% scons -Q --random
cc -o f3.o -c f3.c
cc -o f1.o -c f1.c
cc -o f5.o -c f5.c
cc -o f2.o -c f2.c
cc -o f4.o -c f4.c
cc -o prog f1.o f2.o f3.o f4.o f5.o
Multiple builds using the --random
option
will usually build their dependencies in different,
random orders,
which minimizes the chances for a lot of
contention for same-named files
in the derived-file cache directory.
Multiple simultaneous builds might still race to try to build
the same target file on occasion,
but long sequences of inefficient contention
should be rare.
Note, of course,
the --random
option
will cause the output that SCons prints
to be inconsistent from invocation to invocation,
which may be an issue when
trying to compare output from different build runs.
If you want to make sure dependencies will be built
in a random order without having to specify
the --random
on very command line,
you can use the SetOption
function to
set the random
option
within any SConscript
file:
SetOption('random', 1) Program('prog', ['f1.c', 'f2.c', 'f3.c', 'f4.c', 'f5.c'])