Skip to content
Mats Wichmann edited this page Feb 10, 2022 · 13 revisions

By default SCons goes to great lengths to give you an accurate and reproducible build. If you have a slow computer or a large build, there are various options that can make SCons faster by sacrificing accuracy.

TL;DR: this command will probably execute your build as fast as possible, but please read on to understand what you're doing:

scons --max-drift=1 --implicit-deps-unchanged

Details of various speedups:

  • env.Decider('content-timestamp'): you can choose the decider function for a given environment. content-timestamp differs from the default content by saying if the cached timestamp for a dependency from a previous build is identical to the current timestamp, don't bother recomputing the file's signature, as it's unlikely to have changed. Having to fully read lots of files to compute the hashes only to find they didn't change is expensive. See the man page for more info.
  • --max-drift: By default SCons will calculate the content signature of every source file in your build each time it is run, and will only cache the signature after the file is 2 days old. This default of 2 days is to protect from clock skew from NFS or revision control systems. You can tweak this delay using --max-drift=SECONDS where SECONDS is some number of seconds. Decreasing SECONDS can improve build speed by eliminating superfluous signature calculations. Instead of specifying this on the command line each run, you can set this option inside your SConstruct or SConscript file using SetOption('max_drift', SECONDS). c
  • --implicit-deps-unchanged: By default SCons will rescan all source files for implicit dependencies (e.g. C/C++ header #includes), which can be an expensive process for the same reason computing the content signatures is expensive: you have to do the I/O to read the whole file, and in cases where memory is tight so may cause other file system cache data (not SCons' cache) to flush that you might otherwise have been able to make use of. You can tell SCons to use cached versions of this information using the --implicit-deps-unchanged option. This lets SCons know that you didn't change any of the implicit dependencies since it was last run - which is often the case when you're doing small incremental changes while coding actively on your project - so it doesn't need to bother. If you do change the implicit dependencies, then using --implicit-deps-changed will cause them to be rescaned and cached. You can also set this with a SetOption('implicit_deps_unchanged', 1).
  • --implicit-cache: This option tells SCons to intelligently cache implicit dependencies. It attempts to determine if the implicit dependencies have changed since the last build, and if so it will recalculate them. This is usually slower than using --implicit-deps-unchanged, but is also more accurate. Instead of specifying this on the command line each run, you can set this option inside your SConstruct or SConscript file using SetOption('implicit_cache', 1).
  • CPPPATH: normally you tell Scons about include directories by setting the CPPPATH construction variable, which causes SCons to search those directories when doing implicit dependency scans and also includes those directories in the compile command line. If you have header files that never or rarely change (e.g. system headers, or C run-time headers), then you can exclude them from CPPPATH and include them in the CCFLAGS construction variable instead, which causes SCons to not consider those directories when scanning for implicit dependencies (if put in CCFLAGS, you need to use the whole include syntax, for example -Iincdir rather than just the directory name). Carefully tuning the include directories in this way can usually result in a dramatic speed increase with very little loss of accuracy.
  • Avoid RCS and SCCS scans by using env.SourceCode(".", None) - this is especially interesting if you are using lots of c or c++ headers in your program and that your file system is remote (nfs, samba). Note: this option has been removed from SCons
  • When using VariantdDir, use it with "duplicate" set to 0: VariantDir(variant_dir, src_dir, duplicate=0). This will cause scons to invoke Builders using the path names of source files in src_dir and the path names of derived files within variant_dir. However, this may cause build problems if source files are generated during the build, IF any invoked tools are hard-coded to put derived files in the same directory as the source files.
  • On a multi-processor / multi-core machine it may be beneficial to run multiple jobs at once - use the --jobs N option (where N is the number of processors on your machine), or SetOption)('num_jobs', N) inside your SConstruct or SConscript. On Windows machines, the number of processors is available in the environment variable NUMBER_OF_PROCESSORS. It is worth experimenting with the value - for some build types it works well to have more (perhaps even many more) jobs than processors, in others it is better to have fewer. The jobs number only helps when you're actually building, it doesn't impact much if few or no files are being rebuilt.
  • If you have more than a few dozen preprocessor defines (-DFOO1 -DFOO2) you may find from using --profile that SCons is spending a lot of time in the subst() method, usually just adding the -D string to the defines over and over again. This can really slow down builds where nothing has changed. As an example: with 100+ defines I saw a do-nothing build time drop from 35s to 20s using the idea described in "Caching the CPPDEFINES" elsewhere on this page.
  • Limit tools initialized: SCons always initializes the tool list for the default environment - the one used when you call builders as a global function rather than as a method (e.g. Program instead of env.Program). If you don't ever call builders except as construction environment methods, this is just wasted initialization time that can be skipped. The impact is noticable when you call lots of small builds, like running a testsuite (especially SCons' own testsuite!); it won't have much impact on a single large build. To do this just force initialization of the default environment yourself, with an empty tool list:

Another trick to making things faster is to avoid relinking programs when a shared library has been modified but not rebuilt. See SharedLibrarySignatureOverride

The following was suggested by leanid nazdrynau in scons-users, May 2006:

Note on the CPPPATH trick

The CPPPATH trick mentioned above made a great difference for me, because I have many libraries installed in non-standard directories (e.g. these from Fink), and these directories end up explicitly referenced in compiler command lines.

However, for this trick to work, CCFLAGS (CXXFLAGS) has to come after CPPFLAGS (which are in $_CCCOMCOM) in the compiler command line. If not, the compiler will typically include the installed headers from the library I am developing, instead of including the local headers referenced by, say, env.Prepend(CPPPATH='#mylibheaders']).

Here is the code I use to put CCFLAGS after CPPFLAGS. It also imports software resource paths from env variables. (Any comments to make this easier would be welcome.)

# CPPPATH trick, see

# For this to work, CCFLAGS (CXXFLAGS) have to come *after*
# CPPFLAGS (which are in $_CCCOMCOM)
  # SCons 0.97
  # SCons 0.98
  print("Unexpected default CXXCOM")

  # SCons 0.97
  # SCons 0.98
  print("Unexpected default SHCXXCOM")

# process env variables
  if K in os.environ:
    dict = env.ParseFlags(os.environ[K])
    # These headers are supposed static. Don't check at each build.
    for i in dict['CPPPATH']: 
      dict['CCFLAGS'].append('-I' + i)
    dict['CPPPATH'] = []

Another solution was suggested by Roberto De Vecchi on scons-users:

After the env initialization, I'm overriding _CCINCFLAGS as follow:

env['_CPPINCFLAGS']='$( ${_concat(INCPREFIX, CPPPATH, INCSUFFIX, __env__, RDirs, TARGET, SOURCE)} $)' +\
                    '$( ${_concat(INCPREFIX, CPP3RDPARTYPATH, INCSUFFIX, __env__, RDirs, TARGET, SOURCE)} $)'

So I can selectively add an include path to CCPPATH or CPP3RDPARTYPATH and have both of them passed to the compiler but only having scons scan the CPPPATH dirs..

Caching the CPPDEFINES flags in the compiler command

c_defs = ['FOO1', 'FOO2'. ... , 'FOO100']
import copy
def_str = ' -D'.join(c_defs)

cccom = copy.copy(env['CCCOM'])
cccom = cccom.replace('CCFLAGS', 'CCFLAGS -D%s ' % def_str)

shcccom = copy.copy(env['SHCCCOM'])
shcccom = shcccom.replace('CCFLAGS', 'CCFLAGS -D%s ' % def_str)

cxxcom = copy.copy(env['CXXCOM'])
cxxcom = cxxcom.replace('CCFLAGS', 'CCFLAGS -D%s ' % def_str)

shcxxcom = copy.copy(env['SHCXXCOM'])
shcxxcom = shcxxcom.replace('CCFLAGS', 'CCFLAGS -D%s ' % def_str)

fasterEnv = env.Clone(
    CCCOM = cccom,
    CXXCOM = cxxcom,
    SHCCCOM = shcccom,
    SHCXXCOM = shcxxcom,
Clone this wiki locally