ccache is a utility that caches intermediate files during compilation offering significant speedups. It is especially useful if you are switching between two branches of the same project see below.
To use it, prepend it before the compiler i.e. Use ccache gcc instead of
gcc ... . If you are using cmake with a version greater
that 3.4 you can use the option -DCMAKE_CXX_COMPILER_LAUNCHER=ccache for
older version you can modify the compiler settings or use the variable
RULE_LAUNCH_COMPILE for more see
here.
Example :
Lets try it on the source code of cmake:
First without ccache
$ git clone https://gitlab.kitware.com/cmake/cmake
$ mkdir build
$ cd build
$ cmake ../cmake
$ time make -j
real 7m29.921s
user 25m56.556s
sys 1m46.428s
We switch to another branch and try again:
$ cd ../cmake
$ git checkout v3.15.0
$ cd ../build
$ cmake ../cmake
$ time make -j
real 5m47.679s
user 20m17.790s
sys 1m23.828s
These are our baselines.
Switching back to master and recompiling requires the same time:
$ cd ../cmake
$ git checkout master
$ cd ../build
$ cmake ../cmake
$ time make -j
real 7m29.399s
user 25m56.187s
sys 1m46.214s
Now we enable ccache on a clean build
$ rm -rf ./build/*
$ cmake -DCMAKE_CXX_COMPILER_LAUNCHER=ccache ../cmake
$ time make -j
real 8m32.952s
user 27m36.442s
sys 2m12.991s
There is a slight increase in compilation time. The same when we switch for first time to another branch:
$ cd ../cmake
$ git checkout v3.15.0
$ cd ../build
$ cmake ../cmake
$ time make -j
real 6m42.900s
user 21m48.924s
sys 1m44.644s
However, now that we have populated our cache, builds are significantly faster !
$ cd ../cmake
$ git checkout master
$ cd ../build
$ cmake ../cmake
$ time make -j
real 0m38.711s
user 1m55.778s
sys 0m21.922s
and
$ cd ../cmake
$ git checkout v3.15.0
$ cd ../build
$ cmake ../cmake
$ time make -j
real 0m31.499s
user 1m36.373s
sys 0m18.004s
ccache also offers some statistics:
$ ccache --stats
cache directory /home/user/.cache/ccache
primary config /home/user/.config/ccache/ccache.conf
secondary config (readonly) /etc/ccache.conf
stats updated Tue Jun 23 17:34:08 2020
cache hit (direct) 874
cache hit (preprocessed) 9
cache miss 882
cache hit rate 50.03 %
cleanups performed 0
files in cache 1754
cache size 46.5 MB
max cache size 5.0 GB
Cache size, policies and other configurations are performed by editing ~/.config/ccache/ccache.conf
according to the documentation.