Using ccache when building inside of docker

Question

I am working on moving the build for a C++ project into a docker image. The image will be built and pushed by a Jenkins job. Prior to docker, I made heavy use of ccache to speed up my builds on Jenkins, especially in the case of builds where very little changed. The trouble with docker is that the build now runs in an isolated environment, so I can no longer benefit from ccache. Is there a way to build inside of an ephemeral container while still taking advantage of ccache?

Answer 1

You can still use ccache in conjunction with your build.

Create a Data Volume to allow data to persist between compilations/builds using the following command:

$ docker create -v /mnt/ccache:/ccache --name ccache debian

Then create your container that “mounts” the data container created above using the --volumes-from command line option.

$ docker run -e CCACHE_DIR=/ccache --volumes-from ccache -it debian

Now you'll be in the shell of the debian container and can install the required apps and test ccache:

root@15306d02505a:/# apt-get update && apt-get install -y gcc ccache    

Now at this point you can check the cache, and it will be empty as expected:

root@15306d02505a:/# ccache -s
cache directory                     /ccache
cache hit (direct)                     0
cache hit (preprocessed)               0
cache miss                             0
files in cache                         0
cache size                             0 Kbytes
max cache size                       1.0 Gbytes

The data volume will persist, so even after the container is terminated, the cache is still there. Future builds that mount the volume (and specify the -e ENV variable) will utilize the cache.

Then create a simple app, run it, and check the cache again:

root@15306d02505a:/# cat > foo.c << __EOF__
 int main(int argc, char **argv)
 {
     return 0;
 }
 __EOF__

root@15306d02505a:/# PATH=/usr/lib/ccache:$PATH gcc -o foo.o -c foo.c
root@15306d02505a:/# ccache -s
cache directory                     /ccache
cache hit (direct)                     1
cache hit (preprocessed)               0
cache miss                             1
files in cache                         2
cache size                             8 Kbytes
max cache size                       1.0 Gbytes

You can see the cache is now populated, and further builds will see performance improvements because of it.

The data volume will persist, so even after the container is terminated, the cache is still there. Future builds that mount the volume (and specify the -e ENV variable) will utilize the cache.

This blog post does a good job of explaining it:

Using Ccache with Docker