Hi Folks, I've been wondering why the ccache was that inefficient on the CI. It appear and that is confirmed by local builds that two consecutives builds of the exact same code leads to < 1% of cache hit. This is actually a very bad cache performance : As the cache hit is very low, the needed time to create the cache is not compensated, leading to a 10% loss of performance regarding a build without ccache. C and C++ are usually providing much better results. This kind of misses in the cache are usually tied to some little variations introduced at compilation times like a sha1 or a timestamp (__DATE__ or __TIME__ or similar). Thanks to the wonderful project from debian to get reproducible builds (https://wiki.debian.org/ReproducibleBuilds) it is possible to enforce a particular setup at build time (https://wiki.debian.org/ReproducibleBuilds/TimestampsProposal). I've been trying that on my testing host with a very nice result: - before, make takes 33mn with -j 24 (the user time is reported to 630mn). - after , make takes 7mn with -j 24 (the user time is reported to 77mn) This is a nearly a 5x speedup on the whole process which includes non cachable stuff like linking or npm stuff. To achieve that, I've been setting up SOURCE_DATE_EPOCH & BUILD_DATE like : export SOURCE_DATE_EPOCH=$(date +%D |date -f- +%s); export BUILD_DATE=$(date --utc --date=@${SOURCE_DATE_EPOCH} +%Y-%m-%d) In this example, the SOURCE_DATE_EPOCH is the current day. Unless this variable is redefined, the same source code will hit the cache. This example is taken from a PR for the CI (https://github.com/ceph/ceph-build/pull/1058) where I wanted to get a cache being valid only for a day. Note that if the Ceph code is changing too much, the cache have to be partially or totally repopulated. From the tests I run, it usually goes between 0 & 50% of speedup in such case. To increase the cache efficiency, some could want to call do_cmake with ARGS='-D ENABLE_GIT_VERSION=OFF' to avoid having the git hash in the ceph_ver header. These two little changes improves significantly the local build time and will do soonish also on the CI (https://github.com/ceph/ceph-build/pull/1058) where I expect a great speedup ! Erwan, -- To unsubscribe from this list: send the line "unsubscribe ceph-devel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
