On Thu, Jun 28, 2018 at 3:03 PM Erwan Velu <evelu@xxxxxxxxxx> wrote: > > 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) Wow, that's huge! Nice work! John > > 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 -- 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
