When developing some code for O2 I noticed that the recompilation time of the project were quite noticeable.

While the total build time might also be a concern to be addressed at some point, I’m only considering here the turn-around time. By turn-around I mean the unit development phase, i.e. you change a single file, run the build to see if it compiles and run, then do it again, and again…

Good thing is we are using a build system generator (CMake). While we are currently using its “Unix Makefiles” generator, it can as well generate Ninja files. So let’s try that and run some timings to see how much we can gain with Ninja with respect to make.

All the tests below are made on a Mid 2014 2,8 GHz Intel Core i7 MacBook Pro under macOS 10.13.4 with Apple LLVM version 9.1.0.

Total build time

The starting points are two build directories obtained using the following commands from the same source code (located in $HOME/alice/o2-dev/O2 , commit 912810e785056edcc919472b16171e81d266079a)

> time aliBuild -w ../sw -z time-make --defaults o2 build O2  
3556.49s user 4457.25s system 501% cpu 26:36.96 total
> time CMAKE_GENERATOR=Ninja aliBuild -w ../sw -z time-ninja --defaults o2 build O2  
2980.51s user 1515.54s system 423% cpu 17:41.40 total

So a complete build takes 26 mins with make and 17 mins with Ninja. Not a huge win here, but some gain already.

Recompilation after modifying one single cxx file

First modify an implementation file somewhere deep in the O2 tree. make is taking a “lot” of time figuring out what he’s already done and then recompile that one file and its dependencies. Ninja on the other hand is going straight to recompile/relink what must be compiled/linked.

> touch $HOME/alice/o2-dev/O2/Detectors/MUON/MCH/Mapping/Impl3/src/SegmentationCreator.cxx
> time cmake --build $HOME/alice/sw/BUILD/O2-latest-time-make/O2 -- -j 8
43.72s user 18.18s system 633% cpu 9.769 total
> touch $HOME/alice/o2-dev/O2/Detectors/MUON/MCH/Mapping/Impl3/src/SegmentationCreator.cxx
> time cmake --build $HOME/alice/sw/BUILD/O2-latest-time-ninja/O2 -- -j 8
1.61s user 0.26s system 105% cpu 1.766 total

No-op recompilation

Actually, just trying a build without changing anything already shows how make is behaving poorly, spending its time trying to figure out if there’s something to do, while Ninja “knows” there’s nothing to do :

cmake --build ~/alice/sw/BUILD/O2-latest-time-make/O2 -- -j 8
39.86s user 16.53s system 588% cpu 9.576 total
cmake --build ~/alice/sw/BUILD/O2-latest-time-ninja/O2 -- -j 8
 0.13s user 0.07s system 93% cpu 0.211 total

Recompilation after changing a top-level include file

Now change an implementation file higher in the food chain which will trigger a lot more stuff to be done. Again, Ninja is the clear winner.

> touch $HOME/alice/o2-dev/O2/Detectors/Base/include/DetectorsBase/Detector.h
> time cmake --build $HOME/alice/sw/BUILD/o2-latest-time-make/O2 -- -j 8
200.81s user 40.54s system 508% cpu 47.469 total
> touch $HOME/alice/o2-dev/O2/Detectors/Base/include/DetectorsBase/Detector.h
> time cmake --build $HOME/alice/sw/BUILD/o2-latest-time-ninja/O2 -- -j 8
136.24s user 13.39s system 533% cpu 28.053 total

Install or not install

While rebuilding things with Ninja is a lot faster, installing is still a painfully slow step that should be avoided as much as possible when developing.

> time cmake --build $HOME/alice/sw/BUILD/o2-latest-time-ninja/O2 -- install
4.22s user 5.26s system 80% cpu 11.766 total

And it’s also quite a bit slower with make (and even more so if not using parallelism) :

cmake --build $HOME/alice/sw/BUILD/o2-latest-time-make/O2 -- -j 8 install
42.91s user 21.02s system 313% cpu 20.379 total

cmake --build $HOME/alice/sw/BUILD/o2-latest-time-make/O2 -- -j 1 install
29.35s user 14.08s system 93% cpu 46.614 total


> time aliBuild -w ../sw -z time-dds-make --defaults o2 build DDS
aliBuild -w ../sw -z time-dds-make --defaults o2 build DDS  591.12s user 56.93s system 266% cpu 4:03.10 total

❯ time CMAKE_GENERATOR=Ninja aliBuild -w ../sw -z time-dds-ninja --defaults o2 build DDS
not compiling due to a strange syntax (incorrect ?) in add_custom_target(wn_bin)
which would require a change...