Simultaneous Build Processes

[loglav03]

I'm currently working on converting Yosys to a submodule (see #3156) and this further exposed an issue with multiple builds occurring simultaneously, which can throttle machines when multiple cores are used to compile. This doesn't seem like a simple fix and would need further investigation. Below is an instance of this issue occurring. You can see it begins building yosys at 6%, and then shows 0% just after, meaning it started a new separate build for yosys. After this you can see it continuing the main build at the same time at 6% - 8%. This continues throughout the entire build process.

[vaughnb-cerebras]

@loglav03 : is the issue that we're going too parallel on builds? @AmirhosseinPoolad @AlexandreSinger : any ideas?

[AlexandreSinger]

I can provide more context here.

The issue stems from the fact that Yosys uses Make for its entire build system and we use CMake. These two build systems are not perfectly compatible. From talking with other maintainers of open source projects, they get around this issue by having two build steps:

1. Build Yosys (by calling their Makefile)
2. Build everything else (by calling our CMakefile)

The issue we are seeing here is that we took a different approach. For us, we call Make within our CMake build system; so as CMake goes through and builds each of the targets, it hits this target, builds it by calling Make directly, then continues to build everything else.

This actually works "fine" so long as you are only using one thread; but we usually build with multiple threads. So, if you build VTR with 4 threads for example, one of the threads will hit this target and start making Yosys using their Makefile while the other three threads continue to build VTR. This causes two builds to occur simultaneously. Whats even worse is that I think the Yosys build also uses 4 threads (the number of threads is passed through), so we end up with 8 threads being used in total.

There is no free lunch here. We would either need to split our build step or build our own custom CMake target for Yosys. Both of these have pros and cons, hence I asked Logan to raise this issue.

Yosys is also not the only target we have that has this problem. I have seen tons of threads spinning up from our build system when they are not supposed to.

Something that is worth discussing at a Thursday meeting.

[vaughnbetz]

So the end result is 2x the requested build threads (at least typically)? We could probably just document that ... threads are pretty abundant on most computers these days.

[AlexandreSinger]

From my experience it is a lot more than 2x; but I have not directly tested this. We seem to have multiple targets that do this.