Add concurrency limit

[tonistiigi]

Add ability to control concurrency on low powered machine. Ideally, this is dynamic and based on stats(for distributed mode) with some manual override.

related #1032

[lugeng]

Does buildkit has interface to know home many builds are running?

[tonistiigi]

@lugeng This is not exposed currently through any API

[AkihiroSuda]

I think we should also emulate legacy progress output by default when concurrency=1.

[qdm12]

I have the problem with a Go program when cross compiling for more than 5 architectures with buildkit. This results in my CI (Github Actions) to crash without an error message which confused me for quite some time, to finally realize it would occasionally go out of memory on my machine as well.

Limiting parallelism would be a great option! 👍 Although ideally if we could limit the parallelism for a specific stage only that would be even better (e.g. make only the compilation stage sequential, not other stages that are less cpu/memory intensive).

For the ones struggling with this issue for the cross building case, I coded some little Go tool in a best effort to build sequentially when cross building with buildkit. It sleeps depending on the target platform so that builds don't really run altogether in parallel anymore. See this documentation and a real example here. This is of course glue and sticks to mitigate the problem.

[tonistiigi]

This was added in #2049

Follow-ups tracked in #2108

[qdm12]

Thanks for the intervention 👍 Is there any documentation on how to limit the parallelism?

And I know this is probably far fetched, but do you see any way we could limit the parallelism for a specific stage? Or maybe that can be solved automatically with #2108 🤔

[tonistiigi]

@vladaionescu Could you add some docs about the new config option in https://github.com/moby/buildkit/blob/master/docs/buildkitd.toml.md

Really need this option, my Virtual Machine burning on fire.

Any help?