OOM on MIPS 32-bit

[infinity0]

We previous successfully did a native-compilation of mips rustc 1.30.0 here:

https://buildd.debian.org/status/fetch.php?pkg=rustc&arch=mips&ver=1.30.0%2Bdfsg1-2%2Bb1&stamp=1541331217&raw=0

It was compiling itself using a previously-cross-compiled 1.30.0 rustc mips, cross-compiled from amd64. Only 14 tests failed out of the whole test suite, so we're now shipping it in Debian.

However the build for 1.31.0 is running out of memory in LLVM on stage 1:

https://buildd.debian.org/status/fetch.php?pkg=rustc&arch=mips&ver=1.31.0%2Bdfsg1-1%7Eexp2&stamp=1544918153&raw=0

[nagisa]

Just my two cents, It is not likely to be a high priority to support building the compiler on 32-bit systems. 3GiB of memory is increasingly little for a modern project of rustc’s size, and even if this particular regression is fixed it is certain that at some point down the road the threshold will be exceeded again and again.

There may be some things you can change in the build configuration (e.g. set codegen units to 1, perhaps?) to ensure a lower peak memory use.

[infinity0]

If this is hard to achieve, what you say sounds reasonable. I wanted to file this issue anyways in case >4GB RAM usage is anomalous (e.g. compared to other arches) and/or could be easily fixed.

At any rate it seems like another build attempt succeeded in the meantime so maybe we just have to hope in the future these builds are scheduled on otherwise non-busy machines.

[sanxiyn]

This continued to be a problem with 1.32.0: https://alioth-lists.debian.net/pipermail/pkg-rust-maintainers/2019-January/004844.html

[nagisa]

cc @rust-lang/compiler would be nice to have compiler compilation to fit within 2G or something.

[michaelwoerister]

Reducing the amount of parallelism in LLVM might help. I think -Zno_parallel_llvm would have the effect of never having more than one LLVM module in memory at a time (at least when not using ThinLTO).

[nagisa]

According to the thread linked above -Zno-parallel-llvm is something that has been attempted.