default codegen-units is too high for some devices

[Rudxain]

Problem

helix-editor/helix#14269 (comment)

I had to set codegen-units = 4 on my phone (8cores and 4GB RAM); because the default (for that core-count) codegen-units = 8 triggers the Low-Memory-Killer and forcefully-reboots near the end of cargo build

Steps

1. Clone the Helix repo (or any "big" repo)
2. Build it from source on a device with many CPU cores and little RAM
3. Swapping and thrashing happens, LMK triggered, worst-case auto-reboot (Android)

Possible Solution(s)

No response

Notes

No response

Version

[weihanglo]

Finding a good default for every scenario is hard. There is never one that satisfies everyone. We have some other similar issues around changing the default:

• Debugger support in dev profile slows down builds #15931
• Support a crate flag to recommend optimization #8501
• Add more optimizations to the release build profile. #11298

Some of them want faster builds. Some others favor optimization. Regardless, the current option you have is profile in configuration. You can set a [profile] table in your $CARGO_HOME/config.toml as a global override.

While I personally don't think the default will change to favor development on phones, let's keep the issue open to know there are some use cases.

@rustbot lable +A-profiles -S-triages +S-needs-design

[kepstin]

Note that codegen-units is a memory use tradeoff. With fewer codegen units, each individual codegen unit requires more memory. Setting codegen-units to a number which is too low can also trigger OOM on some systems.

For this particular case, you should leave the number of codegen units the same and instead reduce the number of parallel jobs that are being done during the compile, for example by using cargo build -j4.