Cheng Shao authored Jan 10, 2026 and Marge Bot committed Jan 18, 2026

Most hadrian build flavours pass `-H32m`/`-H64m` to GHC as
conventional wisdom to improve mutator productivity and reduce GC
overhead. They were inherited from the legacy Make build system, and
there used to be make flags to instrument a build process with
`-Rghc-timing` option to collect GC stats of each GHC run from stderr.

It's time to revisit whether there are better defaults for
`-H32m`/`-H64m`, and this patch changes it to `-O64M` which indeed
improves mutator productivity based on real statistics. `-O64M` is
more aggressive than `-H64m`; it allows the old generation to grow to
at least 64M before triggering major GC and reduces major GC runs.

The stats of a clean build with `validate` flavour and `-H64m`:

```
h64m.log
  matched RTS stat lines: 5499
  sum MUT cpu     : 2400.808 s
  sum GC cpu      : 1378.292 s
  sum MUT elapsed : 2788.253 s
  sum GC elapsed  : 1389.233 s
  GC/MUT cpu ratio     : 0.574 (GC is 57.4% of MUT)
  GC/MUT elapsed ratio : 0.498 (GC is 49.8% of MUT)
  GC fraction of (MUT+GC) cpu     : 36.5%
  GC fraction of (MUT+GC) elapsed : 33.3%
  per-line GC/MUT cpu ratio: median 0.691, p90 1.777
  per-line GC/MUT elapsed ratio: median 0.519, p90 1.081
```

The stats of a clean build with `validate` flavour and `-O64M`:

```
o64m.log
  matched RTS stat lines: 5499
  sum MUT cpu     : 2377.383 s
  sum GC cpu      : 1127.146 s
  sum MUT elapsed : 2758.857 s
  sum GC elapsed  : 1135.587 s
  GC/MUT cpu ratio     : 0.474 (GC is 47.4% of MUT)
  GC/MUT elapsed ratio : 0.412 (GC is 41.2% of MUT)
  GC fraction of (MUT+GC) cpu     : 32.2%
  GC fraction of (MUT+GC) elapsed : 29.2%
  per-line GC/MUT cpu ratio: median 0.489, p90 1.099
  per-line GC/MUT elapsed ratio: median 0.367, p90 0.806
```

Mutator time is roughly in the same ballpark, but GC CPU time has
reduced by 18.22%, and mutator productivity has increased from 63.5%
to 67.8%.