Speeding up WASM builds

[adamziel]

A full rebuild of all PHP WASM binaries takes multiple hours on a Mac. That's a long time to wait and everyone dreads doing that, especially when some tasks require multiple rounds of rebuilding. Let's discuss speeding it up.

cc @WordPress/playground-maintainers

[adamziel]

Hypothesis: More parallelization would speed up the builds (✅ confirmed)

The big improvement came from changing -j1 to -j14. I made that -j1 change myself in commit 625a863, though I can't remember why. Whatever the reason was, it's been slowing down our builds significantly.

With parallelization enabled, a single PHP version rebuilds in around a minute and full rebuild of all PHP.wasm binaries dropped to about an hour.

Proposed change

Hardcoding -j8 or -j14 isn't ideal---it could slow things down on machines with fewer cores available to Docker. Instead, we may want to detect available cores at build time:

# NAIVE CHECK:
# Use available cores minus 2, leaving headroom for other processes
JOBS=$(($(nproc) - 2))
# Let's confirm this covers for `min(host cores, max cores from docker engine settings)`

[adamziel]

Hypothesis – Docker on Apple Silicon is slow (❌ refuted)

I've hypothesized that Docker's virtualization layer on macOS was the bottleneck. Perhaps running everything on a beefy Linux server would be faster?

I spun up a 14-core dedicated server on Hetzner (about €40/month) to test this theory. Turns out, i was still much slower than on Mac:

Setup	Single PHP version	Full rebuild
Mac with Docker	~60 seconds	~1 hour
Hetzner (14 cores)	~3.5 minutes	~1 hour

Apple Silicon is remarkably fast. Even with Docker's virtualization overhead, my Mac beat a dedicated 14-core Intel server on per-version build times. The M5 chip currently has the best single-core performance of any consumer CPU---and it's in a laptop.

I was tempted to try an 48-core Intel machine, but assuming a linear speedup, we'd be still around the 60 seconds mark we get on Mac.

It would still be cool to click a button in GitHub and delegate the rebuild to an external server, but I have a hard time justifying spending a few days on a minor convenience improvement.