[NO QA] perf: Increase initial Hermes heap size for faster startup and delay Young-Gen GC

[chrispader]

@mountiny

Explanation of Change

By default, the Hermes JS engine uses an initial heap size of 4MB, which can be used for allocating memory for JS values. Once the heap is fully allocated, Hermes will run garbage collection, which takes time and can slow down other work, which is a problem especially during app startup.

With a small tweak in the creation of the Hermes instance (HermesInstance.cpp) we can increase the initial heap size to a higher value (e.g. 150MB) and delay Young-Gen GC on app start until Old-Gen GC is triggered for the first time. Based on my testing, this saves up around 150-300ms in startup time on low-end Android devices (potentially even more!). The saved startup time heavily depends on the device, the available RAM and memory pressure. The good thing about this change is that it does not introduce any downsides, except for higher RAM usage on startup.

Fixed Issues

$ #76859
PROPOSAL:

Tests

• Verify that no errors appear in the JS console

Profile and benchmark Android app start.

Offline tests

None needed.

QA Steps

• Verify that no errors appear in the JS console

Make sure the app behaves expectedly the same as before.

PR Author Checklist

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Screenshots/Videos

Android: Native

[github-actions]

⚠️ This PR is possibly changing native code and/or updating libraries, it may cause problems with HybridApp. Please check if any patch updates are required in the HybridApp repo and run an AdHoc build to verify that HybridApp will not break. Ask Contributor Plus for help if you are not sure how to handle this. ⚠️

[mountiny]

@chrispader What is your ETA for this one?

[chrispader]

@mountiny @Julesssss i'm finishing up this PR right now.

[melvin-bot]

@brunovjk Please copy/paste the Reviewer Checklist from here into a new comment on this PR and complete it. If you have the K2 extension, you can simply click: [this button]

[chrispader]

The changes in this PR are finished, right now i'm only running some more android build to get reliable numbers.

These build are quite time-consuming, therefore i wasn't able to find time for this earlier. I'm currently testing 4MB, 50MB, 150MB, 300MB initial heap size. I think in general 150mb should be a good value for low-end Android devices, but we can fine-grain this even more.

If we want to get this pushed sooner than later, we can also merge this PR right away and we can adjust the initial heap size value after i have more reliable numbers.

@mountiny @Julesssss what do you think?

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[Julesssss]

If we want to get this pushed sooner than later, we can also merge this PR right away and we can adjust the initial heap size value after i have more reliable numbers.

No worries, happy to add a small delay to help get it right first time. What devices models are you testing with out of interest?

[mountiny]

Fell free to find the best heap size

[chrispader]

What devices models are you testing with out of interest?

I'm benchmarking mainly on a Samsung Galaxy A14 5G (SM-A146DP). This is our baseline low-end Android device at Margelo, which we use for most performance metrics. I've also tested on other low-end devices, which yield similar results.

[chrispader]

@Julesssss @mountiny i'll continue my benchmarks tomorrow, but these are the first reliable numbers that i can provide:

20 iterations each, outliers were ignored

Configuration	Avg. TTI time (ms)	TTI diff (ms)	Improvement (%)
4mb (main)	2,742	—	—
150mb	2,475	267	9.745374229
300mb	2,425	317	11.57380397

As mentioned, i'll also try 50MB and 100MB. As you can see, there is not such a big diff between 150MB and 300MB initial heap size, therefore i would stick with 150MB or lower, depending on my final results. 300MB should still be easily handled by most low-end Android devices, but there's no need for putting that much pressure on memory right from the startup, if the gains are minimal

[Julesssss]

Thanks for sharing, happy with your suggestion of 150 👍

[chrispader]

finishing this up right now!