[coreclr][android] Don't build/ship unused GC variants for Android

[ivanpovazan]

Description

This PR disables building unused GC variants for Android on CoreCLR, reducing the size of built apps and the runtime package.

There are two parts to it, currently:

• we ship both: libclrgc.so~972 KB and libclrgcexp.so~1025 KB as part of CoreCLR runtime pack for Android, which aren't used

• we build server GC into runtime lib which is unlikely to be used on Android taking up ~300 KB in libcoreclr.so:

  .so size (MB)	Before	After	diff
  libcoreclr.so	6,66	6,32	-0,34

Size savings

On a dotnet new android app this change reduces the size of the .apk by ~1MB

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[ivanpovazan]

@Maoni0@janvorli@BrzVlad let me know what do you think about feasibility of the changes.

[janvorli]

I think these changes make sense. On mobile devices, we would not want to use server GC and standalone GC, so it is just a useless added baggage.

[Copilot]

Copilot reviewed 2 out of 4 changed files in this pull request and generated no comments.

Files not reviewed (2)

• src/coreclr/CMakeLists.txt: Language not supported
• src/coreclr/clrdefinitions.cmake: Language not supported

[steveisok]

LGTM!

[janvorli]

LGTM, thank you!

[Maoni0]

(I'm back from vacation)

we ship both: libclrgc.so ~972 KB and libclrgcexp.so ~1025 KB as part of CoreCLR runtime pack for Android, which aren't used

I can see libclrgc.so used occasionally. this contains the old GC implementation that uses segments instead of regions. there were a couple of times when we did ask the customer to use libclrgc.so. it's only for problematic situations, same idea with shipping 2 bridge implementations.

a standalone GC is also very convenient for debugging, eg, if you want to give the customer a GC impl with some additional instrumentation to help track down an issue, a standalone GC just means they need to load it without having to change anything else. if we want this as a viable way for debugging, then at the minimum, we should make sure libcoreclr.so can load a standalone GC. however, I'm unclear what's possible/not possible for debugging on android so someone else who knows please comment.

as far as Server GC goes, again, I have no idea what sort of heap sizes we are seeing on android and if Server GC would be desirable. it's fine to not ship it for now though. just wanted to point out there might be a possibility later we'd want to ship it but we can enable it then.

[BrzVlad]

Mono GC doesn't have great handling of large heap scenarios and I haven't heard it being a problem. We prioritize low pause times and my understanding is that Workstation GC would always be the desirable choice for this purpose.

My understanding is that we're going for static linking on mobile. Also, if the user would need to use some custom runtime binaries, the application would need to be rebuilt and redeployed anyway, so I don't see the standalone GC really of particular help here.

[steveisok]

My understanding is that we're going for static linking on mobile. Also, if the user would need to use some custom runtime binaries, the application would need to be rebuilt and redeployed anyway, so I don't see the standalone GC really of particular help here.

+1 to what Vlad said. It's best to think the main mobile and wasm targets as exclusively self contained. It's easier in any debugging / support scenario to just rebuild / redeploy.

[jkotas]

static linking

Native AOT allows static linking of different GCs. If we see a need to make the GC implementation configurable for mobile, we can use the same pattern. (It does not need to be done as part of this PR. You may want to track it in a list of potential future improvements.)

GC implementation that uses segments

We may want to check whether the segment GC is better fit for typical mobile app. The region GC reserves very large block of address space at startup that may have undesirable perf side-effects on mobile. (Against, it does not need to be done as part this PR.)

[Maoni0]

if you have a large enough heap and allocates frequently, Server GC would be beneficial for pause times. but that doesn't need to be done with this PR - I just wanted to point it out as a potential future consideration.

makes sense to not care about the standalone GC for mobile. thanks for the info.

as far as which GC should be used, it's better to simply reserve less (if it's even a problem) for mobile instead of using the segments GC - we prefer to focus our efforts in fixing problems for regions instead of segments.

[ivanpovazan]

Thank you all for your feedback.

Regarding:

Native AOT allows static linking of different GCs. If we see a need to make the GC implementation configurable for mobile, we can use the same pattern. (It does not need to be done as part of this PR. You may want to track it in a list of potential future improvements.)

I opened: #114737 for tracking, please feel free to include more information there (/cc: @vitek-karas@kotlarmilos)

Regarding:

We may want to check whether the segment GC is better fit for typical mobile app. The region GC reserves very large block of address space at startup that may have undesirable perf side-effects on mobile. (Against, it does not need to be done as part this PR.)

if you have a large enough heap and allocates frequently, Server GC would be beneficial for pause times. but that doesn't need to be done with this PR - I just wanted to point it out as a potential future consideration.

I opened: #114738 for tracking, please feel free to include more information there (/cc: @BrzVlad@Maoni0)