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Update and document Emscripten link-time options #174

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Update and document Emscripten link-time options #174

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15 changes: 15 additions & 0 deletions bazel/defs.bzl
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Original file line number Diff line number Diff line change
Expand Up @@ -89,10 +89,25 @@ def proxy_wasm_cc_binary(
"@proxy_wasm_cpp_sdk//:proxy_wasm_intrinsics_js",
],
linkopts = linkopts + [
# Setting to indicate module is a "reactor library" without a main() entry point:
# https://emscripten.org/docs/tools_reference/settings_reference.html#standalone-wasm
"--no-entry",
# File listing additional functions that Emscripten should expect to be implemented by the host:
# https://emscripten.org/docs/porting/connecting_cpp_and_javascript/Interacting-with-code.html#implement-c-in-javascript
"--js-library=$(location @proxy_wasm_cpp_sdk//:proxy_wasm_intrinsics_js)",
# Emit Wasm module that can run without JavaScript
"-sSTANDALONE_WASM",
# Give host code access to Emscripten's _malloc() function
"-sEXPORTED_FUNCTIONS=_malloc",
# Allow allocating memory past initial heap size
"-sALLOW_MEMORY_GROWTH=1",
# Total stack size (fixed). Emscripten default stack size changed from 5MiB to 64KiB in
# 3.1.27: https://github.com/emscripten-core/emscripten/blob/main/ChangeLog.md,
# https://github.com/emscripten-core/emscripten/pull/18191. We pick 1MiB somewhat
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I'm not a big fan of this reasoning. Perhaps elaborate based on the comments in this thread?

# arbitrarily, since it is gives a little more room and is easy to remember.
"-sSTACK_SIZE=1MB",
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@martijneken martijneken Oct 12, 2024 •
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Feels kind of wasteful for a function/plugin, but that's just a gut feeling.
Real world OS defaults have a wide range: https://stackoverflow.com/a/1826072

Do we have evidence that Emscripten's 64 KiB default is insufficient? If not I would vote to leave the default.

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@PiotrSikora PiotrSikora Oct 12, 2024 •
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Real world OS defaults have a wide range: https://stackoverflow.com/a/1826072

Those are default thread stack sizes, which are often an order of magnitude smaller.

The default process stack size on all my 64-bit systems seems to be 8 MiB (check with ulimit -s).

I believe that historically @kyessenov was opposed to changing this from the 5 MiB default. As far as I recall, the reason was mostly because Istio plugins are using protobuf library, which might have a deep recursive stack? Kuat, could you verify that they still work after the recent changes?

I know this is not apples-to-apples comparison, but Rust uses 1 MiB stack by default, and I don't recall anybody ever running into any issues with that in Proxy-Wasm Rust SDK.

# Initial amount of heap memory
"-sINITIAL_HEAP=1MB",
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FWIW, Rust SDK starts with a minimal 64 KiB heap (64 KiB is 1 Wasm page size), and only because it allocates read-only data with static strings, otherwise it would be zero.

],
tags = tags + [
"manual",
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