Enable Google Benchmark to run in WebAssembly with filesystem disabled #1956
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self test: yxc@yxc-MS-7B89:~/code/2503/benchmark_yxc/build_em$ emcc -O2 test_benchmark.cpp -o test_with_fs.js -I../include -L./src -lbenchmark
cache:INFO: generating system asset: symbol_lists/4ecc501da4e0b22aa538e28f5ac2b2f25c24ad56.json... (this will be cached in "/home/yxc/code/2503/emsdk/upstream/emscripten/cache/symbol_lists/4ecc501da4e0b22aa538e28f5ac2b2f25c24ad56.json" for subsequent builds)
cache:INFO: - ok
yxc@yxc-MS-7B89:~/code/2503/benchmark_yxc/build_em$ emcc -O2 test_benchmark.cpp -o test_without_fs.js -I../include -L./src -lbenchmark -s FILESYSTEM=0
cache:INFO: generating system asset: symbol_lists/9e2ecc3f9ddd9359bfd85a70e497ee587e11349b.json... (this will be cached in "/home/yxc/code/2503/emsdk/upstream/emscripten/cache/symbol_lists/9e2ecc3f9ddd9359bfd85a70e497ee587e11349b.json" for subsequent builds)
cache:INFO: - ok
yxc@yxc-MS-7B89:~/code/2503/benchmark_yxc/build_em$ node test_without_fs.js
***WARNING*** Failed to set thread affinity. Estimated CPU frequency may be incorrect.
2025-03-25T00:08:43+08:00
Running /home/yxc/code/2503/benchmark_yxc/build_em/test_without_fs.js
Run on (1 X 999.986 MHz CPU )
***WARNING*** Library was built as DEBUG. Timings may be affected.
-------------------------------------------------------
Benchmark Time CPU Iterations
-------------------------------------------------------
BM_EmptyLoop 0.000 ns 0.000 ns 1000000000000
yxc@yxc-MS-7B89:~/code/2503/benchmark_yxc/build_em$ node test_with_fs.js
***WARNING*** Failed to set thread affinity. Estimated CPU frequency may be incorrect.
2025-03-25T00:08:57+08:00
Running /home/yxc/code/2503/benchmark_yxc/build_em/test_with_fs.js
Run on (1 X 999.99 MHz CPU )
***WARNING*** Library was built as DEBUG. Timings may be affected.
-------------------------------------------------------
Benchmark Time CPU Iterations
-------------------------------------------------------
BM_EmptyLoop 0.000 ns 0.000 ns 1000000000000
yxc@yxc-MS-7B89:~/code/2503/benchmark_yxc/build_em$ cat test_benchmark.cpp
#include <benchmark/benchmark.h>
static void BM_EmptyLoop(benchmark::State& state) {
for (auto _ : state) {
}
}
BENCHMARK(BM_EmptyLoop);
BENCHMARK_MAIN();yxc@yxc-MS-7B89:~/code/2503/benchmark_yxc/build_em$
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sek test yxc@yxc-MS-7B89:~/code/2503/benchmark_yxc/build_em$ emcc -O2 test_benchmark.cpp -o test_with_fs.js -I../include -L./src -lbenchmark
cache:INFO: generating system asset: symbol_lists/4ecc501da4e0b22aa538e28f5ac2b2f25c24ad56.json... (this will be cached in "/home/yxc/code/2503/emsdk/upstream/emscripten/cache/symbol_lists/4ecc501da4e0b22aa538e28f5ac2b2f25c24ad56.json" for subsequent builds)
cache:INFO: - ok
yxc@yxc-MS-7B89:~/code/2503/benchmark_yxc/build_em$ emcc -O2 test_benchmark.cpp -o test_without_fs.js -I../include -L./src -lbenchmark -s FILESYSTEM=0
cache:INFO: generating system asset: symbol_lists/9e2ecc3f9ddd9359bfd85a70e497ee587e11349b.json... (this will be cached in "/home/yxc/code/2503/emsdk/upstream/emscripten/cache/symbol_lists/9e2ecc3f9ddd9359bfd85a70e497ee587e11349b.json" for subsequent builds)
cache:INFO: - ok
yxc@yxc-MS-7B89:~/code/2503/benchmark_yxc/build_em$ node test_without_fs.js
***WARNING*** Failed to set thread affinity. Estimated CPU frequency may be incorrect.
2025-03-25T00:08:43+08:00
Running /home/yxc/code/2503/benchmark_yxc/build_em/test_without_fs.js
Run on (1 X 999.986 MHz CPU )
***WARNING*** Library was built as DEBUG. Timings may be affected.
-------------------------------------------------------
Benchmark Time CPU Iterations
-------------------------------------------------------
BM_EmptyLoop 0.000 ns 0.000 ns 1000000000000
yxc@yxc-MS-7B89:~/code/2503/benchmark_yxc/build_em$ node test_with_fs.js
***WARNING*** Failed to set thread affinity. Estimated CPU frequency may be incorrect.
2025-03-25T00:08:57+08:00
Running /home/yxc/code/2503/benchmark_yxc/build_em/test_with_fs.js
Run on (1 X 999.99 MHz CPU )
***WARNING*** Library was built as DEBUG. Timings may be affected.
-------------------------------------------------------
Benchmark Time CPU Iterations
-------------------------------------------------------
BM_EmptyLoop 0.000 ns 0.000 ns 1000000000000
yxc@yxc-MS-7B89:~/code/2503/benchmark_yxc/build_em$ cat test_benchmark.cpp
#include <benchmark/benchmark.h>
static void BM_EmptyLoop(benchmark::State& state) {
for (auto _ : state) {
}
}
BENCHMARK(BM_EmptyLoop);
BENCHMARK_MAIN(); |
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This does seem reasonable, but i have a question: is there any way to know https://stackoverflow.com/questions/2224334/gcc-dump-preprocessor-defines |
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@LebedevRI I've investigated the preprocessor macros between builds with and without filesystem support: I received this warning: emcc: warning: linker setting ignored during compilation: 'FILESYSTEM' [-Wunused-command-line-argument] This confirms that -s FILESYSTEM=0 is a linker option rather than a preprocessor definition. During preprocessing, this option is ignored, so there's no specific preprocessor macro that can be used to detect filesystem support. Therefore, using #ifdef EMSCRIPTEN appears to be the most reliable approach, as there's no more specific macro available to detect filesystem support status. The filesystem options are handled at link time rather than compile time in Emscripten. |
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That's unfortunate. Is there some emscripten-native way to query CPU cache properties? |
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After researching, I found that Emscripten doesn't provide a native API to query CPU cache properties, such as cache size, levels, or associativity. This limitation arises because WebAssembly (Wasm), the target output of Emscripten, operates in a sandboxed environment within the browser. This sandboxing restricts access to low-level hardware details, including CPU cache information, for security and portability reasons. Emscripten's public API, as documented in the official reference, does not include functions for retrieving CPU cache properties. Additionally, the Emscripten runtime environment documentation highlights the abstraction from system-specific details, which further limits access to hardware-level information. ([Emscripten][1], [Emscripten][2]) Therefore, if benchmark requires detailed knowledge of CPU cache properties, we may need to consider alternative approaches. |
Thank you!
I don't believe that it //requires// such knowledge, Personally, i think a bug should be filed against Emscripten: it should set a define when in |
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yeah i'm fine with this being merged. it's not ideal, but we're working around a platform and increasing usability. |
3 participants
Problem Description
When Google Benchmark runs in a WebAssembly environment, especially when compiled with Emscripten's -s FILESYSTEM=0 flag (which disables filesystem support), the program aborts when trying to access /sys/devices/system/cpu/cpu0/cache/ to retrieve CPU cache information.
This happens because the current implementation defaults to calling GetCacheSizesFromKVFS() on non-Windows/macOS/QNX/QURT platforms, a function that relies on filesystem access, which is not available in WebAssembly when the filesystem is disabled.
Solution
This PR modifies the GetCacheSizes() function in src/sysinfo.cc to detect Emscripten environments and return an empty cache info vector when running in WebAssembly, avoiding access to a non-existent filesystem.
The change is minimal, simply adding detection for the EMSCRIPTEN macro and merging it with the QURT OS handling, as both return empty cache information.
Related Issue
Fixes #1952