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perf_tune.md

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Tune the performance of running wasm/aot file

Normally there are some methods to tune the performance:

1. Use wasm-opt tool

Download the binaryen release, and use the wasm-opt tool in it to optimize the wasm file, for example:

wasm-opt -O4 -o test_opt.wasm test.wasm

2. Enable simd128 option when compiling wasm source files

WebAssembly 128-bit SIMD is supported by WAMR on x86-64 and aarch64 targets, enabling it when compiling wasm source files may greatly improve the performance. For wasi-sdk and emsdk, please add -msimd128 flag for clang and emcc/em++:

/opt/wasi-sdk/bin/clang -msimd128 -O3 -o <wasm_file> <c/c++ source files>

emcc -msimd128 -O3 -o <wasm_file> <c/c++ source files>

3. Enable segue optimization for wamrc when generating the aot file

Segue is an optimization technology which uses x86 segment register to store the WebAssembly linear memory base address, so as to remove most of the cost of SFI (Software-based Fault Isolation) base addition and free up a general purpose register, by this way it may:

  • Improve the performance of JIT/AOT
  • Reduce the footprint of JIT/AOT, the JIT/AOT code generated is smaller
  • Reduce the compilation time of JIT/AOT

Currently it is supported on linux x86-64, developer can use --enable-segue=[<flags>] for wamrc:

wamrc --enable-segue -o aot_file wasm_file
# or
wamrc --enable-segue=[<flags>] -o aot_file wasm_file

flags can be: i32.load, i64.load, f32.load, f64.load, v128.load, i32.store, i64.store, f32.store, f64.store and v128.store, use comma to separate them, e.g. --enable-segue=i32.load,i64.store, and --enable-segue means all flags are added.

Note: Normally for most cases, using --enable-segue is enough, but for some cases, using --enable-segue=<flags> may be better, for example for CoreMark benchmark, --enable-segue=i32.store may lead to better performance than --enable-segue.

4. Enable segue optimization for iwasm when running wasm file

Similar to segue optimization for wamrc, run:

iwasm --enable-segue wasm_file      (iwasm is built with llvm-jit enabled)
# or
iwasm --enable-segue=[<flags>] wasm_file

5. Use the AOT static PGO method

LLVM PGO (Profile-Guided Optimization) allows the compiler to better optimize code for how it actually runs. WAMR supports AOT static PGO, currently it is tested on Linux x86-64 and x86-32. The basic steps are:

  1. Use wamrc --enable-llvm-pgo -o <aot_file_of_pgo> <wasm_file> to generate an instrumented aot file.

  2. Compile iwasm with cmake -DWAMR_BUILD_STATIC_PGO=1 and run iwasm --gen-prof-file=<raw_profile_file> <aot_file_of_pgo> to generate the raw profile file.

Note: Directly dumping raw profile data to file system may be unsupported in some environments, developer can dump the profile data into memory buffer instead and try outputting it through network (e.g. uart or socket):

uint32_t
wasm_runtime_get_pgo_prof_data_size(wasm_module_inst_t module_inst);

uint32_t
wasm_runtime_dump_pgo_prof_data_to_buf(wasm_module_inst_t module_inst, char *buf, uint32_t len);

  1. Install or compile llvm-profdata tool，refer to here for the details.

  2. Run llvm-profdata merge -output=<profile_file> <raw_profile_file> to merge the raw profile file into the profile file.

  3. Run wamrc --use-prof-file=<profile_file> -o <aot_file> <wasm_file> to generate the optimized aot file.

  4. Run the optimized aot_file: iwasm <aot_file>.

Developer can refer to the test_pgo.sh files under each benchmark folder for more details, e.g. test_pgo.sh of CoreMark benchmark.

6. Disable the memory boundary check

Please notice that this method is not a general solution since it may lead to security issues. And only boost the performance for some platforms in AOT mode and don't support hardware trap for memory boundary check.

  1. Build WAMR with -DWAMR_CONFIGUABLE_BOUNDS_CHECKS=1 option.

  2. Compile AOT module by wamrc with --bounds-check=0 option.

  3. Run the AOT module by iwasm with --disable-bounds-checks option.

Note: The size of AOT file will be much smaller than the default, and some tricks are possible such as let the wasm application access the memory of host os directly. Please notice that if this option is enabled, the wasm spec test will fail since it requires the memory boundary check. For example, the runtime will crash when accessing the memory out of the boundary in some cases instead of throwing an exception as the spec requires.

You should only use this method for well tested wasm applications and make sure the memory access is safe.