Performance Issue in the fd_write Implementation

[hungryzzz]

Summary

Hi, I run the following case in different Wasm runtimes(after being compiled by Emscripten), and I find some performance differences between wasmer and other 3 runtimes: the execution time(collected by perf-tool, probe begins when starting to execute the wasm code(inner_module_run in wasmer) and end in sched:sched_process_exit) in wasmer is 4x slower than which in wasmtime.

• wasmer: 136486.78 us
• wasmtime: 30420.03 us
• wasmedge(AOT): 23816.45 us
• wamr(AOT): 20412.60 us

#include <stdio.h>
#include <sys/time.h>

typedef struct timeval timeval;
timeval tv;

static void repeat(int count) {
  int len = 50;
  do {
    gettimeofday(&tv, NULL);
    count -= len;
    printf("%d\n", tv.tv_usec);
  } while (count >= 0);
}

int main() {
  repeat(500000);
  return 0;
}

Hardware & OS

• Ubuntu 20.04
• CPU: Intel(R) Core(TM) i5-9500T CPU @ 2.20GHz
• Memory: 32GB

Emscripten

• emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 3.1.24 (68a9f990429e0bcfb63b1cde68bad792554350a5) clang version 16.0.0 (https://github.com/llvm/llvm-project 277c382760bf9575cfa2eac73d5ad1db91466d3f) Target: wasm32-unknown-emscripten Thread model: posix

Wasm runtime version

• wasmer: wasmer 3.2.0-alpha.1
• wasmtime: wasmtime-cli 8.0.0
• wasmedge: build from commit 381b7b28049b968297e6a585b92d1cba955def66
• wamr: iwasm 1.1.2

Additional details

I find that if I comment the IO(printf) in source code, the execution time of wasmer will be faster than wasmtime, so I try to replace the fd_write function to an empty function in the wasm binary just as following.

The execution times are totally changed. Because there is an another wasi call (clock_time_get) in this loop, I think the bug doesn't exist in the mechanism of calling a wasi call.

• wasmer: 2792.70 us
• wasmtime: 4796.61 us
• wasmedge: 2508.98 us
• wamr: 1940.16 us

Then, I try to comment the following code(change the written size to a fix value according to my case) and rebuild wasmer, the execution time of the original case is 8675.32 us. https://github.com/wasmerio/wasmer/blob/8a343d0f9a1d3d4b4bfe622ed345253d780a3d76/lib/wasi/src/syscalls/wasi/fd_write.rs#L132-L147

Therefore, I think maybe there are some performance bugs in the implementation of fd_write, especially in the above code.