为什么Node.JS中的V8比我的原生C++插件更快？

Question

为什么我的C++插件中的Google V8 JavaScript引擎的工作速度明显慢于Node.JS？

我试着编写一些简单的代码来生成JavaScript中的素数,并通过我的C++插件在V8中运行,直接在Node.JS中运行.

我非常震惊,因为两者都应该使用相同的JavaScript引擎,并且两者都执行相同的代码(时间以毫秒为单位,越少越好):

V8 in Node.JS:  495517
V8 in Node.JS C++ Addon:  623598

这是JavaScript模块的源代码和运行相同JavaScript代码的C++插件的源代码(我认为问题不在interop中,因为测量时间直接在JS中工作):

index.js:

var jsInNodeJsPrimeGeneratorBenchmark = require("./javascript.js");
var jsInNativePrimeGeneratorBenchmark = require("./native");

console.log("V8 in Node.JS: ", jsInNodeJsPrimeGeneratorBenchmark.primeGeneratorBenchmark());
console.log("V8 in Node.JS C++ Addon: ", jsInNativePrimeGeneratorBenchmark.primeGeneratorBenchmark());

javascript.js:

function primeGeneratorBenchmark() {
    var result, primeNumberCounter, i, j, isPrime, start, end;

    i = 3;
    primeNumberCounter = 1;

    start = Date.now();

    while (primeNumberCounter < 100000) {
        isPrime = true;
        for (j = 2; j < i; j++) {
            if (i % j === 0) {
                isPrime = false;
                break;
            }
        }

        if (isPrime) {
            result = i;
            primeNumberCounter++;
        }

        i++;
    }

    end = Date.now();

    return end - start;
}

exports.primeGeneratorBenchmark = primeGeneratorBenchmark;

native.cpp:

#include <node.h>

v8::Handle<v8::Value> primeGeneratorBenchmark(const v8::Arguments &arguments);
void registerModule(v8::Handle<v8::Object> target);

v8::Handle<v8::Value> primeGeneratorBenchmark(const v8::Arguments &arguments) {
    v8::HandleScope handleScope;

    v8::Local<v8::Context> context = arguments.Holder()->CreationContext();

    v8::Context::Scope scope(context);

    const char *sourceStringC =
        "var result, primeNumberCounter, i, j, isPrime, start, end, time;\n"
        "i = 3;\n"
        "primeNumberCounter = 1;\n"
        "start = Date.now();\n"
        "while (primeNumberCounter < 100000) {\n"
        "    isPrime = true;\n"
        "    for (j = 2; j < i; j++) {\n"
        "        if (i % j === 0) {\n"
        "            isPrime = false;\n"
        "            break;\n"
        "        }\n"
        "    }\n"
        "    if (isPrime) {\n"
        "        result = i;\n"
        "        primeNumberCounter++;\n"
        "    }\n"
        "    i++;\n"
        "}\n"
        "end = Date.now();\n"
        "time = end - start;\n";

    v8::Local<v8::String> sourceStringV8 = v8::String::New(sourceStringC);

    v8::Local<v8::Script> script = v8::Script::Compile(sourceStringV8);
    script->Run();

    v8::Local<v8::Value> timeResult = v8::Context::GetCurrent()->Global()->Get(v8::String::New("time"));

    return handleScope.Close(timeResult);
}

void registerModule(v8::Handle<v8::Object> target) {
    target->Set(v8::String::NewSymbol("primeGeneratorBenchmark"), v8::FunctionTemplate::New(primeGeneratorBenchmark)->GetFunction());
}

NODE_MODULE(native, registerModule);

Answer 1

在C++版本中的脚本源声明的所有变量(result,primeNumberCounter,i,j,isPrime,start,结束time)是全球因为脚本的顶层作用域是全局范围.

为了优化编译器,很容易将局部变量分配到机器寄存器(或堆栈中的溢出槽)并跟踪它们的类型.另一方面,使用全局变量需要恒定的内存访问和类型检查,因为V8(当前)不执行寄存器升级优化.

如果将源包装成一个立即调用的函数,则应该消失.