Go语言中三种不同md5计算方式的性能比较
前言 本文主要介绍的是三种不同的 md5 计算方式,其实区别是读文件的不同,也就是磁盘 I/O,所以也可以举一反三用在网络 I/O 上。下面来一起看看吧。 ReadFile 先看第一种, 简单粗暴: func md5sum1(file string) string { data,err := ioutil.ReadFile(file) if err != nil { return "" } return fmt.Sprintf("%x",md5.Sum(data)) } 之所以说其粗暴,是因为 ReadFile 里面其实调用了一个 readall, 分配内存是最多的。 Benchmark 来一发: var test_path = "/path/to/file" func BenchmarkMd5Sum1(b *testing.B) { for i := 0; i < b.N; i++ { md5sum1(test_path) } } go test -test.run=none -test.bench="^BenchmarkMd5Sum1$" -benchtime=10s -benchmem BenchmarkMd5Sum1-4 300 43704982 ns/op 19408224 B/op 14 allocs/op PASS ok tmp 17.446s 先说明下,这个文件大小是 19405028 字节,和上面的 19408224 B/op 非常接近,因为 readall 确实是分配了文件大小的内存,代码为证: ReadFile 源码 // ReadFile reads the file named by filename and returns the contents. // A successful call returns err == nil,not err == EOF. Because ReadFile // reads the whole file,it does not treat an EOF from Read as an error // to be reported. func ReadFile(filename string) ([]byte,error) { f,err := os.Open(filename) if err != nil { return nil,err } defer f.Close() // It's a good but not certain bet that FileInfo will tell us exactly how much to // read,so let's try it but be prepared for the answer to be wrong. var n int64 if fi,err := f.Stat(); err == nil { // Don't preallocate a huge buffer,just in case. if size := fi.Size(); size < 1e9 { n = size } } // As initial capacity for readAll,use n + a little extra in case Size is zero,// and to avoid another allocation after Read has filled the buffer. The readAll // call will read into its allocated internal buffer cheaply. If the size was // wrong,we'll either waste some space off the end or reallocate as needed,but // in the overwhelmingly common case we'll get it just right. // readAll 第二个参数是即将创建的 buffer 大小 return readAll(f,n+bytes.MinRead) } func readAll(r io.Reader,capacity int64) (b []byte,err error) { // 这个 buffer 的大小就是 file size + bytes.MinRead buf := bytes.NewBuffer(make([]byte,capacity)) // If the buffer overflows,we will get bytes.ErrTooLarge. // Return that as an error. Any other panic remains. defer func() { e := recover() if e == nil { return } if panicErr,ok := e.(error); ok && panicErr == bytes.ErrTooLarge { err = panicErr } else { panic(e) } }() _,err = buf.ReadFrom(r) return buf.Bytes(),err } io.Copy 再看第二种, func md5sum2(file string) string { f,err := os.Open(file) if err != nil { return "" } defer f.Close() h := md5.New() _,err = io.Copy(h,f) if err != nil { return "" } return fmt.Sprintf("%x",h.Sum(nil)) } 第二种的特点是:使用了 func BenchmarkMd5Sum2(b *testing.B) { for i := 0; i < b.N; i++ { md5sum2(test_path) } } $ go test -test.run=none -test.bench="^BenchmarkMd5Sum2$" -benchtime=10s -benchmem BenchmarkMd5Sum2-4 500 37538305 ns/op 33093 B/op 8 allocs/op PASS ok tmp 22.657s 32 * 1024 = 32768,和 上面的 33093 B/op 很接近。 io.Copy + bufio.Reader 然后再看看第三种情况。 这次不仅用了 func md5sum3(file string) string { f,err := os.Open(file) if err != nil { return "" } defer f.Close() r := bufio.NewReader(f) h := md5.New() _,r) if err != nil { return "" } return fmt.Sprintf("%x",h.Sum(nil)) } 看下 Benchmark 的情况: func BenchmarkMd5Sum3(b *testing.B) { for i := 0; i < b.N; i++ { md5sum3(test_path) } } $ go test -test.run=none -test.bench="^BenchmarkMd5Sum3$" -benchtime=10s -benchmem BenchmarkMd5Sum3-4 300 42589812 ns/op 4507 B/op 9 allocs/op PASS ok tmp 16.817s 上面的 4507 B/op 是不是和 4096 很接近? 那为什么 一起看看 io.Copy 相关源码: func Copy(dst Writer,src Reader) (written int64,err error) { return copyBuffer(dst,src,nil) } // copyBuffer is the actual implementation of Copy and CopyBuffer. // if buf is nil,one is allocated. func copyBuffer(dst Writer,src Reader,buf []byte) (written int64,err error) { // If the reader has a WriteTo method,use it to do the copy. // Avoids an allocation and a copy. // hash.Hash 这个 Writer 并没有实现 WriteTo 方法,所以不会走这里 if wt,ok := src.(WriterTo); ok { return wt.WriteTo(dst) } // Similarly,if the writer has a ReadFrom method,use it to do the copy. // 而 bufio.Reader 实现了 ReadFrom 方法,所以,会走这里 if rt,ok := dst.(ReaderFrom); ok { return rt.ReadFrom(src) } if buf == nil { buf = make([]byte,32*1024) } for { nr,er := src.Read(buf) if nr > 0 { nw,ew := dst.Write(buf[0:nr]) if nw > 0 { written += int64(nw) } if ew != nil { err = ew break } if nr != nw { err = ErrShortWrite break } } if er == EOF { break } if er != nil { err = er break } } return written,err } 从上面的源码来看, 用 当然如果你希望 看看是不是这样: // Md5Sum2 用 CopyBufer 重新实现,buf := make([]byte,4096) BenchmarkMd5Sum2-4 500 38484425 ns/op 4409 B/op 8 allocs/op BenchmarkMd5Sum3-4 500 38671090 ns/op 4505 B/op 9 allocs/op 从结果来看, 分配的内存相差不大,毕竟实现不一样,不可能一致。 那下次如果你要写一个下载大文件的程序,你还会用 最后整体对比下 Benchmark 的情况: $ go test -test.run=none -test.bench="." -benchtime=10s -benchmem testing: warning: no tests to run BenchmarkMd5Sum1-4 300 42551920 ns/op 19408230 B/op 14 allocs/op BenchmarkMd5Sum2-4 500 38445352 ns/op 33089 B/op 8 allocs/op BenchmarkMd5Sum3-4 500 38809429 ns/op 4505 B/op 9 allocs/op PASS ok tmp 63.821s 小结 这三种不同的 md5 计算方式在执行时间上都差不多,区别最大的是内存的分配上; bufio 在处理 I/O 还是很有优势的,优先选择; 尽量避免 ReadAll 这种用法。 总结 以上就是这篇文章的全部内容了,希望本文的内容对大家的学习或者工作能带来一定的帮助,如果有疑问大家可以留言交流。 (编辑:李大同) 【声明】本站内容均来自网络,其相关言论仅代表作者个人观点,不代表本站立场。若无意侵犯到您的权利,请及时与联系站长删除相关内容! |