读写文件速度测试的注意事项

测试一组文件读写速度时使用如下dd命令,在多次读取时第二次的速度会明显快于第一次,

这是由于内存中缓存了文件内容

$ dd if=/tmp/1 of=/dev/null bs=1024 count=1024

记录了1024+0 的读入

记录了1024+0 的写出

1048576字节(1.0 MB)已复制，0.0754464 秒，13.9 MB/秒

$ dd if=/tmp/1 of=/dev/null bs=1024 count=1024

记录了1024+0 的读入

记录了1024+0 的写出

1048576字节(1.0 MB)已复制，0.0019523 秒，537 MB/秒

使用如下方式来解决这个问题:

$ sudo sh -c " echo 3 > /proc/sys/vm/drop_caches"

$ dd if=/tmp/1 of=/dev/null bs=1024 count=1024

参考:Documentation/sysctl/vm.txt

drop_caches

Writing to this will cause the kernel to drop clean caches, dentries and

inodes from memory, causing that memory to become free.

To free pagecache:

echo 1 > /proc/sys/vm/drop_caches

To free dentries and inodes:

echo 2 > /proc/sys/vm/drop_caches

To free pagecache, dentries and inodes:

echo 3 > /proc/sys/vm/drop_caches

As this is a non-destructive operation and dirty objects are not freeable, the

user should run `sync' first.