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.