释放Linux系统cache，增加可用内存空间

To free pagecache:

echo 1 > /proc/sys/vm/drop_caches    或sudo sh -c "echo 1 > /proc/sys/vm/drop_caches"

To free reclaimable slab objects (includes dentries and inodes):

echo 2 > /proc/sys/vm/drop_caches   或sudo sh -c "echo 2 > /proc/sys/vm/drop_caches"

To free slab objects and pagecache:

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