command to check the remaining free memory

. 1  Free - m
 2 # - / + buffers / Cache:        6458        1649 
. 3 # 6458M is true for the use of real memory 1649M remaining memory (cache + + remaining memory buffer)
 . 4 #linux will use all of the remaining memory as a cache, so ensure the linux operating speed, we need to ensure that the memory cache size