查看系统memory需要注意

今天查看服务器的内存遇到一个问题，/proc/meminfo里显示是3.9G，但free -g只显示3G，free -m显示3.8G

[root@cdn nginx]# more /proc/meminfo
MemTotal:        3912240 kB
MemFree:          126712 kB
Buffers:           10092 kB
Cached:           255140 kB
SwapCached:       157752 kB
Active:          2714384 kB
Inactive:         922556 kB
Active(anon):    2554248 kB
Inactive(anon):   817792 kB
Active(file):     160136 kB
Inactive(file):   104764 kB
Unevictable:           0 kB
Mlocked:               0 kB
SwapTotal:       6143992 kB
SwapFree:        5241636 kB
Dirty:             16852 kB
Writeback:             0 kB
AnonPages:       3361416 kB
Mapped:            12096 kB
Shmem:               456 kB
Slab:              64128 kB
SReclaimable:      27804 kB
SUnreclaim:        36324 kB
KernelStack:        2592 kB
PageTables:        12804 kB
NFS_Unstable:          0 kB
Bounce:                0 kB
WritebackTmp:          0 kB
CommitLimit:     8100112 kB
Committed_AS:    3107612 kB
VmallocTotal:   34359738367 kB
VmallocUsed:      283160 kB
VmallocChunk:   34359433744 kB
HardwareCorrupted:     0 kB
AnonHugePages:   1994752 kB
HugePages_Total:       0
HugePages_Free:        0
HugePages_Rsvd:        0
HugePages_Surp:        0
Hugepagesize:       2048 kB
DirectMap4k:        6144 kB
DirectMap2M:     4179968 kB
[root@cdn nginx]# free -m
             total       used       free     shared    buffers     cached
Mem:          3820       3702        118          0          9        254
-/+ buffers/cache:       3437        382
Swap:         5999        881       5118
[root@cdn nginx]# free -g
             total       used       free     shared    buffers     cached
Mem:             3          3          0          0          0          0
-/+ buffers/cache:          3          0
Swap:            5          0          4

free -g的时候，为啥不能四舍五入呢？