I try to rearrange the expression to make it more clear:
A = B + C = G + H
G = B - E - F
H = C + E + F
It seems that:
Mem: 1034332 983572 50760 0 149028 530132
shows "buffers" and "cached" as part of "used" memory.
But -/+ buffers/cache: 304412 729920
shows "buffers" and "cached" as part of "free" memory
The -/+ buffers/cache line shows how much memory is used and free from the perspective of the applications.
--- from http://gentoo-wiki.com/FAQ_Linux_Memory_Management#The_difference_between_buffers_and_cache
And I do a test on my box, it shows that when the "free" memory is going to be exhausted,
the kernel will take back all the "buffers" memory, but still left some "cached"memory. Following is the output by "free -m -s 1" when I increasingly created threads to consume the memory:
total used free shared buffers cac
+hed
Mem: 7973 68 7904 0 8
+ 27
-/+ buffers/cache: 33 7939
Swap: 2047 0 2047
total used free shared buffers cac
+hed
Mem: 7973 106 7866 0 8
+ 27
-/+ buffers/cache: 71 7902
Swap: 2047 0 2047
total used free shared buffers cac
+hed
Mem: 7973 112 7860 0 8
+ 27
-/+ buffers/cache: 77 7896
Swap: 2047 0 2047
... ...
total used free shared buffers cac
+hed
Mem: 7973 7750 222 0 8
+ 27
-/+ buffers/cache: 7714 258
Swap: 2047 0 2047
total used free shared buffers cac
+hed
Mem: 7973 7752 220 0 8
+ 27
-/+ buffers/cache: 7716 256
Swap: 2047 0 2047
total used free shared buffers cac
+hed
Mem: 7973 7834 138 0 8
+ 27
-/+ buffers/cache: 7798 174
Swap: 2047 0 2047
total used free shared buffers cac
+hed
Mem: 7973 7932 40 0 7
+ 22
-/+ buffers/cache: 7903 69
Swap: 2047 0 2047
total used free shared buffers cac
+hed
Mem: 7973 7930 42 0 0
+ 11
-/+ buffers/cache: 7917 55
Swap: 2047 2 2045
total used free shared buffers cac
+hed
Mem: 7973 7931 41 0 0
+ 3
-/+ buffers/cache: 7927 45
Swap: 2047 131 1915
total used free shared buffers cac
+hed
Mem: 7973 7925 47 0 0
+ 4
-/+ buffers/cache: 7920 52
Swap: 2047 144 1903
total used free shared buffers cac
+hed
Mem: 7973 7925 47 0 0
+ 4
-/+ buffers/cache: 7920 53
Swap: 2047 144 1903
total used free shared buffers cac
+hed
Mem: 7973 7925 47 0 0
+ 4
-/+ buffers/cache: 7920 53
Swap: 2047 144 1903
