How is available memory counted in top utility in ubuntu 16.04?

Question

ubuntu

Top output is like this
top - 11:52:26 up 2 days, 17:32, 1 user, load average: 0.06, 0.07, 0.02
Tasks: 179 total, 1 running, 178 sleeping, 0 stopped, 0 zombie
%Cpu(s ): 4.2 us, 0.3 sy, 0.0 ni, 95.5 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
KiB Mem : 4045040 total, 2911620 free, 148104 used, 985316 buff/cache
KiB 589364 used. 3768512 avail Mem
avail Mem is not equal to free memory, how is it considered?

Answer 1

$ man top

The avail number on line 2 is an estimation of physical memory available for starting new applications, without swapping. Unlike the free field, it attempts to account for readily reclaimable page cache and memory slabs. It is available on kernels 3.14, emulated on kernels 2.6.27+, otherwise the same as free.