何在golang中正确解释HeapInuse / HeapIdle / HeapReleased内存统计信息

I want to monitor memory usage of my golang program and clean up some internal cache if system lacks of free memory.

The problem is that HeapAlloc / HeapInuse / HeapReleased stats aren't always add up properly (to my understanding).

I'm looking at free system memory (+ buffers/cache) - the value that is shown as available by free utility:

$ free
              total        used        free      shared  buff/cache   available
Mem:       16123232      409248    15113628         200      600356    15398424
Swap:      73242180       34560    73207620

And also I look at HeapIdle - HeapReleased, that, according to comments in https://godoc.org/runtime#MemStats,

HeapIdle minus HeapReleased estimates the amount of memory
that could be returned to the OS, but is being retained by
the runtime so it can grow the heap without requesting more
memory from the OS.

Now the problem: sometimes Available + HeapInuse + HeapIdle - HeapReleased exceeds total amount of system memory. Usually it happens when HeapIdle is quite high and HeapReleased is neither close to HeapIdle nor to zero:

# Start of test
Available: 15379M, HeapAlloc: 49M, HeapInuse: 51M, HeapIdle: 58M, HeapReleased: 0M

# Work is in progress
# Looks good: 11795 + 3593 = 15388
Available: 11795M, HeapAlloc: 3591M, HeapInuse: 3593M, HeapIdle: 0M, HeapReleased: 0M

# Work has been done
# Looks good: 11745 + 45 + 3602 = 15392
Available: 11745M, HeapAlloc: 42M, HeapInuse: 45M, HeapIdle: 3602M, HeapReleased: 0M

# Golang released some memory to OS
# Looks good: 15224 + 14 + 3632 - 3552 = 15318
Available: 15224M, HeapAlloc: 10M, HeapInuse: 14M, HeapIdle: 3632M, HeapReleased: 3552M

# Some other work started
# Looks SUSPICIOUS: 13995 + 1285 + 2360 - 1769 = 15871
Available: 13995M, HeapAlloc: 1282M, HeapInuse: 1285M, HeapIdle: 2360M, HeapReleased: 1769M

# 5 seconds later
# Looks BAD: 13487 + 994 + 2652 - 398 = 16735 - more than system memory
Available: 13487M, HeapAlloc: 991M, HeapInuse: 994M, HeapIdle: 2652M, HeapReleased: 398M

# This bad situation holds for quite a while, even when work has been done
# Looks BAD: 13488 + 14 + 3631 - 489 = 16644
Available: 13488M, HeapAlloc: 10M, HeapInuse: 14M, HeapIdle: 3631M, HeapReleased: 489M

# It is strange that at this moment HeapIdle - HeapReleased = 3142
# > than 2134M of used memory reported by "free" utility.
$ free
              total        used        free      shared  buff/cache   available
Mem:       16123232     2185696    13337632         200      599904    13621988
Swap:      73242180       34560    73207620

# Still bad when another set of work started
# Looks BAD: 13066 + 2242 + 1403 = 16711
Available: 13066M, HeapAlloc: 2240M, HeapInuse: 2242M, HeapIdle: 1403M, HeapReleased: 0M

# But after 10 seconds it becomes good
# Looks good: 11815 + 2325 + 1320 = 15460
Available: 11815M, HeapAlloc: 2322M, HeapInuse: 2325M, HeapIdle: 1320M, HeapReleased: 0M

I do not understand from where this additional "breathing" 1.3GB (16700 - 15400) of memory comes from. Used swap space remained the same during the whole test.