Linux Memory Types
For our purposes there are three types of memory, and one is optional. First is
physical memory, a limited resource where code and data must reside when executed
or referenced. Next is the optional swap file, where modified (dirty) memory can
be saved and later retrieved if too many demands are made on physical memory.
Lastly we have virtual memory, a nearly unlimited resource serving the following
goals:

1. abstraction, free from physical memory addresses/limits
2. isolation, every process in a separate address space
3. sharing, a single mapping can serve multiple needs
4. flexibility, assign a virtual address to a file

Regardless of which of these forms memory may take, all are managed as
pages (typically 4096 bytes) but expressed by default in top as KiB (kibibyte).
The memory discussed under topic `2c. MEMORY Usage' deals with
physical memory and the swap
file for the system as a whole. The memory reviewed in topic `3. FIELDS / Columns
Display' embraces all three memory types, but for individual processes.

For each such process, every memory page is restricted to a single quadrant from
the table below. Both physical memory and virtual memory can include any of the
four, while the swap file only includes #1 through #3. The memory in quadrant #4,
when modified, acts as its own dedicated swap file.

                                     Private | Shared
                                 1           |          2
            Anonymous  . stack               |
                       . malloc()            |
                       . brk()/sbrk()        | . POSIX shm*
                       . mmap(PRIVATE, ANON) | . mmap(SHARED, ANON)
                      -----------------------+----------------------
                       . mmap(PRIVATE, fd)   | . mmap(SHARED, fd)
          File-backed  . pgms/shared libs    |
                                 3           |          4

The following may help in interpreting process level memory values displayed as
scalable columns and discussed under topic `3a. DESCRIPTIONS of Fields'.

%MEM - simply RES divided by total physical memory
CODE - the `pgms' portion of quadrant 3
DATA - the entire quadrant 1 portion of VIRT plus all
explicit mmap file-backed pages of quadrant 3
RES - anything occupying physical memory which, beginning with
Linux-4.5, is the sum of the following three fields:
RSan - quadrant 1 pages, which include any
former quadrant 3 pages if modified
RSfd - quadrant 3 and quadrant 4 pages
RSsh - quadrant 2 pages
RSlk - subset of RES which cannot be swapped out (any quadrant)
SHR - subset of RES (excludes 1, includes all 2 & 4, some 3)
SWAP - potentially any quadrant except 4
USED - simply the sum of RES and SWAP
VIRT - everything in-use and/or reserved (all quadrants)

Note: Even though program images and shared libraries are considered private to a
process, they will be accounted for as shared (SHR) by the kernel.

... later ...
45. VIRT -- Virtual Memory Size (KiB)
The total amount of virtual memory used by the task. It includes all code,
data and shared libraries plus pages that have been swapped out and pages that
have been mapped but not used.

 without any MCM---
top -o VIRT
  shows:
  PID USER      PR  NI    VIRT    RES    SHR S  %CPU  %MEM     TIME+ COMMAND 
 2060 jay       20   0   82.5g 109008  76396 S   0.3   0.9   4:32.17 boincmgr
 2097 jay       20   0   82.0g  40064  33848 S   0.0   0.3   0:00.04 WebKitNetworkPr 
 2096 jay       20   0   81.9g  73772  60600 S   1.0   0.6   5:59.09 WebKitWebProces

top -o USER
  shows:
   PID USER      PR  NI    VIRT    RES    SHR S  %CPU  %MEM     TIME+ COMMAND
 2254 boinc     39  19  835312 732208  27972 R 100.0   6.0 813:08.94 wcgrid_arp1_wrf 
13526 boinc     39  19  147148  82020   6816 R 100.0   0.7 486:52.02 wcgrid_hst1_gro 
13527 boinc     39  19  147252  82032   6816 R 100.3   0.7 486:52.95 wcgrid_hst1_gro 
13530 boinc     39  19  835332 732336  28052 R  99.7   6.0 486:53.96 wcgrid_arp1_wrf
19718 boinc     39  19  385752 318476  48420 R  98.7   2.6 253:02.61 wcgrid_mip1_ros
20993 boinc     39  19  369700 302260  48256 R 100.3   2.5 116:18.61 wcgrid_mip1_ros
23830 boinc     39  19  364512 296860  48044 R  99.7   2.4  49:09.86 wcgrid_mip1_ros 
23940 boinc     30  10 2300288 183680  61364 S   3.7   1.5   1:43.39 hsgamma_FGRPB1G