What are "shrss" / "shswp" in atop?

Question

From atop manual (official):

MEM - Memory occupation.

This line contains the total amount of physical memory (tot), [...] the resident size of shared memory including tmpfs (shmem), the resident size of shared memory (shrss) the amount of shared memory that is currently swapped (shswp) [...]

The description of shrss / shswp is too vague for me. What does it actually cover?

MEM | tot     7.7G  | free    3.6G |  cache   2.1G |  dirty   0.3M | buff  258.1M  | slab  306.4M |  slrec 182.7M | shmem 257.5M  | shrss  12.7M |  shswp   0.0M |  vmbal   0.0M |               | hptot   0.0M |  hpuse   0.0M |

# Or

tot     7.7G  
free    3.6G 
cache   2.1G 
dirty   0.3M 
buff  258.1M  
slab  306.4M 
slrec 182.7M 
shmem 257.5M  
shrss  12.7M 
shswp   0.0M 
vmbal   0.0M 

hptot   0.0M 
hpuse   0.0M |

---

$ rpm -q atop
atop-2.4.0-1.fc30.x86_64

Answer 1

"System V shared memory" only. This is obsolescent.

The most common use is for X Window. Although some X programs could use POSIX shared memory instead.

---

shrss and shswp are obtained (in units of pages) by the following code in atop/photosyst.c:

/*
** get information about the shared memory statistics
*/
if ( shmctl(0, SHM_INFO, (struct shmid_ds *)&shminfo) != -1)
{
    si->mem.shmrss = shminfo.shm_rss;
    si->mem.shmswp = shminfo.shm_swp;
}

man shmctl says:

shmctl - System V shared memory control