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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 |


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

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

The most common use is for X Windows. 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

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