Memory Jargons

/proc/meminfo (free)

$ man proc_meminfo

MemTotal (total)

total usable RAM i.e. total physical RAM minus reserved bits and kernel binary code.

MemAvailable

memory that can bed used if there is demand. Memory could be used for file caches by the kernel and still marked as availabe.

MemFree (free)

the “free” memory per se. Memory that is not used for ANY purpose. This is roughly MemAvailable - Cache - Buffer.

Low/HighFree

amount of free Low/High memory (see notes below)

Low/HighTotal

total amount of Low/High memory

Cached In-memory cache for files read from disk (excl. SwapCached)

Buffers TODO

SwapTotal/Free Amount of total/free swap space

SwapCached Memory that once was swapped out, is swapped back in but still also is in the swap file. (If memory pressure is high, these pages don’t need to be swapped out again because they are already in the swap file. This saves I/O.)

Active / Inactive; also for anon and file Memory recently (/ less) used; ususally not (/ more eligible) to reclaim.

Unevictable

MLocked

• used: memory in use
• free: memory not in use
• shared
• buffers
• cached
• shared / buffers / cached values are included in used

Low and High memory

dogma: with paging enabled, the OS can only access memory via virtual address.

This is rather a legacy term. 32bit kernel can address 2^32 = 4GiB of virtual memory. A typical config is that kernel takes upper 1GiB and user takes lower 3GiB. The kernel can map ~860 MB of physical memory; Physical memory above this line is highmem. Access to highmem requires temporary mapping and is slower.

In contrast, 64bits kernel has a virtual memory space (48 bits = 256 TB) much larger than physical memory, the kernel can map the whole available physical address space to its virtual address space via a fixed mapping e.g.

    VA = PA + offset

/proc/status

$ free -h

      total   used   free  shared  buff/cache  available
Mem:   15Gi  9.7Gi  2.1Gi   2.9Gi       6.8Gi      5.7Gi
Swap:  15Gi  725Mi   15Gi

$ cat /proc/<pid>/ status # only showing memory related

Name:       systemd
VmPeak:    31296 kB
VmSize:    23560 kB
VmLck:         0 kB
VmPin:         0 kB
VmHWM:     20864 kB
VmRSS:     14244 kB
RssAnon:    4948 kB
RssFile:    9296 kB
RssShmem       0 kB
VmData:     4464 kB
VmStk:       132 kB
VmExe:        56 kB
VmLib:     12112 kB
VmPTE:        92 kB
VmSwap:      424 kB
HugetlbPages:  0 kB
Mems_allowed:   [SNIP]
Mems_allowed_list:  0