Memory
free
Parses the /proc/meminfo file and shows total available memory:
# shared - tmpfs to maintain pseudo fs like /sys and /dev
# buff/cache - kernel block I/O
free -h
total used free shared buff/cache available
Mem: 7.8Gi 631Mi 7.0Gi 3.8Mi 373Mi 7.1Gi
Swap: 0B 0B 0B
# MiB with high and low stats
free -lm
total used free shared buff/cache available
Mem: 31704 6752 19689 1023 5263 23472
Low: 31704 12015 19689
High: 0 0 0
Swap: 65535 0 65535
vmstat
Checks swap usage. You can run it over an extended period of time. This gives you 4 readings at 30-second intervals:
# si - swapped into memory
# so - swapped out of memory
vmstat 30 4
procs -----------memory---------- ---swap-- -----io---- -system-- ------cpu-----
r b swpd free buff cache si so bi bo in cs us sy id wa st
0 0 0 4996868 1537492 18966428 0 0 18 61 23 1 4 2 95 0 0
0 0 0 5006424 1537504 18956112 0 0 0 61 6112 7490 4 4 92 0 0
Swap
From All about linux swap space
Linux divides its physical RAM (random access memory) into chunks of memory called pages. Swapping is the process whereby a page of memory is copied to the preconfigured space on the hard disk, called swap space, to free up that page of memory. The combined sizes of the physical memory and the swap space is the amount of virtual memory available.
Virtual memory is RAM (physical memory) and swap space.
- Need swap space when there isn’t enough available physical memory on your machine for all the active processes
- Inactive pages from physical memory are moved into swap to free up physical memory
- Swap space can be a partition, file, or combo of both
- Swap files do not use contiguous disk blocks, so may have performance impact
- If you use swap, make it equal to your physical memory
# check for swap partition (no swap here)
sudo parted -l
Model: KXG60ZNV512G NVMe KIOXIA 512GB (nvme)
Disk /dev/nvme0n1: 512GB
Sector size (logical/physical): 512B/512B
Partition Table: gpt
Disk Flags:
Number Start End Size File system Name Flags
1 1049kB 829MB 828MB fat32 EFI system partition boot, esp
2 829MB 6198MB 5369MB fat32 Basic data partition msftres
3 6198MB 512GB 506GB ext4
# see swap
cat /proc/swaps
Filename Type Size Used Priority
/swapfile file 67108860 0 -2
# in fstab
cat /etc/fstab
# /etc/fstab: static file system information.
...
/swapfile none swap sw 0 0
Swappiness
# 0 - 100. 0 is avoid swapping, 100 is aggressive swappiness
cat /proc/sys/vm/swappiness
60
OOM Killer
Many applications request all their memory up-front and often do not use it. The kernel can over-commit memory for efficiency purposes, so it can provide all the up-front memory. If the applications actually start using all the committed memory, the kernel kills processes to continue running. The out-of-memory (OOM) killer is the mechanism that kills the memory.
High and low memory
- Low memory
- Has a physical address that the kernel can access directly. ON 64-bit systems, all memory is low memory.
- High memory
- No physical address, not really needed in 64-bit systems because these systems don’t need virtual address space.
Use vmstat to monitor free memory over time:
vmstat -SM 5 10
procs -----------memory---------- ---swap-- -----io---- -system-- ------cpu-----
r b swpd free buff cache si so bi bo in cs us sy id wa st
0 0 0 20071 387 4786 0 0 48 53 359 191 5 2 93 0 0
1 0 0 20025 387 4831 0 0 0 9 3245 6709 3 1 96 0 0
2 0 0 19772 388 4915 0 0 184 372 6656 15366 12 3 84 0 0
2 0 0 19708 388 4928 0 0 10 861 8399 27773 19 4 77 0 0
0 0 0 19619 388 4939 0 0 0 558 6890 19543 12 3 84 0 0
0 0 0 19683 388 4894 0 0 0 346 2827 5450 3 1 95 0 0
0 0 0 19680 388 4894 0 0 0 2437 2381 4520 3 1 95 0 0
2 0 0 19725 388 4908 0 0 0 2138 3847 7798 5 2 93 0 0
0 0 0 19709 388 4923 0 0 0 305 3389 7036 4 3 93 0 0
1 0 0 19862 388 4879 0 0 0 13 3277 5510 4 1 95 0 0
# output to file
vmstat -SM 5 10 > memoryusage.out &