Effective system administration requires keeping a close eye on resource usage, and memory monitoring is a critical part of this process. When a system runs low on memory, its performance can degrade significantly. Linux provides several tools to help you track memory consumption, and one of the most versatile is vmstat.
Introduction to vmstat
The vmstat (virtual memory statistics) command is a powerful memory utilization monitor that reports information about processes, memory, paging, block I/O, traps, and CPU activity. Running it without any arguments provides a snapshot of the system's current state since the last boot.
pete@icebox:~$ vmstat
procs -----------memory---------- ---swap-- -----io---- -system-- ------cpu-----
r b swpd free buff cache si so bi bo in cs us sy id wa st
1 0 0 396528 38816 384036 0 0 4 2 38 79 0 0 99 0 0The output is organized into several columns. Let's break down what each field means.
Procs
r: The number of runnable processes waiting for run time.b: The number of processes in uninterruptible sleep, typically waiting for I/O.
Memory
swpd: The amount of virtual memory used (in kilobytes).free: The amount of idle memory (in kilobytes).buff: The amount of memory used as buffers.cache: The amount of memory used as a page cache.
Swap
si: The amount of memory swapped in from disk per second (in kilobytes). High values indicate the system is low on physical memory.so: The amount of memory swapped out to disk per second (in kilobytes). This should ideally be zero.
IO
bi: Blocks received from a block device (blocks/s).bo: Blocks sent to a block device (blocks/s).
System
in: The number of interrupts per second, including the clock.cs: The number of context switches per second.
CPU
These are percentages of total CPU time.
us: Time spent running non-kernel code (user time).sy: Time spent running kernel code (system time).id: Time spent idle.wa: Time spent waiting for I/O.st: Time stolen from a virtual machine (for virtualized environments).