available: Estimation of how much memory is available for starting new applications, without swapping.cache: Memory used by the page cache and slabs (Cached and Slab in /proc/meminfo).buffers: Memory used by kernel buffers (Buffers in /proc/meminfo).shared: Memory used (mostly) by tmpfs (Shmem in /proc/meminfo, available on kernels 2.6.32, displayed as zero if not available).free: Unused memory (MemFree and SwapFree in /proc/meminfo).used: Used memory (calculated as total – free – buffers – cache).total: Total installed memory (MemTotal and SwapTotal in /proc/meminfo).The information is gathered by parsing /proc/meminfo.

The command also displays the total amount of swap memory in the system and buffers and caches used by the Linux kernel. The free command is used to display amount of free and used memory in the system. If you want to make top a bit more memory-friendly, issue the command top -o %MEM, which will cause top to sort all processes by memory used (see figure below).

%MEM - Memory Usage (RES): A task’s currently used share of available physical memory.

In a true SMP environment, if a process is multi-threaded and top is not operating in Threads mode, amounts greater than 100% may be reported.

%CPU - CPU Usage: The task’s share of the elapsed CPU time since the last screen update, expressed as a percentage of total CPU time.

Two of the most important factors displayed by the top command are:

See cpu usage ubuntu