RAM is a finite resource that all processes, like applications and daemons, want a piece of. There\u2019s only so much of it available. The kernel referees the memory squabbles and allocates the rationed memory out to all the hungry processes. It\u2019s kind of like a mother bird with more open beaks pointed at her than she has the grub for.<\/p>\n
Unused RAM is wasted RAM. Linux uses any spare RAM for things like file buffer space, to keep your computer running at optimum performance. It\u2019s easy to get the impression that your system\u2019s RAM has been consumed by some runaway process or memory leak, but that\u2019s rarely the case.<\/p>\n
It\u2019s usually just the kernel tenaciously doing its job in the background. If there are other demands for the RAM that the kernel has poached for its own devices, it relinquishes the memory instantly, so there\u2019s no harm done.<\/p>\n
If the kernel decides it\u2019s more efficient to start using swap space, it brings that into play, as well. There\u2019s a lot of confusion about the swappiness value in Linux and when the kernel will start using swap. It\u2019s untrue that the swappiness value sets a threshold for RAM usage that triggers swap to be enabled.<\/p>\n
But now, let\u2019s look at the different techniques you can use in the terminal window to see the RAM usage on your Linux computer.<\/p>\n
The free Command<\/h4>\n
The free command gives you a table of the total, used, free, shared, buffer\/cache, and available RAM on your computer. It also shows you the total amount of swap space configured.<\/p>\n
In our example, we\u2019ll use the -m (mebibytes) option. However, you could also use -b (bytes), -k (kibibytes), or -g (gibibytes).<\/p>\n
We type the following command:<\/p>\n
free -m<\/code><\/pre> <\/div>\n![\"\"](\"https:\/\/www.wikitechy.com\/technology\/wp-content\/uploads\/2020\/03\/x1-11.png.pagespeed.gpjpjwpjwsjsrjrprwricpmd.ic_.iwEydS9tJM.png\")
<\/p>\n
This is the output we get:<\/p>\n
<\/div> total used free shared buff\/cache availableThe\u00a0Mem\u00a0columns contain the following information:<\/p>\n
\n- Total<\/strong>: The total amount of physical RAM on this computer.<\/li>\n
- Used<\/strong>: The sum of Free+Buffers+Cache subtracted from the total amount.<\/li>\n
- Free<\/strong>: The amount of unused memory.<\/li>\n
- Shared<\/strong>: Amount of memory used by the\u00a0tmpfs\u00a0file systems.<\/li>\n
- Buff\/cache<\/strong>: Amount of memory used for buffers and cache. This can be released quickly by the kernel if required.<\/li>\n
- Available<\/strong>: This is an estimate of the memory that\u2019s available to service memory requests from applications and any other operational software on your computer.<\/li>\n<\/ul>\n
The\u00a0Swap\u00a0columns contain the following information:<\/p>\n
\n- Total<\/strong>: The size of the swap partition or file.<\/li>\n
- Used<\/strong>: The amount of swap space that\u2019s in use.<\/li>\n
- Free<\/strong>: The amount of remaining (unused) swap space.<\/li>\n<\/ul>\n <\/div>
free -m | grep Swap | awk '{print ($3\/$2)*100}'\t<\/code><\/pre> <\/div>\n![\"\"](\"https:\/\/www.wikitechy.com\/technology\/wp-content\/uploads\/2020\/03\/2-9.png\")
<\/p>\n
It\u2019s impossible to have a good understanding of the way RAM is used in your Linux box without an appreciation of the state of your swap space. RAM and swap space work closely together.<\/p>\n
You can use the\u00a0vmstat\u00a0command\u00a0to take a deeper dive into how your swap space (or virtual memory) is being used. It gives you a report on a variety of\u00a0\u00a0based on the average values since the last reboot.<\/p>\n
Type the following:<\/p>\n
<\/div> vmstat<\/code><\/pre> <\/div>\n![\"\"](\"https:\/\/www.wikitechy.com\/technology\/wp-content\/uploads\/2020\/03\/x3-13.png.pagespeed.gpjpjwpjwsjsrjrprwricpmd.ic_.PdNcb0XWqJ.png\")
<\/p>\n
This is the output without the wrap-around:<\/p>\n
<\/div> procs -----------memory---------- ---swap-- -----io---- -system-- ------cpu-----There are a lot of data points in that report, so we\u2019ll break them down:<\/p>\n
\n- Proc:<\/strong><\/li>\n<\/ul>\n
r<\/strong>: The number of \u201crunnable\u201d processes. They\u2019re either running or waiting for their next time-sliced burst of CPU cycles.<\/p>\nb<\/strong>: The number of processes in uninterruptible sleep. These aren\u2019t sleeping, but performing a blocking system call. They can\u2019t be interrupted until they complete their current action. Typically, this process is a device driver waiting for some resource to be free. Any queued interrupts for that process are handled when the process resumes its usual activity.<\/p>\n\n- Memory:<\/strong>\n
\n- swpd<\/strong>: The amount of virtual memory used, i.e., how much memory has been swapped out.<\/li>\n
- free<\/strong>: The amount of idle (unused) memory.<\/li>\n
- buff<\/strong>: The amount of memory used as buffers.<\/li>\n
- cache<\/strong>: The amount of memory used as cache.<\/li>\n<\/ul>\n<\/li>\n
- Swap<\/strong>\u00a0 \u00a0 \u00a0 \u00a0<\/strong><\/li>\n
- si<\/strong>: The amount of virtual memory swapped in from swap space.<\/li>\n
- so<\/strong>: The amount of virtual memory swapped out to swap space.<\/li>\n
- IO:<\/strong><\/li>\n
- \n
\n- bi<\/strong>: Blocks in. The number of data blocks used to swap virtual memory back to RAM.<\/li>\n
- bo<\/strong>: Blocks out. The number of data blocks used to swap virtual memory out of RAM and into swap space.<\/li>\n<\/ul>\n<\/li>\n
- System:<\/strong>\n
\n- in<\/strong>: The number of interrupts per second, including the clock.<\/li>\n
- cs<\/strong>: The number of context switches per second. A context switch is when the kernel swaps from system to user mode processing.<\/li>\n<\/ul>\n<\/li>\n
- CPU:\u00a0<\/strong>These values are all percentages of the total CPU time:\n
\n- us<\/strong>: Time spent running user (non-kernel) code.<\/li>\n
- sy<\/strong>: Time spent running kernel code.<\/li>\n
- id<\/strong>: Time spent idle.<\/li>\n
- wa<\/strong>: Time spent waiting for in- or output.<\/li>\n
- st<\/strong>: The time a virtual machine has to wait for the hypervisor to finish servicing other virtual machines before it can come back and attend to this virtual machine.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n
The top Command<\/strong><\/p>\nThe\u00a0top\u00a0command\u00a0displays a screen packed with information.\u00a0The values are updated every few seconds.<\/p>\n
To use it, we type the following:<\/p>\n
<\/div> top<\/code><\/pre> <\/div>\n![\"\"](\"https:\/\/www.wikitechy.com\/technology\/wp-content\/uploads\/2020\/03\/x5-7.png.pagespeed.gpjpjwpjwsjsrjrprwricpmd.ic_.jHWr3jM1OR.png\")
<\/p>\n
The \u201ce\u201d key was pressed. This changed the display to megabytes, which are easier to visually parse than long strings representing bytes.\u00a0There are five lines of information at the top of the screen and a lower pane with columns of data.<\/p>\n
\n- Line one:<\/strong>The time, how long the computer has been running, how many people are logged in, and what the load average has been over the past one, five, and 15 minutes.<\/li>\n
- Line two:<\/strong>The number of tasks and their states: running, stopped, sleeping, or zombie.<\/li>\n
- Line three:<\/strong>CPU information (see the breakdown of the fields below).<\/li>\n
- Line four:\u00a0<\/strong>The total amount of physical memory, and how much is free, used, buffered, or cached.<\/li>\n
- Line five:<\/strong>Total amount of swap memory, and how much is free, used and available (taking into account memory that\u2019s expected to be recoverable from caches).<\/li>\n<\/ul>\n
The CPU fields on line three are as follows:<\/p>\n
\n- us:<\/strong>Time the CPU spends executing processes for users in user space.<\/li>\n
- sy:<\/strong>Time the CPU spent running system \u201ckernel space\u201d processes.<\/li>\n
- ni:<\/strong>Time the CPU spent executing processes with a manually set nice value.<\/li>\n
- id:<\/strong>CPU idle time.<\/li>\n
- wa:<\/strong>Time the CPU spends waiting for I\/O to complete.<\/li>\n
- hi:<\/strong>Time the CPU spent servicing hardware interrupts.<\/li>\n
- si:<\/strong>Time the CPU spent servicing software interrupts.<\/li>\n
- st (steal time):<\/strong>Time the CPU lost due to running virtual machines.<\/li>\n<\/ul>\n
You might have to press the left or right arrow keys to see all the columns. The values in each column are described below:<\/p>\n
\n- PID:<\/strong>Process ID.<\/li>\n
- USER:<\/strong>Name of the owner of the process.<\/li>\n
- PR:<\/strong>Process priority.<\/li>\n
- NI:<\/strong>The nice value of the process.<\/li>\n
- VIRT:<\/strong>Virtual memory used by the process.<\/li>\n
- RES:<\/strong>Resident memory used by the process.<\/li>\n
- SHR:<\/strong>Shared memory used by the process.<\/li>\n
- S:<\/strong>Status of the process. (See the list of values this field can take below).<\/li>\n
- %CPU:<\/strong>The share of CPU time used by the process since the last update.<\/li>\n
- %MEM:<\/strong>The share of physical memory used.<\/li>\n
- TIME+:<\/strong>Total CPU time used by the task in 100ths of a second.<\/li>\n
- COMMAND:<\/strong>The Command name or line (name + options). (This column is offscreen to the right in the image above.)<\/li>\n<\/ul>\n
The status displayed in the\u00a0S\u00a0column can be one of the following:<\/p>\n
\n- D:<\/strong>Uninterruptible sleep.<\/li>\n
- R:<\/strong><\/li>\n
- S:<\/strong><\/li>\n
- T:<\/strong>Traced (stopped).<\/li>\n
- Z:<\/strong><\/li>\n<\/ul>\n
Press Q to exit\u00a0top.<\/p>\n
Reading \/proc\/meminfo<\/h2>\n <\/div> less \/proc\/meminfo<\/code><\/pre> <\/div>\n![\"\"](\"https:\/\/www.wikitechy.com\/technology\/wp-content\/uploads\/2020\/03\/x6-8.png.pagespeed.gpjpjwpjwsjsrjrprwricpmd.ic_.Kox0D9QWMo.png\")
<\/p>\n