Table of Contents
This chapter will tell you how to manage RAM memory and cache.
We start with some simple tools to display information about memory: free -om, top and cat /proc/meminfo.
We continue with managing swap space, using terms like swapping, paging and virtual memory.
The last part is about using vmstat to monitor swap usage.
Displaying /proc/meminfo will tell you a lot about the memory on your Linux computer.
paul@ubu1010:~$ cat /proc/meminfo MemTotal: 3830176 kB MemFree: 244060 kB Buffers: 41020 kB Cached: 2035292 kB SwapCached: 9892 kB ...
The first line contains the total amount of physical RAM, the second line is the unused RAM. Buffers is RAM used for buffering files, cached is the amount of RAM used as cache and SwapCached is the amount of swap used as cache. The file gives us much more information outside of the scope of this course.
The free tool can display the information provided by /proc/meminfo in a more readable format. The example below displays brief memory information in megabytes.
paul@ubu1010:~$ free -om
total used free shared buffers cached
Mem: 3740 3519 221 0 42 1994
Swap: 6234 82 6152The top tool is often used to look at processes consuming most of the cpu, but it also displays memory information on line four and five (which can be toggled by pressing m).
Below a screenshot of top on the same ubu1010 from above.
top - 10:44:34 up 16 days, 9:56, 6 users, load average: 0.13, 0.09, 0.12 Tasks: 166 total, 1 running, 165 sleeping, 0 stopped, 0 zombie Cpu(s): 5.1%us, 4.6%sy, 0.6%ni, 88.7%id, 0.8%wa, 0.0%hi, 0.3%si, 0.0%st Mem: 3830176k total, 3613720k used, 216456k free, 45452k buffers Swap: 6384636k total, 84988k used, 6299648k free, 2050948k cached
When the operating system needs more memory than physically present in RAM, it can use swap space. Swap space is located on slower but cheaper memory. Notice that, although hard disks are commonly used for swap space, their access times are one hundred thousand times slower.
The swap space can be a file, a partition, or a combination of files and partitions. You can see the swap space with the free command, or with cat /proc/swaps.
paul@ubu1010:~$ free -o | grep -v Mem
total used free shared buffers cached
Swap: 6384636 84988 6299648
paul@ubu1010:~$ cat /proc/swaps
Filename Type Size Used Priority
/dev/sda3 partition 6384636 84988 -1The amount of swap space that you need depends heavily on the services that the computer provides.
You can activate or deactivate swap space with the swapon and swapoff commands. New swap space can be created with the mkswap command. The screenshot below shows the creation and activation of a swap partition.
root@RHELv4u4:~# fdisk -l 2> /dev/null | grep hda Disk /dev/hda: 536 MB, 536870912 bytes /dev/hda1 1 1040 524128+ 83 Linux root@RHELv4u4:~# mkswap /dev/hda1 Setting up swapspace version 1, size = 536702 kB root@RHELv4u4:~# swapon /dev/hda1
Now you can see that /proc/swaps displays all swap spaces separately, whereas the free -om command only makes a human readable summary.
root@RHELv4u4:~# cat /proc/swaps
Filename Type Size Used Priority
/dev/mapper/VolGroup00-LogVol01 partition 1048568 0 -1
/dev/hda1 partition 524120 0 -2
root@RHELv4u4:~# free -om
total used free shared buffers cached
Mem: 249 245 4 0 125 54
Swap: 1535 0 1535Here is one more example showing you how to create a swap file. On Solaris you can use mkfile instead of dd.
root@RHELv4u4:~# dd if=/dev/zero of=/smallswapfile bs=1024 count=4096 4096+0 records in 4096+0 records out root@RHELv4u4:~# mkswap /smallswapfile Setting up swapspace version 1, size = 4190 kB root@RHELv4u4:~# swapon /smallswapfile root@RHELv4u4:~# cat /proc/swaps Filename Type Size Used Priority /dev/mapper/VolGroup00-LogVol01 partition 1048568 0 -1 /dev/hda1 partition 524120 0 -2 /smallswapfile file 4088 0 -3
You can find information about swap usage using vmstat.
Below a simple vmstat displaying information in megabytes.
paul@ubu1010:~$ vmstat -S m procs ---------memory-------- ---swap-- -----io---- -system- ----cpu---- r b swpd free buff cache si so bi bo in cs us sy id wa 0 0 87 225 46 2097 0 0 2 5 14 8 6 5 89 1
Below a sample vmstat when (in another terminal) root launches a find /. It generates a lot of disk i/o (bi and bo are disk blocks in and out). There is no need for swapping here.
paul@ubu1010:~$ vmstat 2 100 procs ----------memory---------- ---swap-- -----io---- -system-- ----cpu---- r b swpd free buff cache si so bi bo in cs us sy id wa 0 0 84984 1999436 53416 269536 0 0 2 5 2 10 6 5 89 1 0 0 84984 1999428 53416 269564 0 0 0 0 1713 2748 4 4 92 0 0 0 84984 1999552 53416 269564 0 0 0 0 1672 1838 4 6 90 0 0 0 84984 1999552 53424 269560 0 0 0 14 1587 2526 5 7 87 2 0 0 84984 1999180 53424 269580 0 0 0 100 1748 2193 4 6 91 0 1 0 84984 1997800 54508 269760 0 0 610 0 1836 3890 17 10 68 4 1 0 84984 1994620 55040 269748 0 0 250 168 1724 4365 19 17 56 9 0 1 84984 1978508 55292 269704 0 0 126 0 1957 2897 19 18 58 4 0 0 84984 1974608 58964 269784 0 0 1826 478 2605 4355 7 7 44 41 0 2 84984 1971260 62268 269728 0 0 1634 756 2257 3865 7 7 47 39
Below a sample vmstat when executing (on RHEL6) a simple memory leaking program. Now you see a lot of memory being swapped (si is 'swapped in').
[paul@rhel6c ~]$ vmstat 2 100 procs ----------memory-------- ---swap-- ----io---- --system-- -----cpu----- r b swpd free buff cache si so bi bo in cs us sy id wa st 0 3 245208 5280 232 1916 261 0 0 42 27 21 0 1 98 1 0 0 2 263372 4800 72 908 143840 128 0 1138 462 191 2 10 0 88 0 1 3 350672 4792 56 992 169280 256 0 1092 360 142 1 13 0 86 0 1 4 449584 4788 56 1024 95880 64 0 606 471 191 2 13 0 85 0 0 4 471968 4828 56 1140 44832 80 0 390 235 90 2 12 0 87 0 3 5 505960 4764 56 1136 68008 16 0 538 286 109 1 12 0 87 0
The code below was used to simulate a memory leak (and force swapping). This code was found on wikipedia without author.
paul@mac:~$ cat memleak.c
#include <stdlib.h>
int main(void)
{
while (malloc(50));
return 0;
}1. Use dmesg to find the total amount of memory in your computer.
2. Use free to display memory usage in kilobytes (then in megabytes).
3. On a virtual machine, create a swap partition (you might need an extra virtual disk for this).
4. Add a 20 megabyte swap file to the system.
5. Put all swap spaces in /etc/fstab and activate them. Test with a reboot that they are mounted.
6. Use free to verify usage of current swap.
7. (optional) Display the usage of swap with vmstat and free -s during a memory leak.
1. Use dmesg to find the total amount of memory in your computer.
dmesg | grep Memory
2. Use free to display memory usage in kilobytes (then in megabytes).
free ; free -m
3. On a virtual machine, create a swap partition (you might need an extra virtual disk for this).
mkswap /dev/sdd1 ; swapon /dev/sdd1
4. Add a 20 megabyte swap file to the system.
dd if=/dev/zero of=/swapfile20mb bs=1024 count=20000 mkswap /swapfile20mb swapon /swapfile20mb
5. Put all swap spaces in /etc/fstab and activate them. Test with a reboot that they are mounted.
root@computer# tail -2 /etc/fstab /dev/sdd1 swap swap defaults 0 0 /swapfile20mb swap swap defaults 0 0
6. Use free to verify usage of current swap.
free -om
7. (optional) Display the usage of swap with vmstat and free -s during a memory leak.