Memory Management Commands
In this lesson you will learn following memory management commands
- free - To check RAM status
- proc/meminfo - Detailed RAM status
- top - Process Management
- htop - Advanced Process Management
- df - Disk Free Space Details
- du - Disk Consumption Details
- vmstat - Virtual Memory Statistics
- demidecode - Systems Hardware Information
- sar - System Activity **Report
- pagesize - Virtual Page size information
free command
Free command is used to find out how much RAM is free and how much is used. It displays physical, swap, cache and buffers status
you can use following options to customize the display
- b - to display output in bytes
- k - to display in kilobytes
- m - to display in mebibytes
- g - to display in gibibytes
You can also use the -s option to display results for given seconds apart
Example1 : Display memory status in megabtes
➜ ~ free -m
total used free shared buff/cache available
Mem: 15702 3442 8185 970 4073 10965
Swap: 37274 0 37274
Meaning of each column
- total : Total full available RAM memory size
- used : Amount of RAM currently used by the processes
- free : Free memory i.e. Available - Used
- shared : memory used in tmpfs to for creating the virtual memory
- buff/cache : Memory used by cache and kernel buffers
- available : memory available to run more applications
Example2: Display memory status in every second in gegabytes format
$ free -g -s 1
➜ ~ free -g -s 1
total used free shared buff/cache available
Mem: 15 3 8 0 3 10
Swap: 36 0 36
/proc/meminfo
Under /proc directory we have meminfo file, this file give lot of information about your systems RAM memory.
You can use cat or nano to display the content of this file. Actually this file is created in the memory by OS to give all the information about the system.
Sample output of meminfo file is shown below
➜ ~ cat /proc/meminfo
MemTotal: 16079272 kB
MemFree: 8352544 kB
MemAvailable: 11210548 kB
Buffers: 194212 kB
Cached: 3848588 kB
SwapCached: 0 kB
Active: 1229996 kB
Inactive: 5143856 kB
Active(anon): 17496 kB
Inactive(anon): 3320736 kB
Active(file): 1212500 kB
Inactive(file): 1823120 kB
Unevictable: 768512 kB
Mlocked: 64 kB
SwapTotal: 38169596 kB
SwapFree: 38169596 kB
Dirty: 404 kB
Writeback: 240 kB
AnonPages: 3099708 kB
Mapped: 1117128 kB
. . .
We will not go through all the attributes of the above file, you need to know th concepts of Operating System to understand the menaing of all the attributes.
In this file you will get total RAM size, total swap memory, consumed, free, available to use, memory page mappings, virtual memory allocations, dirty pages, swapped memory etc.
For more information you can visit : https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/6/html/deployment_guide/s2-proc-meminfo
top
top is used to display the process running in the system. When you run the top command you get following output
Top displays following information on first few lines
- Current time and sinceh when system booted
- Total numers of users logged in
- system load over 1,5 and 15 min
- How many tasks/processes are running and their states
- CPU utilization and their categories of usage as shown following
- us, user : time running un-niced user processes
- sy, system : time running kernel processes
- ni, nice : time running niced user processes
- id, idle : time spent in the kernel idle handler
- wa, IO-wait : time waiting for I/O completion
- hi : time spent servicing hardware interrupts
- si : time spent servicing software interrupts
- st : time stolen from this vm by the hypervisor
- Physical Memory usage information like total, free, used and buff/cache
- Swap memory usage information like total, free, used and avail
As you can see in the output, get following information in the colums
- PID - Process identifier
- USER - User who started or Owner of process
- PRI - Process priority for kernel
- NI - Process priority set by user or root
- VIRT - Vrtual memory consumed by process
- RES - Physical memory consumed by the process
- SHR - Shared memory consumed by the process
- S - State of the process
- CPU% - % CPU utilization by the process
- MEM% - % of Memory consumption by the process
- TIME+ - time since the process started the execution
- Command - Command path which started the execution of the process
Process states :
- D = uninterruptible sleep
- I = idle
- R = running
- S = sleeping
- T = stopped by job control signal
- t = stopped by debugger during trace
- Z = zombie
htop
Htop is advanced process monitoring command siliar to top but it gives more advanced interactive features.
Htop has similar colum out put as top command along with this is shows information about the individual cpu utilizations on top.
You can use the keyboard to do various additional operations like
- Use arrow keys to scroll through processes like left, right, home etc
- Use function keys to search, filter, sort, change priority and kill processes.
df
df is stands for disk space, df can se used to find out all about the consumption of the disk space
Lets see what df gives as a output
dbit@Tayyabali:~$ df
Filesystem 1K-blocks Used Available Use% Mounted on
udev 8004084 0 8004084 0% /dev
tmpfs 1607928 2484 1605444 1% /run
/dev/nvme0n1p2 191197068 11215148 170199904 7% /
tmpfs 8039632 88488 7951144 2% /dev/shm
tmpfs 5120 4 5116 1% /run/lock
tmpfs 8039632 0 8039632 0% /sys/fs/cgroup
/dev/nvme0n1p3 1889292 171152 1604120 10% /boot
/dev/nvme0n1p5 66761128 8368108 54958680 14% /var
/dev/nvme0n1p4 191197068 78045272 103369780 44% /home
/dev/nvme0n1p1 997456 5356 992100 1% /boot/efi
/dev/loop0 128 128 0 100% /snap/bare/5
/dev/loop2 224256 224256 0 100% /snap/gnome-3-34-1804/72
You will get lots of snap applications also in the output you can simple remove it by using any filteration techniques like
dbit@Tayyabali:~$ df | grep -v /snap
Lets now understand the output of df command,
- filesystem type using -t you can specify the which fs to filter in output
- 1K-blocks is nothing but the size of disk space in blocks, Use -H to get in human readable format
- Used and Available are current space stats
- % use is nothing but how much % its used out of availabe and
- Mounted on is nothing but the mount point in which the drive/partition is mounted
Example of filtering based on etx4 type
dbit@Tayyabali:~$ df -t ext4
Filesystem 1K-blocks Used Available Use% Mounted on
/dev/nvme0n1p2 191197068 11215524 170199528 7% /
/dev/nvme0n1p3 1889292 171152 1604120 10% /boot
/dev/nvme0n1p5 66761128 8368112 54958676 14% /var
/dev/nvme0n1p4 191197068 78046824 103368228 44% /home
Use -h to see the file system size in power of 1024
dbit@Tayyabali:~$ df -t ext4 -h
Filesystem Size Used Avail Use% Mounted on
/dev/nvme0n1p2 183G 11G 163G 7% /
/dev/nvme0n1p3 1.9G 168M 1.6G 10% /boot
/dev/nvme0n1p5 64G 8.0G 53G 14% /var
/dev/nvme0n1p4 183G 75G 99G 44% /home
Now lets see how to fnd the space of a particular directory
dbit@Tayyabali:~$ df -h /home/dbit
Filesystem Size Used Avail Use% Mounted on
/dev/nvme0n1p4 183G 75G 99G 44% /home
Dont forget to put -h to get the K, M and G in the output so that you can understand it easily
du
du is disk usage, it is used to find out how much space is used by files and directories. Not df is for disk availablity not for files space estimations.
lets see examples and understand more about the du command
dbit@Tayyabali:~$ du unixlab
4 unixlab/module2
4 unixlab/module3/module_one
4 unixlab/module3/module1
12 unixlab/module3
24 unixlab
du also by default gives output in the blocks format, make it human readable using -h option as shown below
dbit@Tayyabali:~$ du -h unixlab
4.0K unixlab/module2
4.0K unixlab/module3/module_one
4.0K unixlab/module3/module1
12K unixlab/module3
24K unixlab
You can use -c to get the grand total of the directory, Its alsho seen using -h options also
dbit@Tayyabali:~$ du -ch unixlab
4.0K unixlab/module2
4.0K unixlab/module3/module_one
4.0K unixlab/module3/module1
12K unixlab/module3
24K unixlab
24K total
You can use the sort and head to filter out the top biggest directories or files using following
dbit@Tayyabali:~/unixlab$ du -ah | sort -n -r | head -n 5
24K .
12K ./module3
4.0K ./wcdemo
4.0K ./module3/module_one
4.0K ./module3/module1
We use -a to list all files and directries -h to get in human redable format -n to sort using numerical values -r to reverse the result i.e biggest on top -n to specify only 5 lines of output
Using -d you specify the how much depth you want display in the output
dbit@Tayyabali:~/unixlab$ du -hd 1
4.0K ./module2
12K ./module3
24K .
dbit@Tayyabali:~/unixlab$ du -hd 2
4.0K ./module2
4.0K ./module3/module_one
4.0K ./module3/module1
12K ./module3
24K .
Please note -d option needs the depth parameter, other parameters you need to specify before that
vmstat
dbit@Tayyabali:~/unixlab$ 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 6955748 293816 5209896 0 0 35 36 199 160 4 1 95 0 0
vmstat is used to report the virtual memory stats, like reads, writes cpu. Along with it it also gives cpu utilization details.
Read the column informatin in its man pages.
dbit@Tayyabali:~/unixlab$ vmstat -s
16079268 K total memory
3696452 K used memory
1926820 K active memory
5848164 K inactive memory
6938356 K free memory
298420 K buffer memory
5146040 K swap cache
38169596 K total swap
0 K used swap
38169596 K free swap
408569 non-nice user cpu ticks
838 nice user cpu ticks
100900 system cpu ticks
10963639 idle cpu ticks
22982 IO-wait cpu ticks
0 IRQ cpu ticks
9856 softirq cpu ticks
0 stolen cpu ticks
3888411 pages paged in
4010905 pages paged out
0 pages swapped in
0 pages swapped out
22624475 interrupts
62663784 CPU context switches
1647228613 boot time
32933 forks
demidecode
demidecode is used to display Processor, RAM(DIMMs), BIOS detail, Memory, Serial number information.
dmi is nothing but Desktop Management Interface. It reads from DMI table.
dbit@Tayyabali:~/unix-mkdocs$ sudo dmidecode | more
# dmidecode 3.2
Getting SMBIOS data from sysfs.
SMBIOS 3.2.0 present.
Table at 0x7035B000.
Handle 0x0000, DMI type 134, 13 bytes
OEM-specific Type
Header and Data:
86 0D 00 00 18 11 21 20 00 00 00 00 00
Handle 0x0001, DMI type 16, 23 bytes
Physical Memory Array
Location: System Board Or Motherboard
Use: System Memory
Error Correction Type: None
Maximum Capacity: 128 GB
Error Information Handle: Not Provided
Number Of Devices: 1
sar
Sar stands for system activty report. Using sar you can see
- CPU usage
- Memory utilization
- I/O devices consumption
- Network monitoring
- Disk usage
- Process and thread allocation
- Battery performance
- Plug and play devices
- Processor performance
- File system etc
Install sar with sudo apt install sysstat if not installed by default.
After installation we need to enable it using systemctl start sysstat.service
-
Report CPU details total 5 times with the interval of 2 seconds
sar -u 2 5 -
Report about the amount of memory used, amount of memory free, available cache, available buffers total 3 times with the interval of 1 second.
sar -r 1 3 -
report about file systems mounted on the device
sar -F 2 5 -
Report Paging stats
sar -B 2 5 -
Report about the bloc devices
sar -d 1 3
You can explore more about the sar and its output in its man page
pagesize
In Linux RAM is divided into pages, and then based on it pages are allocated to the application programs. Using pagesize command you can get the page size set for your system
If this program is not installed by default then you can install using sudo apt install libhugetlbfs-bin
dbit@Tayyabali:~/unix-mkdocs$ pagesize
4096
dbit@Tayyabali:~/unix-mkdocs$ getconf PAGESIZE
4096
You cna get the same output using the getconf PAGESIZE also
If you use -a then you can get it huge page size also
dbit@Tayyabali:~/unix-mkdocs$ pagesize -a
4096
2097152
1073741824