Vmstat
is a command available in linux which displays information about the
memory, swap and CPU utilization of the system in real time.
A
Basic executions of vmstat gives,
LocalMach:root002-~
$ 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 36570164 1584768 12768668 0 0 0 6 0 1 1 0 99 0 0
The
data provided above was an average usage of system resources since
last reboot. In the above output, there is
0
- nothing is swapped from disk
36570164
– amount of free system memory
1584768
- amount of system data that has yet to be flushed to disk.
12768668
- amount of data that has been read from the disk
LocalMach:root002-~
$ vmstat -a
procs
-----------memory---------- ---swap-- -----io----
--system-- -----cpu------
r
b swpd free inact active si so bi bo in cs us sy id wa st
1
0 0 36442616 3303552 25479784 0 0 0 6 0 0 1 0 99 0 0
In
the above output we asked vmstat to display information about the
number of active and inactive pages. The amount of inactive pages
indicates how much of the memory could be swapped to disk and how
much is currently being used. In this case, we can see that 33.3552
of memory is active, and only 25479784 is considered inactive
Often
vmstat is used with an interval and a count like,
vmstat
[interval] [count]
The
above output provides you a lot of information like
procs
Section
- r field: Total number of runnable process. The More the load on system more number of process are waiting to get CPU
- b field: Total number of blocked process or uninterruptable sleeping process. These processes are most likely waiting for a I/O operations or could be any other task. (should be close to Zero)
The
value for these fields should be either ‘0’ or close to ‘0’.If
there is a high value ,then the system don’t have enough resources
like CPU, memory or I/O.
Memory
section (in KB)
- Swpd field: Used swap space
- Free field: Available free RAM
- Buff field: RAM used for buffers. This is used to store file metadata such as i-node and data from Raw block devices.
- Cache field: RAM used for file system cache .This is used for the file data itself.
Swap
Section (in KB/Sec)
- Si field: Amount of memory swapped from disk per second
- So field: Amount of memory swapped to disk per second
The
data under these fields show the amount of swapping going on. If it
is more than we need to either reduce the memory demand or increase
the Physical Ram.
For
Web servers or Data base Servers its very bad, since a trip to the
disk may often cause a Performance impact for transactions going.
I/O
Section ( in Blocks/Sec)
- Bi field: Blocks received from disk
- Bo field: Blocks sent to disk.
This
shows us the flow of data to and from the disks. if there are large
number of process are blocking and high I/O , then there is an Issue
with the I/O. bi and bo represent the data transfers
between virtual memory and block devices. It depends on your workload
System
Section ( Per Second)
- In field: Number of interrupts per second.
- Cs field: Number of context switches per second.
CPU
Section
- Us field: Time spend running user code. (non-kernel code , application specific code comes under this)
- Sy field: Time spent running kernel code such as root process.
- Id field: Idle time.
- Wa field: Time spent waiting for the IO
The
information is very important in identifying the information about
the CPU. We have one more command ‘top’ which gives the same
information. The ‘top’ command gives information about single CPU
,But vmstat gives information about all the CPU.
We
can also use vmstat like ,
LocalMach:root002-~
$ vmstat -a -n 10 3
procs
-----------memory---------- ---swap-- -----io---- --system--
-----cpu------
r
b swpd free inact active si so bi bo in cs us sy id wa st
1
0 0 36551776 3229768 25445280 0 0 0 6 0 1 1 0 99 0 0
0
0 0 36551972 3229800 25445272 0 0 0 88 277
2602 1 0 99 0 0
0
0 0 36551848 3229856 25445340 0 0 0 100 1265
2545 0 0 100 0 0
The
vmstat example above means we want it to delay 10
seconds before the next update and count the update
3 times. The '-n' option is used so that the header does not
displayed in every update.
In
Order to get the memory Details we can use,
LocalMach:root002-~
$ vmstat -s
65970668
total memory
29418976
used memory
25445424
active memory
3230004
inactive memory
36551692
free memory
1584872
buffer memory
12786880
swap cache
4194296
total swap
0
used swap
4194296
free swap
39488615
non-nice user cpu ticks
178225
nice user cpu ticks
3318192
system cpu ticks
2963059383
idle cpu ticks
41192
IO-wait cpu ticks
94896
IRQ cpu ticks
803239
softirq cpu ticks
0
stolen cpu ticks
10891932
pages paged in
193698804
pages paged out
0
pages swapped in
0
pages swapped out
2664606290
interrupts
723606365
CPU context switches
1346657309
boot time
24413327
forks
In
the default operation, vmstat displays memory statistics in
kilobytes. vmstat considers a single kilobyte equal to 1024
bytes. vmstat can also display reports with memory sizes reported in
megabytes. vmstat reports with the argument "-S m" will
consider a single megabyte equal to 1000 kilobytes as follows:
vmstat
-S k 1 10
vmstat
-S m 1 10
vmstat
can provide information about how the Linux kernel allocates its
memory. The Linux kernel has a series of "slabs" to hold
its dynamic data structures. vmstat displays each of the slabs
(Cache), shows how many of the elements are being used (Num), shows
how many are allocated (Total), shows the size of each element
(Size), and shows the amount of memory in pages (Pages) that the
total slab is using.
LocalMach:root002-~
$ vmstat -S k
procs
-----------memory---------- ---swap-- -----io---- --system--
-----cpu------
r
b swpd free buff cache si so bi bo in cs us sy id wa st
0
0 0 37423276 1622970 1... 0 0 0 6 0 1 1 0 99 0 0
The
same information can be obtained in MB using, vmstat –S M.
LocalMach:root002-~
$ vmstat -f
24447726
forks
Resource
Usage
localMach:root002-sa
$ vmstat -m
Cache
Num Total Size Pages
rpc_buffers
8 8 2048 2
rpc_tasks
8 10 384 10
rpc_inode_cache
6 10 768 5
ip_fib_alias
10 59 64 59
ip_fib_hash
10 59 64 59
iser_descriptors
0 0 128 30
ib_mad
0 0 448 8
fib6_nodes
5 59 64 59
ip6_dst_cache
4 24 320 12
ndisc_cache
1 15 256 15
RAWv6
7 8 960 4
UDPv6
6 8 896 4
tw_sock_TCPv6
0 0 192 20
request_sock_TCPv6
0 0 192 20
Disk
Statistics
localMach:root002-sa
$ vmstat -d
disk-
------------reads------------ ------------writes-----------
-----IO------
total
merged sectors ms total merged sectors ms cur sec
ram0
0 0 0 0 0 0 0 0 0
0
ram1
0 0 0 0 0 0 0 0 0
0
ram2
0 0 0 0 0 0 0 0 0
0
ram3
0 0 0 0 0 0 0 0 0
0
ram4
0 0 0 0 0 0 0 0 0
0
ram5
0 0 0 0 0 0 0 0 0
0
ram6
0 0 0 0 0 0 0 0 0
0
ram7
0 0 0 0 0 0 0 0 0
0
ram8
0 0 0 0 0 0 0 0 0
0
ram9
0 0 0 0 0 0 0 0 0
0
ram10
0 0 0 0 0 0 0 0 0
0
ram11
0 0 0 0 0 0 0 0 0
0
ram12
0 0 0 0 0 0 0 0 0
0
ram13
0 0 0 0 0 0 0 0 0
0
ram14
0 0 0 0 0 0 0 0 0
0
Disk
Table
(!
1018)-> vmstat -D
31
disks
2
partitions
1139802
total reads
214276
merged reads
36688696
read sectors
8146064
milli reading
2168133
writes
742101
merged writes
23021306
written sectors
169686644
milli writing
0
inprogress IO
13502
milli spent IO
Happy
learning , More To Come :-)
