swapinfo

Sajeesh O.K — Mon, 07 Mar 2005 21:57:32 GMT

Hi

Please find the output of swapinfo in one of my server.
Mb Mb Mb PCT START/ Mb
TYPE AVAIL USED FREE USED LIMIT RESERVE PRI NAME
dev 32768 0 24000 0% 0 - 1 /dev/vg00/lvol2
reserve - 9813 -9813
memory 13135 10778 2357 82%
total 45903 20591 16544 45% - 0 -
Please look the Dev Swap Line.
Eventhough used is zero, Free column is showing only 24000 instead of 32768.
Any idea.

Thanks
Sajeesh

Re: swapinfo

Paul_481 — Mon, 07 Mar 2005 22:14:41 GMT

Hi Sajeesh,

From the man pages of swapinfo:

Mb FREE is the amount of space that can be used for future paging. Usually this is the diff. between Mb AVAIL and MB USED. There could be a diff. if some portion of a device paing area is unusable, perhaps because the size of the paing area is not a multiple of the allocation chunk size, or because the tunable parameter maxswapchunks is not set high enough.

Hope that helps,
Regards,

Re: swapinfo

Suraj_2 — Tue, 08 Mar 2005 01:04:17 GMT

Dear Sajeesh

Please check your following kernel tunables

maxswapchunks
swchunk

Total swap the system can see is calculated as follows.

maxswapchunks * swchunk * 1024

This value should be more than the device swap which you have configured.

Hope this helps

Rgds
Suraj

Re: swapinfo

Cem Tugrul — Tue, 08 Mar 2005 01:40:10 GMT

Hi Sajeesh,

By far, the simplest command to use to monitor swap on a 10.X and 11.X system
is swapinfo. With one command the system administrator can see how much swap is
configured, how much swap is being reserved for processes, or even how much
swap is being actively used for swapped processes. Here is an example :

#swapinfo -tam
Mb Mb Mb PCT START/ Mb
TYPE AVAIL USED FREE USED LIMIT RESERVE PRI NAME
dev 128 10 118 8% 0 - 1 /dev/vg00/lvol2
localfs 60 0 60 0% 60 0 4 /var/paging
reserve - 52 -52
memory 91 68 23 75%
total 279 130 149 47% - 0 -

This output shows that device swap(dev), filesystem swap(localfs), and pseudo
swap(memory) are configured on this system. The first thing that I would like
to point out is the total line. From left to right, you can get a quick
reference of how swap is doing on this system. Under the Mb Avail column, it
shows that this system has a total of 279 megs of configured swap. Of the 279
megs configured, 130 megs are used for something. By that, I mean that the
total line does not show if swap is used for reservation or paging. With 130
megs being used for "something", that leaves a total of 149 megs of swap that
are not in use. The Pct Used column just shows a handy percentage of swap that
is already used.

The "memory" line shows pseudo swap usage, and tends to be the most confusing of
all of the swapinfo output. As I documented before, pseudo swap is only used
for reserving a process. So from this example, there is a total of 91 megs of
pseudo swap configured, and of that 91 megs, 68 megs of pseudo swap are being
used by processes running in memory. The remainder of pseudo swap that it is
not used is 23 megs. One of the reasons why pseudo swap is so confusing is the
fact pseudo swap usage does not degrade system performance like device swap or
filesystem swap. In another words, system performance would be the same on a
system that had 3% or 99% percent of pseudo swap used. It is usually
recommended that the "memory" line simpily be ignored when looking at swapinfo.

The "reserve" line only deals with amount of swap we are using for reserving
processes in device and filesystem swap areas. From this example, we have a
total of 188 megs of combined device and filesystem swap and of that, only 52
megs are being used for reserving processes. Now if you take the total of megs
used by memory and reserve, we have 120(52+68) megs of swap allocated to
reserving space for active processes. So, from two lines, we have accounted for
about 92% of swap, which is being used only for reserve for running processes.

The "localfs" line shows some information on much space the system will use
for filesystem swap that was configured on /var. An interesting thing
about that line is the priority of the swap, which is 4. This means that all
of device swap (/dev/vg00/lvol2), which is set to priority 1, will be used
before this filesystem swap area is used.

The "dev" line is one of the most important pieces of information that the
swapinfo command can show. If the percent used line is greater that 0, then the
system is swapping. This is a clear sign that there is not enough physical ram
installed on the system. There are only two methods to make a system stop
swapping, the first is install more physical memory and the other is to reduce
the processes running on the system. The dev line that was used in this example
was altered to show what the output would look like if the system was paging.
It is unlikely that a system would page processes to the swap area if it still
had 149 megs of reserve space left.

If you were interested in learning more about swap, I would suggest reading
through the memory managment white paper. This document is stored on the system
in /usr/shar/doc and is called mem_mgt.txt. There is also information about swap
from the system administration tasks manaul(10.x) and from the configuring
systems for work groups(11.x). Both manuals can be seen at :
http://www.docs.hp.com.

Hope this Helps,

Re: swapinfo

Sajeesh O.K — Tue, 08 Mar 2005 02:20:25 GMT

Suraj, You are right.

Cem, Thanks for the explanation.

Sajeesh

Re: swapinfo

Sajeesh O.K — Tue, 08 Mar 2005 02:28:32 GMT

Cem

Thanks for the explanation. It means if pseudo swap configured, the same memroy is used for user process as well as for reserving ?

Thanks
Sajeesh

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