Swap Utilisation

[Sandip Ghosh]

Hi,

Server = HP K460
Memory = 1 GB
Swap = 1 GB and 0.5 GB
O/S = HP-UX 10.20

swapinfo -tm
Mb Mb Mb PCT START/ Mb
TYPE AVAIL USED FREE USED LIMIT RESERVE PRI NAME
dev 512 0 512 0% 0 - 1 /dev/vg00/lvol2
dev 1024 492 532 48% 0 - 0 /dev/vg01/lvol9
reserve - 853 -853
memory 762 154 608 20%
total 2298 1499 799 65% - 0 -

Whenever the load is increasing in this server the swap utilisation is going high and the memory utilisation is going low. But when the load is low the swap utilisation is low and the memory utilisation is high.

Now my question is that why the memory utilistion is decreasing with the load?

[Ian Dennison_1]

When utilisation of the server starts getting high, the OS will shift portions of memory out to the slower swap space. It has to base its decision to do this on a set of values (LOTSFREE, DESFREE, MINFREE).

When your server is busy, more memory is being asked for than is physically in the box, so the OS simulates more memory by using Swap.

When the server is not as busy, the demands placed on memory are less, and therefore little or no swap is needed. Possibly the server can now fit all the required memory into the physical memory on the system.

Hope this helps. There are plenty of forum postings about how memory and swap works; have a look around.

Cheers, Ian Dennison

[Patrick Wallek]

If you are useing swap space at all, it indicates that you are short on memory. I would seriously look at acquiring more memory for this machine.

I purchased 1GB of RAM (8 x 128MB modules) for $2000/US 2 years ago from a 3rd-party remarketer. I never had any problems with the memory, installation was easy, and the users loved it.

[Sandip Ghosh]

Thanks for your replies.

I know that we need some more memory in the box and that will solve the problem but why the utilisation of main memory going down as the load increases?

[Justo Exposito]

Hi Sandip,

Looking your swapinfo -tm output, why do you have different priority in your swap devices.

I think that for performance is better to have the same priority in all the devices, and the devices in different hardware path, because you can divide the access into two different hardware devices.

Regards,

Justo.

[Helen French]

Hi Sandip,

Check this out:

http://forums.itrc.hp.com/cm/QuestionAnswer/1,,0xdc8a5220af9bd5118ff10090279cd0f9,00.html

HTH,
Shiju

[A. Clay Stephenson · Accepted solution]

Hi:

You are misinterpreting what the memory line in swapinfo means - that is pseudo-swap space (swapmem_on=1). You don't even see it unless that kernel paramter is set. It really should only be enabled when you have much more memory that swap disk. The numbers are behaving exactly as expected.