Swap space usage

[RAJESH GANGADHARAN]

Hi
Our system got 4 GB of RAM and swap sapce of 8GB, now we are facing a problem like the swap space usage is reached almost 95% and when I checkd with Glance Plus, it was showing total swap usage is 95% Avail : 8.0 Gb Resvd 7.6 GB and used 873 Mb. Why it is reserving 7.6 GB when tha actual usage is only 873 Mb, is there any way to reduce the Reserved swap sapce.

Thanks in adv
Rajesh

[Ravi_8]

Hi Rajesh

check the value of 'dbc_max_pct' kernel parameter, default value will be 50, set it to 15 or 20.

[RAJESH GANGADHARAN]

Thanks Ravi,
The current value is "2". Is there any other kernel parameter related to this. What is the significance of dbc_max_pct with reserved swap space?

Glance is showing buffer headers as 100%. Any idea about this?

-Rajesh.

[Artyom Voronchihin]

Hello !
It's UNIX feature. When process starts, HPUX reserves swap space for it. Process doesn't really use reserved space while free memory exist. The reservation of swap space is done so that the operating system knows how much swap space "potentially" may be needed for all the processes currently running on the system.For example, if all processes in memory were to be swapped out, the operation system would know it had enough swap space to perform that function. To decrease disk space for swap you may turn on pseudo swap, but it requires enough memory to place processes and swap in it.

[T G Manikandan]

Rajesh,

It is like this.

Whenever a process is created and before its execution it reserves swap space.
This swap is not used/allocated before the period when the paging happens to the disk.

This is done to prevent the swapper running out of swap space.

For example,
if you are to execute a 4Gb process then this process before execution reserves a 4gb swap area incase it is swapped.

This is a wastage of resource.


you can prevent this by turning on the pseudo-swap from 0 to 1 on the kernel.
by enabling pseudo swap some amount of memory is also used as swap.
When the pseudo swap increases then the amount of lockable memory decreases.

If you have more memory consuming applications then you might think of increasing the memory on the server.

[Artyom Voronchihin]

Hello !
It's UNIX feature. When process starts, HPUX reserves swap space for it. Process doesn't really use reserved space while free memory exist. The reservation of swap space is done so that the operating system knows how much swap space "potentially" may be needed for all the processes currently running on the system.For example, if all processes in memory were to be swapped out, the operation system would know it had enough swap space to perform that function. So if total size of all running processes on your system is 6Gb, swap reservation also will be 6Gb. To decrease disk space for swap you may turn on pseudo swap, but it requires enough memory to place processes and swap in it.

[T G Manikandan · Accepted solution]

Just found out this doc
Good explanation

http://www1.itrc.hp.com/service/cki/docDisplay.do?docLocale=en_US&docId=200000066374044

Also check /usr/share/doc/mem_mgt.txt.

Thanks