vmstat - "si" and "so" are 0 why ?

[Maaz]

OS: SLES 10 SP2 i586

during installation of Oracle10gR2-32bit, vmstat output is attached.
I found that system gradually started using swap (swpd value: 0 then 36..40..44..48..140) but the value of swap-in "si" and swap-out "so" remain 0 .. why ? I think that once system starts using swap the si and so should be greater then 0 .. am I right ?

Regards

[Steven E. Protter]

Shalom,

No, not necessarily.

When a process starts it reserves swap. if it never actually gets swapped you won't see swap-in or swap-out values.

It means you are not out of memory and paging to disk.

This is a good thing.

:-)

SEP

[Court Campbell]

don't try comparing swpd and si/so. swapd is showing how much swap is being used. And the si/so represent pages in/out from the disk.

[Maaz]

Hi SEP and Court Campbell

nice reply. from your reply what I got is that .. that the process simply reserves the swap in anticipation but it does not means that the process is also actually using swap area(virtual memory) ? and it also means that once a process actually starts using swap area(usually due to lack of phys memory).. at that particular time/moment si/so will be greater then 0 .. right ?

[Court Campbell]

The kernel is going to reserve swap for the process. But what you have to be aware of in Linux is that the kernel can over commit it's total memory, ie, physical and virtual. The kernel can promise a lot more memory than it could ever possibly actually give out at any one time. As for the si/so, you need to watch out for that because it is misleading. That is not actually swap in (si) and swap out (so) that is actually pages to and from disk to memory. That is one thing I find fault in with Linux. The documentation is usually old or wrong at times. If anything I would suggest the Red Hat performance tuning class. You get a nice feel for these kinds of questions. I would also suggest this doc:

http://www.redbooks.ibm.com/abstracts/redp4285.html

and here is a good site about memory management in Linux:

http://linux-mm.org/

I realize this is above and beyond vmstat output.