system buffer cache and memory

[Chris Fadrowski]

I have a system with 4 gig of memory. i am running multiple Oracle DB's and am getting close to 98% untilizaition on memory when i view it with glance.

What i noticed was that the system buffer cache is using 2 gig of memory. That is consistant with all my other systems in that the buffer cache is half of total memory.

Is this needed or can i change this and reduce to lets say 1.7 gig so i can use some of that memory for my Oracle SGA's? or is this static and it can't be changed...??

[A. Clay Stephenson]

Hi,

I think you have the default max_dbc_pct (dynamic buffer cache max percent) value of 50% which is way too high. If you want to use dynamic buffer cache I would set the max to about 2% and the min to about 5%. My prerefence would be to disable dynamic buffer cache and set the bufpages to no more than about 81920 (320 MB); that value is quite generous. If you are using raw/io or OnlineJFS mount options to do the equivalent, then I would drop the unix buffers still more (~100MB). You can then use this space in the SGA for buffers.

REgards, Clay

[Chris Fadrowski]

can you tell me how to do this???

[A. Clay Stephenson · Accepted solution]

Hi Chris,

It's not very difficult but if you are not familiar with HP kernel building you might wind up with an unbootable box. It works at least 99% of the time without problem so if a 1% risk is ok, here goes:

Obviously shutdown your databases and any other critical software.

As root, execute SAM.

Kernel Configuration->Configurable Parameters

Scroll down until you see 'bufpages' and set it to the disired value (81920 - 320MB; 40960 180MB; ...)

Then Actions->Process (or build) New Kernel

Accept the defaulkts and allow the machine to reboot automatically.

Regards, Clay

[Michael Tully]

Hi Chris,

One thing I would like to add to Clay's
suggestion would be to look at utilising
memory windows. There are a couple of
documents that may be of assistance.

Doc ID KBAN00000306 and also there should
be a text document on your server called
/usr/share/doc/mem_wndws.txt

My 2 cents worth
Michael

[Celso Medina Kern]

Remember that Oracle SGA is limited to 1.75GB in hp-ux 32 bits. It maybe not worth reduce buffer if you cannot use it in application. Even in 64 bits, Oracle version has to be 64 bits either.

When using Oracle 32 bits, you have to be in version 8.0.5(or 8.0.6?) at least to use memory windows.

Good luck