Setting buffer cache on HP-UX 11.11 system

[Tom Wolf_3]

Hello, we have an rp8420 running HP-UX 11.11 with 64 GB of physical memory installed. I'm trying to set the buffer cache to 400 MB. I'm unable to do this using the maximum and minimum dynamic buffer cache kernel parameters dbc_max_pct and dbc_min_pct (with dbc_max_pct set at 2 and dbc_min_pct set at 1 the lowest value is about 650 MB). I tried setting nbuf to 100 to get buffer cache at 400 MB (assuming 100 X 4096 bytes = 400 MB) but buffer cache was only 1 MB after making this change.

Can anyone offer suggestions on how to set buffer cache to 400 MB on a system with 64 GB of memory installed?

Any assistance would be appreciated.

Thank you.

Tom Wolf

[Steven E. Protter]

Shalom Tom,

Setting this number is a matter of how you use the system.

To set to a particular number is not very helpful.

Memory is defined by HP-UX as ram plus swap

dbc_max_pct is calculated on memory or physical memory I don't remember. Its a percentage. Pick the percentage that brings you closest to 400 MB if that number is so appearling to you.

SEP

[Rita C Workman]

I don't have any boxes running with that much physical memory. And setting up buffer_cache to that low may very likely be an issue~!

But if your wanting to try it, maybe you could get close to it by setting dbc_max to 2 and dbc_min to 1. You'd come out higher than 400M.

Of course it does make me really wonder what the rest of the parms are set at.

Rgrds,
Rita

[Jeeshan]

you can check out this link for more understanding about buffer cache settings
http://forums12.itrc.hp.com/service/forums/questionanswer.do?threadId=419867

[doug hosking]

> (assuming 100 X 4096 bytes = 400 MB) but buffer cache was only 1 MB after making this change.

There's an old saying about what happens when you assume. Isn't 100 X 4096 bytes the same as 400KB, not 400 MB? That would explain your observed "1MB." To be honest, I'm almost surprised your system didn't deadlock (not just peform badly) due to lack of buffers.

Math aside, let's back up a bit. what sort of applications/load are you running, and why do you believe you need to be so careful about the last percent of memory on a system with 64 GB of RAM? I think the answer to that might be worth drilling into a bit.

[Bill Hassell · Accepted solution]

400 MB of buffer cache is appropriate for systems with just a few GB of RAM. With 64 GB, it makes no sense to try to make it this small. At 11.11, the buffer cache handling was improved over previous versions (and vastly improved with 11.23 and 11.31) so good performance will be available with BC set to 1000 MB (dbc 2-3). In the past, it has been recommended to keep the BC under 800 MB but that was a long time ago. You need that 1000 MB just to have good performance with standard files (even if you use a raw database).