Kernel configurable parameters

[Marie Eldridge_1]

I am running HP-UX 10.20 and need to tune the buffer cache. I checked in SAM and couldn't find this paramater. Any suggestions on where to tune it? Also, do you know where I can find a description of what each of the kernel parameters are? Thanks.

[James R. Ferguson]

Hi:

A very good document is:

http://docs.hp.com/hpux/content/KCparams.OverviewAll.html

This will answer many of your questions. Three parameters deal with buffer cache -- bufpages, dbc_min_pct and dbc_max_pct. Setting dbc_max_pct too high (as it sometimes is, by default) can needlessly consume memory with file buffers. The idea, however, is to trade some memory for IO performance. Hope this helps.

...JRF...

[Stacey Rippetoe]

fyi ... if your in sam and need to review the details on kernel parameters, after you highlight the parameter hit the F1 key and you should get a description.


S.

[Insu Kim]

There're two configuable parameters on HPUX for the question that you asked.
Both are dbc_min_pct and dbc_max_pct and you can tune them in SAM.
This parameters indicate that system is going to use dbc_min_pct, Data buffer cache minimum percentage, but it's possible to use up to dbc_max_pct, Data buffer cahce maximum percentage.

For your reference,
On HPUX 9.X, Dynamic buffer cache method has used and about 10% of RAM is reserved for buffer cache.

[CHRIS_ANORUO]

Hi Marie,

I have dbc_max_pct=25;
dbc_min_pct=5
swapmem_on=1
nbuf=0
bufpages=0
This settings will help in your system buffer cache dynamic functions and will give you 75%of Phy Mem as Pseudo memory.
Do swapinfo -tm and run sysdef to find out this information after changes.

[CHRIS_ANORUO]

Hi Marie,

I have these values on my system:
dbc_max_pct=25
dbc_min_pct=5
swapmem_on=1
nbuf=0
bufpages=0
This settings will help in your system buffer cache dynamic functions and will give you 75%of Phy Mem as Pseudo memory for swaapping.
Do swapinfo -tm and run sysdef to find out this information after changes.

[Alan Riggs]

Chris' values work as a baseline, but you should really tune the dbc_max to the needs of your system. In particular, having cache too large can degrade performance as the time it takes sync to traverse the structures increases. Under 10.20, I have been told that 250-350 MB is the balancing point for general performance. Another thing to consider is whether your system is heavily read or heavily write oriented. Also, some applications (such as oracle) perform their own buffering in memory. On an oracle server, it is often more efficient to reduce system buffer cache and rely on the oracle buffering.

[Eileen Millen]

On our K570, we have an EMC disk array that has 1.5Gb of cache. We are running Oracle Financials 11 on it. Oracle recommended that we set our dbc_max_cache to 2-5%. It depends what the system is being used for.

[Bill Hassell]

Alan is quite correct about Oracle doing a better buffering job than the HP-UX buffer cache. After all, the HP-UX buffer cache has all filesystem data rather than just the Oracle data. However, there are two steps: reduce the buffer cache to perhaps 200 megs *AND* after loading Online JFS, add the following options to each OraData disk (only):

convosync=direct,mincache=direct,nodatainlog

These are only available with OnLine JFS and vxfs filesystems. These options bypass the buffer cache and prevent double buffering by HP-UX and Oracle.