topic Re: Tuning Kernel Parameter dbc_max_pct in Operating System - HP-UX

Tuning Kernel Parameter dbc_max_pct

Cliff Lim Kok Hwee — Fri, 26 Nov 2004 00:48:47 GMT

Gd Day Forum,

Currently my HP-UX 11.00 server is having the following value Buf Cache: 4.00gb and Phy Mem:8.00bb

I need to reduce the Buf Cache to 3.00gb because Oracle application requires more memory.

Current dbc_mac_pct = 50
Current dbc_min_pct = 5

Solution: I need to tune dbc_mac_pct = 37 to achieve Buf Cache = 2.96gb.

Query: Can anyone advise me the commands and backup to perform using command mode rather then thru SAM to get this parameter dbc_mac_pct tune?

Quer: Correct me I dun think I need to tune the parameter dbc_min_pct right?

thanks n regards/cliff

Re: Tuning Kernel Parameter dbc_max_pct

Sridhar Bhaskarla — Fri, 26 Nov 2004 00:55:55 GMT

Cliff

Buffer cache of 2.96GB??.. That's too much. For 11.0, 600MB will be the maximum needed. I wouldn't go beyond 600 which is 7%. You can set your min_pct to 2% which is minimum. I use the following commands to tune my kernel.If you have enough space in /stand, backup the current kernel to be space

#cd /stand
#cp system system.112604
#cp vmunix vmunix.112604
#/usr/lbin/sysadm/system_prep -s /stand/system
#kmtune -s dbc_max_pct=7
#kmtune -s dbc_min_pct=2
#grep dbc system
(make sure dbc* values are listed correctly)
#cp system build
(build is a directory under /stand)
#mk_kernel -o /stand/vmunix
#kmupdate
#cd /
#shutdown -r y now

Once the system comes backup, run
#kmtune -l -q dbc_max_pct
#kmtune -l -q dbc_min_pct

to verify.

-Sri

Re: Tuning Kernel Parameter dbc_max_pct

Thierry Poels_1 — Fri, 26 Nov 2004 01:34:45 GMT

Hi,

if this is an only a database server, you can lower dbc_min_pct and dbc_max_pct to something like 2 & 4. Oracle is using its own buffer cache, so there is no need in double caching everything on OS level.

regards,
Thierry.

Re: Tuning Kernel Parameter dbc_max_pct

Jeroen Peereboom — Fri, 26 Nov 2004 01:52:19 GMT

In addition to the commands mentioned:
why not use SAM?

;-)

JP

Re: Tuning Kernel Parameter dbc_max_pct

Geoff Wild — Fri, 26 Nov 2004 09:08:52 GMT

Info about buffer cache in this recent thread:

http://forums1.itrc.hp.com/service/forums/questionanswer.do?threadId=749930

With 8GB of ram - set dbc_max_pct to 8

Rgds...Geoff

Re: Tuning Kernel Parameter dbc_max_pct

Steven E. Protter — Fri, 26 Nov 2004 09:32:51 GMT

My two cents:
WE run:

Current dbc_mac_pct = 7
Current dbc_min_pct = 5

We run very well. Any changes on the fly of the actual system use of these parameters is extremely expensive in cpu terms. Its best to find a couple of numbers that work well for you and stick in a short range.

SEP

Re: Tuning Kernel Parameter dbc_max_pct

Eric Antunes — Fri, 26 Nov 2004 09:34:09 GMT

Hi Cliff,

If you are using Oracle application, take a look at this attachment with all tunable kernel parameters because there is a lot of them that influences apps&db performance: maxswapchunks is another one, etc...

As said before, you shouldn't use more than 10-15 for dbc_max_pct. For db cache, use db_block_buffers init.ora parameter.

Best Regards,

Eric

Re: Tuning Kernel Parameter dbc_max_pct

Fred Ruffet — Fri, 26 Nov 2004 09:51:54 GMT

Cliff,

As I often reply to such questions, I will go into the whole explication and I'll reference this thread in future replies :)

As you mentionned, default buffer cache settings are set to dbc_max_pct=50 and dbc_min_pct=5. This will make this IO Cache using 5% of your RAM and slowly grow to something near 50% of your RAM, which is far too much if you consider the following points.

HP-UX IO Cache works as a cache must, it stores in its memory the blocks you read the more. It also (to anticipate sequential reads) stores a little bit more info that you asked for, in order to give it faster to you on your next read.

Problems start here : Oracle does a lot of reads. It also stores those most read blocks. And it also stores a little bit more info to prevent sequential reads. So, when you need some data, Oracle will ask OS to read this data and a little more and will cache it. OS will read this "data and little more" and little more and will cache it. So your system will store more than twice the data it needs, and will consume your memory this fast. In case of an app with lots of IO and few RAM, it will make your system start swapping dramaticaly and make your OS unusable (but it's only extreme case).

Solution reside in these points :
. Reduce OS IO Cache by tunning these parameters. If your system is only running Oracle, precedent advice of setting dbc_min_pct and dbc_max_pct to 2 and 4 is good. This will prevent OS cache to grow and will offer opportunity to you to increase Oracle SGA settings.
. If you have Online JFS on your system, make dedicated filesystems for your DB files and mount them with options convosync=direct and mincache=direct. This will bypass the OS cache for this files and will eliminate the problem.

For more info and other points of tunning for Oracle on HP-UX, read this excellent doc :
http://h21007.www2.hp.com/dspp/files/unprotected/database/HP3KOracle.ppt

Regards,

Fred

Re: Tuning Kernel Parameter dbc_max_pct

Fred Ruffet — Fri, 26 Nov 2004 09:54:39 GMT

As a complement to Eric's answer, for versions 9i and upper of Oracle, use db_cache_size parameter to set database blocks buffer.

Regards,

Fred

Re: Tuning Kernel Parameter dbc_max_pct

Eric Antunes — Fri, 26 Nov 2004 10:05:10 GMT

And don't forget to start the house by the foundations and not by the roof ;-) :

- Tune SQL (custumizations, reports, etc...) first! This is where you will see great achievments!
- Tune init.ora: you may change the following parameters from defaults:

processes = 200
sort_area_size = 512000
log_checkpoint_interval = 20000
log_checkpoint_timeout = 3600
log_buffer = 1048576
timed_statistics = true # set this one to collect performance statistics but consider some overhead it will bring...
shared_pool_size = 192M
db_block_buffers = 3000
- Tune Kernel at the end

Eric

Re: Tuning Kernel Parameter dbc_max_pct

Cliff Lim Kok Hwee — Fri, 26 Nov 2004 23:51:58 GMT

Guys,

Thanks for all the valuable advices!

Apreciated n Cheers to ITRC/cliff

Re: Tuning Kernel Parameter dbc_max_pct

Ted Buis — Sun, 28 Nov 2004 10:49:22 GMT

To avoid double buffering by Oracle and the operating system, OnLineJFS is valuable. See the Performance Cookbook attached for details.

Re: Tuning Kernel Parameter dbc_max_pct

Cliff Lim Kok Hwee — Tue, 30 Nov 2004 23:46:12 GMT

Gd Day Forum,

I just performed tuning of dbc_max_pct=10, rebuilt kernel and rebooted the server.

Upon a bootup and run glance, I realised that the buffer_cache is not at 800MB instead at other values.

kmtune -l -q dbc_max_pct

Parameter: dbc_max_pct
Current: 10
Planned: 10
Default: 50
Minimum: -
Module: -
Version: -
Dynamic: No

Query: Correct me if I am wrong, the setting is for maximum buffer cache and as such will not reflect the true/current cache buffer as shown on glance memory reports?

thanks/cliff

Re: Tuning Kernel Parameter dbc_max_pct

Patrick Wallek — Wed, 01 Dec 2004 00:07:54 GMT

Your buffer cache will not be at its maximum value right after a reboot. Your buffer cache will start out very small and slowly grow to the dbc_max_pct value.

Re: Tuning Kernel Parameter dbc_max_pct

Fred Ruffet — Wed, 01 Dec 2004 01:59:33 GMT

initial buffer cache size at boot time is fixed by dbc_min_pct. If you only changed max, you always start with your setting of 5% (i.e. 400MB).

If it is already 800MB (10%) a low time after boot, it shows your buffer cache is growing rapidly. How many time did you wait for you measure ?

Did you add mounting options to your Oracle filesystems ?

Regards,

Fred

Re: Tuning Kernel Parameter dbc_max_pct

Cliff Lim Kok Hwee — Wed, 01 Dec 2004 02:13:13 GMT

Hi Fred,

The cache_buffer is not growing currently since booted up.

regards/cliff

Re: Tuning Kernel Parameter dbc_max_pct

Fred Ruffet — Wed, 01 Dec 2004 04:11:34 GMT

Cliff,

What I meant isn't that it still grows, but it has come fast to its maximal value (800MB).

What about mount options ?

Regards,

Fred