HP-UX memory issue

[Sureshbabu Nair_1]

Hi,
After monitoring our HP server, HP 5470 running HP-UX, I found that my server is using very high memory. Since its a DB server, ideally most of the memory should be used by the oracle process. We found that the "buffer_cache" is about 50% of the memory. We found that kernel parameter "dbc_max_pct" is set to 50. Please advice me whether or not we should reduce this?.. Is there a undesirable side effects to reducing this parameter?

thanks in advance,
Suresh

[T G Manikandan · Accepted solution]

There are lot of discussions on this topic,you can search the forum.

The recommendation is to have set the dynamic buffer cache to around 300-400 MB on your server.

by default it is 50% of your memory,which is spared for your buffer cache.

When you reduce this you can use the freed up percentage to improve the memory performance.

[Isralyn Manalac_1]

50 is the default value for dbc_max_pct. We have lowered our dbc_max_pct and dbc_min_pct because we encountered a similar problem. But it is not to say that this should be the same as your case. Would you mind posting the output of:

#sar -b 5 30

Which UX version are you using? When you mentioned it was using high memory, which is it? Usr or sys? Which Oracle DB version do you have installed on this server?


Regards,

Isralyn

[Steven E. Protter]

In real world production:

dbc_max_pct 7
dbc_min_pct 5

Reasoning:

1) Double buffering Oracle db does nothing to help performance

2) Its very expensive in terms of CPU terms to change the values on a system. When the change is occuring, there isn't much else the system can do.

SEP

[Eric Antunes]

Hi,

As SEP said you should be buffering just on Oracle side with a confortable db_block_buffers (something like 3000). Also avoid very large log_checkpoint_interval...

Best Regards,

Eric Antunes

[Paul Bardeskar]

Hi Suresh,

You can set the dbc_max_pct to 15 and dbc_max_pct to 5

This will certainly give you the result.

regards,
Paul

[Sureshbabu Nair_1]

hi Isralyn,
As per your request, I am attaching the sar report

I really thankfull for the fast reponses I got.

Thanks

Suresh

[sreejith_4]

Hi,

setting buffer cache to 500 would be ideal.

also check ur shmmax value..this shuld not be very high on a low memory system

Thanks
Sreejith M

[Geoff Wild]

Also - you can disable the buffering at the lvm level for your Oracle db files like so:

convosync=direct,mincache=direct as well as rw,suid,largefiles,delaylog,datainlog

NOTE: that is only for database files! Any commands like tar, awk...etc will suffer a performance hit - because that will bypass the OS buffer cache.

Depending on how you back up the database - you could suffer there as well....

Main thing for you is - to decrease dbc_max_pct - depending on how much ram you have...

2GB - try 25%
4GB - 14%
10GB - 7%

Rgds...Geoff

[Devender Khatana]

Hi,

I also would suggest that you can not set to a fix value on all systems. Just set it according to the physical memory available in your system.

Generally dbc_min_pct should be 5 & dbc_max_pct should be 8 if your system has sufficient memory. But calculation of your sufficient memory is done having look at your application requirements.

50% is too much in any sense and we should always reduce it.

Apart from the parameters what Geoff suggested you can try be using read_nstream=1 ,write_nstream=1. This will surely help if sequentional reads are not very high.

HTH,
Devender

[Isralyn Manalac_1]

Hi Suresh,

Here's an excerpt from the intro to perf tuning that I've attached here:

Ideally we want to see a %wcache of 95 or greater. If the system consistenly shows %wcache less than 75 it would be advisable to lower the value of dbc_max_pct.

As it is your case, you might want to consider lowering both values of dbc_max_pct and dbc_min_pct.

Regards,

Isralyn

[Darrel Louis]

Hi,

When setting your nbuf and bufpages to zero
and by setting dbc_min and max to a lower percentage, you will make use of Dynamic buffer cache.
Depends on the amount of physical memory and your applications..

dbc_max_pct Max dynamic buffer cache between 3 and 10 % of memory
dbc_min_pct Min dynamic buffer cache between 2 and 5% of memory
See the following Best practices document
http://h21007.www2.hp.com/dspp/files/unprotected/database/HP3KOracle.ppt

GoodLuck

Darrel