low memory server and dbc_max_pct

[Darren Murray_1]

I have a D Class server with 256Mb installed and currently the dbc_max_pct kernel parameter is set at 50.

The server only has the 5 internal disk drives.

The file systems are currently HFS and I plan to change them to VXFS.

It currently runs a old version of Ingres on the server.

What are your thoughts on reducing the dbc_max_pct parameter?

Cheers
Darren

[Michael Tully · Accepted solution]

For a system with such small amount of RAM, I wouldn't reduce it at all. This of course may depend on how well your application runs and how much swap space you have and the overall performance of your server. Generally speaking the recommended amount of dbc_max_pct should represent no more than 300-500mb.

[Con O'Kelly]

Hi Darren

Generally the default setting of 50% for dbc_max_pct is way too high for most systems.
However as you only have 256MB of RAM then I personally wouldn't recommend reducing it.

Cheers
Con

[Darren Murray_1]

Thanks Michael

Good to hear as I am now 1-0 in the debate I am having with someone.

Any other suggestions on what to tweek on the kernel to improve performance on the server. It currently runs 10.20

[Darren Murray_1]

In saying you wouldnt reduce it from 50 , if you had exact replica of the server would you increase it from 20 to 50?
Cheers
Darren

[Michael Tully]

Just so you can get to at least 1 1/2 - 0 we need to know a little more about your system.

application/database any current performance issues that stick out like a sore thumb, heavy disk IO, how much swap/and utilisation and current kernel parameters. Different systems require different kernel settings ...

[Steven E. Protter]

A agree with Michael but:

what about dbc_min_pct

If that is a nice low number like 5, you have a very expensive performance problem.

Whenever the system changes the actual dbc value, its extremely expensive in terms of CPU.

Therefore you might want to consider having those numbers closer to each other to avoid expensive changes.

Also look at vx_ninode after you convert to vxfs.

Going with the system default, zero means the system sets it to whatever it thinks it should. This is EXTREMELY expensive in terms of CPU.

Now I have to change that value ,because the ssytem I just checked so I'd spell the parmameter right is not set correctly.

vx_ninode is more of a HFS feature.

Good Luck,

SEP

[Darren Murray_1]

Michael,

The machine is quite i/o intensive as they do a backup to disk which copies from what file system to another.


Steve,

The dbc_min_pct is set to 5

Im not quite sure from your last section

about vx_ninode after you convert to vxfs.

How do u determine what vx_ninode should be?

Cheers
Darren

[Con O'Kelly]

With only 256MB of RAM you will be very limited in the tuning you can do. I'd firstly check if you have a high page out rate and the amount of free memory.
Use:
# vmstat 2 5

Look for the 'po' value to show pageouts.
Also look at the 'free' value. This shows amount of free memory in 4K blocks (therefore multiply by 4 to get free memory in KB.)

If you are i/o intensive then you definitely want to keep the value of dbc_max_pct at 50%. i/o activities make extensive use of the buffer cache.

The following doco might help a bit in tuning your system (especially the bit on max_dbc_pct):
http://docs.hp.com/hpux/onlinedocs/os/11.0/tuningwp.html

Cheers
Con

[Mike Stroyan]

| The machine is quite i/o intensive as they do
| a backup to disk which copies from what file
| system to another.

Note that a big backup is not likely to gain performance from a 128Mb buffer cache. The backup will be handling lots of file data exactly once, so it won't get any cache reuse.
The only hope for reuse would be directory data, which may be squeezed out of the buffer cache by file contents anyway.

[Paula J Frazer-Campbell]

Hi
On a average server (if there is such a beast)

The dbc_max should be about 400 meg and dbc_min about 80 meg.

you are low on memory - search the www for HP compatable memory and get "The Management" to dip into their pockets.




Paula