changing `dynamic buffer cache` to free up memory

[Jerry Sims]

I'm in the process of installing HP's `SCM3.0` on our 11i system. SCM3.0
requires a minimum of 256MB of memory. From the swapinfo listed below, it seems
I have over 500MB of memory free? When I review measureware it shows 'total
system memory at 83%'. The current `dbc_max_pct`=30, and the `dbc_min_pct`=5.
My question is, if I change the `dbc_max_pct` from 30 to 20 will this deliver
the additional 256MB of memory needed to install SCM3.0 ?

Following is some info the 11i system I want load SCM3.0 on:

System info for host, model, OS:
root@DALUXCLE # uname -a;model
HP-UX DALUXCLE B.11.11 U 9000/800 1104474626 unlimited-user license
9000/800/L3000-7x

Dmesg info for memory:
Jun 5 17:52:23 DALUXCLE vmunix: Memory Information:
Jun 5 17:52:23 DALUXCLE vmunix: physical page size = 4096 bytes, logical
page size = 4096 bytes
Jun 5 17:52:23 DALUXCLE vmunix: Physical: 2097152 Kbytes, lockable:
1546296 Kbytes, available: 1782664 Kbytes

Swapinfo for swap:root@DALUXCLE # swapinfo -ta
Kb Kb Kb PCT START/ Kb
TYPE AVAIL USED FREE USED LIMIT RESERVE PRI NAME
dev 2097152 0 2097152 0% 0 - 1 /dev/vg00/lvol2
dev 2097152 0 2097152 0% 0 - 1 /dev/vg01/lvol1
reserve - 701204 -701204
memory 1548820 998720 550100 64%
total 5743124 1699924 4043200 30% - 0 -

root@DALUXCLE # sysdef | grep db
dbc_max_pct 30
dbc_min_pct 5

[Patrick Wallek · Accepted solution]

I seriously doubt that you would have a problem installing SCM on this box with 2GB of RAM.

Have you tried installing it? Did you get an error?

I would try the installation first since resetting a kernel parm requires a reboot.

There is no guarantee that changing the dbc_max_pct will give you the additional RAM. It will only free up space if you buffer cache is at it's maximum size. BUT, the way buffer cache is supposed to work is that if your system comes under memory pressure, the buffer cache will be reduced to provide more RAM for your programs.

30% may be a bit high for buffer cache, but I seriously doubt that it would require a change in order to install SCM.

[steven Burgess_2]

Hi Jerry

Recommendations are for a max of 10% for dbc.
Reduce it to 10 and you should free up 20% of your memory usage ? Thats how I would read it but!

There are a couple of things I suppose need clarification, which, now the question is raised I am also interested.

when you set your max value is the space allocated not used by other processes other than buffer cache ? Ie , I have a process that is taking up x% of memory. Data is being written to disk and the buffer cache is growing to the point where it needs all of its allocation. Is this allocation available when needed or are other processes required to be swapped out to serve the requirments of the buffer cache ?

Are you trying the install on a busy system ? Is the buffer cache high when you try the install ? if 'yes' then a reduction should certainly work

Interesting

HTH

Steve

[Con O'Kelly]

Hi Jerry

You can't use swapinfo -ta to determine free memory. Use glance if its installed (-m option) or alternatively
# vmstat 1 3
(Look for the "free" column and multiply this value by 4 to give free memory in bytes).

If you have don't have much free memory on the system, then you could reduce dbc_maxt_pct to 20% but with only 2GB of RAM I wouldn't reduce further than this.

Alternatively you can determine which processes are using the memory & if possible shutdown them down while trying to do the install.

To determine top memory processes use the following:
# UNIX95= ps -el -o pid,comm,sz | grep -i -v pid | sort -nr -k 3,3|head -25

Cheers
Con

[Steven E. Protter]

I think you should go further.

dbc_max_pct 30
dbc_min_pct 5

to

dbc_max_pct 10
dbc_min_pct 5

Maybe even lower that max figure to 7. It is very expensive in terms of CPU time to change these settings and the bigger the change, the bigger the whallop on your system performance.

This document, written by a sharp HP-UX performance guy goes into this and other topics.

http://www1.itrc.hp.com/service/cki/search.do?category=c0&docType=Security&docType=Patch&docType=EngineerNotes&docType=BugReports&docType=Hardware&docType=ReferenceMaterials&docType=ThirdParty&searchString=UPERFKBAN00000726&search.y=8&search.x=28&mode=id&admit=-682735245+1064363952456+28353475&searchCrit=allwords
Doc ID: UPERFKBAN00000726

It has recently been updated.

SEP

[Con O'Kelly]

Hi

I'd be careful setting dbc_max_pct to only 7%. With 2GB of RAM that only gives you 140MB of Buffer Cache. General recommendations are 200MB-400MB. From what I've heard 11i can also better utilize higher buffer cache settings.

Cheers
Con