SysGen changes

[Douglass Anderson]

I was told to increase npag_aggressive to 100, and npag_gentle to 100. Is this a good idea? How do I change them, and then how do I verify the changes?

[Dale A. Marcy · Accepted solution]

I cannot comment on whether the change is a good idea or not, but there are several ways to change. To change temporarily on the running system do the following:

$ mcr sysgen
SYSGEN> USE ACTIVE
SYSGEN> SET NPAG_AGGRESSIVE 100
SYSGEN> SET NPAG_GENTLE 100
SYSGEN> WRITE ACTIVE
SYSGEN> EXIT

I would suggest doing a SHOW NPAG_ before changing values to see the current settings.

If you want to make the change permanent across reboots, then there are two methods. The best method is to edit SYS$SYSTEM:MODPARAMS.DAT and add the lines:

NPAG_AGGRESSIVE=100
NPAG_GENTLE=100

The other method is to perform the same block of commands as shown at the start of this message, but replace the word ACTIVE with the word CURRENT in both places. Active affects the currently running system and CURRENT modifies the structures that are read during the boot process.

[Dale A. Marcy]

Sorry, I read my post and realized I did not complete my answer. After editing the SYS$SYSTEM:MODPARAMS.DAT file, you will need to run Autogen and reboot to make the changes take effect.

[Andy Bustamante]

You and run

@sys$update:autogen help

for quick information about autogen. Generally you'd change these values (or insert) into modparams.dat then run autogen. Each set is input into the next step. If you modify modparams.dat you need to go back to GETDATA step.

After the SETPARMS phase you will have a copy of your old system parameters in sys$system:ALPHAVMSSYS.OLD. To restore the previous configuration rename this file to sys$system:ALPHAVMSSYS.PAR.

Whether or not these changes are a good idea depends on your system, application and so on. Sometimes you need to try a change and be prepared to return to a known state. OpenVMS makes this possible.

[John Gillings]

Douglass,

>Is this a good idea?

I guess it depends on who told you and why. It also depends on what version you're running. The defaults for V7.3 are 50 and 85, but for V8.2 they've been changed to 100 & 100.

NPAG_AGGRESSIVE and NPAG_GENTLE affect the way the system manages Non Paged Dynamic Memory lookaside lists. Some workloads can cause pool fragmentation, leading to excessive expansion. This can be alleviated by adjusting those parameters.

For details, see SYSGEN> HELP SYS NPAG_

If your system is exhibiting signs of pool expansion and/or fragmentation, then this might be worthwhile. On the other hand, if it's being done without regard to symptoms, I'd recommend against it.

Both parameters are dynamic, so they can be modified without a reboot, as described by Dale:

$ mcr sysgen
SYSGEN> USE ACTIVE
SYSGEN> SET NPAG_AGGRESSIVE 100
SYSGEN> SET NPAG_GENTLE 100
SYSGEN> WRITE ACTIVE
SYSGEN> EXIT


To check that they've changed, use

$ mcr sysgen show npag_

look at the "Current" column.

To see the effect of the changes, use SDA

SDA> SHOW POOL/NONPAGED/STAT

and look at the lookaside lists. Do this before you change the parameters. After changing them, wait at least NPAG_INTERVAL seconds and look again.


It surprises me that someone would recommend such a change without telling you why, how to do it, and whether it should be a permanent or temporary change. I'd strongly recommend going back and asking the whys and hows.

[Jim Frederick_1]

Just FYI,

I was also advised to set npag_aggressive and gentle to 100 by an HP technician that looked at a crash dump for us (OpenVms 7.2-1).

He stated :
-------------------------------------------
I suspect that the root cause of the reset and crash is that we ran out of
non-paged pool. We had expanded npagedyn from 4882432 to 24412160 (npagevir)
bytes and the largest packet of non-paged pool available to us is 1024 bytes.

I would recommend increasing npagedyn to 30MB, pagepyn to 8MB and upgrade the
console firmware to the latest version. I would also set npag_gentle and
npag_aggressive to 100 and smp_spinwait to 800000 to avoid potential pool
reclaimation problems.
-------------------------------------------
Hope that helps.

[Douglass Anderson]

change suggested not needed.