e i g h t - c u b e d . c o m 

Interrupt time

For the last couple of versions, OpenVMS has offered "Fastpath", a way to offload most disk interrupt procesing from the primary CPU on SMP systems.

There are a couple of SYSGEN parameters controlling this functionality, one of which could give you a performance boost, especially if you have a lot of disk.

To check that you have Fastpath enabled, you can look at the FAST_PATH SYSGEN parameter:

SYSGEN> show fast_path
Parameter Name           Current    Default     Min.      Max.     Unit  Dynamic
--------------           -------    -------    -------   -------   ----  -------
FAST_PATH                       1          1         0         -1

If it's not set to 1, I'll wait while you modify it and reboot...

The interesting parameter is IO_PREFER_CPUS, which is a bitmask specifying which CPUs can perform Fastpath processing. By default, this is set to -1, which means all CPUs will perform Fastpath, including the primary processor.

If your primary processor is saturated with interrupt processing, excluding your primary CPU from the bitmask may be a significant win.

As IO_PREFERED_CPUS is a dynamic parameter, you can just tweak it without a reboot, however don't forget to add the new value to MODPARAMS.DAT and run AUTOGEN.