[Info-vax] What choices are available to the OpenVMS Process Scheduler once all the CPUs are in use?

[Stephen Hoffman]

On 2016-02-10 19:09:14 +0000, abrsvc said:

> On Wednesday, February 10, 2016 at 1:40:06 PM UTC-5, Carl Friedberg wrote:
>> I would try turning off hyper-threads. I've never seen it make more 
>> than a few percent better perofrmance, and it can make things worse. 
>> Remember the VAX 782 (with 2CPUs, performance, IIRC, was of order .8 
>> VUP. Hoff will remember better.
>> 
> 
> I remember that well.  There was an analysis tool that would indicate 
> whether or not your particular application would benefit from the 
> additional CPU.  Please note that the 782 was an ASMP machine 
> (Asynchronous) and would only benefit compute bound applications.

The VAX-11/782 tool was "QUALIFY".   That tool was because the 
secondary was limited in what it could run.   The secondary processor 
was reasonably good at running compiles for VMS Engineering and for 
other similar computable and user-mode tasks for customers, but was not 
so good at flinging around I/O or various other kernel activities.   
The secondary didn't have access to the I/O, for that matter.   Hence 
the A in the ASMP nomenclature.  But I digress.

Hyperthreads is a different matter entirely, and the performance 
improvement can be hit or miss depending on details of the application 
load.

I'm not aware of any analog to QUALIFY here.   It was the application 
load that was used for this determination.

But if I have a 16-core box with threads lit and ~32 processes CUR and 
probably a few spare processes stuck in COM, then I'd look for a big 
issue somewhere in the system configuration and — in the absence of 
that — would start working toward an upgrade or load sharding or 
offloading, etc.     Beyond a good baseline review and on-going trend 
collections, and cases involving identifying and resolving the 
aforementioned big issues, extensive efforts around system tuning 
usually don't find substantial improvements commensurate with the 
effort involved.  It's usually ten kilos of software loaded into a 
five-kilo server.


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