[Info-vax] How to obtain percentage of a given CPU consumed by a process

[Johnny Billquist]

On 2016-06-06 13:23, johnwallace4 at yahoo.co.uk wrote:
> On Monday, 6 June 2016 08:46:37 UTC+1, lawren... at gmail.com  wrote:
>> On Sunday, June 5, 2016 at 8:56:35 PM UTC+12, johnwa... at yahoo.co.uk wrote:
>>> I can't remember whether the approach I'd use can be done just with
>>> non-priv code...
>>
>> It was non-privileged code, that much I can remember. :)
>>
>> It had to do with the fact that CPU accumulation was done in a 100Hz interrupt routine that simply incremented a counter in the PCB of the current process. My code was running for just short of 10ms each time, before doing something that triggered a reschedule (might have been a $QIOW or a $HIBER, I can’t remember now). Only that “something” completed very quickly, putting my process back to being runnable again.
>>
>> I think I was actually consuming about 75% of the CPU on that VAX-11/750, without it showing up.
>>
>> I never tried the same trick again, so I have no idea if it would still work on later hardware or versions of VMS...
>
> That's basically the trick I was thinking of.
>
> Lots of OSes (not just VMS) calculate their performance/accounting
> data based on periodic sampling of which processes are 'current'.
> The current processes are then 'charged' for the whole interval.
> Most of the time this approach is plenty good enough. However...
>
> A process which somehow arranges to not be 'current' when the
> sampling is done, but eats CPU the rest of the time, can give a
> devloper, sysadmin, etc, a bit of a headache.
>
> Not specific to VAX, not specific to VMS.
>
> Don't tell anyone :)

Won't work on RSX. If a process yields the CPU before the time quanta is 
up, that is counted in a special bucket. So that counter would go up 
instead on RSX systems...
(And by yield I mean any kind of event that cause the OS to schedule 
another task.)

	Johnny