topic Re: What will happen when a system reaches the maximum of PID? in Operating System - OpenVMS

What will happen when a system reaches the maximum of PID?

Zsuzsa — Mon, 13 Oct 2014 09:00:53 GMT

Dear Colleagues,

I would like to know what will happen when a system reaches the maximum of PID (Process Identification Number).

PID is an 8-digit hexadecimal number and VMS generates an ordinal number for every new process in the 3-8. digits. So some 16 million processes can come off during the life of a VMS from boot to shutdown - it could be overstepped.

What will happen in this case?

Thanks for listening to me!

Best regards,

Zsuzsa Fodor

Q: What will happen when a system reaches the maximum of PID?

Ruslan R. Laishev — Mon, 13 Oct 2014 09:32:34 GMT

A:Apocalypse.

Re: What will happen when a system reaches the maximum of PID?

Volker Halle — Mon, 13 Oct 2014 09:43:13 GMT

Zsuzsa,

if you believe the OpenVMS Wizard, this is not a problem. The PID is not just 'a number', it consists of an index into the PCB vector and a sequence number.

Please see:

http://h71000.www7.hp.com/wizard/wiz_5774.html

Volker.

Re: What will happen when a system reaches the maximum of PID?

Hoff — Mon, 13 Oct 2014 16:47:24 GMT

@Volker Halle wrote:

if you believe the OpenVMS Wizard, this is not a problem. The PID is not just 'a number', it consists of an index into the PCB vector and a sequence number.

The Wizard's statement is correct.

The PID is best viewed as an opaque longword value with AFAIK an officially-undocumented internal format, and one that is subject to change without notice. (Yes, I know about the PID format description in the VMS Internals and Data Structures Manual, etc.) Like the roughly-analogous job controller queue entry ID, no dependencies on the current format or current behavior should be implemented in application code. The PID format hasn't changed within VMS in a very long time, but it has changed.

The PID value is guaranteed to be unique across all processes active on a single host and (if the OpenVMS system is clustered) across all processes within the same cluster at any moment in time, but no PID value is unique across all time; not across the lifetime of the host nor across the cluster, nor across any arbitrary clusters, nor across the lifetime of the contents of auditing or accounting files, nor across the PID values that might be stored elsewhere.

A more interesting question is where the OP might headed with this question, if what was posed is not the literal question. If this question is seeking some sort of reasonably unique value, then generating a UUID/GUID is a more typical mechanism, for instance.

Re: What will happen when a system reaches the maximum of PID?

Zsuzsa — Wed, 15 Oct 2014 17:07:06 GMT

Dear Volker, Hoff, Ruslan,

Thank you for answer and help. I have to accept this fact but I am not like it too much: I used to proudly mention to my students the lifetime uniqueness of PID, now I have to give up.

Zsuzsa