topic Re: How OS picks the PID in Operating System - HP-UX

How OS picks the PID

skt_skt — Thu, 04 Jun 2009 10:15:04 GMT

HP-UX 11.11/PA RISC
I happened to trace a Oracleprocess which is suppose to be terminating at a particular time( i was expecting them to be finish before 10.oo Hrs; see the extract o/p below). But i could see the same PID in the extract o/p more time running than expected. I am not sure if the same process was still running or a new one with same PID.

06/03/2009|07:45| 23367|oraclep500 |oracle | 0.00| 0.7| 4.5| 1.25| 0.25| 17704|
06/03/2009|07:46| 23367|oraclep500 |oracle | 0.00| 0.0| 0.7| 0.82| 0.44| 4988|
06/03/2009|09:16| 23367|oraclep500 |oracle | 0.00| 0.0| 0.0| 0.00| 0.00| 9592|
06/03/2009|09:33| 23367|oraclep500 |oracle | 0.00| 0.0| 0.0| 0.00| 0.00| 32|
06/03/2009|11:12| 23367|oraclep500 |oracle | 0.00| 0.0| 0.0| 0.00| 0.00| 3068|

Re: How OS picks the PID

Pete Randall — Thu, 04 Jun 2009 10:21:22 GMT

PID is sequentially assigned from 1 (the init process) up to PID_MAX (defined in limits.h) before they wrap around again. It is unlikely, unless you have an extremely busy system, that you would see a new process with the same PID.

Pete

Re: How OS picks the PID

Steven E. Protter — Thu, 04 Jun 2009 10:40:06 GMT

Shalom,

There is a table out there and PID's are picked sequentially.

ITs very simple.

You might find a leak detector script I wrote useful in this hunt

http://www.hpux.ws/?p=8

It identifies processes by memory use and is very flexible.

SEP

Re: How OS picks the PID

Dennis Handly — Fri, 05 Jun 2009 02:22:25 GMT

In order to provide the least surprise to users, free PIDs are kept around, instead of being reused right away.

Re: How OS picks the PID

Dennis Handly — Fri, 05 Jun 2009 03:11:37 GMT

>ME: provide the least surprise to users

It seems in order to do this, PIDs can't be assigned sequentially, they would use a FIFO algorithm, which is initially sequential.

Re: How OS picks the PID

Dennis Handly — Mon, 08 Jun 2009 23:15:23 GMT

Oops, I was wrong. PIDs are assigned sequentially but skipped if in use.
So on one ps(1) output, it is possible to see the same PID used for two processes.

Re: How OS picks the PID

V. Nyga — Tue, 09 Jun 2009 10:45:58 GMT

Well, with a 'ps -ef|grep ' you'll see when the process started, so it's simple to say if it's a new one.

Volkmar