I was going to ask the same question, and appears that this is your culprit.
_______________________________ Dave Jousma Vice President | Director, Platform Engineering Fifth Third Bank | 1830 East Paris Ave, SE | Grand Rapids, MI 49546 From: IBM Mainframe Discussion List <[email protected]> on behalf of Paul Feller <[email protected]> Date: Wednesday, December 17, 2025 at 2:48 PM To: [email protected] <[email protected]> Subject: Re: Nasty IPL Boost surprise CAUTION EXTERNAL EMAIL This message came from outside your organization. DO NOT open attachments or click on links from unknown senders or unexpected emails. Report Suspicious<https://us-phishalarm-ewt.proofpoint.com/EWT/v1/MwwqYLOC6b6whF7V!rrOKGRUpDmBLO4SzTfKs95fJiV_fA1EcU7yUvsH7Zac6maUB1HEQ2AiEy3gOhiL1ZAEf3WgUg5w2VfodrcqNwatFI8IM$> Jay, I have to ask. Are the zIIPs set in the lpar profile to be online at IPL time? By chance are you normally bringing the zIIPs online after the IPL is done? If I remember correctly the boost process by default will only keep online the zIIPs it see as being online at IPL time. That would be those zIIPs you have configured in the lpar profile to be online. So, if the zIIP setting for INITIAL is set to 0 and RESERVED is set to 2 after the boost process is done it would take the zIIPs offline. On the flip side from what I remember if INITIAL is set to 2 and RESERVED is set to 2 then when boost is done there should be 2 zIIP online. I don’t recall ever using the PRESCPU option. From the MVS Initialization and Tuning Reference PRESCPU This parameter causes system initialization CPU processing to bring logically online those CPUs (and only those CPUs) that are physically online when the IPL is initiated, without regard to the number of CPUs defined to be initially online in the logical partition profile. The purpose is to ensure that all CPUs that are online when an IPL is initiated are online when the IPL completes. CPUs that are offline when an IPL is initiated remains offline. You might not want to use PRESCPU when WLM CPU management is active because that processing might be configured CPUs offline. If PRESCPU is not coded, CPU initialization takes CPUs offline or brings CPUs online as needed to make the number of online CPUs equal to the number of initial CPUs defined for the partition, when that information is provided by the machine (as it should be on model 2064 (GA-3), model 2066, and later machines) and is not 0. If the number of initial CPUs is not provided or is 0, CPU initialization attempts to bring online all CPUs that are physically online. On most models, those are the CPUs that were online just before the IPL was initiated. However, models 2064 and 2066 can configure additional CPUs online when an IPL is initiated, and CPU initialization also brings those CPUs online. in System Recovery Boost. For information about how PRESCPU interacts with BOOST, see Interaction of Shutdown Boost an Paul -----Original Message----- From: IBM Mainframe Discussion List <[email protected]> On Behalf Of Jay Maynard Sent: Wednesday, December 17, 2025 10:07 AM To: [email protected] Subject: Nasty IPL Boost surprise We cut our production LPAR over from a z14 to a z16 this morning, yay! We did discover one thing the hard way, and it came as a nasty shock. The z16's IPL Boost feature is a nice thing. It basically unleashes the two zIIPs we have configured (alongside the two CPs on a z16-M02) to do CPU workloads as well. It did make our system come up much quicker. But when the hour was up, the system ended the boost by configuring the two zIIPs completely offline. We didn't discover this until almost three hours later when we realized that no work was getting dispatched on the zIIPs. We recovered by configuring the two zIIPs online. We're an Adabas and Natural shop, and essentially our entire application workload is zIIP eligible and normally runs that way. Not having the zIIPs running would have severely impacted our system's performance. Is there a way to have the system end boost not by taking the zIIPs completely offline, but making them zIIPs again? Or is the best approach here to automate a response to the IEF678I or IWM064I messages to issue the CF CPU command? -- Jay Maynard ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] <mailto:[email protected]> with the message: INFO IBM-MAIN ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO IBM-MAIN This e-mail transmission contains information that is confidential and may be privileged. 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