On 06/08/2018 02:50 AM, Parwez Hamid wrote: > Re the comment: > > How prevalent are installations today where the CPs run at top speed, in > other words at the same speed as zIIP engines? In other words, Is it that > valid to assume equal speed processors? Clearly guidelines for lower zIIP > utilization matter more when there is a difference, as offloading any zIIP > work to a slower CP would elongate processing and response time, even if > there is no delay waiting for a processor. > > For any given Z system, the 'speed' (GHz) is always the same for all types to > (PUs) processing units (CPs, IFLs, zIIPs, ICFs, IFP and SAP). In case of CPs, > these can be different 'sub capacity settings' All the rest are always full > capacity setting. Speed is always the same. I can't find a decent explanation of how the sub capacity enforcement on a CP is managed beyond "the clock frequency... remains unchanged ... adjustment is achieved through other means". That statement doesn't actually say that instruction execution speed in unaffected, only that clock frequency is constant: one way to enforce 50% capacity at the same clock frequency would be the old IBM 407 approach of running at full speed but doing no productive work on half the clock cycles; but, considering all the parallelism in today's processors, I suspect it would be much simpler to just force the entire CP to be idle or non-dispatchable for an interval of time when the utilization exceeds allowable levels.
Would be curious if anyone has seen a more complete description about how the sub-capacity CP enforcement works. On 06/08/2018 02:36 PM, Peter Hunkeler wrote: > >> .... the workload on the CP is totally different. > > > Is it? If you think about Java, maybe. But when it comes to workload such as > DB2, Sort, Monitors, that have shifted more and more of its task towards > zIIPs, isn't this still the same workload? > -- > Peter Hunkeler The zIIP-eligible criteria for choosing a subset of tasks to run on zIIP engines, as I understand it, has nothing to do with installation defined service classes but is totally based on IBM marketing strategy. There is no reason to expect the mix of tasks eligible for zIIP resources to have the same service-class mix and CPU/IO usage patterns as those restricted at that same time to CP resources --the zIIP utilization may even peak at a totally different time of day. The total system workload may be the same as before things were made zIIP eligible, but with artificial separation into those that prefer a zIIP and those that must run on a CP, that workload is now artificially subdivided into two distinct and different workload subsets when competing for CPU resources. If zIIP utilization forces something that would normally run on zIIP onto a CP, it is now competing for CPU resources with a different subset of that total workload, and it would be surprising if that shift didn't affect response time. Joel C. Ewing -- Joel C. Ewing, Bentonville, AR [email protected] ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO IBM-MAIN
