This seems to come up a lot. I'm going to start by taking the opposite tack: you probably shouldn't run your GCPs at 90-100% busy either. Busier CPUs are generally going to have more cache contention which means the work is generally going to run "somewhat" less efficiently (i.e. more CPU time / unit of work) than if the CPUs were running less busy. Apparently some IBMer at some point measured that as a 4% increase in CPU time / unit of work per 10% increase in overall utilization. Alas I have no further details about the source of that number. I believe that it's at least directionally correct though.
Now if you have a mix of different importances / priorities for the work that is driving the machine to 100% busy, then likely the most (but not only) impacted work is the lower importance work. So maybe that's ok. But, in all likelihood, if the machine had more capacity and was running at only say 70% busy, then likely the same work would consume fewer MSUs. Which may be a good thing. From purely a performance perspective, running less busy is always better as there's less chance for queueing for a processor. But rarely is "as fast as possible" the required and most cost effective answer. But this question is about zIIPs. But zIIPs are the same processors as GCPs and the aforementioned discussion is mostly the same: you can run the zIIPs busy, but things may not be as efficient. Of course "less efficient" matters a little less on the zIIPs given that the hardware is cheaper and they don't increase software costs. The primary issue as I see it comes in where the zIIPs are running busier than the GCPs and so work is more delayed trying to get through the zIIP queue than if they had been just dispatched directly to the GCP. If that work is very important work (such as perhaps DB2 system tasks) then that could have relatively widespread negative impacts. Are those impacts greater than if there were no zIIPs in the configuration at all and the GCPs were running a similarly busy level? Maybe a bit due to the extra overhead of (unsuccessfully) scheduling work onto the zIIP. But I believe the potential for harm is very situation dependent. In particular with the way the rules are today, and with the mix of the different workloads that are zIIP eligible, if you have zIIP capacity (both in terms of MIPS and engines) greater or equal to your GCP capacity, I'm hard pressed to believe that there's a significant risk to running the zIIPs as busy as you're comfortable running your GCPs. But my belief is also that many are comfortable running their GCPs hotter than is really ideal. In Kathy Walsh's presentation from Orlando one of her slides has the statement: "Can run zIIPs very busy IF there are multiple classes of work with different response time objectives, but watch IIPCP time". I think that's a very reasonable statement. If you're starting to see significant crossover from the zIIPs to the GCPs, you're probably running the zIIPs too busy for that particular workload. Note that the crossover amounts are not necessarily well-correlated with zIIP busy, although generally when zIIPs are busy they are more likely to see significant crossover. A potential issue is that some systems only have a single zIIP. When you have a single CP, your threshold for where you'll start feeling pain is significantly lower vs. having evern 2 CPs. The situation of having a 710 with a single zIIP is a significantly different situation than having a say a 505 with 4 zIIPs. I'm going to be a lot more concerned about zIIP utilization in the former vs. the latter. In the former, that zIIP very well might become a bottleneck that could be more problematic than not having a zIIP at all. That would seem to be much less likely in the second case. My personal belief is that given the amount of work that's zIIP eligible today, most systems should have at least 2 zIIPs, and ideally the system should have zIIP capacity at least similar to the GCP capacity. Yes, there's a hardware cost to that, but in the grand scheme of things, the costs are not nearly as significant as GCP capacity, so err on the side of having too much zIIP capacity. That would be an interesting study: what's common ratio of zIIP capacity to GCP capacity? I suspect that that ratio has been creeping up over the years. Scott Chapman ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to lists...@listserv.ua.edu with the message: INFO IBM-MAIN
