From: Trevor Perry To: All
I would like to raise a question regarding AS/400 (and S/38) performance. It seems that everyone I talk to is scared of running their machine at a high CPU usage percentage. There are many stories about running at a number like 99 percent or even the magical +++++. Why is everyone scared about this?
The Bible says to run over 90 percent. Often, when some or all of the other tuning factors are not quite right, then a high CPU percentage and a slow machine happen at the same time. But this does not mean that the high CPU percentage is the cause. Shouldn't all AS/400s run at 100 percent--the reason being that if you run lower than the full capability of your machine, then you are wasting some of the investment?
Even so, isn't the aim of performance tuning to even out the performance and make it a fair spread for all users, with a side benefit of speed? So, shouldn't all AS/400s run at 100 percent CPU usage all day long?
I would like to hear some comments on this subject and maybe, someone has some ideas about how to teach the average AS/400 shop what performance tuning is really about!
From: Toby Miller To: Trevor Perry
I would tend to agree with you. Our AS/400 B30 runs at 97.8 percent to '+++++' during regular business hours. Initially we were getting fairly poor response times, but after some adjustment (changing pools, max active, etc.) we got a relatively decent response even though we were still running at '+++++.' We are going to go to a faster machine (D45), but it is only because we are increasing the workload on the machine. I think that if you aren't getting above 90 percent, you are wasting your investment in the machine.
From: Michael Russell To: Trevor Perry
Read the Work Management Guide and the Performance Tools manuals. The guidelines are in terms of workload type. Basically, you want the sum of CPU utilization for interactive jobs and all jobs with higher priority (less than or equal to 20) to be less than 60 percent. Actually, I prefer 50 percent. It is those jobs with same or higher priority which will interfere or affect interactive response time. The remaining 40 percent of the CPU is available for batch jobs.
There is nothing wrong with running at 90-100 percent of CPU. (By the way, the +++++ is a result of rounding errors which causes the CPU utilization to add up to greater than 100). Filling in the gap, so to speak, is just taking full advantage of your resources. The only possible problem that may result from running at 90-100 percent is if the batch jobs causes the DASD controller or arm utilization to approach 40 percent, which will in turn have some impact on all other jobs.
I think the fear of running at 90 percent or higher is caused by not understanding work management. I think articles in both MC and News 3X/400 have addressed this issue. Also, the performance analysis feature part of V2R1M0's Performance Tool helps evaluate actual measured performance relative to the IBM guidelines.
From: David Romaine To: Trevor Perry
The performance curve of OS/400 is very interesting--unlike many other operating systems/platforms, response time remains fairly flat until you reach a highly saturated system. I'm not sure I agree with the "run it at 100 percent" theory. If you have your system well tuned, then the 30 percent margin you need for good interactive performance is used for batch processing. Same way, it seems to me, that highways aren't designed for peak loads, but for average loads.