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Hello Jacob,
Am 21.08.2024 um 23:08 schrieb Jacob Banda via MIDRANGE-L <midrange-l@xxxxxxxxxxxxxxxxxx>:
1. How the heck do I identify just how much memory is required for my workloads in an IBM i partition?
AFAIK there is no easy way do to this without going through adjusting-observing cycles. This is basically the same topic which has been explained over and over again in numerous articles and books about performance tuning all those decades. But just the other way 'round.
The nature of SLIC makes everything from DASD going through paging code to be chewed upon by the CPU. RAM is merely cache of stuff from DASD. While this is a gross oversimplification, it's sufficient for the purpose of explaining "how to meaningfully cut RAM".
Reading things from DASD means, the CPU will have to wait more often than "instantly" accessing things from RAM. Your paging stats in wrk-/dspsyssts will go up as long as the machine has actual processing to do. The latter is the crucial point.
How would I do this? Well, I'd estimate the size of data objects being part of the daily work routine for the machine. I'd take this amount as a starting point, set the partition's RAM to that and closely watch paging statistics and answer times from applications (bei it 5250 or be it web based stuff) as well as run time of batch jobs.
If the there is a lot of data (in a table) being necessary for daily work but rarely accessed, it's safe to take that out of the equation.
There is some overhead from loading application programs and their work space at IPL time which I don't know how to estimate. It all comes down to watch paging activity, measure answer times and decide if the status quo is barely tolerable, okay, or luxuriously small.
Also, for the current, luxurious amounts of RAM and possible I/Os per second in modern iron, I'd take some care to review and set system values per my experiences:
https://try-as400.pocnet.net/wiki/Post-Install_Optimizations#(Memory)_Allocation — this allows the system to adjust/optimize itself to changing workload over the day and dynamically assign available RAM to where it's needed.
Note: I have no first hand experience with modern machinery being part of big shops. My experiences are based on older machines which are severely CPU- and sometimes I/O constrained, and mostly waiting for work to do (idle). Your mileage may vary.
:wq! PoC
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