It Depends: SMF 30 Job Timings And CPU Time
MartinPacker 11000094DH Comments (4) Visits (13037)
This may be stating the obvious - but I wonder to whom it actually is obvious...
I've been doing quite a lot of work with batch job timings and CPU recently. (Everything I'm about to say is equally true of steps.) It's interesting to think about the effects of faster engines versus more engines (a question I haven't been asked recently) and whether a customer needs more capacity or just faster engines (a question that has come into play).
There's nothing very new about this but it is worth thinking about. And in particular what we can glean from SMF 30 Step- and Job-end records...
We know lots of things about a job's timing and related stuff, most notably:
Suppose we eliminated queuing (by and large). We can't really say what the effect would be - except in our example job's case it's some fraction of the 70% that isn't CPU. With the EXCP count we can "hand wave" a number, but it's precisely that: a hand wave.
Some of the above is part of why I don't like to give elapsed time speed up estimates. And why I'm not overly keen on answering the "faster versus more engines" question for batch. It's actually worse than I've stated because individual job speed is hard to factor into the overall window's outcome when migrating to a newer processor. (This same problem applies no matter which speed up you apply to individual jobs and steps, of course.)
But the "unknowable CPU queuing" fact plays into how to interpret other facts like (as in our example) the job is "only" 30% CPU. We don't know whether it would've been 100% CPU without queuing or 30%. (Probably somewhere in between.) But we can use EXCP count, as I said, to help us guess.
For what it's worth I rarely see jobs much above 30% or much below 10% CPU. I'd say it was a bell curve around 20%. If true, this means most of the leverage is either in I/O time or CPU queuing. Though as a contrary data point I hear quite frequently of customers upgrading to faster engines and seeing their overall batch gets faster.
Welcome to my world of "it depends".