Analysing A WLM Policy - Part 2
MartinPacker 11000094DH Visits (4069)
This is the second part, following on from Part 1.
Importance Versus Velocity
After drawing out the hierarchy you have to set actual goals - whether velocity or some form of response time. And you have to set importances.
The importances should now be easy as they flow from the hierarchy. IRLM should be in SYSSTC - which serves as an anchor at the top. It’s not quite as simple, though, as assigning from 1 downwards - perhaps with gaps. You might find there are too many hierarchical steps and you have to decide how to conflate some.
It’s important to understand that Importance trumps Velocity: Importance 1 goals are satisfied first, then 2 and so on.
But a low Velocity service class period with Importance 1 might well have its goal satisfied too easily and WLM will then go on to satisfy less important service class periods rather than trying to overachieve the Imp 1 goal.
Further, a velocity goal that is always overachieved provides no protection on those occasions when resources become constrained: The attainment could well be dragged all the way down to the goal.
At the other extreme an overly aggressive velocity goal can lead to WLM giving up on the goal.
It sounds against the spirit of WLM but I’d set a goal at roughly normal attainment - assuming this level provides acceptable performance,
Actually the same things apply to response time goals: Importance overrides and setting the tightness of the goal right is important.
What’s In A Name?
I’ve seen enough WLM policies now to know they fall into three categories:
I know these because of the names therein. (By the way it’s the third category I see the most problems in.)
The first two contain names which are rhetorically useful such as “STCHI”. It’s better not to either name them something too specific - in case you have to repurpose them - or to encode the goal values in the name - in case you have to adjust them (as you probably will).
By the way the same applies to the descriptions - which appear in SMF 72–3. If I ever learn Serbo-Croat it’ll probably be from SMF.
The Importance Of Instrumentation
Having just mentioned SMF let me talk about instrumentation.
Recall I was asked to look at a WLM policy.
Initially I was sent a WLM policy print. I then asked for (and swiftly got) appropriate SMF.
The point is it’s both you need:
Note the SMF doesn’t give you classification rules but the policy obviously does.
As an aside I’ve posited to Development it would be useful to instrument which classification rules fire with what frequency. Do you agree?
The most obvious use case is figuring out which rules are actually worthwhile, not that that’s a major theme in WLM tuning. I suspect there are others.
I’d like to thank Dougie Lawson and Colin Paice for their help in thinking about certain subsystems they are more conversant with than I am. This whole discussion would’ve been a lot worse without their input.