I'd made the assumption (in my code) that IRD would vary offline the top logical engines when it decided to vary an LPAR's logical engine offline.

In fact that isn't the case. I have data from a US company where it's engines 2 and 3 (out of the range 0-11) that tend to be offline more than all the others.

In principle even Logical CP 0 can be offline. (Indeed some of the test systems in Poughkeepsie have no Logical CP 0.) Now this was a problem for us because I used the "Online Time" (SMF70ONT) for Logical CP 0 to determine the RMF interval length in STCK format. I use that to determine how much of the interval any Logical CP is online (using their SMF70ONT). In theory I could be dividing by 0. In practice I haven't seen that.

Now APAR OA05798 documents a change in the order in which Logical CPs are varied offline. It talks about starting with CP 1, not CP 0.

But it does show how the implementation has already evolved and how wrong basic assumptions can be. :-(

Now I feel a new PMMVS chart coming on. :-) I have several times now plotted how much of the time each logical CP is online. This is, at this stage, of pedagogical interest. But clients do ask me to explain how IRD works. It

**is** counter-intuitive.