We're looking at the possibility of a very similar move, so if you come across 
the actual reason, I would be interested.  Some of my thoughts though...

1) The smaller CPs on the zEC12 may keep a single high importance TCB from 
dominating as much of the machine.  E.G. such a TCB could consume about twice 
as much CPU on the 504 as it can on the 408.

2) Because you have more CPs, you have more total L1 and L1.5/L2 cache 
(although less per core) and certainly a lot more cache overall.  That may be 
substantially beneficial to certain workloads/configurations.  

3) It would be natural (at least for us) when doing a processor upgrade to add 
memory.  If you're avoiding paging and I/O due to having more memory, that 
could be some small improvement in CPU as well.  

It would be interesting to look at SMF 113 data before and after the upgrade.  

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