I previously wrote about SMT in Born With A Measuring Spoon In It’s Mouth in 2016 – before z14 was announced. I also wrote about it again in 2016 in SMT – Some Actual Graphs. It’s been a year since z15 was announced so enough time has passed for me to want to write about SMT once more.
But actually there isn’t any real SMT news.
But there’s something I thought I’d written about before, but I hadn’t: With z14, IOPs are always enabled for SMT. Actually one of them isn’t, but the rest are. So, in SMF 78–3 you get an odd number of IOPs – and therefore an odd number of IOP Initiative Queue and Utilization Data Sections. One is not SMT-enabled and the rest are.
So, if you have 10 IOP cores you have 19 IOP sections.
It would be interesting to see how they behave. So I took data from a two-drawer z14. (It’s a M02 hardware model, with a software designation 507, with 7 GCPs, 4 zIIPs, and 5 ICFs. It has lots of LPARs.)
So, I used the 78–3 data to plot two metrics:
- Processed I/O Interrupts per second
- IOP Busy %
Here is the graph, with IOP Busy on the right-hand axis and I/O Interrupts on the left.
The numbers are interesting but there is no clear pattern:
- The I/O Interrupt rate varies wildly – and I suspect it has something to do with the devices and channels the IOP is handling.
- The IOP Busy % doesn’t necessarily correspond to the I/O Interrupt rate.
Probably the more important and useful metric is the IOP Busy number.
When I say “no clear pattern” I mean it would be difficult to say something like “IOP 4 is busier because of its position in the machine”.
I do think it’s worth keeping an eye on IOP Busy %. This particular set of data shows very low IOP utilisations – which is a go thing.
For a 2-drawer z14, 10 IOPs is the standard number but you can buy more. For z13 it was 12 and for z15 it’s 8. there’s a clear trend here. I do think that having SMT as standard on IOPs will have contributed to the possibility of reducing the number of standard IOPs. Obviously them getting a little bit faster with each generation helps, but you have to balance that against other processor types also getting faster. Another factor might be the historical trend towards more memory in a machine and fewer I/Os, relatively speaking.
My code knows that it’s standard for a 2-drawer z14 to have 10 IOPs. It has to calculate – especially from z14 onwards – the number of IOPs as this isn’t recorded. SMT is part of that calculation. So I report standard IOPs and additional IOPs – though I haven’t seen a case of the latter yet.
And this is in the “Engineering” series of blog posts as we’re dealing with individual processors, even if they are IOPs.
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