Where's Axon?

Last year, before a rampaging virus ate the globe (we're told the locusts are coming as this article goes to press), IBM announced more details on what would be the last iteration of POWER9 at the OpenPOWER Summit, the "Advanced I/O" flavour variously codenamed POWER9 AIO, POWER9 Prime, Axon and (in a few places) Axone. As POWER10 neared availability, the final POWER9 generation was due to come out in 2020, but now into the beginning of December there's no such chip. So what happened?

Axon, as it happens, was in the roadmap under various names for quite awhile. IBM has always prioritized bandwidth as a market discriminant against commodity x86 hardware, and memory bandwidth and I/O are two things IBM POWER chips have in spades. To maintain this competitive advantage, in 2018 IBM announced POWER9 AIO with OpenCAPI 4.0 (up from 3.0 in the Nimbus-class POWER9 CPUs in this Raptor Talos II workstation), NVLink 3.0 (up from 2.0), plus CAPI 2.0, 48 PCIe 4.0 lanes and up to 350GB/s of memory bandwidth (compared to "just" 150GB/s in Nimbus, and 210GB/s in Cumulus with Centaur memory buffers). Back then it was slated for 2019; POWER10 was due in "2020+".

That date obviously slipped, so IBM came back in 2019 and announced AIO remained on the roadmap but this time for 2020 with a memory bandwidth of 650 GB/s using the new Open Memory Interface; instead of putting the Centaur buffers on the board, OMI now allows RAM vendors to put them right on the DIMMs. Again with an eye to their biggest competitive advantage, IBM promoted it as a "Bandwidth Beast" and gave it the name "AXON": "the ‘AX’ representing [symmetric multiprocessing and up to 24 SMT-4 cores], ‘O’ representing OpenCAPI and the ‘N’ representing NVLink. Think neuron-to-neuron axon connections in the brain," according to Jeff Stuecheli, POWER hardware architect. The variety of available interconnects was clearly positioned to succeed both the current scale-out Nimbus and scale-up Cumulus POWER9s and squeeze one more processor generation out of the hardware.

IBM may sometimes be overly bureaucratic but they don't generally leave money on the table, and if demand existed for such a product odds are they'd deliver. However, while the memory bandwidth is considerably greater, this would only happen with an OMI system which would put stress on both IBM and vendors like Raptor and Tyan to produce them, so blame COVID-19: no one really wants to be in the business of producing a stopgap design in the middle of a pandemic when their currently shipping systems are already imperiled by supply chain issues. Likewise, while OpenCAPI 4.0 and NVLink 3.0 are a nice bump, they're not enough on its own to justify that sort of investment when the identical node size suggests no improvements in raw compute (and, as probable confirmation, both Cumulus and Axon are described with the identical phrase "enhanced microarchitecture").

So what happened to Axon? 2020 happened. Ordinarily one could phone in such an upgrade but by now there's obviously not enough money in another go-around and the systems that would take best advantage of it don't and won't exist. For those of us on OpenPOWER workstations, our upgrade path for at least the next year is more cores with differently binned chips that don't require anything more than a board that can support their power and cooling requirements. POWER10 would require more design investment than POWER9 Axon, but not a lot more by comparison, and that investment is justified by the more and better reasons to buy a POWER10 server (assuming the openness issues are resolved) than there would be to buy this; furthermore, IBM is already integrating Axon's improvements into POWER10's communication fabric and marketing it as PowerAXON, making POWER9's iteration of it superfluous. These are powerful systems that vendors can actually sell, even in a down economy. At 7nm and with ISA enhancements, PCIe 5 and OMI out of the box, POWER10 should be faster and beefier all around, and it will certainly be more than Axon would have been.