IBM i Scalability Stays The Same With 7.3
April 25, 2016 Timothy Prickett Morgan
All operating systems are not created equal, at least not when it comes to NUMA scalability. For a long time now, the IBM i operating system has trailed the scalability of its peers, AIX and Linux, on Power processors when it comes to spanning the large number of cores and threads that IBM forges into its Power machinery.
Back in the dawn of time, and many of you were there with me, it was a very exciting thing to even be able to have a two-processor, two-thread machine like the AS/400-D80, which made its debut in April 1991 and which was the first multiprocessor machine IBM created in the midrange line and which set the stage for scalability improvements for years to come. (Getting two processors to cooperate is hard, but once you crack that, getting four or six or eight or 12 or whatever gets a bit easier if you architect it correctly.)
We did not expect to see a scalability increase with the new IBM i 7.3 release, but it would have been a nice indicator that the largest IBM i shops were bumping up against the ceiling and needed to be able to span more iron. As it turns out, IBM i 7.3 has basically the same scalability limits as the prior IBM i 7.2 release, which is at the Technology Refresh level right now. Take a look:
As you can see, the Power6 and Power6+ chips were stuck at two threads per core, and were capped at a total of 32 cores. With Power7 and Power7+ chips, IBM i was still capped at a maximum of 32 cores, but the operating system was tweaked so it could see all four threads on each core (what IBM calls SMT4 mode for the chips). Power8 machines can’t support the older IBM i 6.1.X releases, but with IBM i 7.1 TR8 the operating system can run in SMT8 mode and see all eight threads on a Power8 core and can support up to 32 cores, for a total of 256 threads. With IBM i 7.2 TR4, the operating system can see up to 48 cores and 384 threads, and as it turns out this is the same limit that the new IBM i 7.3 release supports on Power8 machines.
The funny bit, of course, is that IBM can deploy much larger Power8 iron than this. The Power E880, for instance, has up to 16 sockets with either 10-core or 12-core Power8 chips, for a maximum of either 128 cores or 192 cores, respectively. With SMT8 mode activated, these machines can have 1,024 or 1,536 threads, which is 2.7X to 4X the threads than a single instance of IBM i can span.
The fact that IBM supports 384 threads with IBM i 7.2 and 7.3 is obviously better than the situation was two years ago, when it topped out at half that number. So IBM continues to make improvements. And, of course, historically IBM has allowed customers to ask for special PRPQ support that allows for the thread count to be doubled over the published upper limits, so it is likely that if you need 768 threads for a big Power E870 or Power E880 system, you can get it. As a matter of principle, though, we think that IBM i should always span the full Power Systems iron, just like AIX and Linux do every time there is a new set of hardware.