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Comment by newman314 | Hacker News Reader

A few changes to note since the P7/P8 days. As of P9, the default is now SMT8 instead of SMT4.

Changing the profile from say P8 to P9_base to P9 changes things under the hood in terms of processor capabilities. This roughly translates into VM hardware versions in VMware-speak.

There are cores and then there are virtual processors as well as the notion of processor folding. Here's a starting point: https://www.ibm.com/support/pages/aix-virtual-processor-fold...

Some P9 specific changes re folding: https://www.ibm.com/downloads/cas/39XWR7YM

Source: I have to deal with POWER systems day to day.

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> There are cores and then there are virtual processors as well as the notion of processor folding.

I'm really not sure if I understood that "processor folding" correctly -> would that translate in the case of Intel/AMD/Linux to something like a "dynamic hyperthreading" done under-the-hood by the OS?

I would say that folding is not a consequence of SMT, but of the assignment between the VP (virtual processors) assigned to the LPAR (VM) and the processing units assigned. So more a effect resulting from the virtualization with consequences in conjunction with SMT.

Furthermore, the folding behavior can be adjusted. From the second link that I posted:

In addition to the reduction of VPs, you can consider setting the schedo tunable “vpm_throughput_mode” to a value of 2. By default, running in raw throughput mode (i.e. vpm_throughput_mode=0) AIX spreads all available work across as many VPs as available, dispatching work first to the primary SMT thread of each VP, then the secondary SMT threads, and so on, thus providing the best performance in most workloads. This configuration, however, can lead to a higher PC (processor capacity) utilized by the LPAR. Depending the workload, you can reduce the PC by setting the vpm_throughput_mode tunable to 2, thus having AIX to schedule work to the primary and secondary threads equally.

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