ncpu -p8 -z 25 -h1 -s 900

• This reads 8 processes, sleeping 25% of the time but pause for 1 second after each 1 second of CPU time and then stop after 900 seconds.
• This gives us a bunch of programs running and starting and stopping fairly randomly. The also provides a safety net, so this artificial workload will stop - even if I forget to kill the program!

  nmon -f -s5 -c60

• This command means: collect a snapshot every 5 seconds for 60 snapshots (5 minutes worth).

• Virtual machine called blue.
• Running AIX 6.1 TL6 on latest firmware.
• With Entitlement=0.4, uncapped, SMT=2 and virtual processes = 4.

• Virtual machine called diamond5.
• Running AIX 7.1 TL1 on latest firmware.
• With Entitlement=0.4, uncapped, SMT=4 and virtual processes = 4.

• The POWER7 virtual machine has twice the logical CPUs as expected as it is running SMT=4 instead on SMT=2
• The POWER6 graphs show a fairly even split of work between logical CPU CPU001 and CPU002 (these two combined make up the first POWER6 physical CPU-core) - this is because it is in SMT=2 mode and there is no real favourite between the SMT threads. One thread is as good as the other.
• The POWER7 graph show that for logical CPUs CPU001, CPU002, CPU002, CPU003 (these four combined make up the first POWER7 physical CPU-core), that the first logical CPU is very much more favoured than the second and third and fourth logical CPUs are not used much at all.
• The POWER7 behaviour is Intelligent SMT Threading mode switching in action. It knows there are not enough processes running (low run queue) to use SMT=4 so it has switched to SMT=2 and moves the processes to the first two logical CPUs. Then it notices that there are not even enough processes running for a fair chunk of the time to need SMT=2 so it switches to SMT=1 and moves the processes to the first logical CPU. This means the single running progress is getting the internal resources for the whole physical CPU-core with no contention from other threads and so gets a speed boost.
• Both POWER6 and POWER7 were using roughly 2.5 physical CPUs but it is clear with POWER7 that we could remove a physical CPU-core or even two physical CPU-cores as you can clearly see there are plenty of unused SMT logical CPUs to run work on.
• Once more for the record for shared CPUs: It is impossible to average the logical CPU utilisation stats to work out how busy are your physical CPU-cores because the logical CPUs are con-currently executing on the shared internal compute units of the physical CPU-core. You can't find the 2.5 in the graphs above.