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List SMT xx
On power 8 with AIX 7.1 you can have SMT8, on power 7 or AIX 6.1 only SMT4.

# lsattr -El proc0
frequency     3525000000
smt_enabled   true
smt_threads   8
state         enable
type          PowerPC POWER8

In order to change SMT, verify in the partition profile on HMC in processor section the compatibility mode. If power7 then you can have a maximum of SMT 4

# smtctl
This system is SMT capable.
This system supports up to 8 SMT threads per processor.
SMT is currently enabled.
SMT boot mode is set to enabled.
SMT threads are bound to the same virtual processor.

proc0 has 4 SMT threads.

Change SMT:

# smtctl -m on
# smtctl -t 8 -w boot
# bosboot -ad hdisk0

Here you can see which smt level is used

# lparstat -E 1 4

System configuration: type=Shared mode=Uncapped smt=8 lcpu=8 mem=6144MB ent=0.40 Power=Disabled

Physical Processor Utilisation:

 --------Actual--------              ------Normalised------
 user   sys  wait  idle      freq    user   sys  wait  idle
 ----  ----  ----  ----   ---------  ----  ----  ----  ----
0.138 0.029 0.000 0.233 3.5GHz[100%] 0.138 0.029 0.000 0.233
0.123 0.025 0.000 0.252 3.5GHz[100%] 0.123 0.025 0.000 0.252
0.155 0.031 0.000 0.214 3.5GHz[100%] 0.155 0.031 0.000 0.213
0.144 0.027 0.000 0.229 3.5GHz[100%] 0.144 0.027 0.000 0.229