Intel AS/400 RISC Server manual HMT and SMT Compared and Contrasted, HMT Feature SMT Feature

HMT and SMT Compared and Contrasted

Some key similarities and differences are:

HMT Feature	SMT Feature
yHMT is can be turned on and off only by a	ySMT can be turned on and off dynamically at
whole system IPL.	any time. No IPL required
yAll partitions have the same value for HMT	ySMT, because it is more dynamic, the SMT state
	need not be identical across partitions.
yHMT executes only one instruction stream at a	ySMT allows multiple streams of execution
time.	simultaneously.
yCPU utilization measurements are not greatly	ySMT complicates the question of measuring
affected by HMT.	CPU utilization.
ySystem performance counters and CPU	ySMT machines continue to report data on a
utilization values continue to be reported on a	physical processor basis, but some of the
physical CPU basis.	measurements are harder to interpret (reporting on
	a logical CPU basis would be no better).
yHMT operation is controlled by the system value	ySMT has three values for QPRCMLTTSK ("0"
QPRCMLTTSK ("1" means active, "0" means	for off, also called "ST mode", "1" for on, and "2"
inactive)	for "controlled" where OS/400 decides,
	dynamically, whether to be in ST or SMT mode.
yHMT needs a full IPL for the change to	ySMT can allow QPRCMLTTSK to change at any
QPRCMLTTSK to be activated.	time.
yHMT typically improves throughput by 10 to 25	ySMT can improve throughput up to 40 per cent,
per cent.	in rare cases, higher.

Models With/Without HMT

Not all prior models have HMT. In fact, some recent models have neither HMT nor SMT. The following models have HMT available:

y270, 800, 810, 820, 830, 840

The following have neither SMT nor HMT:

y825, 870, 890

Earlier models than the 270 or 820 series (e.g. 170, 7xx, etc.) did not have either HMT nor SMT.

IBM i 6.1 Performance Capabilities Reference - January/April/October 2008
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