SMT Vocabulary Tips
Last revised: 2020-02-04, BKW
In the pre-SMT days we had all kinds of words we would use to talk about "a CPU". We would use processor and engine interchangeably. We would talk about an LPAR being a four-way. We would use IFL, CP, zIIP, zAAP, and ICF as nouns. And we would talk about utilization without needing to be very clear about what we meant. We could usually get away with our slang without confusing anyone.
With SMT we now have the notions of cores and processors. Cores contain processors: sometimes one, sometimes two. Because of this distinction we now need to be very careful about how we talk to one another. Here are some guidelines.
The machine is equipped with physical cores. Not engines, not
processors, not CPUs, not IFLs, not CPs.
An LPAR is equipped with logical cores. Not engines, not
processors, not CPUs, not CPs, not IFLs.
Cores, whether physical or logical,
come in different types: CP, IFL, and so on. Those
type-words are always adjectives:
a physical IFL core,
a logical CP core,
and the like.
No longer are the type-words ever nouns:
two CPs, and
are now phrases to be avoided.
On the Support Element's
LPAR activation profile configuration window, there is a choice
called Processors, but be assured what you are defining there are
the numbers and types of logical cores that
should be present in the LPAR.
No longer can we use the word way to talk about the
equippage of the LPAR. If I say the LPAR is a four-way,
do I mean it has four logical cores, or do I mean it has
four logical processors? Rather, we now should say
it is a four-core
LPAR, and then we can say whether it is running non-SMT, or SMT-1,
and physical cores have a percent-busy metric called
core utilization. For a logical core, this is the percent of
time the logical core is dispatched on a physical core. For a physical
core, this is the percent of time the physical core has a logical core
dispatched upon it.
FCX126 LPAR, FCX299 PUCFGLOG,
FCX302 PHYSLOG, and FCX306 LSHARACT depict core
Within a given scope (physical or logical) and type (IFL, etc.),
we can add up the individual cores' core utilizations to get a total
core utilization for the type. For example, we can say that
the physical IFL pool has a total core utilization of 325%. But note
that we never add up these values cross-type.
In a similar way, a physical type-pool or an LPAR's logical cores
of a type have an associated core capacity. This ranges
from 0% up to N*100%, where N is the number of cores. And we never
add up these values cross-type.
Contained within a core, either physical or logical, are
instruction execution units called
contain physical processors or physical CPUs.
contain logical processors or logical CPUs.
- If the core-type is IFL there can be either one or two CPUs contained in the core, depending upon the SMT level: non-SMT and SMT-1 configurations have one processor in the core, but SMT-2 configurations have two processors in the core.
- All other core-types always have one processor in the core, regardless of the SMT level.
Logical processors have a percent-busy metric called
This is the percent of elapsed time the processor
has a non-wait PSW loaded, just as
it always has been. Processor
utilization shows up
in PerfKit on
FCX144 PROCLOG, and
All of the following terms are synonyms for processor utilization:
percent busy is ambiguous. We don't know whether we're
talking about a core, or a processor, or an LPAR, or a CEC.
avoid the phrase, unless you make it very clear what exactly
you mean. Logical processor 6 is 35% busy is surely talking
about processor utilization.
System GDLVM7 is 81% busy
is very unclear. Core busy? Processor busy? Who really knows.
Though in some minute way it exists, nobody talks about physical CPU
utilization in an SMT environment.
Within a given LPAR and
type (IFL, etc.),
we can add up the individual logical
processors' CPU utilizations
to get a total
CPU utilization for the type. For example, we can say that
the logical IFL CPUs of our LPAR are 325% busy. But note
that we never add up these values cross-type.
In a similar way, an LPAR's logical processors
of a type have an associated CPU capacity.
from 0% up to N*100%, where N is the number of logical processors
of the type. And we never add up these values cross-type.
- Guests have virtual processors or virtual CPUs.
- z/VM dispatches guests' virtual processors on logical processors of the LPAR.
- Perfkit user-busy reports, such as FCX112 USER, report virtual CPU utilization.
Further, regarding SMT,
- z/VM always tells guests the SMT facility is absent, and so
- Guests always have virtual processors, never virtual cores.
- It is possible for virtual processors from two different guests to inhabit the two logical processors of an SMT-2 logical IFL core.
If you remain true to the above vocabulary rules, you will never confuse your conversation partner. And if he says his LPAR is a six-way with two CPs and four IFLs, ask him what he means, and then give him a link to this article :) .
For more information about how z/VM reports utilization, take a look at this article.