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NAME | C SYNOPSIS | DESCRIPTION | SEMANTIC CHECKS AND RULES | EXPRESSION EVALUATION | PMIDs AND MASKING | CAVEATS | DIAGNOSTICS | SEE ALSO | COLOPHON |
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PMREGISTERDERIVED(3) Library Functions Manual PMREGISTERDERIVED(3)
pmRegisterDerived, pmRegisterDerivedMetric - register a global
derived metric name and definition
#include <pcp/pmapi.h>
char *pmRegisterDerived(char *name, char *expr);
int pmRegisterDerivedMetric(char *name, char *expr,
char **errmsg);
cc ... -lpcp
Derived metrics provide a way of extending the Performance Metrics
Name Space (PMNS) with new metrics defined at the PCP client-side
using expressions over the existing performance metrics.
Typical uses would be to aggregate a number of similar metrics to
provide a higher-level summary metric or to support the ``delta
value-a over delta value-b'' class of metrics that are not possi‐
ble in the base data semantics of PCP. An example of the latter
class would be the average I/O size, defined as
delta(disk.dev.total_bytes) / delta(disk.dev.total)
where both of the disk.dev metrics are counters, and what is re‐
quired is to sample both metrics, compute the difference between
the current and previous values and then calculate the ratio of
these differences.
The arguments to pmRegisterDerived are the name of the new derived
metric and expr is an expression defining how the values of name
should be computed.
pmRegisterDerivedMetric is the exact functional equivalent to pm‐
RegisterDerived except that it provides a simplified model of er‐
ror handling, where a formatted message is returned via the errmsg
parameter.
Syntactic checking is performed at the time pmRegisterDerived is
called, but semantic checking is deferred until each new PMAPI
context is created with pmNewContext(3) or re-established with
pmReconnectContext(3), at which time the PMNS and metadata is
available to allow semantic checking and the metadata of the de‐
rived metrics to be determined.
If pmRegisterDerived is called after one or more PMAPI contexts
has been opened, then the newly registered metrics will be avail‐
able in those contexts, however the more normal use would be to
make all calls to pmRegisterDerived (possibly via
pmLoadDerivedConfig(3)) or pmRegisterDerivedMetric before calling
pmNewContext(3).
All of the defined global derived metrics are available in all
PMAPI contexts.
It is also possible to define per-context derived metrics once a
PMAPI context has been established. These derived metrics are
private to the context in which they are defined using the allied
routines pmAddDerived(3) and pmAddDerivedMetric(3).
name should follow the syntactic rules for the names of perfor‐
mance metrics, namely one or more components separated with a dot
(``.''), and each component must begin with an alphabetic followed
by zero or more characters drawn from the alphabetics, numerics
and underscore (``_''). For more details, refer to PCPIntro(1)
and PMNS(5).
name must be unique across all derived metrics and should not
match the name of any regular metric in the PMNS. It is accept‐
able for name to share some part of its prefix with an existing
subtree of the PMNS, e.g. the average I/O size metric above could
be named disk.dev.avgsz which would place it amongst the other
disk.dev metrics in the PMNS. Alternatively, derived metrics
could populate their own subtree of the PMNS, e.g. the average I/O
size metric above could be named my.summary.disk.avgsz.
The expression expr follows these syntactic rules:
• Terminal elements are either names of existing metrics or numer‐
ic constants. Recursive definitions are not allowed, so only
the names of regular metrics (not other derived metrics) may be
used. Numeric constants are either integers constrained to the
precision of 32-bit unsigned integers or double precision float‐
ing point numbers.
• The usual binary arithmetic operators are supported, namely ad‐
dition (``+''), subtraction (``-''), multiplication (``*'') and
division (``/'') with the normal precedence rules where multi‐
plication and division have higher precedence than addition and
subtraction, so a+b*c is evaluated as a+(b*c).
• Unary negation may be used, e.g. -3*some.metric.
• C-style relational operators are supported, namely ``<'',
``<='', ``=='', ``>='', ``>'' and ``!=''. Relational expres‐
sions return a value as a 32-bit unsigned number being 0 for
false and 1 for true. The expected operator precedence rules
apply, so arithmetic operators have higher precedence than rela‐
tional operators, and a-b>c+d is evaluated as (a-b)>(c+d). All
the relational operators have equal precedence, so the (slightly
odd) expression involving consecutive relational operators
a>b!=c is evaluated as (a>b)!=c.
• C-style boolean operators are supported, namely and (``&&'') and
or (``||''). Boolean expressions return a value as a 32-bit un‐
signed number being 0 for false and 1 for true. The expected
operator precedence rules apply, so relational operators have
higher precedence than boolean operators, and a>b*c&&d<=e+f is
evaluated as (a>(b*c))&&(d<=(e+f)). Both the boolean operators
have equal precedence, so the expression involving consecutive
boolean operators a>=b||b>c&&d!=e||f>g is evaluated as
(((a>=b)||(b>c))&&(d!=e))||(f>g).
• Additionally, the ``!'' operator may be used to negate a boolean
or relational expression, returning a value as a 32-bit unsigned
number being 0 for false and 1 for true. The expected operator
precedence rules apply, so boolean (and relational) operators
have higher precedence than boolean negation, and !a>b||c<d is
evaluated as !((a>b)||(c<d)), while !a<b+c is evaluated as
!(a<(b+c)).
• C-style ternary conditional expressions are supported. In gener‐
al terms the expression check ? foo : bar is evaluated as foo
(the ``true'' operand) if check (the ``guard'') is true, else
the expression evaluates to bar (the ``false'' operand). Some
special semantic rules apply to the ``guard'' expression and the
other two operand expressions:
(a) Each expression may involve a singular value or a set of
values (when the expression involves one or more metrics
with an instance domain).
(b) All expressions with a set of values must be defined over
the same instance domain.
(c) Both operand expressions must have the same metadata, so the
same metric type, semantics and units (dimension and scale).
(d) The ``guard'' expression must have an arithmetic or rela‐
tional or boolean value, so that it can be evaluated as 0
for false, else true.
(e) If the ``guard'' expression has a singular value and one or
more of the other operand expressions involves an instance
domain, the ``guard'' applies to all instances.
(f) If the ``guard'' expression has a set of values and one or
more of the other operand expressions involves an instance
domain, the ``guard'' is evaluated once for each instance
(there must be one instance domain as per rule (b) above).
(g) If one of the operand expressions has a singular value and
the other has a set of values, and the singular value is se‐
lected based on the evaluation of the ``guard'', then the
result is a set of values (all the same) with instance enu‐
meration being taken from the other operand expression. For
example in the expression: check ? foo : bar, if foo is an
expression with a singular value and bar is a set-valued ex‐
pression, then if check is true, then the result is a set of
values (all having the same value, foo) over the instance
domain of bar.
(h) As a special case, either (but not both) of the expressions
foo or bar may be the special constructor novalue(). If no‐
value() needs to be evaluated, it will return no values.
The most common use of novalue() is in conjunction with de‐
fined() as in the following example:
mumble = defined(abc.def) ? count(abc.def) : novalue()
The metadata for novalue() is taken from the peer operand of
the : operator, making conditions (b) and (c) above trivial‐
ly true; if the peer operand cannot be evaluated, then the
default metadata for novalue() is a singular discrete inte‐
ger with no units, and conditions (b) and (c) are waived.
An extension of novalue() is novalue(<metadata>) where the
<metadata> can be specified using one or more parameters of
the form tag=value or tag="value" as described for the mk‐
const() constructor below, for example:
novalue(type=float, semantics=instant,
units="Kbytes/sec")
(i) As an additional further case, if check is of the form de‐
fined(somemetric) then references to undefined metrics are
allowed in whichever of foo or bar is not required once the
existence if somemetric has been established. This allows
uses of the form:
fumble = defined(new.metric) ? new.metric : old.metric
which is valid when new.metric is defined and when new.met‐
ric is not defined, although this does mean rules (b) and
(c) are relaxed in this case, which further means novalue()
may have no peer operand to provide metadata.
A generalization of this construct is supported for any
check that can be evaluated statically, so a boolean expres‐
sion involving defined() predicates, for example:
bar = !defined(a) || !defined(b) ? novalue() : a + b
• Selection of a single instance can be specified by the construct
``[instance_name]'' which may be appended to a metric name or a
parenthesized expression. For example:
fw.bytes = network.interface.in.bytes[eth1] + \
network.interface.out.bytes[eth1]
or (equivalently):
fw.bytes = (network.interface.in.bytes + \
network.interface.out.bytes)[eth1]
All characters between the ``['' and ``]'' are considered to be
part of the (external) instance name, so be careful to avoid any
spurious white space. A backslash may be used as an escape pre‐
fix in the (unlikely) event that the external instance name con‐
tains a ``]''.
• Numeric constants can also be specified using the mkconst() con‐
structor which takes a number of arguments: the first is a nu‐
meric constant (either integer or floating point), then follow
one or more parameters of the form tag=value or tag="value"
where the allowed values of tag and value are as follows:
┌───────────┬──────────────────────────────────────────────────┐
│ tag │ value │
├───────────┼──────────────────────────────────────────────────┤
│ type │ one of the numeric metric types from │
│ │ <pcp/pmapi.h>, stripped of the PM_TYPE_ prefix, │
│ │ so 32, U32, 64, U64, FLOAT or DOUBLE. │
├───────────┼──────────────────────────────────────────────────┤
│ semantics │ one of the semantic types from <pcp/pmapi.h>, │
│ │ stripped of the PM_SEM_ prefix, so COUNTER, IN‐ │
│ │ STANT or DISCRETE. │
├───────────┼──────────────────────────────────────────────────┤
│ units │ a specification of dimension and scale (together │
│ │ forming the units), in the syntax accepted by │
│ │ pmParseUnitsStr(3). │
├───────────┼──────────────────────────────────────────────────┤
│ meta │ a metric name and that metric provides the base │
│ │ metadata that may be modified by other parame‐ │
│ │ ters │
└───────────┴──────────────────────────────────────────────────┘
The value may optionally be enclosed in double quotes, and may
appear in any mix of upper and/or lower case. The tag must be
in lower case as shown in the table above.
This is most useful when the expression semantics require match‐
ing type and/or semantics and/or units for operands, e.g.
idle = mem.util.free > mkconst(10485760, units=Kbyte)
avg_io_size = delta(disk.dev.total) == 0 ? \
mkconst(1.0, semantics=instant, units="kbyte / count") :
\
delta(disk.dev.total_bytes) / delta(disk.dev.total)
• Expressions may be rescaled using the rescale function that
takes two arguments. The first is an arithmetic expression to
be rescaled, and the second is the desired units after rescaling
that is a string value in the syntax accepted by
pmParseUnitsStr(3). For example:
rescale(network.interface.total.bytes, "Mbytes/hour")
The expression and the desired units must both have the same di‐
mension, e.g Space=1, Time=-1 and Count=0 in the example above.
• The following unary functions operate on a single performance
metric and return one or more values. For all functions (except
count() and defined()), the type of the operand metric must be
arithmetic (integer of various sizes and signedness, float or
double).
┌────────────┬─────────────────────────────────────────────────┐
│ Function │ Value │
├────────────┼─────────────────────────────────────────────────┤
│ avg(x) │ A singular instance being the average value │
│ │ across all instances for the metric x. │
├────────────┼─────────────────────────────────────────────────┤
│ count(x) │ A singular instance being the count of the num‐ │
│ │ ber of instances for the metric x. As a spe‐ │
│ │ cial case, if fetching the metric x returns an │
│ │ error, then count(x) will be 0. │
├────────────┼─────────────────────────────────────────────────┤
│ defined(x) │ A boolean value that is true (``1'') if the │
│ │ metric x is defined in the PMNS, else false │
│ │ (``0''). The function is evaluated when a new │
│ │ PMAPI context is created with pmNewContext(3) │
│ │ or re-established with pmReconnectContext(3). │
│ │ So any subsequent changes to the PMNS after the │
│ │ PMAPI context has been established will not │
│ │ change the value of this function in the ex‐ │
│ │ pression evaluation. │
├────────────┼─────────────────────────────────────────────────┤
│ max(x) │ A singular instance being the maximum value │
│ │ across all instances for the metric x. │
├────────────┼─────────────────────────────────────────────────┤
│ min(x) │ A singular instance being the minimum value │
│ │ across all instances for the metric x. │
├────────────┼─────────────────────────────────────────────────┤
│ sum(x) │ A singular instance being the sum of the values │
│ │ across all instances for the metric x. │
└────────────┴─────────────────────────────────────────────────┘
• The following unary function returns the instantaneous value of
an expression, not the rate-converted value that is the default
for expressions with the semantics of PM_SEM_COUNTER.
┌───────────────┬──────────────────────────────────────────────┐
│ Function │ Value │
├───────────────┼──────────────────────────────────────────────┤
│ instant(expr) │ Returns the current value of the expression, │
│ │ even it has the semantics of a counter, i.e. │
│ │ PM_SEM_COUNTER. The semantics of the de‐ │
│ │ rived metric are based on the semantics of │
│ │ the expression expr; if expr has semantics │
│ │ PM_SEM_COUNTER, the semantics of instant(ex‐ │
│ │ pr) is PM_SEM_INSTANT, otherwise the seman‐ │
│ │ tics of the derived metric is the same as │
│ │ the semantics of expr. │
└───────────────┴──────────────────────────────────────────────┘
• The following unary functions return values computed from the
value of an expression on consecutive samples, or pmFetch(3)
calls. The expression (expr below) may involve one or more met‐
rics but must have an arithmetic value (integer of various sizes
and signedness, float or double) for all instances.
If expr is a set-valued expression then only those instances
that appear in both samples will appear in the result.
┌─────────────┬────────────────────────────────────────────────┐
│ Function │ Value │
├─────────────┼────────────────────────────────────────────────┤
│ delta(expr) │ Returns the difference in values for the ex‐ │
│ │ pression between one call to pmFetch(3) and │
│ │ the next. There is one value in the result for │
│ │ each instance that appears in both the current │
│ │ and the previous sample. If the expression is │
│ │ unsigned, then the type of the result is con‐ │
│ │ verted to ensure as much precision as possible │
│ │ can be retained, so if the expression has type │
│ │ PM_TYPE_U32 then the result is of type │
│ │ PM_TYPE_64, else if the expression has type │
│ │ PM_TYPE_U64 then the result is of type │
│ │ PM_TYPE_DOUBLE. Otherwise the type of the re‐ │
│ │ sult is the same as the type of the expres‐ │
│ │ sion. │
├─────────────┼────────────────────────────────────────────────┤
│ rate(expr) │ Returns the difference in values for the ex‐ │
│ │ pression between one call to pmFetch(3) and │
│ │ the next divided by the elapsed time between │
│ │ the calls to pmFetch(3). The semantics of the │
│ │ derived metric are based on the semantics of │
│ │ the expression with the dimension in the time │
│ │ domain decreased by one and scaling if re‐ │
│ │ quired in the time utilization case where the │
│ │ operand is in units of time, and the derived │
│ │ metric is unitless. There is one value in the │
│ │ result for each instance that appears in both │
│ │ the current and the previous sample, except in │
│ │ the case where the expression has the seman‐ │
│ │ tics of a counter, i.e. PM_SEM_COUNTER, and │
│ │ current value of an instance is smaller than │
│ │ the previous value of the same instance then │
│ │ no value is returned for this instance (this │
│ │ corresponds to a ``counter wrap'' or a │
│ │ ``counter reset''). These rules mimic the │
│ │ rate conversion applied to counter metrics by │
│ │ tools such as pmval(1), pmie(1) and │
│ │ pmchart(1). │
└─────────────┴────────────────────────────────────────────────┘
• The matchinst function may be used to select a subset of the in‐
stances from an instance domain for a metric or expression. The
function takes two arguments:
(a) A instance filter that consists of an optional negation op‐
erator ``!'' followed by a regular expression delimited by
``/'' characters. The regular expression follows the POSIX
Extended Regular Expression syntax as described in regex(3).
A single backslash may be used to escape the regular expres‐
sion delimiter ``/'', but double backslashes are required to
escape any regular expression special characters, e.g. for
the (extremely unlikely) case of wanting to match instance
names like ``some*text/other[text]'' a regular expression of
the form /some\\*text\/other\\[text]/ would be required. If
present, the negation operator reverses the sense of the
filtering, so all instances not matching the regular expres‐
sion will be selected.
(b) A metric or expression that must be defined over an instance
domain.
For example, the following expression will have values for the
metric network.interface.in.bytes for all network interfaces ex‐
cept the loopback and virtual bridge devices:
matchinst(!/^(lo)|(vbir)/, network.interface.in.bytes)
• The scalar function may be used convert a metric or expression
defined over an instance domain into a scalar value that can be
used in other expressions. For example:
net.in.bytes = scalar(network.interface.in.bytes[eth0]) + \
scalar(network.interface.in.bytes[eth1])
The instance domain is removed from the metadata for the result
and the instance identifier is removed from the value during
fetching.
If the metric or expression involves more than one instance then
the result is formed by picking the first instance - this is ar‐
bitrary and implies the scalar function should only be used for
metrics or expressions that are expected to contain zero or one
instances, e.g. the construct ``[instance_name]'' or the
matchinst function with a pattern that matches at most one in‐
stance.
• Parenthesis may be used for explicit grouping.
• A line ending with ``\'' is treated as ``to be continued'' and
the following line is appended after stripping the ``\'' and the
embedded newline.
• Lines beginning with ``#'' are treated as comments and ignored.
• White space is ignored.
There are a number of conversions required to determine the meta‐
data for a derived metric and to ensure the semantics of the ex‐
pressions are sound.
In an arithmetic expression or a relational expression, if the se‐
mantics of both operands is not a counter (i.e. PM_SEM_INSTANT or
PM_SEM_DISCRETE) then the result will have semantics PM_SEM_IN‐
STANT unless both operands are PM_SEM_DISCRETE in which case the
result is also PM_SEM_DISCRETE.
For an arithmetic expression, the dimension of each operand must
be the same. For a relational expression, the dimension of each
operand must be the same, except that numeric constants (with no
dimension) are allowed, e.g. in the expression network.inter‐
face.in.drops > 0 .
To prevent arbitrary and non-sensical combinations some restric‐
tions apply to expressions that combine metrics with counter se‐
mantics to produce a result with counter semantics. For an arith‐
metic expression, if both operands have the semantics of a
counter, then only addition or subtraction is allowed, or if the
left operand is a counter and the right operand is not, then only
multiplication or division are allowed, or if the left operand is
not a counter and the right operand is a counter, then only multi‐
plication is allowed.
Because relational expressions use the current value only and pro‐
duce a result that is not a counter, either or both operands of a
relational expression may be counters.
The mapping of the pmUnits of the metadata uses the following
rules:
• If both operands have a dimension of Count and the scales are
not the same, use the larger scale and convert the values of the
operand with the smaller scale.
• If both operands have a dimension of Time and the scales are not
the same, use the larger scale and convert the values of the
operand with the smaller scale.
• If both operands have a dimension of Space and the scales are
not the same, use the larger scale and convert the values of the
operand with the smaller scale.
• For addition and subtraction all dimensions for each of the
operands and result are identical.
• For multiplication, the dimensions of the result are the sum of
the dimensions of the operands.
• For division, the dimensions of the result are the difference of
the dimensions of the operands.
Scale conversion involves division if the dimension is positive
else multiplication if the dimension is negative. If scale conver‐
sion is applied to either of the operands, the result is promoted
to type PM_TYPE_DOUBLE.
Putting all of this together in an example, consider the derived
metric defined as follows:
x = network.interface.speed - delta(network.interface.in.bytes) /
delta(sample.milliseconds)
The type, dimension and scale settings would propagate up the ex‐
pression tree as follows.
┌─────────────────────────┬────────┬───────────────┬─────────────────┐
│ Expression │ Type │ Dimension & │ Scale Factor(s) │
│ │ │ Scale │ │
├─────────────────────────┼────────┼───────────────┼─────────────────┤
│ sample.milliseconds │ DOUBLE │ millisec │ │
│ delta(...) │ DOUBLE │ millisec │ │
│ network...bytes │ U64 │ byte │ │
│ delta(...) │ U64 │ byte │ │
│ delta(...) / delta(...) │ DOUBLE │ byte/millisec │ /1048576 and │
│ │ │ │ *1000 │
│ network...speed │ FLOAT │ Mbyte/sec │ │
│ x │ DOUBLE │ Mbyte/sec │ │
└─────────────────────────┴────────┴───────────────┴─────────────────┘
Expressions involving single instance selection or the matchinst
function must be associated with underlying metrics that have an
instance domain. These constructors make no sense for singular
metrics.
Because semantic checking cannot be done at the time pmRegister‐
Derived is called, errors found during semantic checking (when any
subsequent calls to pmNewContext(3) or pmReconnectContext(3) suc‐
ceed) are reported using pmprintf(3). These include:
Error: derived metric <name1>: operand: <name2>: <reason>
There was a problem calling pmLookupName(3) to identify the
operand metric <name2> used in the definition of the de‐
rived metric <name1>.
Error: derived metric <name1>: operand (<name2> [<pmid2>]): <rea‐
son>
There was a problem calling pmLookupDesc(3) to identify the
operand metric <name2> with PMID <pmid2> used in the defin‐
ition of the derived metric <name1>.
Semantic error: derived metric <name>: <operand> : <operand> Dif‐
ferent <metadata> for ternary operands
For a ternary expression, the ``true'' operand and the
``false'' operand must have exactly the same metadata, so
type, semantics, instance domain, and units (dimension and
scale).
Semantic error: derived metric <name>: <operand> <op> <operand>:
Dimensions are not the same
Operands must have the same units (dimension and scale) for
each of addition, subtraction, the relational operators and
the boolean ``and'' or ``or'' operators.
Semantic error: derived metric <name>: <operand> <op> <operand>:
Illegal operator for counter and non-counter
Only multiplication or division are allowed if the left
operand has the semantics of a counter and the right
operand is not a counter.
Semantic error: derived metric <name>: <operand> <op> <operand>:
Illegal operator for counters
If both operands have the semantics of counter, only addi‐
tion or subtraction make sense, so multiplication and divi‐
sion are not allowed.
Semantic error: derived metric <name>: <operand> <op> <operand>:
Illegal operator for non-counter and counter
Only multiplication is allowed if the right operand has the
semantics of a counter and the left operand is not a
counter.
Semantic error: derived metric <metric> <expr> RESCALE <units>:
Incompatible dimensions
The parameters <expr> and <units> to the rescale function
must have the same dimension along the axes of Time, Space
and Count.
Semantic error: derived metric <name>: Incorrect time dimension
for operand
Rate conversion using the rate() function is only possible
for operand metrics with a Time dimension of 0 or 1 (see
pmLookupDesc(3)). If the operand metric's Time dimension
is 0, then the derived metrics has a value "per second"
(Time dimension of -1). If the operand metric's Time di‐
mension is 1, then the derived metrics has a value of time
utilization (Time dimension of 0).
Semantic error: derived metric <name>: <function>(<operand>): Non-
arithmetic operand for function
The unary functions are only defined if the operand has
arithmetic type. Similarly the first argument to the
rescale function must be of arithmetic type.
Semantic error: derived metric <name>: <expr> ? ...: Non-arith‐
metic operand for ternary guard
The first expression for a ternary operator must have an
arithmetic type.
Semantic error: derived metric <name>: ... - ...: Non-arithmetic
operand for unary negation
Unary negation only makes sense if the following expression
has an arithmetic type.
Semantic error: derived metric <name>: <operand> <op> <operand>:
Non-arithmetic type for <left-or-right> operand
The binary arithmetic operators are only allowed with
operands with an arithmetic type (integer of various sizes
and signedness, float or double).
Semantic error: derived metric <name>: <operand> <op> <operand>:
Non-counter and not dimensionless for <left-or-right> operand
For multiplication or division or any of the relational op‐
erators, if one of the operands has the semantics of a
counter and the other has the semantics of a non-counter
(instantaneous or discrete) then the non-counter operand
must have no units (dimension and scale).
Semantic error: derived metric <name>: <expr> ? <expr> : <expr>:
Non-scalar ternary guard with scalar expressions
If the ``true'' and ``false'' operands of a ternary expres‐
sion have a scalar value, then the ``guard'' expression
must also have a scalar value.
Semantic error: derived metric <name>: <expr> <op> <expr>:
Operands should have the same instance domain
For all of the binary operators (arithmetic and relation‐
al), if both operands have non-scalar values, then they
must be defined over the same instance domain.
Semantic error: derived metric <name>: operand <dname>: Illegal
nested derived metric
A derived metric (<dname>) cannot be nested (i.e. used) in
the definition of another derived metric (<name>).
Semantic error: derived metric <name>: <expr> <op> <expr>: Illegal
operator for non-counters
Metrics with counter semantics may be added or subtracted,
but <op> (division or multiplication) is not allowed.
Semantic error: derived metric <name>: operand <bname>: Unknown
metric for ternary expression
When a new context was established, the metric <bname> was
not in the PMNS of the new context and <bname> is a re‐
quired operand in the definition of the derived metric
<name>.
For the binary arithmetic operators, if either operand must be
scaled (e.g. convert bytes to Kbytes) then the result is promoted
to PM_TYPE_DOUBLE. Otherwise the type of the result is determined
by the types of the operands, as per the following table which is
evaluated from top to bottom until a match is found.
┌──────────────────────────┬──────────┬────────────────┐
│ Operand Types │ Operator │ Result Type │
├──────────────────────────┼──────────┼────────────────┤
│ either is PM_TYPE_DOUBLE │ any │ PM_TYPE_DOUBLE │
├──────────────────────────┼──────────┼────────────────┤
│ any │ division │ PM_TYPE_DOUBLE │
├──────────────────────────┼──────────┼────────────────┤
│ either is PM_TYPE_FLOAT │ any │ PM_TYPE_FLOAT │
├──────────────────────────┼──────────┼────────────────┤
│ either is PM_TYPE_U64 │ any │ PM_TYPE_U64 │
├──────────────────────────┼──────────┼────────────────┤
│ either is PM_TYPE_64 │ any │ PM_TYPE_64 │
├──────────────────────────┼──────────┼────────────────┤
│ either is PM_TYPE_U32 │ any │ PM_TYPE_U32 │
├──────────────────────────┼──────────┼────────────────┤
│ otherwise (both are │ any │ PM_TYPE_32 │
│ PM_TYPE_32) │ │ │
└──────────────────────────┴──────────┴────────────────┘
Within PCP each metric is assigned a unique Performance Metric
Identifier (PMID) and internally a PMID is constructed from 3
fields: the domain number (of the associated Performance Metrics
Domain Agent, or PMDA), the cluster number and the item number.
Derived metrics use the reserved domain number 511 and special
PMIDs as described in the following table, where the PMID is shown
as domain.cluster.item:
┌────────────────┬───────────────┬───────────────────────────────┐
│ Derived Metric │ Starting PMID │ Description │
├────────────────┼───────────────┼───────────────────────────────┤
│ global │ 511.0.1 │ Metrics are assigned │
│ │ │ ascending PMIDs as they are │
│ │ │ registered via │
│ │ │ pmRegisterDerived, │
│ │ │ pmRegisterDerivedMetric or │
│ │ │ pmLoadDerivedConfig(3). │
├────────────────┼───────────────┼───────────────────────────────┤
│ per-context │ 511.2047.1023 │ Metrics are assigned │
│ │ │ descending PMIDs as they are │
│ │ │ registered via │
│ │ │ pmAddDerived(3) or │
│ │ │ pmAddDerivedMetric(3). │
├────────────────┼───────────────┼───────────────────────────────┤
│ remapped │ 511.c.i │ When a derived metric is │
│ │ │ recorded in a PCP archive by │
│ │ │ pmlogger(1) or one of the │
│ │ │ related archive creation │
│ │ │ tools, the PMID of the │
│ │ │ derived metric is remapped in │
│ │ │ the archive so that c is the │
│ │ │ cluster of derived metric │
│ │ │ plus 2048, and i is the item │
│ │ │ of the derived metric. For │
│ │ │ example, a derived metric │
│ │ │ with PMID 511.0.13 will have │
│ │ │ the remapped PMID 511.2048.13 │
│ │ │ in an archive. See │
│ │ │ pmlogger(1) for a discussion │
│ │ │ about adding derived metrics │
│ │ │ to a PCP archive. │
└────────────────┴───────────────┴───────────────────────────────┘
For base metrics from a host or archive context, there is a re‐
quirement that each metric name is unique. But derived metrics
require this restriction to be relaxed, so that the same metric
name could associated with a base metric, and/or a per-context de‐
rived metric, and/or a global metric. This means that one or more
of the metrics with the same name may be masked by other metrics
of the same name. The following table describes which metric will
be used when one of these multiply-defined names is presented to
the PMAPI.
┌─────────────┬─────────────────────────────────┬─────────────────────┐
│ │ derived metric │ │
│ base metric ├──────────┬─────────────┬────────┤ chosen metric │
│ │ remapped │ per-context │ global │ │
├─────────────┼──────────┼─────────────┼────────┼─────────────────────┤
│ no │ no │ no │ no │ none; the metric is │
│ │ │ │ │ undefined │
├─────────────┼──────────┼─────────────┼────────┼─────────────────────┤
│ no │ no │ no │ yes │ global derived │
├─────────────┼──────────┼─────────────┼────────┼─────────────────────┤
│ no │ no │ yes │ no │ per-context derived │
├─────────────┼──────────┼─────────────┼────────┼─────────────────────┤
│ no │ no │ yes │ yes │ per-context derived │
├─────────────┼──────────┼─────────────┼────────┼─────────────────────┤
│ no │ yes │ no │ no │ remapped │
├─────────────┼──────────┼─────────────┼────────┼─────────────────────┤
│ no │ yes │ no │ yes │ remapped │
├─────────────┼──────────┼─────────────┼────────┼─────────────────────┤
│ no │ yes │ yes │ no │ remapped │
├─────────────┼──────────┼─────────────┼────────┼─────────────────────┤
│ no │ yes │ yes │ yes │ remapped │
├─────────────┼──────────┼─────────────┼────────┼─────────────────────┤
│ yes │ no │ no │ no │ base metric │
├─────────────┼──────────┼─────────────┼────────┼─────────────────────┤
│ yes │ no │ no │ yes │ base metric │
├─────────────┼──────────┼─────────────┼────────┼─────────────────────┤
│ yes │ no │ yes │ no │ base metric │
├─────────────┼──────────┼─────────────┼────────┼─────────────────────┤
│ yes │ no │ yes │ yes │ base metric │
├─────────────┼──────────┼─────────────┼────────┼─────────────────────┤
│ yes │ yes │ - │ - │ cannot happen; for │
│ │ │ │ │ any name there can │
│ │ │ │ │ be at most one of a │
│ │ │ │ │ base metric or a │
│ │ │ │ │ remapped metric │
└─────────────┴──────────┴─────────────┴────────┴─────────────────────┘
When masking happens, it is done silently. Most PCP applications
support -Dderive on the command line and this pmdbg(1) debug op‐
tion will report derived metric operations (including masking) on
stderr.
Derived metrics are not available when using pmFetchArchive(3) as
this routine does not use a target list of PMIDs that could be
remapped (as is done for pmFetch(3)).
There is no pmUnregisterDerived method, so once registered a de‐
rived metric persists for the life of the application.
On success, pmRegisterDerived returns NULL.
If a syntactic error is found at the time of registration, the
value returned by pmRegisterDerived is a pointer into expr indi‐
cating where the error was found. To identify what the error was,
the application should call pmDerivedErrStr(3) to retrieve the
corresponding parser error message.
pmRegisterDerivedMetric returns 0 and errmsg is undefined if the
parsing is successful.
If the given expr does not conform to the required syntax pmRegis‐
terDerivedMetric returns -1 and a dynamically allocated error mes‐
sage string in errmsg. The error message is terminated with a
newline and includes both the input name and expr, along with an
indicator of the position at which the error was detected. e.g.
Error: pmRegisterDerivedMetric("my.disk.rates", ...)
syntax error
4rat(disk.dev.read)
^
The position indicator line may be followed by an additional diag‐
nostic line describing the nature of the error, when available.
In the case of an error, the pmRegisterDerivedMetric caller is re‐
sponsible for calling free(3) to release the space allocated for
errmsg.
PCPIntro(1), pmlogger(1), PMAPI(3), free(3), pmAddDerived(3),
pmAddDerivedMetric(3), pmDerivedErrStr(3), pmFetch(3),
pmLoadDerivedConfig(3), pmNewContext(3), pmReconnectContext(3),
pmprintf(3) and PMNS(5).
This page is part of the PCP (Performance Co-Pilot) project. In‐
formation about the project can be found at ⟨http://www.pcp.io/⟩.
If you have a bug report for this manual page, send it to
pcp@groups.io. This page was obtained from the project's upstream
Git repository ⟨https://github.com/performancecopilot/pcp.git⟩ on
2025-08-11. (At that time, the date of the most recent commit
that was found in the repository was 2025-08-11.) If you discover
any rendering problems in this HTML version of the page, or you
believe there is a better or more up-to-date source for the page,
or you have corrections or improvements to the information in this
COLOPHON (which is not part of the original manual page), send a
mail to man-pages@man7.org
Performance Co-Pilot PMREGISTERDERIVED(3)
Pages that refer to this page: pcp2elasticsearch(1), pcp2graphite(1), pcp2influxdb(1), pcp2json(1), pcp2openmetrics(1), pcp2spark(1), pcp2template(1), pcp2xlsx(1), pcp2xml(1), pcp2zabbix(1), pminfo(1), pmlogctl(1), pmlogger(1), pmrep(1), pmaddderived(3), pmaddderivedtext(3), pmderivederrstr(3), pmfetchgroup(3), pmgetderivedcontrol(3), pmloadderivedconfig(3), pmreconnectcontext(3), pcp-dstat(5), pmrep.conf(5)
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