Provided by: libpcp4-dev_7.0.2-1_amd64 
NAME
pmRegisterDerived, pmRegisterDerivedMetric - register a global derived metric name and definition
C SYNOPSIS
#include <pcp/pmapi.h>
char *pmRegisterDerived(char *name, char *expr);
int pmRegisterDerivedMetric(char *name, char *expr, char **errmsg);
cc ... -lpcp
DESCRIPTION
Derived metrics provide a way of extending the Performance Metrics Name Space (PMNS) with new metrics de‐
fined 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 possible 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 required 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 pmRegisterDerived except that it provides a
simplified model of error 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 de‐
ferred until each new PMAPI context is created with pmNewContext(3) or re-established with pmReconnect‐
Context(3), at which time the PMNS and metadata is available to allow semantic checking and the metadata
of the derived metrics to be determined.
If pmRegisterDerived is called after one or more PMAPI contexts has been opened, then the newly regis‐
tered metrics will be available in those contexts, however the more normal use would be to make all calls
to pmRegisterDerived (possibly via pmLoadDerivedConfig(3)) or pmRegisterDerivedMetric before calling pm‐
NewContext(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 performance metrics, namely one or more compo‐
nents 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 acceptable 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 numeric constants. Recursive definitions are
not allowed, so only the names of regular metrics (not other derived metrics) may be used. Numeric con‐
stants are either integers constrained to the precision of 32-bit unsigned integers or double precision
floating point numbers.
• The usual binary arithmetic operators are supported, namely addition (``+''), subtraction (``-''), mul‐
tiplication (``*'') and division (``/'') with the normal precedence rules where multiplication and di‐
vision 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 expressions 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 eval‐
uated as (a>b)!=c.
• C-style boolean operators are supported, namely and (``&&'') and or (``||''). Boolean expressions re‐
turn a value as a 32-bit unsigned 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 general 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'' expres‐
sion 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 relational 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 selected based on the evaluation of the ``guard'', then the result is a set of
values (all the same) with instance enumeration 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 expression, 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 construc‐
tor novalue(). If novalue() needs to be evaluated, it will return no values. The most common use
of novalue() is in conjunction with defined() 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 trivially true; if the peer operand cannot be evaluated, then the default metadata
for novalue() is a singular discrete integer 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 mkconst() constructor be‐
low, for example:
novalue(type=float, semantics=instant, units="Kbytes/sec")
(i) As an additional further case, if check is of the form defined(somemetric) then references to unde‐
fined metrics are allowed in whichever of foo or bar is not required once the existence if somemet‐
ric 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.metric 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 expression 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 ap‐
pended 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 prefix in the
(unlikely) event that the external instance name contains a ``]''.
• Numeric constants can also be specified using the mkconst() constructor which takes a number of argu‐
ments: the first is a numeric constant (either integer or floating point), then follow one or more pa‐
rameters 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, INSTANT or DISCRETE. │
├───────────┼─────────────────────────────────────────────────────────────────────────────────────────┤
│ units │ a specification of dimension and scale (together forming the units), in the syntax ac‐ │
│ │ cepted by pmParseUnitsStr(3). │
├───────────┼─────────────────────────────────────────────────────────────────────────────────────────┤
│ meta │ a metric name and that metric provides the base metadata that may be modified by other │
│ │ parameters │
└───────────┴─────────────────────────────────────────────────────────────────────────────────────────┘
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 matching 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 dimension, 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 number of instances for the metric x. As a │
│ │ special 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 pm‐ │
│ │ NewContext(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 expression 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 derived metric are based on the │
│ │ semantics of the expression expr; if expr has semantics PM_SEM_COUNTER, the seman‐ │
│ │ tics of instant(expr) is PM_SEM_INSTANT, otherwise the semantics 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 metrics 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 expression 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 converted 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. Oth‐ │
│ │ erwise the type of the result is the same as the type of the expression. │
├─────────────┼───────────────────────────────────────────────────────────────────────────────────────┤
│ rate(expr) │ Returns the difference in values for the expression between one call to pmFetch(3) │
│ │ and the next divided by the elapsed time between the calls to pmFetch(3). The seman‐ │
│ │ tics of the derived metric are based on the semantics of the expression with the di‐ │
│ │ mension in the time domain decreased by one and scaling if required in the time uti‐ │
│ │ lization case where the operand is in units of time, and the derived metric is unit‐ │
│ │ less. 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 se‐ │
│ │ mantics of a counter, i.e. PM_SEM_COUNTER, and current value of an instance is small‐ │
│ │ er 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 instances from an instance domain for a
metric or expression. The function takes two arguments:
(a) A instance filter that consists of an optional negation operator ``!'' followed by a regular ex‐
pression 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
expression 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 expression 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 except 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 arbitrary 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 instance.
• 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.
SEMANTIC CHECKS AND RULES
There are a number of conversions required to determine the metadata for a derived metric and to ensure
the semantics of the expressions are sound.
In an arithmetic expression or a relational expression, if the semantics of both operands is not a
counter (i.e. PM_SEM_INSTANT or PM_SEM_DISCRETE) then the result will have semantics PM_SEM_INSTANT un‐
less 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 expres‐
sion, the dimension of each operand must be the same, except that numeric constants (with no dimension)
are allowed, e.g. in the expression network.interface.in.drops > 0 .
To prevent arbitrary and non-sensical combinations some restrictions apply to expressions that combine
metrics with counter semantics to produce a result with counter semantics. For an arithmetic 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 multiplication is
allowed.
Because relational expressions use the current value only and produce a result that is not a counter, ei‐
ther 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 conversion 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 expression tree as follows.
┌─────────────────────────┬────────┬───────────────────┬────────────────────┐
│ Expression │ Type │ Dimension & Scale │ Scale Factor(s) │
├─────────────────────────┼────────┼───────────────────┼────────────────────┤
│ 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 under‐
lying metrics that have an instance domain. These constructors make no sense for singular metrics.
Because semantic checking cannot be done at the time pmRegisterDerived is called, errors found during se‐
mantic checking (when any subsequent calls to pmNewContext(3) or pmReconnectContext(3) succeed) are re‐
ported 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 de‐
finition of the derived metric <name1>.
Error: derived metric <name1>: operand (<name2> [<pmid2>]): <reason>
There was a problem calling pmLookupDesc(3) to identify the operand metric <name2> with PMID
<pmid2> used in the definition of the derived metric <name1>.
Semantic error: derived metric <name>: <operand> : <operand> Different <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 re‐
lational 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 addition or subtraction make sense, so multi‐
plication and division 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 dimen‐
sion of 0 or 1 (see pmLookupDesc(3)). If the operand metric's Time dimension is 0, then the de‐
rived 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 ar‐
gument to the rescale function must be of arithmetic type.
Semantic error: derived metric <name>: <expr> ? ...: Non-arithmetic 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 operators, if one of the operands has the
semantics of a counter and the other has the semantics of a non-counter (instantaneous or dis‐
crete) 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 ex‐
pressions
If the ``true'' and ``false'' operands of a ternary expression 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 relational), if both operands have non-scalar val‐
ues, 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 met‐
ric (<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 required operand in the definition of the derived metric <name>.
EXPRESSION EVALUATION
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) │ │ │
└──────────────────────────────┴──────────┴────────────────┘
PMIDs AND MASKING
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 PM‐
DA), the cluster number and the item number. Derived metrics use the reserved domain number 511 and spe‐
cial 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 or pmRegister-DerivedMetric, or pmLoad- │
│ │ │ DerivedConfig(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 requirement 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 derived 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 option will report derived metric operations (including masking) on stderr.
CAVEATS
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 derived metric persists for the life of the
application.
DIAGNOSTICS
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 indicating 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 pmRegisterDerivedMetric returns -1 and a dynam‐
ically allocated error message string in errmsg. The error message is terminated with a newline and in‐
cludes both the input name and expr, along with an indicator of the position at which the error was de‐
tected. e.g.
Error: pmRegisterDerivedMetric("my.disk.rates", ...) syntax error
4rat(disk.dev.read)
^
The position indicator line may be followed by an additional diagnostic line describing the nature of the
error, when available.
In the case of an error, the pmRegisterDerivedMetric caller is responsible for calling free(3) to release
the space allocated for errmsg.
SEE ALSO
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).
Performance Co-Pilot PMREGISTERDERIVED(3)