PMREGISTERDERIVED(3)

NAME
pmRegisterDerived, pmRegisterDerivedMetric - register a 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 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 V over delta V'' 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 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 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 derived metrics to be determined. This means pmRegisterDerived does not apply retrospectively to any open PMAPI contexts, nor to any PMAPI contexts already open at the time pmRegisterDerived is called, so the normal use would be to make all calls to pmRegisterDerived (possibly via pmLoadDerivedConfig(3)) or pmRegisterDerivedMetric and then call pmNewContext(3).

name should follow the syntactic rules for the names of performance 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 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 constants 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 (``-''), multiplication (``*'') and division (``/'') with the normal precedence rules where multiplication 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 expresssions 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 relational 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 expresssions return 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 : baris 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 aritmetic 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: foo ? scalar : set, if foo is true, then the result is a set of values (all having the same value, scalar) over the instance domain of set.

*   Numeric constants can also be specified using the mkconst() constructor which takes a number of arguments: the first is a numeric constant (either integer or floating point), then follow one or more parameters of the form tag=value or tag= 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 accepted by

    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(), defined() and instant()), 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 or re-established with 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.
    delta(x) | Returns the difference in values for the metric x between one call to and the next. There is one value in the result for each instance that appears in both the current and the previous sample.
    rate(x) | Returns the difference in values for the metric x between one call to and the next divided by the elapsed time between the calls to The semantics of the derived metric are based on the semantics of the operand (x) with the dimension in the domain decreased by one and scaling if required in the time utilization case where the operand is in units of time, and the derived metric is unitless. This mimics the rate conversion applied to counter metrics by tools such as and There is one value in the result for each instance that appears in both the current and the previous sample.
    instant(x) | Returns the current value of the metric x, 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 operand (x); if x has semantics PM_SEM_COUNTER, the semantics of instant(x) is PM_SEM_INSTANT, otherwise the semantics of the derived metric is the same as the semantics of the metric x.
    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.

*   Parenthesis may be used for explicit grouping.

*   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 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.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 subraction 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, 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 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

Because semantic checking cannot be done at the time pmRegisterDerived is called, errors found during semantic checking (when any subsequent calls to pmNewContext(3) or pmReconnectContext(3) succeed) 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 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 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 addition or subtraction make sense, so multiplication 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 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 dimension 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-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 <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 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 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 values, then they must be defined over the same instance domain.

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 PM_TYPE_32) | any | PM_TYPE_32

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 dynamically allocated error message 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

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 caller is responsible for calling free(3) to release the space allocated for errmsg.

SEE ALSO
PCPIntro(1), free(3), PMAPI(3), pmDerivedErrStr(3), pmFetch(3), pmLoadDerivedConfig(3), pmNewContext(3), pmprintf(3) and pmReconnectContext(3).