NAME
pmRegisterDerived - register a derived metric name and definition
C SYNOPSIS
#include <pcp/pmapi.h>
char *pmRegisterDerived(char *name, char *expr)
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
arithmetic 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 arithmetic expression defining how the values of
name should be computed.
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(4).
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 integer
constants. Recursive definitions are not allowed, so only the names
of regular metrics (not other derived metrics) may be used. Integer
constants are constrained to the precision of 32-bit unsigned
integers.
* 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)
* Parenthesis may be used for grouping.
* The following unary functions operate on a single performance metric
and return one or more values. For all functions (except count()),
the type of the operand metric must be arithmetic (integer of various
sizes and signedness, float or double).
* White space is ignored.
Syntactic checking is performed at the time pmRegisterDerived is
called, but semantic checking is deferred until each new context is
created with pmNewContext(3) or re-establised 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 established.
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 a binary 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.
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 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 | |
+------------------------+--------+---------------+-----------------+
Because semantic checking cannot be done at the time pmRegisterDerived
is called, errors found during semantic checking 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> <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 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 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 <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>: <function>(<operand>): Non-
arithmetic operand for function
The unary functions are only defined if the operand has
arithmetic type.
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) | | |
+-------------------------+----------+----------------+
CAVEATS
Unary negation is not supported, so the following expressions would be
syntactically incorrect, -3*abc and -this.number
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)).
Derived metrics are not available with the routines
pmRequestTraversePMNS(3) and pmReceiveTraversePMNS(3) because the by
the time the list of names is received, the original name at the root
of the search is no longer available.
pmRegisterDerived does not apply retrospectively to any open contexts,
so the normal use would be to make all calls to pmRegisterDerived
(possibly via pmLoadDerivedConfig(3)) and then call pmNewContext(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.
SEE ALSO
PCPIntro(1), PMAPI(3), pmDerivedErrStr(3), pmFetch(3),
pmLoadDerivedConfig(3), pmNewContext(3) and pmReconnectContext(3).