PMREGISTERDERIVED - Online Linux Manual Page

Section : 3
Updated :
Source : Performance Co-Pilot

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 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.

If pmRegisterDerived is called after one or more PMAPI contexts has been opened, then the newly registered metrics will be avaiable 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 establised. 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 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 : 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 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

* 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 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 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 pmParseUnitsStr(3).

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 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 expression evaluation.

delta(x)	Returns the difference in values for the metric x 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.

rate(x)	Returns the difference in values for the metric x 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 operand (x) with the dimension in the time 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 pmval(1), pmie(1) and pmchart(1). 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.

* 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 expression delimited by ``/'' characters. The regular expression follows the POSIX Extended Regular Expression syntax as described in regex(3). Backslashes may be used as escape prefixes, but double backslash is 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.

* 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 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, 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.

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