1PMREGISTERDERIVED(3) Library Functions Manual PMREGISTERDERIVED(3)
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6 pmRegisterDerived - register a derived metric name and definition
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9 #include <pcp/pmapi.h>
10 char *pmRegisterDerived(char *name, char *expr)
12 cc ... -lpcp
14
16 Derived metrics provide a way of extending the Performance Metrics Name
17 Space (PMNS) with new metrics defined at the PCP client-side using
18 arithmetic expressions over the existing performance metrics.
19 Typical uses would be to aggregate a number of similar metrics to pro‐
21 vide a higher-level summary metric or to support the ``delta V over
22 delta V'' class of metrics that are not possible in the base data
23 semantics of PCP. An example of the latter class would be the average
24 I/O size, defined as
25 delta(disk.dev.total_bytes) / delta(disk.dev.total)
26 where both of the disk.dev metrics are counters, and what is required
27 is to to sample both metrics, compute the difference between the cur‐
28 rent and previous values and then calculate the ratio of these differ‐
29 ences.
30 The arguments to pmRegisterDerived are the name of the new derived met‐
32 ric and expr is an arithmetic expression defining how the values of
33 name should be computed.
34 name should follow the syntactic rules for the names of performance
36 metrics, namely one or more components separated with a dot (``.''),
37 and each component must begin with an alphabetic followed by zero or
38 more characters drawn from the alphabetics, numerics and underscore
39 (``_''). For more details, refer to PCPIntro(1) and pmns(4).
40 name must be unique across all derived metrics and should not match the
42 name of any regular metric in the PMNS. It is acceptable for name to
43 share some part of its prefix with an existing subtree of the PMNS,
44 e.g. the average I/O size metric above could be named disk.dev.avgsz
45 which would place it amongst the other disk.dev metrics in the PMNS.
46 Alternatively, derived metrics could populate their own subtree of the
47 PMNS, e.g. the average I/O size metric above could be named my.sum‐
48 mary.disk.avgsz.
49 The expression expr follows these syntactic rules:
51 * Terminal elements are either names of existing metrics or integer
53 constants. Recursive definitions are not allowed, so only the names
54 of regular metrics (not other derived metrics) may be used. Integer
55 constants are constrained to the precision of 32-bit unsigned inte‐
56 gers.
57 * The usual binary arithmetic operators are supported, namely - addi‐
59 tion (``+''), subtraction (``-''), multiplication (``*'') and divi‐
60 sion (``/'') with the normal precedence rules where multiplication
61 and division have higher precedence than addition and subtraction, so
62 a+b*c is evaluated as a+(b*c)
63 * Parenthesis may be used for grouping.
65 * The following unary functions operate on a single performance metric
67 and return one or more values. For all functions (except count()),
68 the type of the operand metric must be arithmetic (integer of various
69 sizes and signedness, float or double).
70 ┌─────────┬──────────────────────────────────────────────────────────┐
72 │Function │ Value │
73 ├─────────┼──────────────────────────────────────────────────────────┤
74 │avg(x) │ A singular instance being the average value across all │
75 │ │ instances for the metric x. │
76 ├─────────┼──────────────────────────────────────────────────────────┤
77 │count(x) │ A singular instance being the count of the number of │
78 │ │ instances for the metric x. │
79 ├─────────┼──────────────────────────────────────────────────────────┤
80 │delta(x) │ Returns the difference in values for the metric x │
81 │ │ between one call to pmFetch(3) and the next. There is │
82 │ │ one value in the result for each instance that appears │
83 │ │ in both the current and the previous sample. │
84 ├─────────┼──────────────────────────────────────────────────────────┤
85 │max(x) │ A singular instance being the maximum value across all │
86 │ │ instances for the metric x. │
87 ├─────────┼──────────────────────────────────────────────────────────┤
88 │min(x) │ A singular instance being the minimum value across all │
89 │ │ instances for the metric x. │
90 ├─────────┼──────────────────────────────────────────────────────────┤
91 │sum(x) │ A singular instance being the sum of the values across │
92 │ │ all instances for the metric x. │
93 └─────────┴──────────────────────────────────────────────────────────┘
94 * White space is ignored.
95 Syntactic checking is performed at the time pmRegisterDerived is
97 called, but semantic checking is deferred until each new context is
98 created with pmNewContext(3) or re-establised with pmReconnectCon‐
99 text(3), at which time the PMNS and metadata is available to allow
100 semantic checking and the metadata of the derived metrics to be estab‐
101 lished.
102
104 There are a number of conversions required to determine the metadata
105 for a derived metric and to ensure the semantics of the expressions are
106 sound.
107 In a binary expression, if the semantics of both operands is not a
109 counter (i.e. PM_SEM_INSTANT or PM_SEM_DISCRETE) then the result will
110 have semantics PM_SEM_INSTANT unless both operands are PM_SEM_DISCRETE
111 in which case the result is also PM_SEM_DISCRETE.
112 The mapping of the pmUnits of the metadata uses the following rules:
114 * If both operands have a dimension of COUNT and the scales are not the
116 same, use the larger scale and convert the values of the operand with
117 the smaller scale.
118 * If both operands have a dimension of TIME and the scales are not the
120 same, use the larger scale and convert the values of the operand with
121 the smaller scale.
122 * If both operands have a dimension of SPACE and the scales are not the
124 same, use the larger scale and convert the values of the operand with
125 the smaller scale.
126 * For addition and subtraction all dimensions for each of the operands
128 and result are identical.
129 * For multiplication, the dimensions of the result are the sum of the
131 dimensions of the operands.
132 * For division, the dimensions of the result are the difference of the
134 dimensions of the operands.
135 Scale conversion involves division if the dimension is positive else
137 multiplication if the dimension is negative. If scale conversion is
138 applied to either of the operands, the result is promoted to type
139 PM_TYPE_DOUBLE.
140 Putting all of this together in an example, consider the derived metric
142 defined as follows:
143 x = network.interface.speed - delta(network.interface.in.bytes) /
144 delta(sample.milliseconds)
145 The type, dimension and scale settings would propagate up the expres‐
146 sion tree as follows.
147 ┌────────────────────────┬────────┬───────────────┬─────────────────┐
149 │ Expression │ Type │ Dimension & │ Scale Factor(s) │
150 │ │ │ Scale │ │
151 ├────────────────────────┼────────┼───────────────┼─────────────────┤
152 │sample.milliseconds │ DOUBLE │ millisec │ │
153 │delta(...) │ DOUBLE │ millisec │ │
154 │network...bytes │ U64 │ byte │ │
155 │delta(...) │ U64 │ byte │ │
156 │delta(...) / delta(...) │ DOUBLE │ byte/millisec │ /1048576 and │
157 │ │ │ │ *1000 │
158 │network...speed │ FLOAT │ Mbyte/sec │ │
159 │x │ DOUBLE │ Mbyte/sec │ │
160 └────────────────────────┴────────┴───────────────┴─────────────────┘
161 Because semantic checking cannot be done at the time pmRegisterDerived
162 is called, errors found during semantic checking are reported using
163 pmprintf(3). These include:
164 Error: derived metric <name1>: operand: <name2>: <reason>
166 There was a problem calling pmLookupName(3) to identify the op‐
167 erand metric <name2> used in the definition of the derived met‐
168 ric <name1>.
169 Error: derived metric <name1>: operand (<name2> [<pmid2>]): <reason>
171 There was a problem calling pmLookupDesc(3) to identify the op‐
172 erand metric <name2> with PMID <pmid2> used in the definition of
173 the derived metric <name1>.
174 Semantic error: derived metric <name>: <operand> <op> <operand>: Ille‐
176 gal operator for counters
177 If both operands have the semantics of counter, only addition or
178 subtraction make sense, so multiplication and division are not
179 allowed.
180 Semantic error: derived metric <name>: <operand> <op> <operand>: Ille‐
182 gal operator for counter and non-counter
183 Only multiplication or division are allowed if the left operand
184 has the semantics of a counter and the right operand is not a
185 counter.
186 Semantic error: derived metric <name>: <operand> <op> <operand>: Ille‐
188 gal operator for non-counter and counter
189 Only multiplication is allowed if the right operand has the
190 semantics of a counter and the left operand is not a counter.
191 Semantic error: derived metric <name>: <operand> <op> <operand>: Non-
193 arithmetic type for <left-or-right> operand
194 The binary arithmetic operators are only allowed with operands
195 with an arithmetic type (integer of various sizes and signed‐
196 ness, float or double).
197 Semantic error: derived metric <name>: <function>(<operand>): Non-
199 arithmetic operand for function
200 The unary functions are only defined if the operand has arith‐
201 metic type.
202
204 For the binary arithmetic operators, if either operand must be scaled
205 (e.g. convert bytes to Kbytes) then the result is promoted to
206 PM_TYPE_DOUBLE. Otherwise the type of the result is determined by the
207 types of the operands, as per the following table which is evaluated
208 from top to bottom until a match is found.
209 ┌─────────────────────────┬──────────┬────────────────┐
211 │ Operand Types │ Operator │ Result Type │
212 ├─────────────────────────┼──────────┼────────────────┤
213 │either is PM_TYPE_DOUBLE │ any │ PM_TYPE_DOUBLE │
214 ├─────────────────────────┼──────────┼────────────────┤
215 │any │ division │ PM_TYPE_DOUBLE │
216 ├─────────────────────────┼──────────┼────────────────┤
217 │either is PM_TYPE_FLOAT │ any │ PM_TYPE_FLOAT │
218 ├─────────────────────────┼──────────┼────────────────┤
219 │either is PM_TYPE_U64 │ any │ PM_TYPE_U64 │
220 ├─────────────────────────┼──────────┼────────────────┤
221 │either is PM_TYPE_64 │ any │ PM_TYPE_64 │
222 ├─────────────────────────┼──────────┼────────────────┤
223 │either is PM_TYPE_U32 │ any │ PM_TYPE_U32 │
224 ├─────────────────────────┼──────────┼────────────────┤
225 │otherwise (both are │ any │ PM_TYPE_32 │
226 │PM_TYPE_32) │ │ │
227 └─────────────────────────┴──────────┴────────────────┘
229 Unary negation is not supported, so the following expressions would be
230 syntactically incorrect, -3*abc and -this.number
231 Derived metrics are not available when using pmFetchArchive(3) as this
233 routine does not use a target list of PMIDs that could be remapped (as
234 is done for pmFetch(3)).
235 Derived metrics are not available with the routines pmRequestTra‐
237 versePMNS(3) and pmReceiveTraversePMNS(3) because the by the time the
238 list of names is received, the original name at the root of the search
239 is no longer available.
240 pmRegisterDerived does not apply retrospectively to any open contexts,
242 so the normal use would be to make all calls to pmRegisterDerived (pos‐
243 sibly via pmLoadDerivedConfig(3)) and then call pmNewContext(3).
244 There is no pmUnregisterDerived method, so once registered a derived
246 metric persists for the life of the application.
247
249 On success, pmRegisterDerived returns NULL.
250 If a syntactic error is found at the time of registration, the value
252 returned by pmRegisterDerived is a pointer into expr indicating where
253 the error was found. To identify what the error was, the application
254 should call pmDerivedErrStr(3) to retrieve the corresponding parser
255 error message.
256
258 PCPIntro(1), PMAPI(3), pmDerivedErrStr(3), pmFetch(3), pmLoadDerived‐
259 Config(3), pmNewContext(3) and pmReconnectContext(3).
260Performance Co-Pilot PMREGISTERDERIVED(3)