1RRDGRAPH_RPN(1) rrdtool RRDGRAPH_RPN(1)
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6 rrdgraph_rpn - About RPN Math in rrdtool graph
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9 RPN expression:=vname|operator|value[,RPN expression]
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12 If you have ever used a traditional HP calculator you already know RPN.
13 The idea behind RPN is that you have a stack and push your data onto
14 this stack. Whenever you execute an operation, it takes as many ele‐
15 ments from the stack as needed. Pushing is done implicitly, so whenever
16 you specify a number or a variable, it gets pushed onto the stack auto‐
17 matically.
18 At the end of the calculation there should be one and only one value
20 left on the stack. This is the outcome of the function and this is
21 what is put into the vname. For CDEF instructions, the stack is pro‐
22 cessed for each data point on the graph. VDEF instructions work on an
23 entire data set in one run. Note, that currently VDEF instructions only
24 support a limited list of functions.
25 Example: "VDEF:maximum=mydata,MAXIMUM"
27 This will set variable "maximum" which you now can use in the rest of
29 your RRD script.
30 Example: "CDEF:mydatabits=mydata,8,*"
32 This means: push variable mydata, push the number 8, execute the oper‐
34 ator *. The operator needs two elements and uses those to return one
35 value. This value is then stored in mydatabits. As you may have
36 guessed, this instruction means nothing more than mydatabits = mydata *
37 8. The real power of RPN lies in the fact that it is always clear in
38 which order to process the input. For expressions like "a = b + 3 * 5"
39 you need to multiply 3 with 5 first before you add b to get a. However,
40 with parentheses you could change this order: "a = (b + 3) * 5". In
41 RPN, you would do "a = b, 3, +, 5, *" without the need for parentheses.
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44 Boolean operators
45 LT, LE, GT, GE, EQ, NE
46 Pop two elements from the stack, compare them for the selected con‐
48 dition and return 1 for true or 0 for false. Comparing an unknown
49 or an infinite value will always result in 0 (false).
50 UN, ISINF
52 Pop one element from the stack, compare this to unknown respec‐
54 tively to positive or negative infinity. Returns 1 for true or 0
55 for false.
56 IF
58 Pops three elements from the stack. If the element popped last is
60 0 (false), the value popped first is pushed back onto the stack,
61 otherwise the value popped second is pushed back. This does,
62 indeed, mean that any value other than 0 is considered to be true.
63 Example: "A,B,C,IF" should be read as "if (A) then (B) else (C)"
65 Comparing values
67 MIN, MAX
68 Pops two elements from the stack and returns the smaller or larger,
70 respectively. Note that infinite is larger than anything else. If
71 one of the input numbers is unknown then the result of the opera‐
72 tion will be unknown too.
73 LIMIT
75 Pops two elements from the stack and uses them to define a range.
77 Then it pops another element and if it falls inside the range, it
78 is pushed back. If not, an unknown is pushed.
79 The range defined includes the two boundaries (so: a number equal
81 to one of the boundaries will be pushed back). If any of the three
82 numbers involved is either unknown or infinite this function will
83 always return an unknown
84 Example: "CDEF:a=alpha,0,100,LIMIT" will return unknown if alpha is
86 lower than 0 or if it is higher than 100.
87 Arithmetics
89 +, -, *, /, %
90 Add, subtract, multiply, divide, modulo
92 SIN, COS, LOG, EXP, SQRT
94 Sine and cosine (input in radians), log and exp (natural loga‐
96 rithm), square root.
97 ATAN
99 Arctangent (output in radians).
101 ATAN2
103 Arctangent of y,x components (output in radians). This pops one
105 element from the stack, the x (cosine) component, and then a sec‐
106 ond, which is the y (sine) component. It then pushes the arctan‐
107 gent of their ratio, resolving the ambiguity between quadrants.
108 Example: "CDEF:angle=Y,X,ATAN2,RAD2DEG" will convert "X,Y" compo‐
110 nents into an angle in degrees.
111 FLOOR, CEIL
113 Round down or up to the nearest integer.
115 DEG2RAD, RAD2DEG
117 Convert angle in degrees to radians, or radians to degrees.
119 ABS
121 Take the absolute value.
123 Set Operations
125 SORT, REV
126 Pop one element from the stack. This is the count of items to be
128 sorted (or reversed). The top count of the remaining elements are
129 then sorted (or reversed) in place on the stack.
130 Example: "CDEF:x=v1,v2,v3,v4,v5,v6,6,SORT,POP,5,REV,POP,+,+,+,4,/"
132 will compute the average of the values v1 to v6 after removing the
133 smallest and largest.
134 AVG
136 Pop one element (count) from the stack. Now pop count elements and
138 build the average, ignoring all UNKNOWN values in the process.
139 Example: "CDEF:x=a,b,c,d,4,AVG"
141 TREND
143 Create a "sliding window" average of another data series.
145 Usage: CDEF:smoothed=x,1800,TREND
147 This will create a half-hour (1800 second) sliding window average
149 of x. The average is essentially computed as shown here:
150 +---!---!---!---!---!---!---!---!--->
152 now
153 delay t0
154 <--------------->
155 delay t1
156 <--------------->
157 delay t2
158 <--------------->
159 Value at sample (t0) will be the average between (t0-delay) and (t0)
161 Value at sample (t1) will be the average between (t1-delay) and (t1)
162 Value at sample (t2) will be the average between (t2-delay) and (t2)
163 Special values
165 UNKN
166 Pushes an unknown value on the stack
168 INF, NEGINF
170 Pushes a positive or negative infinite value on the stack. When
172 such a value is graphed, it appears at the top or bottom of the
173 graph, no matter what the actual value on the y-axis is.
174 PREV
176 Pushes an unknown value if this is the first value of a data set or
178 otherwise the result of this CDEF at the previous time step. This
179 allows you to do calculations across the data. This function can‐
180 not be used in VDEF instructions.
181 PREV(vname)
183 Pushes an unknown value if this is the first value of a data set or
185 otherwise the result of the vname variable at the previous time
186 step. This allows you to do calculations across the data. This
187 function cannot be used in VDEF instructions.
188 COUNT
190 Pushes the number 1 if this is the first value of the data set, the
192 number 2 if it is the second, and so on. This special value allows
193 you to make calculations based on the position of the value within
194 the data set. This function cannot be used in VDEF instructions.
195 Time
197 Time inside RRDtool is measured in seconds since the epoch. The
198 epoch is defined to be "Thu Jan 1 00:00:00 UTC 1970".
199 NOW
201 Pushes the current time on the stack.
203 TIME
205 Pushes the time the currently processed value was taken at onto the
207 stack.
208 LTIME
210 Takes the time as defined by TIME, applies the time zone offset
212 valid at that time including daylight saving time if your OS sup‐
213 ports it, and pushes the result on the stack. There is an elabo‐
214 rate example in the examples section below on how to use this.
215 Processing the stack directly
217 DUP, POP, EXC
218 Duplicate the top element, remove the top element, exchange the two
220 top elements.
221
223 These operators work only on VDEF statements. Note that currently ONLY
224 these work for VDEF.
225 MAXIMUM, MINIMUM, AVERAGE
227 Return the corresponding value, MAXIMUM and MINIMUM also return the
228 first occurrence of that value in the time component.
229 Example: "VDEF:avg=mydata,AVERAGE"
231 LAST, FIRST
233 Return the last/first value including its time. The time for FIRST
234 is actually the start of the corresponding interval, whereas LAST
235 returns the end of the corresponding interval.
236 Example: "VDEF:first=mydata,FIRST"
238 TOTAL
240 Returns the rate from each defined time slot multiplied with the
241 step size. This can, for instance, return total bytes transfered
242 when you have logged bytes per second. The time component returns
243 the number of seconds.
244 Example: "VDEF:total=mydata,TOTAL"
246 PERCENT
248 This should follow a DEF or CDEF vname. The vname is popped,
249 another number is popped which is a certain percentage (0..100).
250 The data set is then sorted and the value returned is chosen such
251 that percentage percent of the values is lower or equal than the
252 result. Unknown values are considered lower than any finite number
253 for this purpose so if this operator returns an unknown you have
254 quite a lot of them in your data. Infinite numbers are lesser, or
255 more, than the finite numbers and are always more than the Unknown
256 numbers. (NaN < -INF < finite values < INF)
257 Example: "VDEF:perc95=mydata,95,PERCENT"
259 LSLSLOPE, LSLINT, LSLCORREL
261 Return the parameters for a Least Squares Line (y = mx +b) which
262 approximate the provided dataset. LSLSLOPE is the slope (m) of the
263 line related to the COUNT position of the data. LSLINT is the
264 y-intercept (b), which happens also to be the first data point on
265 the graph. LSLCORREL is the Correlation Coefficient (also know as
266 Pearson's Product Moment Correlation Coefficient). It will range
267 from 0 to +/-1 and represents the quality of fit for the approxima‐
268 tion.
269 Example: "VDEF:slope=mydata,LSLSLOPE"
271
273 rrdgraph gives an overview of how rrdtool graph works. rrdgraph_data
274 describes DEF,CDEF and VDEF in detail. rrdgraph_rpn describes the RPN
275 language used in the ?DEF statements. rrdgraph_graph page describes
276 all of the graph and print functions.
277 Make sure to read rrdgraph_examples for tips&tricks.
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281 Program by Tobias Oetiker <tobi@oetiker.ch>
282 This manual page by Alex van den Bogaerdt <alex@ergens.op.het.net>
2841.2.27 2008-02-17 RRDGRAPH_RPN(1)