DTrace stores the results of aggregating functions in objects called aggregations. In D, the syntax for an aggregation is as follows:
@name[keys] =aggfunc(args);
The aggregation name is a D identifier
that is prefixed with the special character @.
All aggregations that are named in your D programs are global
variables. There are no thread-local or clause-local aggregations.
The aggregation names are kept in an identifier namespace that is
separate from other D global variables. If you reuse names,
remember that a and @a are
not the same variable. The special
aggregation name @ can be used to name an
anonymous aggregation in simple D programs. The D compiler treats
this name as an alias for the aggregation name
@_.
Aggregations are indexed with keys, where
keys are a comma-separated list of D
expressions, similar to the tuples of expressions used for
associative arrays. Keys can also be actions with
non-void return values, such as
stack, func,
sym, mod,
ustack, uaddr, and
usym.
The aggfunc is one of the DTrace
aggregating functions, and args is a
comma-separated list of arguments that is appropriate to that
function. The DTrace aggregating functions are described in the
following table. Most aggregating functions take just a single
argument that represents the new datum.
Table 3.1 DTrace Aggregating Functions
Function Name | Arguments | Result |
|---|---|---|
| None | Number of times called. |
| Scalar expression | Total value of the specified expressions. |
| Scalar expression | Arithmetic average of the specified expressions. |
| Scalar expression | Smallest value among the specified expressions. |
| Scalar expression | Largest value among the specified expressions. |
| Scalar expression | Standard deviation of the specified expressions. |
| Scalar expression [, increment] | Power-of-two frequency distribution (histogram) of the values of the specified expressions. An optional increment (weight) can be specified. |
| Scalar expression, lower bound, upper bound [, step value [, increment]] | Lnear frequency distribution of the values of the specified expressions, sized by the specified range.
Note that the default step value is
|
| Scalar expression, base, lower exponent, upper exponent, number of steps per order of magnitude [, increment] | Log-linear frequency distribution. The logarithmic base is specified, along with lower and upper exponents and the number of steps per order of magnitude. |