When we said above that values can only be strings or numbers, we lied a little. There is a third type: statistics aggregates, or aggregates for short. Instances of this type are used to collect statistics on numerical values, where it is important to accumulate new data quickly (without exclusive locks) and in large volume (storing only aggregated stream statistics). This type only makes sense for global variables, and may be stored individually or as elements of an array.
To add a value to a statistics aggregate, systemtap uses the special
operator <<<. Think of it like C++'s << output
streamer: the left hand side object accumulates the data sample given
on the right hand side. This operation is efficient (taking a shared
lock) because the aggregate values are kept separately on each
processor, and are only aggregated across processors on request.
a <<< delta_timestamp writes[execname()] <<< count
To read the aggregate value, special functions are available to
extract a selected statistical function. The aggregate value
cannot be read by simply naming it as if it were an ordinary
variable. These operations take an exclusive lock on the respective
globals, and should therefore be relatively rare. The simple ones
are: @min, @max, @count, @avg, and
@sum, and evaluate to a single number. In addition, histograms
of the data stream may be extracted using the @hist_log and
@hist_linear. These evaluate to a special sort of array that
may at present2 only be printed.
@avg(a) |
the average of all the values accumulated
into a |
print(@hist_linear(a,0,100,10)) |
print an ``ascii art'' linear histogram of the same data stream, |
bounds  , bucket width is  |
|
@count(writes["zsh"]) |
the number of times ``zsh'' ran the probe handler |
print(@hist_log(writes["zsh"])) |
print an ``ascii art'' logarithmic histogram of the same data stream |