3.5.7. Computing for Statistical Aggregates

Statistical aggregates are used to collect statistics on numerical values where it is important to accumulate new data quickly and in large volume (that is storing only aggregated stream statistics). Statistical aggregates can be used in global variables or as elements in an array.

To add value to a statistical aggregate, use the operator <<< value.

global reads	
probe vfs.read
{
  reads[execname()] <<< count
}

In Example 3.19, “stat-aggregates.stp”, the operator <<< count stores the amount returned by count to the associated value of the corresponding execname() in the reads array. Remember, these values are stored; they are not added to the associated values of each unique key, nor are they used to replace the current associated values. In a manner of speaking, think of it as having each unique key (execname()) having multiple associated values, accumulating with each probe handler run.

To extract data collected by statistical aggregates, use the syntax format @extractor(variable/array index expression). extractor can be any of the following integer extractors:

When using statistical aggregates, you can also build array constructs that use multiple index expressions (to a maximum of 5). This is helpful in capturing additional contextual information during a probe. For example:

global reads
probe vfs.read
{
  reads[execname(),pid()] <<< 1
}
probe timer.s(3)
{
  foreach([var1,var2] in reads)
    printf("%s (%d) : %d \n", var1, var2, @count(reads[var1,var2]))
}

In Example 3.20, “Multiple Array Indexes”, the first probe tracks how many times each process performs a VFS read. What makes this different from earlier examples is that this array associates a performed read to both a process name and its corresponding process ID.

The second probe in Example 3.20, “Multiple Array Indexes” demonstrates how to process and print the information collected by the array reads. Note how the foreach statement uses the same number of variables (that is var1 and var2) contained in the first instance of the array reads from the first probe.