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Diffstat (limited to 'doc/SystemTap_Beginners_Guide/en-US/Array-Operations.xml')
| -rw-r--r-- | doc/SystemTap_Beginners_Guide/en-US/Array-Operations.xml | 38 |
1 files changed, 38 insertions, 0 deletions
diff --git a/doc/SystemTap_Beginners_Guide/en-US/Array-Operations.xml b/doc/SystemTap_Beginners_Guide/en-US/Array-Operations.xml index 987e2b29..8fe46721 100644 --- a/doc/SystemTap_Beginners_Guide/en-US/Array-Operations.xml +++ b/doc/SystemTap_Beginners_Guide/en-US/Array-Operations.xml @@ -1148,6 +1148,44 @@ reads[execname()] <<< count --> </variablelist> + +<para> + 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: +</para> + +<example id="multiplearrayindices"> + <title>Multiple Array Indexes</title> +<programlisting> +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]))
+} +</programlisting> +</example> + +<para> + In <xref linkend="multiplearrayindices"/>, 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 <emphasis>and</emphasis> its corresponding process ID. +</para> + +<para> + The second probe in <xref linkend="multiplearrayindices"/> demonstrates how to process and print + the information collected by the array <literal>reads</literal>. Note how the + <command>foreach</command> statement uses the same number of variables (i.e. + <literal>var1</literal> and <literal>var2</literal>) contained in the first instance of the array + <literal>reads</literal> from the first probe. +</para> + + </section> </section>
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