xref: /src/external/cddl/dtracetoolkit/dist/Docs/Notes/ALLflow_notes.txt

**************************************************************************
* Notes for all scripts that print a function or method flow.
*
* $Id: ALLflow_notes.txt,v 1.1 2015/09/30 22:01:08 christos Exp $
*
* COPYRIGHT: Copyright (c) 2007 Brendan Gregg.
**************************************************************************


* What is a flow?

Output that has some meaningful indent, such as function flow indented by
stack depth. eg,

   # ./pl_flow.d 
     C TIME(us)          FILE             -- SUB
     0 2963130861619     func_abc.pl      -> func_a
     0 2963131870998     func_abc.pl        -> func_b
     0 2963132871121     func_abc.pl          -> func_c
     0 2963133881150     func_abc.pl          <- func_c
     0 2963133881166     func_abc.pl        <- func_b
     0 2963133881174     func_abc.pl      <- func_a
   ^C


* The output looks shuffled?

Eg,

   # ./pl_flow.d 
     C TIME(us)          FILE             -- SUB
     0 2963130861619     func_abc.pl      -> func_a
     0 2963131870998     func_abc.pl        -> func_b
     0 2963132871121     func_abc.pl          -> func_c
     0 2963133881166     func_abc.pl        <- func_b
     0 2963133881174     func_abc.pl      <- func_a
     1 2963133881150     func_abc.pl          <- func_c
   ^C

Yes, this is shuffled. DTrace has been designed with a number of important
goals in mind - including minimising the enabled performance overhead. To do
this, per-CPU kernel buffers have been used to collect output, which are
(currently) dumped in sequence by /usr/sbin/dtrace whenever it wakes
up ("switchrate" tunable). So, on multi-CPU servers, there is always the
possibility that any DTrace script can print out-of-order data.

To deal with this behaviour, the flow scripts may,

- print a "C" CPU column. If this changes from one line to the next then
  the output is probably shuffled around that point. This is why the "C"
  column appears in these flow scripts.
- print a "TIME(us)" column. You can eyeball this for shuffles, or just
  post sort the dtrace output.

Now have a closer look at the pl_flow.d output above. The change in C 
indicates that a shuffle may have happened, and the out-of-order TIME(us)
shows that it did happen.

It is possible that DTrace will be enhanced to always sort output before
printing, and this behaviour is no longer an issue.

See "The output seems shuffled?" in Notes/ALLsnoop_notes.txt for more
notes on this behaviour.