suitability of PCP for event tracing

["Frank Ch. Eigler" <fche at redhat dot com>]

Hi -

We're investigating to what extent the PCP suite may be suitable for
more general low-level event tracing.  Just from docs / source gazing
(so please excuse my terminology errors), a few challenges would seem
to be:


* poll-based data gathering

  It seems as though PMDAs are used exclusively in 'polling' mode,
  meaning that underlying system statistics are periodically queried
  and summary results stored.  In our context, it would be useful if
  PMDAs could push event data into the stream as they occur - perhaps
  hundreds of times a second.

* relatively static pmns

  It would be desirable if PMNS metrics were parametrizable with
  strings/numbers, so that a PMDA engine could use it to synthesize
  metrics on demand from a large space.  (Example: have a
  "kernel-probe" PMNS namespace, parametrized by function name, which
  returns statistics of that function's execution.  There are too many
  kernel functions, and they vary from host to host enough, so that
  enumerating them as a static PMNS table would be impractical.)

* scalar payloads

  It seems as though each metric value provided by PMDAs is
  necessarily a scalar value, as opposed to some structured type.  For
  event tracing, it would be useful to have tuples.  Front-ends could
  choose the interesting fields to render.  (Example: tracing NFS
  calls, complete with decoded payloads.)

* filtering

  It would be desirable for the apps fetching metric values to
  communicate a filtering predicate associated with them, perhaps as
  per pmie rules.  This is to allow the data server daemon to reduce
  the amount of data sent to the gui frontends.  Perhaps also it could
  use them to inform PMDAs as a form of subscription, and in turn they
  could reduce the amount of data flow.

* no web-based frontends

  In our usage, it would be desirable to have some mini pcp-gui that
  is based on web technologies rather than QT.


To what extent could/should PCP be used/extended to cover this space?


- FChE