Tom Lane <tgl@sss.pgh.pa.us> writes:
> Greg Stark <gsstark@mit.edu> writes:
> > What would be useful is instrumenting high level calls that can't be traced
> > without application guidance. For example, inserting a dtrace probe for each
> > SQL and each plan node. That way someone could get the same info as EXPLAIN
> > ANALYZE from a production server without having to make application
> > modifications (or suffer the gettimeofday overhead).
>
> My bogometer just went off again. How is something like dtrace going to
> magically get realtime information without reading the clock?
Sorry, I meant get the same info as EXPLAIN ANALYZE minus the timing.
I'm not familiar with DTrace first-hand but I did have the impression it was
possible to get timing information though. I don't know how much overhead it
has but I wouldn't be surprised if it was lower for a kernel-based profiling
elapsed time counter on Sun hardware than a general purpose gettimeofday call
on commodity PC hardware.
For example it could use a cpu instruction counter and have hooks in the
scheduler for saving and restoring the counter to avoid the familiar gotchas
with being rescheduled across processors.
--
greg
