[Ksummit-2008-discuss] DTrace

[James Bottomley]

On Fri, 2008-06-27 at 09:04 -0600, Matthew Wilcox wrote:
> I went to a Postgresql conference a few weeks ago.  There was a lot of
> buzz around DTrace.  Sun and a couple of other companies have put DTrace
> hooks into postgres, so they now have some really useful canned queries.
> If you're running Solaris or MacOS, of course.
> 
> So there was a lot of talk about switching away from Linux.  This can't
> possibly be a good thing for us.  I don't personally know what the state
> of our competing projects are, but clearly they haven't got their hooks
> into postgres ... at least not upstream.
> 
> I don't think I'd be the right person to lead a discussion on this, but
> maybe we can find someone to talk about it.

DTrace is more a piece of sun marketing coolaid which they use to beat
us up at every opportunity.

We actually have a reasonably functional equivalent piece of technology
called systemtap.

When I go around end users, I find people in two camps:  The ones who've
drunk the sun coolaid and won't take anything on linux that isn't a
fully replicated dtrace (sort of like windows people who demand the
availability of outlook on linux) and people who are migrating to Linux
and trying to use systemtap for tracing.  These latter seem to have a
number of genuine concerns including latency, the time it takes to
actually go from command executing to functional trace, the inability to
trace user programs (dtrace can) and concerns about the amount of
perturbation the probes actually place inside the kernel.

I have to say after receiving my last list of complaints about
systemtap, I actually went back home and vowed to use it to track a SCSI
regression (actually the cdparanoia problem Geert reported) and
immediately ran into a systemtap failing:  You can't probe ata_qc_issue
and ask for $qc->scsicmd->device->host without it throwing semantic
errors.

I finally (after an unhappy week spend with dwarf) worked out how to fix
this particular annoyance, but my general impression is that systemtap
is klunky and prone to break in unexpected ways.  Although we could have
a topic about systemtap at KS, I think a better way to improve it might
be if more kernel developers started actually using it (and fixing the
bugs they find).

James