Demo Perils

One of the downsides of being an operating systems developer is that the
demos of the technology that you develop often suck. (“Look, it boots!
And hey, we can even run programs and it doesn’t crash!”) So it’s been a
pleasant change to develop
DTrace,
a technology that packs a jaw-dropping
demo. In demonstrating DTrace for customers around the world, I have had
the distinct (and rare) pleasure of impressing the most technically adept
(and often jaded) audiences. My typical demonstration is on my

Solaris x86 laptop, where I use DTrace to
instrument the running system — exploring
with the audience the peculiarities that exist even on an idle laptop.
(This usually involves discovering and understanding the unnecessary work
being done by
acroread,
dhcpagent,
sendmail, etc.)
This ad hoc demo
shows DTrace as it’s meant to be used: dynamically answering questions
that themselves were formed on-the-fly.

And when I demonstrate DTrace, I always do so on the absolute latest
Solaris 10 build. Our mantra in Solaris Kernel Development is “FCS Quality
All the Time” — we believe that the product should always be
ready to be run in production. And if we’re going to tell a customer that
it’s ready to be run in production, we damn well better run it in
production ourselves. This has the added advantage that we tend to run
into bugs before our customers do, allowing us to ship a final product that
is that much more solid. Over the past year, I have given hundreds of
DTrace demonstrations in front of customers running latest bits, and before
last week, it had always gone off without a hitch…¹

Last week, I had the opportunity to give a DTrace demonstration for a
highly technical — and highly influential — audience at a Fortune 100
company. When I demonstrate DTrace, I typically do a couple of invocations
on the command line before things become sufficiently complicated to merit
writing a DTrace
script. And it was when I went to run
the first such script (a script that explored the activity of
xclock) that
it happened:

# dtrace -s ./xclock.d
Segmentation Fault (core dumped)
#

If you’ve never had it, there’s no feeling quite like having a demo blow up
on you: it’s as if you peed your pants, failed an exam and were punched in
the gut — all at the same horrifying instant. It’s a feeling that every
software developer should have exactly once in their lives: that unique
rush of shock,
and then humiliation and then despair, followed by the adrenal surge of a
fight-or-flight reaction. In the time it takes a single process to dump
core, you go from an (over)confident technologist to a frightened woodland
creature, transfixed by the light of an oncoming freight train. For the
woodland creature, at least it all ends mercifully quickly; the creature is
spared the suffering of trying to explain away its foolishness. The
hapless technologist, on the other hand, is left with several options:

1. Pretend that you didn’t write the software: “Boy, will you get a load
   of those fancy-pants software engineers? Overpriced underworked morons,
   every last one!”

2. Explain that this is demo software and isn’t expected to work:
   “Well, that’s why we haven’t shipped it yet! I mean, what fool would
   run this stuff anyway? Other than me, that is.”

3. Make light of it: “Hey, knock knock! Who’s there? Not my
   software, that’s for sure! Wocka wocka wocka!”

4. Suck it up: “That’s a serious problem. If you can excuse me for
   a second, let me get a handle on what we’ve got here that we can demo.”

I always aim for this last option, but
on the rare occasion that this has happened to me (and this is — honest —
probably the worst that a customer-facing demo has gone for me)
I usually end up with
some combination of the last three, often with plenty of stuttering,
some mild swearing (“Damn! Damn!”) and profuse sweating.

In my particular case, the worst part was not knowing the exact pathology
of the bug that I had just
run into. Was there something basic that was broken or toxic about
my machine? Would all scripts that I tried to run dump
core? And if this was broken, what else was broken? Would I panic the
machine or crash a target app if I continued? (Much more serious
problems, both.) In an effort to get a handle on it, I did a quick

pstack on the
core file:

0804718f ???????? (8046604, 2)
d137c839 dt_instr_size (82d051a, 8067320, 223, d1380fe2) + 59
d137c0c2 dt_pid_create_return_probe (81651b8, 8067320, 8046af0, 8047170, 80472d
d137370d dt_pid_per_sym (80472ac, 8047170, d087b02c) + 15b
d13739ae dt_pid_sym_filt (80472ac, 8047170, d087b02c, 804715c) + 7c
d13152ca Psymbol_iter_com (81651b8, ffffffff, 8069060, 1, 407, 1) + 1e0
d13153ae Psymbol_iter_by_addr (81651b8, 8069060, 1, 407, d1373932, 80472ac) + 1
d1373b81 dt_pid_per_mod (80472ac, 82cf600, 8069060) + 191
d1373d56 dt_pid_mod_filt (80472ac, 82cf600, 8069060) + a3
d1314fe4 Pobject_iter (81651b8, d1373cb3, 80472ac) + 4f
d13740b4 dt_pid_create_probes (82cafa0, 8067320) + 344
d1353af8 dt_setcontext (8067320, 82cafa0) + 42
d13537d4 dt_compile_one_clause (8067320, 82be430, 82cdae0) + 32
d1353a9c dt_compile_clause (8067320, 82be430) + 26
d1354d66 dt_compile (8067320, 16a, 3, 0, 80, 1) + 3d9
d1355263 dtrace_program_strcompile (8067320, 8047ec2, 3, 80, 1, 8066848) + 23
080526ef ???????? (8066e48)
0805370e main     (3, 8047df8, 8047e08) + 8fc
0805177a ???????? (3, 8047eb8, 8047ebf, 8047ec2, 0, 8047edf)

This was dying in the code that analyzes a target binary as part of
creating
pid
provider probes. There was at least a chance that
this problem was localized to something specific about the xclock
program text — it was worth trying a similar script on a different
process.
Fortunately, I was able to stave off total panic long
enough to write such a script and — even better —
this one worked. The problem did indeed seem to be localized to something
specific in xclock. And thanks to my
coreadm
settings, the core file from the seg faulting
dtrace
had been stashed away for later analysis; the best thing I could do at
that point was
drive on with the rest of the demo.

And this is what I did. The rest of the demo went well, and the
audience was ultimately impressed with the technology. And while I never quite
regained my stride (in part because my mind was racing
about which change to DTrace could have introduced the problem), I
was at least sufficiently effective — we achieved the goals of the
meeting.²
On the plane back home, I root-caused the problem and developed a fix.
The next day, I integrated the fix into Solaris — and I don’t think
I’ve ever been so relieved to put latest bits on my laptop!

In the end, having the demo blow up certainly wasn’t a pleasant experience —
but I wouldn’t change my
decision to demo on the latest bits. Not only did we discover a serious
bug, we discovered the hole in our test suite that prevented us from
finding the bug before it integrated. So who am I to get upset about a
little personal humiliation if the upshot is a better product? 😉

¹ This is a slight exaggeration. I had actually run into
DTrace bugs in front of customers, but they were always sufficiently
small that only a trained eye would realize that something was amiss —
things like slightly incorrect error messages.

² The primary goal of such a demo is often to get the customer
sufficiently excited about Solaris 10
to download Solaris Express
(usually for x86) and start playing around
with the technology themselves.
We are nearly always successful in this — and I have
even had a few customers start downloading Solaris Express before
the end of the meeting!