Piping `telemetry.py --run` into anything that stops reading first — head, or
the campaign shutting the sampler down — raised BrokenPipeError and printed a
traceback. Under ab_repeat that traceback lands in telemetry.err, where it
reads as "telemetry is broken" rather than "telemetry ended normally", and
telemetry.err is exactly the file someone checks when the clock record looks
short.
Swallowing the exception alone is not enough: the interpreter flushes stdout
again on exit and emits a second "Exception ignored" to stderr, so stdout is
pointed at devnull the way CPython documents.
A 6-round A/B here reports side noise at the 0.01% level. Clock throttling and
thermal drift move latency by far more than that and leave no trace in
run.log, so the report comes out looking exactly as clean as a run on a stable
machine. The NV run that reproduced PR763 sampled nvidia-smi every 5s
alongside the campaign; the MUSA campaigns had no equivalent, and the parallel
plan makes it worse by heating the whole box.
telemetry.py samples through the backend, so cuda and musa emit the same
columns (index, sm_clock_mhz, mem_clock_mhz, temp_c, power_w, util_pct) and
two machines' campaigns can be read side by side. mthreads-gmi offers no CSV
query mode, so the MUSA path parses --json and strips the units glued onto
each value ("36C", "1888MHz", "477.68W"); its "Power Draw " key carries a
trailing space, matched by stripping rather than by literal.
The device comes from the backend's own visibility variable when not pinned. A
list of devices resolves to no -i at all, sampling every one, rather than
silently picking the first. Dropping the placeholder drops its flag too: a
bare trailing -i fails the sampler instead of widening it.
Telemetry is evidence, not a gate. A missing or unpermitted sampler exits
quietly with its reason in telemetry.err and the campaign runs unchanged.
Aharrypotter