Whose Now Is It?
Copyright: Sanjay Basu I run into a timestamp problem most weeks, and it has nothing to do with philosophy. A training run kicks off across a few hundred GPUs. The nodes agree, more or less, on what time it is, because we keep them disciplined with PTP and a grandmaster clock, and even then they drift by tens of nanoseconds against each other. A gradient all-reduce that finishes “at the same moment” on eight nodes did not finish at the same moment on any of them. The word “simultaneous” is a convenience we grant ourselves so the log files line up. When something goes wrong at that scale, the first thing an engineer learns is to stop trusting the wall clock and start trusting the causal order. What depended on what. Which message could not have arrived before which other message left. I bring this up because I read a lovely piece this morning, from the Starts With a Bang newsletter, asking whether time really exists. It is a good question asked well, and the answer it lands on is ...