Probably Dance

I submitted a tiny patch to flash attention. The necessary typing for the change takes less ten seconds, but the overall change took more than ten hours So where does the time go?

It started when coworker had a bug where cudnn attention would crash randomly. We looked at his unreleased changes and concluded that they couldn’t possibly cause this, so we suspected that we had a lingering bug that was exposed by making harmless changes to related code.

Step 1, a few hours: My coworker tried to figure this out just by running the code repeatedly, trying out various theories. The bug was hard to reproduce so this took hours without much progress.

Step 2, 1 hour: I thought this is a good reason to try out compute sanitizer. It would be easiest to just run it on our existing tests to see if it finds any issues without my coworker’s changes. But the tests run on another box because they require certain GPUs, which means you have to run the tests through some layers. Unfortunately compute sanitizer really wants to be in charge of the program, so we have to convince those layers to let compute sanitizer run the whole thing. It keeps on failing and we can’t figure out why, until eventually I suspect that the issue is that the tests run in a sandbox, and the sandbox is strict enough that it breaks compute sanitizer somehow. This turned out to be true and we probably wasted an hour together.