Bah. Wanted to take a peek at something running on my laptop but JVisualVM grumbled about unable being able to detect local apps. Helpfully had a link to the trouble-shooting page and the underlying issue, which is that HotSpot won't write the necessary perf stuff to the temp dir if it's on a FAT partition because it'd be world readable. Thanks Acer.
The trick is to run both the app you want to look at and JVisualVM with the argument