On Sat, 07 Sep 2019 09:14:49 -0700, Theodore Dubois said:

If I’m reading this right, this is a sampling event which overflows 4000 times a second. But perf then does a poll call which wakes up on this FD with POLLIN after 1.637 seconds, instead of 0.00025 seconds.

No, it *takes a sample* 4,000 times a second. For instance, number of cache line misses since the last sample. You get an overflow when the counter wraps because there have been more than 2^32 events since you read the counter. At least that's my understanding of it.