[twotwotwo]

I'm too late to the thread, but wonder if locking might have to do with updating access times on the dirs on read--that would help justify a read-only workload taking some sort of exclusive lock. The "fix" there would be something like relatime to get atimes set infrequently.

There're def a ton of other possibilities. Some locking could be overbroad, like an exclusive lock where a reader lock would be enough. I don't see super-strong evidence that any locks involved are actually global, just that they're under contention here. And I can understand shipping a product with somewhat excessive locking because perf issues with specific workloads are a better problem to have with your shiny new FS rollout than data loss.

Anyway it's mostly shooting in the dark for us here (though cool some folks disassembled the functions), but there are some shots in the dark that look different from what I could find in the comments already here :)