I don't think that's a fair characterization of the trade offs. Acquiring uncontended mutexes is basically free (and fairly side-effect free) so single-threaded performance will not be noticeably impacted.

Every large C project I'm aware of (read: kernels) that has publicly switched from coarse locks to fine-grained locks has considered it to be a huge win with little to no impact on single-threaded performance. You can even gain performance if you chop up objects or allocations into finer-grained blobs to fit your finer-grained locking strategy because it can play nicer with cache friendliness (accessing one bit of code doesn't kick the other bits of code out of the cache).

1) I don't buy this a priori. Almost everybody who removed a gigantic lock suddenly realizes that there was more contention than they thought and that atomizing it made performance improve.

2) Had Python bitten the bullet and removed the GIL back at Python 3.0, the performance would likely already be back to normal or better. You can't optimize hypothetically. Optimization on something like Python is an accumulation of lots of small wins.

You don’t have to buy anything, that’s been the result of every attempt so far and a big reason for their rejection. The latest effort only gained some traction because the backers also did optimisation work which compensated (and then was merged separately).

> Almost everybody who removed a gigantic lock

See that’s the issue with your response, you’re not actually reading the comment you’re replying to.

And the “almost” is a big tell.

> suddenly realizes that there was more contention than they thought and that atomizing it made performance improve.

There is no contention on the gil in single threaded workloads.

> Had Python bitten the bullet and removed the GIL back at Python 3.0

It would have taken several more years and been completely DOA.