Reduce function calling overhead by ~25% in ReferencePool::update_counts:

[alex]

1. Place both increfs and decrefs behind a single mutex, rather than two. Even uncontended mutexes aren't free to acquire.
2. Keep a dirty tracking bool to avoid acquiring any mutexes at all in the common case of no modifications (because everything happened with the GIL held)

Benchmarks

Before

(.venv) alex@media-01:~/p/pyo3$ python3 -mtimeit -s "from pyo3_benchmarks import no_args" "no_args()"
5000000 loops, best of 5: 63.6 nsec per loop
(.venv) alex@media-01:~/p/pyo3$ python3 -mtimeit -s "from pyo3_benchmarks import no_args" "no_args()"
5000000 loops, best of 5: 63.7 nsec per loop
(.venv) alex@media-01:~/p/pyo3$ python3 -mtimeit -s "from pyo3_benchmarks import no_args" "no_args()"
5000000 loops, best of 5: 64.1 nsec per loop
(.venv) alex@media-01:~/p/pyo3$ python3 -mtimeit -s "from pyo3_benchmarks import no_args" "no_args()"
5000000 loops, best of 5: 63.6 nsec per loop
(.venv) alex@media-01:~/p/pyo3$ python3 -mtimeit -s "from pyo3_benchmarks import no_args" "no_args()"
5000000 loops, best of 5: 63.5 nsec per loop

With single lock

(.venv) alex@media-01:~/p/pyo3$ python3 -mtimeit -s "from pyo3_benchmarks import no_args" "no_args()"
5000000 loops, best of 5: 51.9 nsec per loop
(.venv) alex@media-01:~/p/pyo3$ python3 -mtimeit -s "from pyo3_benchmarks import no_args" "no_args()"
5000000 loops, best of 5: 52 nsec per loop
(.venv) alex@media-01:~/p/pyo3$ python3 -mtimeit -s "from pyo3_benchmarks import no_args" "no_args()"
5000000 loops, best of 5: 52.9 nsec per loop
(.venv) alex@media-01:~/p/pyo3$ python3 -mtimeit -s "from pyo3_benchmarks import no_args" "no_args()"
5000000 loops, best of 5: 51.8 nsec per loop
(.venv) alex@media-01:~/p/pyo3$ python3 -mtimeit -s "from pyo3_benchmarks import no_args" "no_args()"
5000000 loops, best of 5: 54.7 nsec per loop

With both single lock and dirty flag

(.venv) alex@media-01:~/p/pyo3$ python3 -mtimeit -s "from pyo3_benchmarks import no_args" "no_args()"
5000000 loops, best of 5: 47.2 nsec per loop
(.venv) alex@media-01:~/p/pyo3$ python3 -mtimeit -s "from pyo3_benchmarks import no_args" "no_args()"
5000000 loops, best of 5: 46.7 nsec per loop
(.venv) alex@media-01:~/p/pyo3$ python3 -mtimeit -s "from pyo3_benchmarks import no_args" "no_args()"
5000000 loops, best of 5: 46.6 nsec per loop
(.venv) alex@media-01:~/p/pyo3$ python3 -mtimeit -s "from pyo3_benchmarks import no_args" "no_args()"
5000000 loops, best of 5: 46.6 nsec per loop
(.venv) alex@media-01:~/p/pyo3$ python3 -mtimeit -s "from pyo3_benchmarks import no_args" "no_args()"
5000000 loops, best of 5: 47.1 nsec per loop

[davidhewitt]

Awesome! These are some really great optimizations which many people will be happy with. I think there's a potential deadlock though (which we can avoid), see comment below.

[davidhewitt]

We need to be careful here. Decrementing references can run arbitrary Python drop code, which can lead to GIL release which may eventually deadlock because we hold the lock here. (I think this mutex is also non reentrant so may deadlock on a single thread.)

After swapping ops.1 we should release the lock before decreasing any references.

[alex]

Good call, should be good now.

[davidhewitt]

Looks great! Thanks very much 😌

[kngwyu]

Looks like I was late for the party... Very interesting to see this optimization works, thanks.