Make pending request channel size configurable

[muzarski]

This would directly correspond to cass_cluster_set_queue_size_io.

We even have a TODO in our code regarding this: https://github.com/scylladb/scylla-rust-driver/blob/v1.1.0/scylla/src/network/connection.rs#L453-L454.

By default we currently set the size to 1024. OTOH, cpp-driver sets this value to 8192 by detault. Maybe it is something to consider.

[dhvll]

Hey I would like to contribute to this issue.

[Lorak-mmk]

Great! It should not be too difficult. If you need any help feel free to ask.

[Lorak-mmk]

OTOH, cpp-driver sets this value to 8192 by detault. Maybe it is something to consider.

@muzarski is it also per-connection queue there?

[muzarski]

OTOH, cpp-driver sets this value to 8192 by detault. Maybe it is something to consider.

@muzarski is it also per-connection queue there?

Not really. The architecture is a bit different in cpp-driver. The queue is per-RequestProcessor. There are as many RequestProcessors as specified by cass_cluster_set_threads_io. In rust-driver we have as many tokio tasks as there are connections - (Connection::writer task per connection).

So the semantics will not be exactly the same. This will need to be mentioned in cpp-rust-driver API documentation (same for cass_cluster_set_threads_io - we plan to configure the number of threads used by the tokio runtime using this, and not the number of tokio tasks handling the requests).

The default values for these two will probably need to be adjusted on cpp-rust-driver side as well. 8192 sounds like a bit too much for a per-connection queue size.