Metered Weights in the Polkadot-SDK

[shawntabrizi]

Metered Weights in the Polkadot-SDK

[gui1117]

The DB Layer could provide very specific details like exactly where the item exists in the merkle trie (depth, size, neighboring children, if it or other neighboring children have already been cached, etc..). Then with really comprehensive database benchmarks, we can dynamically meter how much weight each data operation would be.

This would require to specify an abstract database architecture with a specific cache size, and enforce all clients to have a database compatible with this abstraction.
Maybe just enforcing one cache (without all the neighboring children, depth, size, information) can already be a huge improvement.

Also if we have more memory in the runtime with PVM we can also implement some caching inside the runtime itself I guess.

I see we can do this RFC in multiple step:

• 1- Tracking operations: have a better automatic refund.

  • inside the runtime: keep track of most time-intensive runtime-interface call: crypto, hashing, Storage, Trie.

  • benchmark them independantly from the ref_time. (we already do it for Storage)

  • have the weight related to them separated from ref_time in the dispatch info (e.g.: tracked_operation_ref_time).

  • tracked_operation_ref_time gets an automatic precise refund at the end of the transaction by calculating the actual weight from the tracked call.

  • (maybe we could even track wasm execution in the tracked_logic_ref_time, as I heard the expensive part of metering was branching when gas is exceeded, not actually counting the number of operation, but it feels difficult for less gain, also considering PVM)

• 2- Tracking Storage more precisely: even better refund

  • enforce an architecture on the database: maybe just a cache size or what you propose with neighbor, depth etc..

  • add this information in the Storage runtime interface.

  • use this information in the tracking and refund.

• 3- Have metering and execution stopped when max weight is reached.