perf: maintain a cache of block information in ChainStore

[nagisa]

Currently all ChainStore queries such as get_block_header, get_block_hash_by_height, get_block_height, get_block_header_on_chain_by_height and so on go straight to rocksdb.

In practice most of the queries will involve just a couple dozen most recent heights¹, so in theory it should be possible to improve certain workloads significantly. If ChainStore maintained deserialized data for these recent blocks in warm memory, these queries could avoid a trip through the rocksdb layers (all the way to its block cache,) deserialization overhead, etc.

A particular workload where such queries appear to have a significant overhead is a native token transfer benchmark with a lower number of accounts. A profile of such a workload can be seen here.

Footnotes

1. verify this theory! both in a benchmark and a real node -- I imagine we could log how far behind HEAD queries reach to for each query and then plot a frequency graph. My intuition is that it should take a shape of a normal distribution bell. ↩

[mooori]

Can this lead to issues with stateless validation? For instance in the following scenario:

• The node generating the state witness has Block B in it's cache, so the corresponding storage access is not recorded.
• B is not included in the state witness.
• A chunk validator cannot process a transaction that requires info related to B

If yes, one workaround might be clearing the cache before starting to produce a new state witness. Then the first access to B always goes through storage, but subsequent requests hit the cache.

[mooori]

Can this lead to issues with stateless validation?

It's not an issue because chunk validators maintain chain state by themselves and the related getters go through chain instead of state_witness (example).

Confirmed with @pugachAG that chunk validators can answer ChainStore::get_* queries by themselves without relying on the state witness, as long as they've processed the corresponding block.