## Issue Addressed
Closes#3963 (hopefully)
## Proposed Changes
Compute attestation selection proofs gradually each slot rather than in a single `join_all` at the start of each epoch. On a machine with 5k validators this replaces 5k tasks signing 5k proofs with 1 task that signs 5k/32 ~= 160 proofs each slot.
Based on testing with Goerli validators this seems to reduce the average time to produce a signature by preventing Tokio and the OS from falling over each other trying to run hundreds of threads. My testing so far has been with local keystores, which run on a dynamic pool of up to 512 OS threads because they use [`spawn_blocking`](https://docs.rs/tokio/1.11.0/tokio/task/fn.spawn_blocking.html) (and we haven't changed the default).
An earlier version of this PR hyper-optimised the time-per-signature metric to the detriment of the entire system's performance (see the reverted commits). The current PR is conservative in that it avoids touching the attestation service at all. I think there's more optimising to do here, but we can come back for that in a future PR rather than expanding the scope of this one.
The new algorithm for attestation selection proofs is:
- We sign a small batch of selection proofs each slot, for slots up to 8 slots in the future. On average we'll sign one slot's worth of proofs per slot, with an 8 slot lookahead.
- The batch is signed halfway through the slot when there is unlikely to be contention for signature production (blocks are <4s, attestations are ~4-6 seconds, aggregates are 8s+).
## Performance Data
_See first comment for updated graphs_.
Graph of median signing times before this PR:
Graph of update attesters metric (includes selection proof signing) before this PR:
Median signing time after this PR (prototype from 12:00, updated version from 13:30):
99th percentile on signing times (bounded attestation signing from 16:55, now removed):
Attester map update timing after this PR:
Selection proof signings per second change:
## Link to late blocks
I believe this is related to the slow block signings because logs from Stakely in #3963 show these two logs almost 5 seconds apart:
> Feb 23 18:56:23.978 INFO Received unsigned block, slot: 5862880, service: block, module: validator_client::block_service:393
> Feb 23 18:56:28.552 INFO Publishing signed block, slot: 5862880, service: block, module: validator_client::block_service:416
The only thing that happens between those two logs is the signing of the block:
0fb58a680d/validator_client/src/block_service.rs (L410-L414)
Helpfully, Stakely noticed this issue without any Lighthouse BNs in the mix, which pointed to a clear issue in the VC.
## TODO
- [x] Further testing on testnet infrastructure.
- [x] Make the attestation signing parallelism configurable.
57dfcfd83a
