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## Issue Addressed
NA
## Proposed Changes
Missed head votes on attestations is a well-known issue. The primary cause is a block getting set as the head *after* the attestation deadline.
This PR aims to shorten the overall time between "block received" and "block set as head" by:
1. Persisting the head and fork choice *after* setting the canonical head
- Informal measurements show this takes ~200ms
1. Pruning the op pool *after* setting the canonical head.
1. No longer persisting the op pool to disk during `BeaconChain::fork_choice`
- Informal measurements show this can take up to 1.2s.
I also add some metrics to help measure the effect of these changes.
Persistence changes like this run the risk of breaking assumptions downstream. However, I have considered these risks and I think we're fine here. I will describe my reasoning for each change.
## Reasoning
### Change 1: Persisting the head and fork choice *after* setting the canonical head
For (1), although the function is called `persist_head_and_fork_choice`, it only persists:
- Fork choice
- Head tracker
- Genesis block root
Since `BeaconChain::fork_choice_internal` does not modify these values between the original time we were persisting it and the current time, I assert that the change I've made is non-substantial in terms of what ends up on-disk. There's the possibility that some *other* thread has modified fork choice in the extra time we've given it, but that's totally fine.
Since the only time we *read* those values from disk is during startup, I assert that this has no impact during runtime.
### Change 2: Pruning the op pool after setting the canonical head
Similar to the argument above, we don't modify the op pool during `BeaconChain::fork_choice_internal` so it shouldn't matter when we prune. This change should be non-substantial.
### Change 3: No longer persisting the op pool to disk during `BeaconChain::fork_choice`
This change *is* substantial. With the proposed changes, we'll only be persisting the op pool to disk when we shut down cleanly (i.e., the `BeaconChain` gets dropped). This means we'll save disk IO and time during usual operation, but a `kill -9` or similar "crash" will probably result in an out-of-date op pool when we reboot. An out-of-date op pool can only have an impact when producing blocks or aggregate attestations/sync committees.
I think it's pretty reasonable that a crash might result in an out-of-date op pool, since:
- Crashes are fairly rare. Practically the only time I see LH suffer a full crash is when the OOM killer shows up, and that's a very serious event.
- It's generally quite rare to produce a block/aggregate immediately after a reboot. Just a few slots of runtime is probably enough to have a decent-enough op pool again.
## Additional Info
Credits to @macladson for the timings referenced here.
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Lighthouse: Ethereum 2.0
An open-source Ethereum 2.0 client, written in Rust and maintained by Sigma Prime.
Overview
Lighthouse is:
- Ready for use on Eth2 mainnet.
- Fully open-source, licensed under Apache 2.0.
- Security-focused. Fuzzing techniques have been continuously applied and several external security reviews have been performed.
- Built in Rust, a modern language providing unique safety guarantees and excellent performance (comparable to C++).
- Funded by various organisations, including Sigma Prime, the Ethereum Foundation, ConsenSys, the Decentralization Foundation and private individuals.
- Actively involved in the specification and security analysis of the Ethereum 2.0 specification.
Eth2 Deposit Contract
The Lighthouse team acknowledges
0x00000000219ab540356cBB839Cbe05303d7705Fa
as the canonical Eth2 deposit contract address.
Documentation
The Lighthouse Book contains information for users and developers.
The Lighthouse team maintains a blog at lighthouse.sigmaprime.io which contains periodical progress updates, roadmap insights and interesting findings.
Branches
Lighthouse maintains two permanent branches:
stable: Always points to the latest stable release.- This is ideal for most users.
unstable: Used for development, contains the latest PRs.- Developers should base their PRs on this branch.
Contributing
Lighthouse welcomes contributors.
If you are looking to contribute, please head to the Contributing section of the Lighthouse book.
Contact
The best place for discussion is the Lighthouse Discord server. Alternatively, you may use the sigp/lighthouse gitter.
Sign up to the Lighthouse Development Updates mailing list for email notifications about releases, network status and other important information.
Encrypt sensitive messages using our PGP key.
Donations
Lighthouse is an open-source project and a public good. Funding public goods is hard and we're grateful for the donations we receive from the community via:
- Gitcoin Grants.
- Ethereum address:
0x25c4a76E7d118705e7Ea2e9b7d8C59930d8aCD3b(donation.sigmaprime.eth).