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lighthouse/book/src/late-block-re-orgs.md
Michael Sproul 775d222299 Enable proposer boost re-orging (#2860) 
## Proposed Changes

With proposer boosting implemented (#2822) we have an opportunity to re-org out late blocks.

This PR adds three flags to the BN to control this behaviour:

* `--disable-proposer-reorgs`: turn aggressive re-orging off (it's on by default).
* `--proposer-reorg-threshold N`: attempt to orphan blocks with less than N% of the committee vote. If this parameter isn't set then N defaults to 20% when the feature is enabled.
* `--proposer-reorg-epochs-since-finalization N`: only attempt to re-org late blocks when the number of epochs since finalization is less than or equal to N. The default is 2 epochs, meaning re-orgs will only be attempted when the chain is finalizing optimally.

For safety Lighthouse will only attempt a re-org under very specific conditions:

1. The block being proposed is 1 slot after the canonical head, and the canonical head is 1 slot after its parent. i.e. at slot `n + 1` rather than building on the block from slot `n` we build on the block from slot `n - 1`.
2. The current canonical head received less than N% of the committee vote. N should be set depending on the proposer boost fraction itself, the fraction of the network that is believed to be applying it, and the size of the largest entity that could be hoarding votes.
3. The current canonical head arrived after the attestation deadline from our perspective. This condition was only added to support suppression of forkchoiceUpdated messages, but makes intuitive sense.
4. The block is being proposed in the first 2 seconds of the slot. This gives it time to propagate and receive the proposer boost.


## Additional Info

For the initial idea and background, see: https://github.com/ethereum/consensus-specs/pull/2353#issuecomment-950238004

There is also a specification for this feature here: https://github.com/ethereum/consensus-specs/pull/3034

Co-authored-by: Michael Sproul <micsproul@gmail.com>
Co-authored-by: pawan <pawandhananjay@gmail.com>
2022-12-13 09:57:26 +00:00

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Late Block Re-orgs

Since v3.4.0 Lighthouse will opportunistically re-org late blocks when proposing.

This feature is intended to disincentivise late blocks and improve network health. Proposing a re-orging block is also more profitable for the proposer because it increases the number of attestations and transactions that can be included.

Command line flags

There are three flags which control the re-orging behaviour:

  • --disable-proposer-reorgs: turn re-orging off (it's on by default).
  • --proposer-reorg-threshold N: attempt to orphan blocks with less than N% of the committee vote. If this parameter isn't set then N defaults to 20% when the feature is enabled.
  • --proposer-reorg-epochs-since-finalization N: only attempt to re-org late blocks when the number of epochs since finalization is less than or equal to N. The default is 2 epochs, meaning re-orgs will only be attempted when the chain is finalizing optimally.

All flags should be applied to lighthouse bn. The default configuration is recommended as it balances the chance of the re-org succeeding against the chance of failure due to attestations arriving late and making the re-org block non-viable.

Safeguards

To prevent excessive re-orgs there are several safeguards in place that limit when a re-org will be attempted.

The full conditions are described in the spec but the most important ones are:

  • Only single-slot re-orgs: Lighthouse will build a block at N + 1 to re-org N by building on the parent N - 1. The result is a chain with exactly one skipped slot.
  • No epoch boundaries: to ensure that the selected proposer does not change, Lighthouse will not propose a re-orging block in the 0th slot of an epoch.

Logs

You can track the reasons for re-orgs being attempted (or not) via Lighthouse's logs.

A pair of messages at INFO level will be logged if a re-org opportunity is detected:

INFO Attempting re-org due to weak head threshold_weight: 45455983852725, head_weight: 0, parent: 0x09d953b69041f280758400c671130d174113bbf57c2d26553a77fb514cad4890, weak_head: 0xf64f8e5ed617dc18c1e759dab5d008369767c3678416dac2fe1d389562842b49

INFO Proposing block to re-org current head head_to_reorg: 0xf64f…2b49, slot: 1105320

This should be followed shortly after by a WARN log indicating that a re-org occurred. This is expected and normal:

WARN Beacon chain re-org reorg_distance: 1, new_slot: 1105320, new_head: 0x72791549e4ca792f91053bc7cf1e55c6fbe745f78ce7a16fc3acb6f09161becd, previous_slot: 1105319, previous_head: 0xf64f8e5ed617dc18c1e759dab5d008369767c3678416dac2fe1d389562842b49

In case a re-org is not viable (which should be most of the time), Lighthouse will just propose a block as normal and log the reason the re-org was not attempted at debug level:

DEBG Not attempting re-org reason: head not late

If you are interested in digging into the timing of forkchoiceUpdated messages sent to the execution layer, there is also a debug log for the suppression of forkchoiceUpdated messages when Lighthouse thinks that a re-org is likely:

DEBG Fork choice update overridden slot: 1105320, override: 0x09d953b69041f280758400c671130d174113bbf57c2d26553a77fb514cad4890, canonical_head: 0xf64f8e5ed617dc18c1e759dab5d008369767c3678416dac2fe1d389562842b49