lighthouse/eth2/fork_choice/src/optimized_lmd_ghost.rs

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//! The optimised bitwise LMD-GHOST fork choice rule.
extern crate bit_vec;
use crate::{ForkChoice, ForkChoiceError};
use db::{
stores::{BeaconBlockStore, BeaconStateStore},
ClientDB,
};
use log::{debug, trace};
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use std::cmp::Ordering;
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use std::collections::HashMap;
use std::sync::Arc;
use types::{
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validator_registry::get_active_validator_indices, BeaconBlock, ChainSpec, Hash256, Slot,
SlotHeight,
};
//TODO: Pruning - Children
//TODO: Handle Syncing
// NOTE: This uses u32 to represent difference between block heights. Thus this is only
// applicable for block height differences in the range of a u32.
// This can potentially be parallelized in some parts.
/// Compute the base-2 logarithm of an integer, floored (rounded down)
#[inline]
fn log2_int(x: u64) -> u32 {
if x == 0 {
return 0;
}
63 - x.leading_zeros()
}
fn power_of_2_below(x: u64) -> u64 {
2u64.pow(log2_int(x))
}
/// Stores the necessary data structures to run the optimised lmd ghost algorithm.
pub struct OptimizedLMDGhost<T: ClientDB + Sized> {
/// A cache of known ancestors at given heights for a specific block.
//TODO: Consider FnvHashMap
cache: HashMap<CacheKey<u64>, Hash256>,
/// Log lookup table for blocks to their ancestors.
//TODO: Verify we only want/need a size 16 log lookup
ancestors: Vec<HashMap<Hash256, Hash256>>,
/// Stores the children for any given parent.
children: HashMap<Hash256, Vec<Hash256>>,
/// The latest attestation targets as a map of validator index to block hash.
//TODO: Could this be a fixed size vec
latest_attestation_targets: HashMap<u64, Hash256>,
/// Block storage access.
block_store: Arc<BeaconBlockStore<T>>,
/// State storage access.
state_store: Arc<BeaconStateStore<T>>,
max_known_height: SlotHeight,
}
impl<T> OptimizedLMDGhost<T>
where
T: ClientDB + Sized,
{
pub fn new(
block_store: Arc<BeaconBlockStore<T>>,
state_store: Arc<BeaconStateStore<T>>,
) -> Self {
OptimizedLMDGhost {
cache: HashMap::new(),
ancestors: vec![HashMap::new(); 16],
latest_attestation_targets: HashMap::new(),
children: HashMap::new(),
max_known_height: SlotHeight::new(0),
block_store,
state_store,
}
}
/// Finds the latest votes weighted by validator balance. Returns a hashmap of block_hash to
/// weighted votes.
pub fn get_latest_votes(
&self,
state_root: &Hash256,
block_slot: Slot,
spec: &ChainSpec,
) -> Result<HashMap<Hash256, u64>, ForkChoiceError> {
// get latest votes
// Note: Votes are weighted by min(balance, MAX_DEPOSIT_AMOUNT) //
// FORK_CHOICE_BALANCE_INCREMENT
// build a hashmap of block_hash to weighted votes
let mut latest_votes: HashMap<Hash256, u64> = HashMap::new();
// gets the current weighted votes
let current_state = self
.state_store
.get_deserialized(&state_root)?
.ok_or_else(|| ForkChoiceError::MissingBeaconState(*state_root))?;
let active_validator_indices = get_active_validator_indices(
&current_state.validator_registry[..],
block_slot.epoch(spec.slots_per_epoch),
);
for index in active_validator_indices {
let balance = std::cmp::min(
current_state.validator_balances[index],
spec.max_deposit_amount,
) / spec.fork_choice_balance_increment;
if balance > 0 {
if let Some(target) = self.latest_attestation_targets.get(&(index as u64)) {
*latest_votes.entry(*target).or_insert_with(|| 0) += balance;
}
}
}
trace!("Latest votes: {:?}", latest_votes);
Ok(latest_votes)
}
/// Gets the ancestor at a given height `at_height` of a block specified by `block_hash`.
fn get_ancestor(
&mut self,
block_hash: Hash256,
target_height: SlotHeight,
spec: &ChainSpec,
) -> Option<Hash256> {
// return None if we can't get the block from the db.
let block_height = {
let block_slot = self
.block_store
.get_deserialized(&block_hash)
.ok()?
.expect("Should have returned already if None")
.slot;
block_slot.height(spec.genesis_slot)
};
// verify we haven't exceeded the block height
if target_height >= block_height {
if target_height > block_height {
return None;
} else {
return Some(block_hash);
}
}
// check if the result is stored in our cache
let cache_key = CacheKey::new(&block_hash, target_height.as_u64());
if let Some(ancestor) = self.cache.get(&cache_key) {
return Some(*ancestor);
}
// not in the cache recursively search for ancestors using a log-lookup
if let Some(ancestor) = {
let ancestor_lookup = self.ancestors
[log2_int((block_height - target_height - 1u64).as_u64()) as usize]
.get(&block_hash)
//TODO: Panic if we can't lookup and fork choice fails
.expect("All blocks should be added to the ancestor log lookup table");
self.get_ancestor(*ancestor_lookup, target_height, &spec)
} {
// add the result to the cache
self.cache.insert(cache_key, ancestor);
return Some(ancestor);
}
None
}
// looks for an obvious block winner given the latest votes for a specific height
fn get_clear_winner(
&mut self,
latest_votes: &HashMap<Hash256, u64>,
block_height: SlotHeight,
spec: &ChainSpec,
) -> Option<Hash256> {
// map of vote counts for every hash at this height
let mut current_votes: HashMap<Hash256, u64> = HashMap::new();
let mut total_vote_count = 0;
trace!("Clear winner at block height: {}", block_height);
// loop through the latest votes and count all votes
// these have already been weighted by balance
for (hash, votes) in latest_votes.iter() {
if let Some(ancestor) = self.get_ancestor(*hash, block_height, spec) {
let current_vote_value = current_votes.get(&ancestor).unwrap_or_else(|| &0);
current_votes.insert(ancestor, current_vote_value + *votes);
total_vote_count += votes;
}
}
// Check if there is a clear block winner at this height. If so return it.
for (hash, votes) in current_votes.iter() {
if *votes > total_vote_count / 2 {
// we have a clear winner, return it
return Some(*hash);
}
}
// didn't find a clear winner
None
}
// Finds the best child (one with highest votes)
fn choose_best_child(&self, votes: &HashMap<Hash256, u64>) -> Option<Hash256> {
if votes.is_empty() {
return None;
}
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// Iterate through hashmap to get child with maximum votes
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let best_child = votes.iter().max_by(|(child1, v1), (child2, v2)| {
let mut result = v1.cmp(v2);
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// If votes are equal, choose smaller hash to break ties deterministically
if result == Ordering::Equal {
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// Reverse so that max_by chooses smaller hash
result = child1.cmp(child2).reverse();
}
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result
});
Some(*best_child.unwrap().0)
}
}
impl<T: ClientDB + Sized> ForkChoice for OptimizedLMDGhost<T> {
fn add_block(
&mut self,
block: &BeaconBlock,
block_hash: &Hash256,
spec: &ChainSpec,
) -> Result<(), ForkChoiceError> {
// get the height of the parent
let parent_height = self
.block_store
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.get_deserialized(&block.previous_block_root)?
.ok_or_else(|| ForkChoiceError::MissingBeaconBlock(block.previous_block_root))?
.slot
.height(spec.genesis_slot);
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let parent_hash = &block.previous_block_root;
// add the new block to the children of parent
(*self
.children
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.entry(block.previous_block_root)
.or_insert_with(|| vec![]))
.push(block_hash.clone());
// build the ancestor data structure
for index in 0..16 {
if parent_height % (1 << index) == 0 {
self.ancestors[index].insert(*block_hash, *parent_hash);
} else {
// TODO: This is unsafe. Will panic if parent_hash doesn't exist. Using it for debugging
let parent_ancestor = self.ancestors[index][parent_hash];
self.ancestors[index].insert(*block_hash, parent_ancestor);
}
}
// update the max height
self.max_known_height = std::cmp::max(self.max_known_height, parent_height + 1);
Ok(())
}
fn add_attestation(
&mut self,
validator_index: u64,
target_block_root: &Hash256,
spec: &ChainSpec,
) -> Result<(), ForkChoiceError> {
// simply add the attestation to the latest_attestation_target if the block_height is
// larger
trace!(
"Adding attestation of validator: {:?} for block: {}",
validator_index,
target_block_root
);
let attestation_target = self
.latest_attestation_targets
.entry(validator_index)
.or_insert_with(|| *target_block_root);
// if we already have a value
if attestation_target != target_block_root {
trace!("Old attestation found: {:?}", attestation_target);
// get the height of the target block
let block_height = self
.block_store
.get_deserialized(&target_block_root)?
.ok_or_else(|| ForkChoiceError::MissingBeaconBlock(*target_block_root))?
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.slot
.height(spec.genesis_slot);
// get the height of the past target block
let past_block_height = self
.block_store
.get_deserialized(&attestation_target)?
.ok_or_else(|| ForkChoiceError::MissingBeaconBlock(*attestation_target))?
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.slot
.height(spec.genesis_slot);
// update the attestation only if the new target is higher
if past_block_height < block_height {
trace!("Updating old attestation");
*attestation_target = *target_block_root;
}
}
Ok(())
}
/// Perform lmd_ghost on the current chain to find the head.
fn find_head(
&mut self,
justified_block_start: &Hash256,
spec: &ChainSpec,
) -> Result<Hash256, ForkChoiceError> {
debug!(
"Starting optimised fork choice at block: {}",
justified_block_start
);
let block = self
.block_store
.get_deserialized(&justified_block_start)?
.ok_or_else(|| ForkChoiceError::MissingBeaconBlock(*justified_block_start))?;
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let block_slot = block.slot;
let state_root = block.state_root;
let mut block_height = block_slot.height(spec.genesis_slot);
let mut current_head = *justified_block_start;
let mut latest_votes = self.get_latest_votes(&state_root, block_slot, spec)?;
// remove any votes that don't relate to our current head.
latest_votes
.retain(|hash, _| self.get_ancestor(*hash, block_height, spec) == Some(current_head));
// begin searching for the head
loop {
debug!(
"Iteration for block: {} with vote length: {}",
current_head,
latest_votes.len()
);
// if there are no children, we are done, return the current_head
let children = match self.children.get(&current_head) {
Some(children) => children.clone(),
None => {
debug!("Head found: {}", current_head);
return Ok(current_head);
}
};
// logarithmic lookup blocks to see if there are obvious winners, if so,
// progress to the next iteration.
let mut step =
power_of_2_below(self.max_known_height.saturating_sub(block_height).as_u64()) / 2;
while step > 0 {
trace!("Current Step: {}", step);
if let Some(clear_winner) = self.get_clear_winner(
&latest_votes,
block_height - (block_height % step) + step,
spec,
) {
current_head = clear_winner;
break;
}
step /= 2;
}
if step > 0 {
trace!("Found clear winner: {}", current_head);
}
// if our skip lookup failed and we only have one child, progress to that child
else if children.len() == 1 {
current_head = children[0];
trace!(
"Lookup failed, only one child, proceeding to child: {}",
current_head
);
}
// we need to find the best child path to progress down.
else {
trace!("Searching for best child");
let mut child_votes = HashMap::new();
for (voted_hash, vote) in latest_votes.iter() {
// if the latest votes correspond to a child
if let Some(child) = self.get_ancestor(*voted_hash, block_height + 1, spec) {
// add up the votes for each child
*child_votes.entry(child).or_insert_with(|| 0) += vote;
}
}
// check if we have votes of children, if not select the smallest hash child
if child_votes.is_empty() {
current_head = *children
.iter()
.min_by(|child1, child2| child1.cmp(child2))
.expect("Must be children here");
trace!(
"Children have no votes - smallest hash chosen: {}",
current_head
);
} else {
// given the votes on the children, find the best child
current_head = self
.choose_best_child(&child_votes)
.ok_or(ForkChoiceError::CannotFindBestChild)?;
trace!("Best child found: {}", current_head);
}
}
// didn't find head yet, proceed to next iteration
// update block height
block_height = self
.block_store
.get_deserialized(&current_head)?
.ok_or_else(|| ForkChoiceError::MissingBeaconBlock(current_head))?
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.slot
.height(spec.genesis_slot);
// prune the latest votes for votes that are not part of current chosen chain
// more specifically, only keep votes that have head as an ancestor
for hash in latest_votes.keys() {
trace!(
"Ancestor for vote: {} at height: {} is: {:?}",
hash,
block_height,
self.get_ancestor(*hash, block_height, spec)
);
}
latest_votes.retain(|hash, _| {
self.get_ancestor(*hash, block_height, spec) == Some(current_head)
});
}
}
}
/// Type for storing blocks in a memory cache. Key is comprised of block-hash plus the height.
#[derive(PartialEq, Eq, Hash)]
pub struct CacheKey<T> {
block_hash: Hash256,
block_height: T,
}
impl<T> CacheKey<T> {
pub fn new(block_hash: &Hash256, block_height: T) -> Self {
CacheKey {
block_hash: *block_hash,
block_height,
}
}
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
pub fn test_power_of_2_below() {
assert_eq!(power_of_2_below(4), 4);
assert_eq!(power_of_2_below(5), 4);
assert_eq!(power_of_2_below(7), 4);
assert_eq!(power_of_2_below(24), 16);
assert_eq!(power_of_2_below(32), 32);
assert_eq!(power_of_2_below(33), 32);
assert_eq!(power_of_2_below(63), 32);
}
#[test]
pub fn test_power_of_2_below_large() {
let pow: u64 = 1 << 24;
for x in (pow - 20)..(pow + 20) {
assert!(power_of_2_below(x) <= x, "{}", x);
}
}
}