Remove grandparents from snapshot cache (#2917)
## Issue Addressed NA ## Proposed Changes In https://github.com/sigp/lighthouse/pull/2832 we made some changes to the `SnapshotCache` to help deal with the one-block reorgs seen on mainnet (and testnets). I believe the change in #2832 is good and we should keep it, but I think that in its present form it is causing the `SnapshotCache` to hold onto states that it doesn't need anymore. For example, a skip slot will result in one more `BeaconSnapshot` being stored in the cache. This PR adds a new type of pruning that happens after a block is inserted to the cache. We will remove any snapshot from the cache that is a *grandparent* of the block being imported. Since we know the grandparent has two valid blocks built atop it, it is not at risk from a one-block re-org. ## Additional Info NA
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@ -2796,6 +2796,7 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
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beacon_block_root: block_root,
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},
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None,
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&self.spec,
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)
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})
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.unwrap_or_else(|e| {
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@ -3740,6 +3741,12 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
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.try_write_for(BLOCK_PROCESSING_CACHE_LOCK_TIMEOUT)
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.map(|mut snapshot_cache| {
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snapshot_cache.prune(new_finalized_checkpoint.epoch);
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debug!(
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self.log,
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"Snapshot cache pruned";
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"new_len" => snapshot_cache.len(),
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"remaining_roots" => ?snapshot_cache.beacon_block_roots(),
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);
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})
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.unwrap_or_else(|| {
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error!(
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@ -4,8 +4,12 @@ use crate::{BeaconChain, BeaconChainError, BeaconChainTypes};
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use lazy_static::lazy_static;
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pub use lighthouse_metrics::*;
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use slot_clock::SlotClock;
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use std::time::Duration;
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use types::{BeaconState, Epoch, EthSpec, Hash256, Slot};
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/// The maximum time to wait for the snapshot cache lock during a metrics scrape.
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const SNAPSHOT_CACHE_TIMEOUT: Duration = Duration::from_millis(100);
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lazy_static! {
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/*
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* Block Processing
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@ -18,6 +22,10 @@ lazy_static! {
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"beacon_block_processing_successes_total",
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"Count of blocks processed without error"
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);
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pub static ref BLOCK_PROCESSING_SNAPSHOT_CACHE_SIZE: Result<IntGauge> = try_create_int_gauge(
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"beacon_block_processing_snapshot_cache_size",
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"Count snapshots in the snapshot cache"
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);
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pub static ref BLOCK_PROCESSING_SNAPSHOT_CACHE_MISSES: Result<IntCounter> = try_create_int_counter(
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"beacon_block_processing_snapshot_cache_misses",
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"Count of snapshot cache misses"
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@ -913,6 +921,16 @@ pub fn scrape_for_metrics<T: BeaconChainTypes>(beacon_chain: &BeaconChain<T>) {
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let attestation_stats = beacon_chain.op_pool.attestation_stats();
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if let Some(snapshot_cache) = beacon_chain
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.snapshot_cache
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.try_write_for(SNAPSHOT_CACHE_TIMEOUT)
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{
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set_gauge(
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&BLOCK_PROCESSING_SNAPSHOT_CACHE_SIZE,
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snapshot_cache.len() as i64,
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)
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}
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set_gauge_by_usize(
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&OP_POOL_NUM_ATTESTATIONS,
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attestation_stats.num_attestations,
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@ -1,4 +1,5 @@
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use crate::BeaconSnapshot;
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use itertools::process_results;
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use std::cmp;
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use std::time::Duration;
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use types::{
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@ -164,9 +165,25 @@ impl<T: EthSpec> SnapshotCache<T> {
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}
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}
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/// The block roots of all snapshots contained in `self`.
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pub fn beacon_block_roots(&self) -> Vec<Hash256> {
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self.snapshots.iter().map(|s| s.beacon_block_root).collect()
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}
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/// The number of snapshots contained in `self`.
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pub fn len(&self) -> usize {
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self.snapshots.len()
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}
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/// Insert a snapshot, potentially removing an existing snapshot if `self` is at capacity (see
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/// struct-level documentation for more info).
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pub fn insert(&mut self, snapshot: BeaconSnapshot<T>, pre_state: Option<BeaconState<T>>) {
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pub fn insert(
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&mut self,
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snapshot: BeaconSnapshot<T>,
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pre_state: Option<BeaconState<T>>,
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spec: &ChainSpec,
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) {
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let parent_root = snapshot.beacon_block.message().parent_root();
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let item = CacheItem {
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beacon_block: snapshot.beacon_block,
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beacon_block_root: snapshot.beacon_block_root,
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@ -174,6 +191,25 @@ impl<T: EthSpec> SnapshotCache<T> {
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pre_state,
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};
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// Remove the grandparent of the block that was just inserted.
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//
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// Assuming it's unlikely to see re-orgs deeper than one block, this method helps keep the
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// cache small by removing any states that already have more than one descendant.
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//
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// Remove the grandparent first to free up room in the cache.
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let grandparent_result =
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process_results(item.beacon_state.rev_iter_block_roots(spec), |iter| {
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iter.map(|(_slot, root)| root)
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.find(|root| *root != item.beacon_block_root && *root != parent_root)
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});
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if let Ok(Some(grandparent_root)) = grandparent_result {
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let head_block_root = self.head_block_root;
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self.snapshots.retain(|snapshot| {
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let root = snapshot.beacon_block_root;
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root == head_block_root || root != grandparent_root
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});
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}
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if self.snapshots.len() < self.max_len {
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self.snapshots.push(item);
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} else {
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@ -384,7 +420,7 @@ mod test {
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*snapshot.beacon_state.slot_mut() =
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Slot::from(i * MainnetEthSpec::slots_per_epoch() + 1);
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cache.insert(snapshot, None);
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cache.insert(snapshot, None, &spec);
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assert_eq!(
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cache.snapshots.len(),
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@ -402,7 +438,7 @@ mod test {
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// 2 2
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// 3 3
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assert_eq!(cache.snapshots.len(), CACHE_SIZE);
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cache.insert(get_snapshot(42), None);
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cache.insert(get_snapshot(42), None, &spec);
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assert_eq!(cache.snapshots.len(), CACHE_SIZE);
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assert!(
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@ -462,7 +498,7 @@ mod test {
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// Over-fill the cache so it needs to eject some old values on insert.
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for i in 0..CACHE_SIZE as u64 {
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cache.insert(get_snapshot(u64::max_value() - i), None);
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cache.insert(get_snapshot(u64::max_value() - i), None, &spec);
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}
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// Ensure that the new head value was not removed from the cache.
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