Use BTreeMap for state.validators pending updates (#9017)

Closes:

- https://github.com/sigp/lighthouse/issues/9003


  Milhouse `List`s use a map in front of the binary tree to cache updates. Ever since we adopted Milhouse, we've been using `VecMap`, which is essentially `Vec<Option<T>>`. Turns out, when you've got 2M indices and only 2 non-`None` entries (changes), this is inefficient.

Milhouse is generic in the choice of map (`U: UpdateMap`) and has always supported `BTreeMap`, so this PR switches us over to `BTreeMap`. In previous benchmarks (years ago) it had been slower than `VecMap`, but now it is vastly superior.


Co-Authored-By: Michael Sproul <michael@sigmaprime.io>

consensus/state_processing/src/per_epoch_processing/epoch_processing_summary.rs

@@ -4,8 +4,8 @@ use milhouse::List;
 use std::sync::Arc;
 use types::{
     BeaconStateError, Epoch, EthSpec, ParticipationFlags, ProgressiveBalancesCache, SyncCommittee,
-    Validator,
     consts::altair::{TIMELY_HEAD_FLAG_INDEX, TIMELY_SOURCE_FLAG_INDEX, TIMELY_TARGET_FLAG_INDEX},
+    state::Validators,
 };
 
 /// Provides a summary of validator participation during the epoch.
@@ -26,7 +26,7 @@ pub enum EpochProcessingSummary<E: EthSpec> {
 #[derive(PartialEq, Debug)]
 pub struct ParticipationEpochSummary<E: EthSpec> {
     /// Copy of the validator registry prior to mutation.
-    validators: List<Validator, E::ValidatorRegistryLimit>,
+    validators: Validators<E>,
     /// Copy of the participation flags for the previous epoch.
     previous_epoch_participation: List<ParticipationFlags, E::ValidatorRegistryLimit>,
     /// Copy of the participation flags for the current epoch.
@@ -37,7 +37,7 @@ pub struct ParticipationEpochSummary<E: EthSpec> {
 
 impl<E: EthSpec> ParticipationEpochSummary<E> {
     pub fn new(
-        validators: List<Validator, E::ValidatorRegistryLimit>,
+        validators: Validators<E>,
         previous_epoch_participation: List<ParticipationFlags, E::ValidatorRegistryLimit>,
         current_epoch_participation: List<ParticipationFlags, E::ValidatorRegistryLimit>,
         previous_epoch: Epoch,

consensus/types/src/state/beacon_state.rs

@@ -14,6 +14,7 @@ use serde::{Deserialize, Deserializer, Serialize};
 use ssz::{Decode, DecodeError, Encode, ssz_encode};
 use ssz_derive::{Decode, Encode};
 use ssz_types::{BitVector, FixedVector};
+use std::collections::BTreeMap;
 use superstruct::superstruct;
 use swap_or_not_shuffle::compute_shuffled_index;
 use test_random_derive::TestRandom;
@@ -58,7 +59,8 @@ pub const CACHED_EPOCHS: usize = 3;
 const MAX_RANDOM_BYTE: u64 = (1 << 8) - 1;
 const MAX_RANDOM_VALUE: u64 = (1 << 16) - 1;
 
-pub type Validators<E> = List<Validator, <E as EthSpec>::ValidatorRegistryLimit>;
+pub type Validators<E> =
+    List<Validator, <E as EthSpec>::ValidatorRegistryLimit, BTreeMap<usize, Validator>>;
 pub type Balances<E> = List<u64, <E as EthSpec>::ValidatorRegistryLimit>;
 
 #[derive(Debug, PartialEq, Clone)]
@@ -453,7 +455,7 @@ where
     // Registry
     #[compare_fields(as_iter)]
     #[test_random(default)]
-    pub validators: List<Validator, E::ValidatorRegistryLimit>,
+    pub validators: Validators<E>,
     #[serde(with = "ssz_types::serde_utils::quoted_u64_var_list")]
     #[compare_fields(as_iter)]
     #[test_random(default)]

consensus/types/src/state/mod.rs

@@ -17,7 +17,7 @@ pub use balance::Balance;
 pub use beacon_state::{
     BeaconState, BeaconStateAltair, BeaconStateBase, BeaconStateBellatrix, BeaconStateCapella,
     BeaconStateDeneb, BeaconStateElectra, BeaconStateError, BeaconStateFulu, BeaconStateGloas,
-    BeaconStateHash, BeaconStateRef, CACHED_EPOCHS,
+    BeaconStateHash, BeaconStateRef, CACHED_EPOCHS, Validators,
 };
 pub use committee_cache::{
     CommitteeCache, compute_committee_index_in_epoch, compute_committee_range_in_epoch,