perf: reserve deposit balance capacity

src/state_transition/epoch/process_pending_deposits.zig

@@ -152,7 +152,7 @@ fn applyPendingDeposit(
             // this is to fix the double deposit issue found in mekong
             // see https://github.com/ChainSafe/lodestar/pull/7255
             if (cache.balances) |*balances| {
-                try balances.append(cache.allocator, amount);
+                balances.appendAssumeCapacity(amount);
             }
         } else |_| {
             // invalid deposit signature, ignore the deposit

src/state_transition/epoch/process_rewards_and_penalties.zig

@@ -5,6 +5,7 @@ const BeaconState = @import("fork_types").BeaconState;
 const EpochCache = @import("../cache/epoch_cache.zig").EpochCache;
 const EpochTransitionCache = @import("../cache/epoch_transition_cache.zig").EpochTransitionCache;
 const GENESIS_EPOCH = @import("preset").GENESIS_EPOCH;
+const preset = @import("preset").preset;
 const getAttestationDeltas = @import("./get_attestation_deltas.zig").getAttestationDeltas;
 const getRewardsAndPenaltiesAltair = @import("./get_rewards_and_penalties.zig").getRewardsAndPenaltiesAltair;
 
@@ -25,8 +26,20 @@ pub fn processRewardsAndPenalties(
     const penalties = cache.penalties;
     try getRewardsAndPenalties(fork, config, epoch_cache, state, cache, rewards, penalties);
 
-    const balances = try state.balancesSlice(cache.allocator);
-    errdefer cache.allocator.free(balances);
+    var balances_view = try state.balances();
+    try balances_view.commit();
+    const validator_count = try balances_view.length();
+
+    // applyPendingDeposit appends with appendAssumeCapacity for each validator it adds, at most
+    // MAX_PENDING_DEPOSITS_PER_EPOCH per epoch, so this headroom is required.
+    var new_balances: std.ArrayList(u64) = try .initCapacity(
+        cache.allocator,
+        validator_count + preset.MAX_PENDING_DEPOSITS_PER_EPOCH,
+    );
+    errdefer new_balances.deinit(cache.allocator);
+
+    const balances = new_balances.addManyAsSliceAssumeCapacity(validator_count);
+    _ = try balances_view.getAllInto(balances);
 
     for (rewards, penalties, balances) |reward, penalty, *balance| {
         balance.* = (try std.math.add(u64, balance.*, reward)) -| penalty;
@@ -38,15 +51,14 @@ pub fn processRewardsAndPenalties(
         }
     }
 
-    // Populate cache.balances for reuse by the validator monitor and
-    // more importantly processEffectiveBalanceUpdates() doesn't need to
-    // get from tree view state which has to commit.
-    var new_balances: std.ArrayList(u64) = .fromOwnedSlice(balances);
     try state.setBalances(&new_balances);
 
     if (cache.balances) |*old_balances| {
         old_balances.deinit(cache.allocator);
     }
+    // Populate cache.balances for reuse by the validator monitor and
+    // more importantly processEffectiveBalanceUpdates() doesn't need to
+    // get from tree view state which has to commit.
     cache.balances = new_balances;
 }
 

src/state_transition/epoch/process_rewards_and_penalties_test.zig

@@ -2,6 +2,7 @@
 
 const std = @import("std");
 const Node = @import("persistent_merkle_tree").Node;
+const preset = @import("preset").preset;
 const TestCachedBeaconState = @import("../test_utils/root.zig").TestCachedBeaconState;
 const processRewardsAndPenalties = @import("process_rewards_and_penalties.zig").processRewardsAndPenalties;
 
@@ -70,3 +71,29 @@ test "processRewardsAndPenalties maps compact slashing penalties to validator in
     try std.testing.expectEqual(first_balance_without_slashing, try updated_balances.get(0));
     try std.testing.expectEqual(slashed_balance_without_slashing - slashing_penalty, try updated_balances.get(slashed_index));
 }
+
+test "processRewardsAndPenalties should reserve cached balance capacity for pending deposits" {
+    const allocator = std.testing.allocator;
+    const pool_size = 200_000;
+    var pool = try Node.Pool.init(.{ .page_allocator = allocator, .allocator = allocator, .pool_size = pool_size });
+    defer pool.deinit();
+
+    const validator_count = 256;
+    var test_state = try TestCachedBeaconState.init(allocator, &pool, validator_count);
+    defer test_state.deinit();
+
+    const cache = test_state.epoch_transition_cache;
+    try processRewardsAndPenalties(
+        .electra,
+        test_state.cached_state.config,
+        test_state.cached_state.epoch_cache,
+        test_state.cached_state.state.castToFork(.electra),
+        cache,
+        null,
+    );
+
+    const balances = cache.balances.?;
+    try std.testing.expectEqual(validator_count, balances.items.len);
+    const spare_capacity = balances.unusedCapacitySlice().len;
+    try std.testing.expect(spare_capacity >= preset.MAX_PENDING_DEPOSITS_PER_EPOCH);
+}