Merge pull request #1627 from ethereum/hysteris-fix

adjust hysteresis to avoid initial over-deposit incentive

configs/mainnet.yaml

@@ -21,6 +21,12 @@ SHUFFLE_ROUND_COUNT: 90
 MIN_GENESIS_ACTIVE_VALIDATOR_COUNT: 16384
 # Jan 3, 2020
 MIN_GENESIS_TIME: 1578009600
+# 4
+HYSTERESIS_QUOTIENT: 4
+# 1 (minus 0.25)
+HYSTERESIS_DOWNWARD_MULTIPLIER: 1
+# 5 (plus 1.25)
+HYSTERESIS_UPWARD_MULTIPLIER: 5
 
 
 # Fork Choice

configs/minimal.yaml

@@ -20,6 +20,13 @@ SHUFFLE_ROUND_COUNT: 10
 MIN_GENESIS_ACTIVE_VALIDATOR_COUNT: 64
 # Jan 3, 2020
 MIN_GENESIS_TIME: 1578009600
+# 4
+HYSTERESIS_QUOTIENT: 4
+# 1 (minus 0.25)
+HYSTERESIS_DOWNWARD_MULTIPLIER: 1
+# 5 (plus 1.25)
+HYSTERESIS_UPWARD_MULTIPLIER: 5
+
 
 
 # Fork Choice

specs/phase0/beacon-chain.md

@@ -183,6 +183,10 @@ The following values are (non-configurable) constants used throughout the specif
 | `SHUFFLE_ROUND_COUNT` | `90` |
 | `MIN_GENESIS_ACTIVE_VALIDATOR_COUNT` | `2**14` (= 16,384) |
 | `MIN_GENESIS_TIME` | `1578009600` (Jan 3, 2020) |
+| `HYSTERESIS_QUOTIENT` | `4` |
+| `HYSTERESIS_DOWNWARD_MULTIPLIER` | `1` |
+| `HYSTERESIS_UPWARD_MULTIPLIER` | `5` |
+
 
 - For the safety of committees, `TARGET_COMMITTEE_SIZE` exceeds [the recommended minimum committee size of 111](http://web.archive.org/web/20190504131341/https://vitalik.ca/files/Ithaca201807_Sharding.pdf); with sufficient active validators (at least `SLOTS_PER_EPOCH * TARGET_COMMITTEE_SIZE`), the shuffling algorithm ensures committee sizes of at least `TARGET_COMMITTEE_SIZE`. (Unbiasable randomness with a Verifiable Delay Function (VDF) will improve committee robustness and lower the safe minimum committee size.)
 
@@ -1402,8 +1406,13 @@ def process_final_updates(state: BeaconState) -> None:
     # Update effective balances with hysteresis
     for index, validator in enumerate(state.validators):
         balance = state.balances[index]
-        HALF_INCREMENT = EFFECTIVE_BALANCE_INCREMENT // 2
-        if balance < validator.effective_balance or validator.effective_balance + 3 * HALF_INCREMENT < balance:
+        HYSTERESIS_INCREMENT = EFFECTIVE_BALANCE_INCREMENT // HYSTERESIS_QUOTIENT
+        DOWNWARD_THRESHOLD = HYSTERESIS_INCREMENT * HYSTERESIS_DOWNWARD_MULTIPLIER
+        UPWARD_THRESHOLD = HYSTERESIS_INCREMENT * HYSTERESIS_UPWARD_MULTIPLIER
+        if (
+            balance + DOWNWARD_THRESHOLD < validator.effective_balance
+            or validator.effective_balance + UPWARD_THRESHOLD < balance
+        ):
             validator.effective_balance = min(balance - balance % EFFECTIVE_BALANCE_INCREMENT, MAX_EFFECTIVE_BALANCE)
     # Reset slashings
     state.slashings[next_epoch % EPOCHS_PER_SLASHINGS_VECTOR] = Gwei(0)

tests/core/pyspec/eth2spec/test/phase_0/epoch_processing/test_process_final_updates.py

@@ -51,19 +51,25 @@ def test_effective_balance_hysteresis(spec, state):
     max = spec.MAX_EFFECTIVE_BALANCE
     min = spec.EJECTION_BALANCE
     inc = spec.EFFECTIVE_BALANCE_INCREMENT
-    half_inc = inc // 2
+    div = spec.HYSTERESIS_QUOTIENT
+    hys_inc = inc // div
+    down = spec.HYSTERESIS_DOWNWARD_MULTIPLIER
+    up = spec.HYSTERESIS_UPWARD_MULTIPLIER
     cases = [
         (max, max, max, "as-is"),
-        (max, max - 1, max - inc, "round down, step lower"),
+        (max, max - 1, max, "round up"),
         (max, max + 1, max, "round down"),
+        (max, max - down * hys_inc, max, "lower balance, but not low enough"),
+        (max, max - down * hys_inc - 1, max - inc, "lower balance, step down"),
+        (max, max + (up * hys_inc) + 1, max, "already at max, as is"),
         (max, max - inc, max - inc, "exactly 1 step lower"),
-        (max, max - inc - 1, max - (2 * inc), "just 1 over 1 step lower"),
+        (max, max - inc - 1, max - (2 * inc), "past 1 step lower, double step"),
         (max, max - inc + 1, max - inc, "close to 1 step lower"),
-        (min, min + (half_inc * 3), min, "bigger balance, but not high enough"),
-        (min, min + (half_inc * 3) + 1, min + inc, "bigger balance, high enough, but small step"),
-        (min, min + (half_inc * 4) - 1, min + inc, "bigger balance, high enough, close to double step"),
-        (min, min + (half_inc * 4), min + (2 * inc), "exact two step balance increment"),
-        (min, min + (half_inc * 4) + 1, min + (2 * inc), "over two steps, round down"),
+        (min, min + (hys_inc * up), min, "bigger balance, but not high enough"),
+        (min, min + (hys_inc * up) + 1, min + inc, "bigger balance, high enough, but small step"),
+        (min, min + (hys_inc * div * 2) - 1, min + inc, "bigger balance, high enough, close to double step"),
+        (min, min + (hys_inc * div * 2), min + (2 * inc), "exact two step balance increment"),
+        (min, min + (hys_inc * div * 2) + 1, min + (2 * inc), "over two steps, round down"),
     ]
     current_epoch = spec.get_current_epoch(state)
     for i, (pre_eff, bal, _, _) in enumerate(cases):