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transform a few quadratic algorithms from processEpoch into linear or otherwise subquadratic algorithms (#155)

This commit is contained in:
Dustin Brody 2019-03-06 14:21:13 +00:00 committed by Jacek Sieka
parent b8402e9809
commit 5d0de00168
2 changed files with 68 additions and 11 deletions
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4
beacon_chain/spec/beaconstate.nim
View File

@ -308,7 +308,7 @@ func get_attestation_participants*(state: BeaconState,
filterIt(crosslink_committees, it.shard == attestation_data.shard),
it.committee)[0]
assert verify_bitfield(bitfield, len(crosslink_committee))
doAssert verify_bitfield(bitfield, len(crosslink_committee))
# Find the participating attesters in the committee
result = @[]
@ -328,7 +328,7 @@ func process_ejections*(state: var BeaconState) =
exit_validator(state, index)
# https://github.com/ethereum/eth2.0-specs/blob/v0.3.0/specs/core/0_beacon-chain.md#get_total_balance
func get_total_balance*(state: BeaconState, validators: seq[ValidatorIndex]): Gwei =
func get_total_balance*(state: BeaconState, validators: auto): Gwei =
# Return the combined effective balance of an array of validators.
foldl(validators, a + get_effective_balance(state, b), 0'u64)

73
beacon_chain/state_transition.nim
View File

@ -31,7 +31,7 @@
# now.
import
algorithm, chronicles, math, options, sequtils,
algorithm, collections/sets, chronicles, math, options, sequtils, tables,
./extras, ./ssz,
./spec/[beaconstate, crypto, datatypes, digest, helpers, validator]
@ -632,7 +632,7 @@ func processEpoch(state: var BeaconState) =
previous_epoch == slot_to_epoch(it.data.slot))
previous_epoch_attester_indices =
get_attester_indices(state, previous_epoch_attestations)
toSet(get_attester_indices(state, previous_epoch_attestations))
previous_epoch_attesting_balance =
get_total_balance(state, previous_epoch_attester_indices)
@ -646,7 +646,7 @@ func processEpoch(state: var BeaconState) =
previous_epoch_attestations)
previous_epoch_boundary_attester_indices =
get_attester_indices(state, previous_epoch_boundary_attestations)
toSet(get_attester_indices(state, previous_epoch_boundary_attestations))
previous_epoch_boundary_attesting_balance =
get_total_balance(state, previous_epoch_boundary_attester_indices)
@ -659,7 +659,7 @@ func processEpoch(state: var BeaconState) =
it.data.beacon_block_root == get_block_root(state, it.data.slot))
previous_epoch_head_attester_indices =
get_attester_indices(state, previous_epoch_head_attestations)
toSet(get_attester_indices(state, previous_epoch_head_attestations))
previous_epoch_head_attesting_balance =
get_total_balance(state, previous_epoch_head_attester_indices)
@ -777,6 +777,9 @@ func processEpoch(state: var BeaconState) =
INACTIVITY_PENALTY_QUOTIENT div 2
# https://github.com/ethereum/eth2.0-specs/blob/v0.3.0/specs/core/0_beacon-chain.md#justification-and-finalization
## TODO remove inclusion_{slot,distance} when fully replaced, but note
## intentional absence, for spec sync purposes. Both positively invite
## quadratic behavior.
func inclusion_slot(state: BeaconState, v: ValidatorIndex): uint64 =
for a in previous_epoch_attestations:
if v in get_attestation_participants(state, a.data, a.aggregation_bitfield):
@ -789,6 +792,15 @@ func processEpoch(state: var BeaconState) =
return a.inclusion_slot - a.data.slot
doAssert false
func inclusion_distances(state: BeaconState): auto =
result = initTable[ValidatorIndex, uint64]()
for a in previous_epoch_attestations:
for v in get_attestation_participants(
state, a.data, a.aggregation_bitfield):
if v notin result:
result[v] = a.inclusion_slot - a.data.slot
block: # Justification and finalization
let
active_validator_indices =
@ -796,7 +808,7 @@ func processEpoch(state: var BeaconState) =
state.validator_registry, slot_to_epoch(state.slot))
epochs_since_finality = next_epoch - state.finalized_epoch
proc update_balance(attesters: openArray[ValidatorIndex], attesting_balance: uint64) =
proc update_balance(attesters: HashSet[ValidatorIndex], attesting_balance: uint64) =
# TODO Spec - add helper?
for v in attesters:
statePtr.validator_balances[v] +=
@ -826,13 +838,25 @@ func processEpoch(state: var BeaconState) =
previous_epoch_head_attesting_balance)
# Inclusion distance
let distances = inclusion_distances(state)
for v in previous_epoch_attester_indices:
statePtr.validator_balances[v] +=
base_reward(state, v) *
MIN_ATTESTATION_INCLUSION_DELAY div inclusion_distance(state, v)
MIN_ATTESTATION_INCLUSION_DELAY div distances[v]
when false:
doAssert inclusion_distance(state, v) == distances[v]
else:
# Case 2: epochs_since_finality > 4
let distances =
if previous_epoch_attester_indices.len > 0:
inclusion_distances(state)
else:
## Last case will not occur. If this assumption becomes false,
## indexing into distances will fail.
initTable[ValidatorIndex, uint64]()
for index in active_validator_indices:
if index notin previous_epoch_attester_indices:
reduce_balance(
@ -855,13 +879,44 @@ func processEpoch(state: var BeaconState) =
state.validator_balances[index],
base_reward(state, index) -
base_reward(state, index) * MIN_ATTESTATION_INCLUSION_DELAY div
inclusion_distance(state, index))
distances[index])
when false:
doAssert inclusion_distance(state, index) == distances[index]
# https://github.com/ethereum/eth2.0-specs/blob/v0.3.0/specs/core/0_beacon-chain.md#attestation-inclusion
block:
## Minimally transform spec algorithm into something non-quadratic, while
## retaining enough resemblance to both verify equivalence and update, if
## it changes. inclusion_slot iterates across previous_epoch_attestations
## until it finds the attester index `v` represented. Instead construct a
## (reverse) index based on one O(n) scan of previous_epoch_attestations,
## then perform O(n) O(1) lookups. Keep same iteration order and rules in
## which first match wins and terminates for each ValidatorIndex.
var proposer_indexes = initTable[ValidatorIndex, uint64]()
# This is the loop from inclusion_slot(...)
for a in previous_epoch_attestations:
for v in get_attestation_participants(
state, a.data, a.aggregation_bitfield):
## Here, though, collect all results (but only, though it shouldn't
## happen regardless, the first proposer index per ValidatorIndex),
## which avoids the quadratic behavior.
if v notin proposer_indexes:
proposer_indexes[v] = a.inclusion_slot
for v in previous_epoch_attester_indices:
## inclusion_slot(...) doAsserts false if it doesn't find anything, so
## equivalently, simply look up table element, intentionally fragilely
## such that a useful stack trace is produced.
let proposer_index =
get_beacon_proposer_index(state, proposer_indexes[v])
# Ensure this keeps getting compiled to some extent to help compare.
when false:
let proposer_index_slow =
get_beacon_proposer_index(state, inclusion_slot(state, v))
doAssert proposer_index == proposer_index_slow
state.validator_balances[proposer_index] +=
base_reward(state, v) div ATTESTATION_INCLUSION_REWARD_QUOTIENT
@ -870,8 +925,10 @@ func processEpoch(state: var BeaconState) =
for slot in get_epoch_start_slot(previous_epoch) ..< get_epoch_start_slot(current_epoch):
let crosslink_committees_at_slot = get_crosslink_committees_at_slot(state, slot)
for crosslink_committee in crosslink_committees_at_slot:
let committee_attesting_validators =
toSet(attesting_validators(crosslink_committee))
for index in crosslink_committee.committee:
if index in attesting_validators(crosslink_committee):
if index in committee_attesting_validators:
state.validator_balances[index.int] +=
base_reward(state, index) *
total_attesting_balance(crosslink_committee) div