Rate limiting backfill sync (#3936)

## Issue Addressed

#3212 

## Proposed Changes

- Introduce a new `rate_limiting_backfill_queue` - any new inbound backfill work events gets immediately sent to this FIFO queue **without any processing**
- Spawn a `backfill_scheduler` routine that pops a backfill event from the FIFO queue at specified intervals (currently halfway through a slot, or at 6s after slot start for 12s slots) and sends the event to `BeaconProcessor` via a `scheduled_backfill_work_tx` channel
- This channel gets polled last in the `InboundEvents`, and work event received is  wrapped in a `InboundEvent::ScheduledBackfillWork` enum variant, which gets processed immediately or queued by the `BeaconProcessor` (existing logic applies from here)

Diagram comparing backfill processing with / without rate-limiting: 
https://github.com/sigp/lighthouse/issues/3212#issuecomment-1386249922

See this comment for @paulhauner's  explanation and solution: https://github.com/sigp/lighthouse/issues/3212#issuecomment-1384674956

## Additional Info

I've compared this branch (with backfill processing rate limited to to 1 and 3 batches per slot) against the latest stable version. The CPU usage during backfill sync is reduced by ~5% - 20%, more details on this page:

https://hackmd.io/@jimmygchen/SJuVpJL3j

The above testing is done on Goerli (as I don't currently have hardware for Mainnet), I'm guessing the differences are likely to be bigger on mainnet due to block size.

### TODOs

- [x] Experiment with processing multiple batches per slot. (need to think about how to do this for different slot durations)
- [x] Add option to disable rate-limiting, enabed by default.
- [x] (No longer required now we're reusing the reprocessing queue) Complete the `backfill_scheduler` task when backfill sync is completed or not required

beacon_node/beacon_chain/src/beacon_chain.rs

@@ -2893,7 +2893,7 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
                 metrics::start_timer(&metrics::FORK_CHOICE_PROCESS_BLOCK_TIMES);
             let block_delay = self
                 .slot_clock
-                .seconds_from_current_slot_start(self.spec.seconds_per_slot)
+                .seconds_from_current_slot_start()
                 .ok_or(Error::UnableToComputeTimeAtSlot)?;
 
             fork_choice
@@ -3746,7 +3746,7 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
 
         let slot_delay = self
             .slot_clock
-            .seconds_from_current_slot_start(self.spec.seconds_per_slot)
+            .seconds_from_current_slot_start()
             .or_else(|| {
                 warn!(
                     self.log,

beacon_node/beacon_chain/src/chain_config.rs

@@ -68,6 +68,8 @@ pub struct ChainConfig {
     ///
     /// This is useful for block builders and testing.
     pub always_prepare_payload: bool,
+    /// Whether backfill sync processing should be rate-limited.
+    pub enable_backfill_rate_limiting: bool,
 }
 
 impl Default for ChainConfig {
@@ -94,6 +96,7 @@ impl Default for ChainConfig {
             optimistic_finalized_sync: true,
             shuffling_cache_size: crate::shuffling_cache::DEFAULT_CACHE_SIZE,
             always_prepare_payload: false,
+            enable_backfill_rate_limiting: true,
         }
     }
 }

beacon_node/network/src/beacon_processor/mod.rs

@@ -61,6 +61,7 @@ use std::time::Duration;
 use std::{cmp, collections::HashSet};
 use task_executor::TaskExecutor;
 use tokio::sync::mpsc;
+use tokio::sync::mpsc::error::TrySendError;
 use types::{
     Attestation, AttesterSlashing, Hash256, LightClientFinalityUpdate, LightClientOptimisticUpdate,
     ProposerSlashing, SignedAggregateAndProof, SignedBeaconBlock, SignedBlsToExecutionChange,
@@ -77,7 +78,9 @@ mod tests;
 mod work_reprocessing_queue;
 mod worker;
 
-use crate::beacon_processor::work_reprocessing_queue::QueuedGossipBlock;
+use crate::beacon_processor::work_reprocessing_queue::{
+    QueuedBackfillBatch, QueuedGossipBlock, ReprocessQueueMessage,
+};
 pub use worker::{ChainSegmentProcessId, GossipAggregatePackage, GossipAttestationPackage};
 
 /// The maximum size of the channel for work events to the `BeaconProcessor`.
@@ -218,6 +221,7 @@ pub const GOSSIP_LIGHT_CLIENT_FINALITY_UPDATE: &str = "light_client_finality_upd
 pub const GOSSIP_LIGHT_CLIENT_OPTIMISTIC_UPDATE: &str = "light_client_optimistic_update";
 pub const RPC_BLOCK: &str = "rpc_block";
 pub const CHAIN_SEGMENT: &str = "chain_segment";
+pub const CHAIN_SEGMENT_BACKFILL: &str = "chain_segment_backfill";
 pub const STATUS_PROCESSING: &str = "status_processing";
 pub const BLOCKS_BY_RANGE_REQUEST: &str = "blocks_by_range_request";
 pub const BLOCKS_BY_ROOTS_REQUEST: &str = "blocks_by_roots_request";
@@ -738,6 +742,9 @@ impl<T: BeaconChainTypes> std::convert::From<ReadyWork<T>> for WorkEvent<T> {
                     seen_timestamp,
                 },
             },
+            ReadyWork::BackfillSync(QueuedBackfillBatch { process_id, blocks }) => {
+                WorkEvent::chain_segment(process_id, blocks)
+            }
         }
     }
 }
@@ -893,6 +900,10 @@ impl<T: BeaconChainTypes> Work<T> {
             Work::GossipLightClientFinalityUpdate { .. } => GOSSIP_LIGHT_CLIENT_FINALITY_UPDATE,
             Work::GossipLightClientOptimisticUpdate { .. } => GOSSIP_LIGHT_CLIENT_OPTIMISTIC_UPDATE,
             Work::RpcBlock { .. } => RPC_BLOCK,
+            Work::ChainSegment {
+                process_id: ChainSegmentProcessId::BackSyncBatchId { .. },
+                ..
+            } => CHAIN_SEGMENT_BACKFILL,
             Work::ChainSegment { .. } => CHAIN_SEGMENT,
             Work::Status { .. } => STATUS_PROCESSING,
             Work::BlocksByRangeRequest { .. } => BLOCKS_BY_RANGE_REQUEST,
@@ -1054,23 +1065,23 @@ impl<T: BeaconChainTypes> BeaconProcessor<T> {
             FifoQueue::new(MAX_BLS_TO_EXECUTION_CHANGE_QUEUE_LEN);
 
         let mut lcbootstrap_queue = FifoQueue::new(MAX_LIGHT_CLIENT_BOOTSTRAP_QUEUE_LEN);
+
+        let chain = match self.beacon_chain.upgrade() {
+            Some(chain) => chain,
+            // No need to proceed any further if the beacon chain has been dropped, the client
+            // is shutting down.
+            None => return,
+        };
+
         // Channels for sending work to the re-process scheduler (`work_reprocessing_tx`) and to
         // receive them back once they are ready (`ready_work_rx`).
         let (ready_work_tx, ready_work_rx) = mpsc::channel(MAX_SCHEDULED_WORK_QUEUE_LEN);
-        let work_reprocessing_tx = {
-            if let Some(chain) = self.beacon_chain.upgrade() {
-                spawn_reprocess_scheduler(
+        let work_reprocessing_tx = spawn_reprocess_scheduler(
             ready_work_tx,
             &self.executor,
             chain.slot_clock.clone(),
             self.log.clone(),
-                )
-            } else {
-                // No need to proceed any further if the beacon chain has been dropped, the client
-                // is shutting down.
-                return;
-            }
-        };
+        );
 
         let executor = self.executor.clone();
 
@@ -1083,12 +1094,55 @@ impl<T: BeaconChainTypes> BeaconProcessor<T> {
                 reprocess_work_rx: ready_work_rx,
             };
 
+            let enable_backfill_rate_limiting = chain.config.enable_backfill_rate_limiting;
+
             loop {
                 let work_event = match inbound_events.next().await {
                     Some(InboundEvent::WorkerIdle) => {
                         self.current_workers = self.current_workers.saturating_sub(1);
                         None
                     }
+                    Some(InboundEvent::WorkEvent(event)) if enable_backfill_rate_limiting => {
+                        match QueuedBackfillBatch::try_from(event) {
+                            Ok(backfill_batch) => {
+                                match work_reprocessing_tx
+                                    .try_send(ReprocessQueueMessage::BackfillSync(backfill_batch))
+                                {
+                                    Err(e) => {
+                                        warn!(
+                                            self.log,
+                                            "Unable to queue backfill work event. Will try to process now.";
+                                            "error" => %e
+                                        );
+                                        match e {
+                                            TrySendError::Full(reprocess_queue_message)
+                                            | TrySendError::Closed(reprocess_queue_message) => {
+                                                match reprocess_queue_message {
+                                                    ReprocessQueueMessage::BackfillSync(
+                                                        backfill_batch,
+                                                    ) => Some(backfill_batch.into()),
+                                                    other => {
+                                                        crit!(
+                                                            self.log,
+                                                            "Unexpected queue message type";
+                                                            "message_type" => other.as_ref()
+                                                        );
+                                                        // This is an unhandled exception, drop the message.
+                                                        continue;
+                                                    }
+                                                }
+                                            }
+                                        }
+                                    }
+                                    Ok(..) => {
+                                        // backfill work sent to "reprocessing" queue. Process the next event.
+                                        continue;
+                                    }
+                                }
+                            }
+                            Err(event) => Some(event),
+                        }
+                    }
                     Some(InboundEvent::WorkEvent(event))
                     | Some(InboundEvent::ReprocessingWork(event)) => Some(event),
                     None => {

beacon_node/network/src/beacon_processor/tests.rs

@@ -9,7 +9,7 @@ use crate::{service::NetworkMessage, sync::SyncMessage};
 use beacon_chain::test_utils::{
     AttestationStrategy, BeaconChainHarness, BlockStrategy, EphemeralHarnessType,
 };
-use beacon_chain::{BeaconChain, MAXIMUM_GOSSIP_CLOCK_DISPARITY};
+use beacon_chain::{BeaconChain, ChainConfig, MAXIMUM_GOSSIP_CLOCK_DISPARITY};
 use lighthouse_network::{
     discv5::enr::{CombinedKey, EnrBuilder},
     rpc::methods::{MetaData, MetaDataV2},
@@ -23,8 +23,8 @@ use std::sync::Arc;
 use std::time::Duration;
 use tokio::sync::mpsc;
 use types::{
-    Attestation, AttesterSlashing, EthSpec, MainnetEthSpec, ProposerSlashing, SignedBeaconBlock,
-    SignedVoluntaryExit, SubnetId,
+    Attestation, AttesterSlashing, Epoch, EthSpec, MainnetEthSpec, ProposerSlashing,
+    SignedBeaconBlock, SignedVoluntaryExit, SubnetId,
 };
 
 type E = MainnetEthSpec;
@@ -70,6 +70,10 @@ impl Drop for TestRig {
 
 impl TestRig {
     pub async fn new(chain_length: u64) -> Self {
+        Self::new_with_chain_config(chain_length, ChainConfig::default()).await
+    }
+
+    pub async fn new_with_chain_config(chain_length: u64, chain_config: ChainConfig) -> Self {
         // This allows for testing voluntary exits without building out a massive chain.
         let mut spec = E::default_spec();
         spec.shard_committee_period = 2;
@@ -78,6 +82,7 @@ impl TestRig {
             .spec(spec)
             .deterministic_keypairs(VALIDATOR_COUNT)
             .fresh_ephemeral_store()
+            .chain_config(chain_config)
             .build();
 
         harness.advance_slot();
@@ -261,6 +266,14 @@ impl TestRig {
         self.beacon_processor_tx.try_send(event).unwrap();
     }
 
+    pub fn enqueue_backfill_batch(&self) {
+        let event = WorkEvent::chain_segment(
+            ChainSegmentProcessId::BackSyncBatchId(Epoch::default()),
+            Vec::default(),
+        );
+        self.beacon_processor_tx.try_send(event).unwrap();
+    }
+
     pub fn enqueue_unaggregated_attestation(&self) {
         let (attestation, subnet_id) = self.attestations.first().unwrap().clone();
         self.beacon_processor_tx
@@ -873,3 +886,49 @@ async fn test_rpc_block_reprocessing() {
     // cache handle was dropped.
     assert_eq!(next_block_root, rig.head_root());
 }
+
+/// Ensure that backfill batches get rate-limited and processing is scheduled at specified intervals.
+#[tokio::test]
+async fn test_backfill_sync_processing() {
+    let mut rig = TestRig::new(SMALL_CHAIN).await;
+    // Note: to verify the exact event times in an integration test is not straight forward here
+    // (not straight forward to manipulate `TestingSlotClock` due to cloning of `SlotClock` in code)
+    // and makes the test very slow, hence timing calculation is unit tested separately in
+    // `work_reprocessing_queue`.
+    for _ in 0..1 {
+        rig.enqueue_backfill_batch();
+        // ensure queued batch is not processed until later
+        rig.assert_no_events_for(Duration::from_millis(100)).await;
+        // A new batch should be processed within a slot.
+        rig.assert_event_journal_with_timeout(
+            &[CHAIN_SEGMENT_BACKFILL, WORKER_FREED, NOTHING_TO_DO],
+            rig.chain.slot_clock.slot_duration(),
+        )
+        .await;
+    }
+}
+
+/// Ensure that backfill batches get processed as fast as they can when rate-limiting is disabled.
+#[tokio::test]
+async fn test_backfill_sync_processing_rate_limiting_disabled() {
+    let chain_config = ChainConfig {
+        enable_backfill_rate_limiting: false,
+        ..Default::default()
+    };
+    let mut rig = TestRig::new_with_chain_config(SMALL_CHAIN, chain_config).await;
+
+    for _ in 0..3 {
+        rig.enqueue_backfill_batch();
+    }
+
+    // ensure all batches are processed
+    rig.assert_event_journal_with_timeout(
+        &[
+            CHAIN_SEGMENT_BACKFILL,
+            CHAIN_SEGMENT_BACKFILL,
+            CHAIN_SEGMENT_BACKFILL,
+        ],
+        Duration::from_millis(100),
+    )
+    .await;
+}

beacon_node/network/src/beacon_processor/work_reprocessing_queue.rs

@@ -11,21 +11,25 @@
 //! Aggregated and unaggregated attestations that failed verification due to referencing an unknown
 //! block will be re-queued until their block is imported, or until they expire.
 use super::MAX_SCHEDULED_WORK_QUEUE_LEN;
+use crate::beacon_processor::{ChainSegmentProcessId, Work, WorkEvent};
 use crate::metrics;
 use crate::sync::manager::BlockProcessType;
 use beacon_chain::{BeaconChainTypes, GossipVerifiedBlock, MAXIMUM_GOSSIP_CLOCK_DISPARITY};
 use fnv::FnvHashMap;
 use futures::task::Poll;
 use futures::{Stream, StreamExt};
+use itertools::Itertools;
 use lighthouse_network::{MessageId, PeerId};
 use logging::TimeLatch;
 use slog::{crit, debug, error, trace, warn, Logger};
 use slot_clock::SlotClock;
 use std::collections::{HashMap, HashSet};
+use std::future::Future;
 use std::pin::Pin;
 use std::sync::Arc;
 use std::task::Context;
 use std::time::Duration;
+use strum::AsRefStr;
 use task_executor::TaskExecutor;
 use tokio::sync::mpsc::{self, Receiver, Sender};
 use tokio::time::error::Error as TimeError;
@@ -65,7 +69,21 @@ const MAXIMUM_QUEUED_ATTESTATIONS: usize = 16_384;
 /// How many light client updates we keep before new ones get dropped.
 const MAXIMUM_QUEUED_LIGHT_CLIENT_UPDATES: usize = 128;
 
+// Process backfill batch 50%, 60%, 80% through each slot.
+//
+// Note: use caution to set these fractions in a way that won't cause panic-y
+// arithmetic.
+pub const BACKFILL_SCHEDULE_IN_SLOT: [(u32, u32); 3] = [
+    // One half: 6s on mainnet, 2.5s on Gnosis.
+    (1, 2),
+    // Three fifths: 7.2s on mainnet, 3s on Gnosis.
+    (3, 5),
+    // Four fifths: 9.6s on mainnet, 4s on Gnosis.
+    (4, 5),
+];
+
 /// Messages that the scheduler can receive.
+#[derive(AsRefStr)]
 pub enum ReprocessQueueMessage<T: BeaconChainTypes> {
     /// A block that has been received early and we should queue for later processing.
     EarlyBlock(QueuedGossipBlock<T>),
@@ -84,6 +102,8 @@ pub enum ReprocessQueueMessage<T: BeaconChainTypes> {
     UnknownBlockAggregate(QueuedAggregate<T::EthSpec>),
     /// A light client optimistic update that references a parent root that has not been seen as a parent.
     UnknownLightClientOptimisticUpdate(QueuedLightClientUpdate<T::EthSpec>),
+    /// A new backfill batch that needs to be scheduled for processing.
+    BackfillSync(QueuedBackfillBatch<T::EthSpec>),
 }
 
 /// Events sent by the scheduler once they are ready for re-processing.
@@ -93,6 +113,7 @@ pub enum ReadyWork<T: BeaconChainTypes> {
     Unaggregate(QueuedUnaggregate<T::EthSpec>),
     Aggregate(QueuedAggregate<T::EthSpec>),
     LightClientUpdate(QueuedLightClientUpdate<T::EthSpec>),
+    BackfillSync(QueuedBackfillBatch<T::EthSpec>),
 }
 
 /// An Attestation for which the corresponding block was not seen while processing, queued for
@@ -144,6 +165,40 @@ pub struct QueuedRpcBlock<T: EthSpec> {
     pub should_process: bool,
 }
 
+/// A backfill batch work that has been queued for processing later.
+#[derive(Clone)]
+pub struct QueuedBackfillBatch<E: EthSpec> {
+    pub process_id: ChainSegmentProcessId,
+    pub blocks: Vec<Arc<SignedBeaconBlock<E>>>,
+}
+
+impl<T: BeaconChainTypes> TryFrom<WorkEvent<T>> for QueuedBackfillBatch<T::EthSpec> {
+    type Error = WorkEvent<T>;
+
+    fn try_from(event: WorkEvent<T>) -> Result<Self, WorkEvent<T>> {
+        match event {
+            WorkEvent {
+                work:
+                    Work::ChainSegment {
+                        process_id: process_id @ ChainSegmentProcessId::BackSyncBatchId(_),
+                        blocks,
+                    },
+                ..
+            } => Ok(QueuedBackfillBatch { process_id, blocks }),
+            _ => Err(event),
+        }
+    }
+}
+
+impl<T: BeaconChainTypes> From<QueuedBackfillBatch<T::EthSpec>> for WorkEvent<T> {
+    fn from(queued_backfill_batch: QueuedBackfillBatch<T::EthSpec>) -> WorkEvent<T> {
+        WorkEvent::chain_segment(
+            queued_backfill_batch.process_id,
+            queued_backfill_batch.blocks,
+        )
+    }
+}
+
 /// Unifies the different messages processed by the block delay queue.
 enum InboundEvent<T: BeaconChainTypes> {
     /// A gossip block that was queued for later processing and is ready for import.
@@ -155,6 +210,8 @@ enum InboundEvent<T: BeaconChainTypes> {
     ReadyAttestation(QueuedAttestationId),
     /// A light client update that is ready for re-processing.
     ReadyLightClientUpdate(QueuedLightClientUpdateId),
+    /// A backfill batch that was queued is ready for processing.
+    ReadyBackfillSync(QueuedBackfillBatch<T::EthSpec>),
     /// A `DelayQueue` returned an error.
     DelayQueueError(TimeError, &'static str),
     /// A message sent to the `ReprocessQueue`
@@ -191,6 +248,8 @@ struct ReprocessQueue<T: BeaconChainTypes> {
     queued_lc_updates: FnvHashMap<usize, (QueuedLightClientUpdate<T::EthSpec>, DelayKey)>,
     /// Light Client Updates per parent_root.
     awaiting_lc_updates_per_parent_root: HashMap<Hash256, Vec<QueuedLightClientUpdateId>>,
+    /// Queued backfill batches
+    queued_backfill_batches: Vec<QueuedBackfillBatch<T::EthSpec>>,
 
     /* Aux */
     /// Next attestation id, used for both aggregated and unaggregated attestations
@@ -200,6 +259,8 @@ struct ReprocessQueue<T: BeaconChainTypes> {
     rpc_block_debounce: TimeLatch,
     attestation_delay_debounce: TimeLatch,
     lc_update_delay_debounce: TimeLatch,
+    next_backfill_batch_event: Option<Pin<Box<tokio::time::Sleep>>>,
+    slot_clock: Pin<Box<T::SlotClock>>,
 }
 
 pub type QueuedLightClientUpdateId = usize;
@@ -287,6 +348,20 @@ impl<T: BeaconChainTypes> Stream for ReprocessQueue<T> {
             Poll::Ready(None) | Poll::Pending => (),
         }
 
+        if let Some(next_backfill_batch_event) = self.next_backfill_batch_event.as_mut() {
+            match next_backfill_batch_event.as_mut().poll(cx) {
+                Poll::Ready(_) => {
+                    let maybe_batch = self.queued_backfill_batches.pop();
+                    self.recompute_next_backfill_batch_event();
+
+                    if let Some(batch) = maybe_batch {
+                        return Poll::Ready(Some(InboundEvent::ReadyBackfillSync(batch)));
+                    }
+                }
+                Poll::Pending => (),
+            }
+        }
+
         // Last empty the messages channel.
         match self.work_reprocessing_rx.poll_recv(cx) {
             Poll::Ready(Some(message)) => return Poll::Ready(Some(InboundEvent::Msg(message))),
@@ -323,12 +398,15 @@ pub fn spawn_reprocess_scheduler<T: BeaconChainTypes>(
         queued_unaggregates: FnvHashMap::default(),
         awaiting_attestations_per_root: HashMap::new(),
         awaiting_lc_updates_per_parent_root: HashMap::new(),
+        queued_backfill_batches: Vec::new(),
         next_attestation: 0,
         next_lc_update: 0,
         early_block_debounce: TimeLatch::default(),
         rpc_block_debounce: TimeLatch::default(),
         attestation_delay_debounce: TimeLatch::default(),
         lc_update_delay_debounce: TimeLatch::default(),
+        next_backfill_batch_event: None,
+        slot_clock: Box::pin(slot_clock.clone()),
     };
 
     executor.spawn(
@@ -679,6 +757,14 @@ impl<T: BeaconChainTypes> ReprocessQueue<T> {
                     }
                 }
             }
+            InboundEvent::Msg(BackfillSync(queued_backfill_batch)) => {
+                self.queued_backfill_batches
+                    .insert(0, queued_backfill_batch);
+                // only recompute if there is no `next_backfill_batch_event` already scheduled
+                if self.next_backfill_batch_event.is_none() {
+                    self.recompute_next_backfill_batch_event();
+                }
+            }
             // A block that was queued for later processing is now ready to be processed.
             InboundEvent::ReadyGossipBlock(ready_block) => {
                 let block_root = ready_block.block.block_root;
@@ -786,6 +872,33 @@ impl<T: BeaconChainTypes> ReprocessQueue<T> {
                     }
                 }
             }
+            InboundEvent::ReadyBackfillSync(queued_backfill_batch) => {
+                let millis_from_slot_start = slot_clock
+                    .millis_from_current_slot_start()
+                    .map_or("null".to_string(), |duration| {
+                        duration.as_millis().to_string()
+                    });
+
+                debug!(
+                    log,
+                    "Sending scheduled backfill work";
+                    "millis_from_slot_start" => millis_from_slot_start
+                );
+
+                if self
+                    .ready_work_tx
+                    .try_send(ReadyWork::BackfillSync(queued_backfill_batch.clone()))
+                    .is_err()
+                {
+                    error!(
+                        log,
+                        "Failed to send scheduled backfill work";
+                        "info" => "sending work back to queue"
+                    );
+                    self.queued_backfill_batches
+                        .insert(0, queued_backfill_batch);
+                }
+            }
         }
 
         metrics::set_gauge_vec(
@@ -809,4 +922,95 @@ impl<T: BeaconChainTypes> ReprocessQueue<T> {
             self.lc_updates_delay_queue.len() as i64,
         );
     }
+
+    fn recompute_next_backfill_batch_event(&mut self) {
+        // only recompute the `next_backfill_batch_event` if there are backfill batches in the queue
+        if !self.queued_backfill_batches.is_empty() {
+            self.next_backfill_batch_event = Some(Box::pin(tokio::time::sleep(
+                ReprocessQueue::<T>::duration_until_next_backfill_batch_event(&self.slot_clock),
+            )));
+        } else {
+            self.next_backfill_batch_event = None
+        }
+    }
+
+    /// Returns duration until the next scheduled processing time. The schedule ensure that backfill
+    /// processing is done in windows of time that aren't critical
+    fn duration_until_next_backfill_batch_event(slot_clock: &T::SlotClock) -> Duration {
+        let slot_duration = slot_clock.slot_duration();
+        slot_clock
+            .millis_from_current_slot_start()
+            .and_then(|duration_from_slot_start| {
+                BACKFILL_SCHEDULE_IN_SLOT
+                    .into_iter()
+                    // Convert fractions to seconds from slot start.
+                    .map(|(multiplier, divisor)| (slot_duration / divisor) * multiplier)
+                    .find_or_first(|&event_duration_from_slot_start| {
+                        event_duration_from_slot_start > duration_from_slot_start
+                    })
+                    .map(|next_event_time| {
+                        if duration_from_slot_start >= next_event_time {
+                            // event is in the next slot, add duration to next slot
+                            let duration_to_next_slot = slot_duration - duration_from_slot_start;
+                            duration_to_next_slot + next_event_time
+                        } else {
+                            next_event_time - duration_from_slot_start
+                        }
+                    })
+            })
+            // If we can't read the slot clock, just wait another slot.
+            .unwrap_or(slot_duration)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use beacon_chain::builder::Witness;
+    use beacon_chain::eth1_chain::CachingEth1Backend;
+    use slot_clock::TestingSlotClock;
+    use store::MemoryStore;
+    use types::MainnetEthSpec as E;
+    use types::Slot;
+
+    type TestBeaconChainType =
+        Witness<TestingSlotClock, CachingEth1Backend<E>, E, MemoryStore<E>, MemoryStore<E>>;
+
+    #[test]
+    fn backfill_processing_schedule_calculation() {
+        let slot_duration = Duration::from_secs(12);
+        let slot_clock = TestingSlotClock::new(Slot::new(0), Duration::from_secs(0), slot_duration);
+        let current_slot_start = slot_clock.start_of(Slot::new(100)).unwrap();
+        slot_clock.set_current_time(current_slot_start);
+
+        let event_times = BACKFILL_SCHEDULE_IN_SLOT
+            .map(|(multiplier, divisor)| (slot_duration / divisor) * multiplier);
+
+        for &event_duration_from_slot_start in event_times.iter() {
+            let duration_to_next_event =
+                ReprocessQueue::<TestBeaconChainType>::duration_until_next_backfill_batch_event(
+                    &slot_clock,
+                );
+
+            let current_time = slot_clock.millis_from_current_slot_start().unwrap();
+
+            assert_eq!(
+                duration_to_next_event,
+                event_duration_from_slot_start - current_time
+            );
+
+            slot_clock.set_current_time(current_slot_start + event_duration_from_slot_start)
+        }
+
+        // check for next event beyond the current slot
+        let duration_to_next_slot = slot_clock.duration_to_next_slot().unwrap();
+        let duration_to_next_event =
+            ReprocessQueue::<TestBeaconChainType>::duration_until_next_backfill_batch_event(
+                &slot_clock,
+            );
+        assert_eq!(
+            duration_to_next_event,
+            duration_to_next_slot + event_times[0]
+        );
+    }
 }

beacon_node/src/cli.rs

@@ -261,6 +261,14 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
             .min_values(0)
             .hidden(true)
         )
+        .arg(
+            Arg::with_name("disable-backfill-rate-limiting")
+                .long("disable-backfill-rate-limiting")
+                .help("Disable the backfill sync rate-limiting. This allow users to just sync the entire chain as fast \
+                    as possible, however it can result in resource contention which degrades staking performance. Stakers \
+                    should generally choose to avoid this flag since backfill sync is not required for staking.")
+                .takes_value(false),
+        )
         /* REST API related arguments */
         .arg(
             Arg::with_name("http")

beacon_node/src/config.rs

@@ -759,6 +759,10 @@ pub fn get_config<E: EthSpec>(
         client_config.always_prefer_builder_payload = true;
     }
 
+    // Backfill sync rate-limiting
+    client_config.chain.enable_backfill_rate_limiting =
+        !cli_args.is_present("disable-backfill-rate-limiting");
+
     Ok(client_config)
 }
 

common/slot_clock/src/lib.rs

@@ -104,12 +104,23 @@ pub trait SlotClock: Send + Sync + Sized + Clone {
         self.slot_duration() * 2 / INTERVALS_PER_SLOT as u32
     }
 
-    /// Returns the `Duration` since the start of the current `Slot`. Useful in determining whether to apply proposer boosts.
-    fn seconds_from_current_slot_start(&self, seconds_per_slot: u64) -> Option<Duration> {
+    /// Returns the `Duration` since the start of the current `Slot` at seconds precision. Useful in determining whether to apply proposer boosts.
+    fn seconds_from_current_slot_start(&self) -> Option<Duration> {
         self.now_duration()
             .and_then(|now| now.checked_sub(self.genesis_duration()))
             .map(|duration_into_slot| {
-                Duration::from_secs(duration_into_slot.as_secs() % seconds_per_slot)
+                Duration::from_secs(duration_into_slot.as_secs() % self.slot_duration().as_secs())
+            })
+    }
+
+    /// Returns the `Duration` since the start of the current `Slot` at milliseconds precision.
+    fn millis_from_current_slot_start(&self) -> Option<Duration> {
+        self.now_duration()
+            .and_then(|now| now.checked_sub(self.genesis_duration()))
+            .map(|duration_into_slot| {
+                Duration::from_millis(
+                    (duration_into_slot.as_millis() % self.slot_duration().as_millis()) as u64,
+                )
             })
     }
 

lighthouse/tests/beacon_node.rs

@@ -1052,6 +1052,19 @@ fn disable_upnp_flag() {
         .with_config(|config| assert!(!config.network.upnp_enabled));
 }
 #[test]
+fn disable_backfill_rate_limiting_flag() {
+    CommandLineTest::new()
+        .flag("disable-backfill-rate-limiting", None)
+        .run_with_zero_port()
+        .with_config(|config| assert!(!config.chain.enable_backfill_rate_limiting));
+}
+#[test]
+fn default_backfill_rate_limiting_flag() {
+    CommandLineTest::new()
+        .run_with_zero_port()
+        .with_config(|config| assert!(config.chain.enable_backfill_rate_limiting));
+}
+#[test]
 fn default_boot_nodes() {
     let mainnet = vec![
     // Lighthouse Team (Sigma Prime)

testing/ef_tests/src/cases/fork_choice.rs

@@ -425,7 +425,7 @@ impl<E: EthSpec> Tester<E> {
                     .harness
                     .chain
                     .slot_clock
-                    .seconds_from_current_slot_start(self.spec.seconds_per_slot)
+                    .seconds_from_current_slot_start()
                     .unwrap();
 
                 let result = self