Adjust requests costs for light client (#9925)

* PIP Table Cost relative to average peers instead of max peers

* Add tracing in PIP new_cost_table

* Update stat peer_count

* Use number of leeching peers for Light serve costs

* Fix test::light_params_load_share_depends_on_max_peers (wrong type)

* Remove (now) useless test

* Remove `load_share` from LightParams.Config
Prevent div. by 0

* Add LEECHER_COUNT_FACTOR

* PR Grumble: u64 to u32 for f64 casting

* Prevent u32 overflow for avg_peer_count

* Add tests for LightSync::Statistics

ethcore/light/src/net/mod.rs

@@ -28,7 +28,7 @@ use parking_lot::{Mutex, RwLock};
 use provider::Provider;
 use request::{Request, NetworkRequests as Requests, Response};
 use rlp::{RlpStream, Rlp};
-use std::collections::{HashMap, HashSet};
+use std::collections::{HashMap, HashSet, VecDeque};
 use std::fmt;
 use std::ops::{BitOr, BitAnd, Not};
 use std::sync::Arc;
@@ -38,7 +38,7 @@ use std::time::{Duration, Instant};
 use self::request_credits::{Credits, FlowParams};
 use self::context::{Ctx, TickCtx};
 use self::error::Punishment;
-use self::load_timer::{LoadDistribution, NullStore};
+use self::load_timer::{LoadDistribution, NullStore, MOVING_SAMPLE_SIZE};
 use self::request_set::RequestSet;
 use self::id_guard::IdGuard;
 
@@ -70,6 +70,16 @@ const PROPAGATE_TIMEOUT_INTERVAL: Duration = Duration::from_secs(5);
 const RECALCULATE_COSTS_TIMEOUT: TimerToken = 3;
 const RECALCULATE_COSTS_INTERVAL: Duration = Duration::from_secs(60 * 60);
 
+const STATISTICS_TIMEOUT: TimerToken = 4;
+const STATISTICS_INTERVAL: Duration = Duration::from_secs(15);
+
+/// Maximum load share for the light server
+pub const MAX_LIGHTSERV_LOAD: f64 = 0.5;
+
+/// Factor to multiply leecher count to cater for
+/// extra sudden connections (should be >= 1.0)
+pub const LEECHER_COUNT_FACTOR: f64 = 1.25;
+
 // minimum interval between updates.
 const UPDATE_INTERVAL: Duration = Duration::from_millis(5000);
 
@@ -256,18 +266,18 @@ pub trait Handler: Send + Sync {
 pub struct Config {
 	/// How many stored seconds of credits peers should be able to accumulate.
 	pub max_stored_seconds: u64,
-	/// How much of the total load capacity each peer should be allowed to take.
+	/// The network config median peers (used as default peer count)
-	pub load_share: f64,
+	pub median_peers: f64,
 }
 
 impl Default for Config {
 	fn default() -> Self {
-		const LOAD_SHARE: f64 = 1.0 / 25.0;
+		const MEDIAN_PEERS: f64 = 25.0;
 		const MAX_ACCUMULATED: u64 = 60 * 5; // only charge for 5 minutes.
 
 		Config {
 			max_stored_seconds: MAX_ACCUMULATED,
-			load_share: LOAD_SHARE,
+			median_peers: MEDIAN_PEERS,
 		}
 	}
 }
@@ -335,6 +345,42 @@ mod id_guard {
 	}
 }
 
+/// Provides various statistics that could
+/// be used to compute costs
+pub struct Statistics {
+	/// Samples of peer count
+	peer_counts: VecDeque<usize>,
+}
+
+impl Statistics {
+	/// Create a new Statistics instance
+	pub fn new() -> Self {
+		Statistics {
+			peer_counts: VecDeque::with_capacity(MOVING_SAMPLE_SIZE),
+		}
+	}
+
+	/// Add a new peer_count sample
+	pub fn add_peer_count(&mut self, peer_count: usize) {
+		while self.peer_counts.len() >= MOVING_SAMPLE_SIZE {
+			self.peer_counts.pop_front();
+		}
+		self.peer_counts.push_back(peer_count);
+	}
+
+	/// Get the average peer count from previous samples. Is always >= 1.0
+	pub fn avg_peer_count(&self) -> f64 {
+		let len = self.peer_counts.len();
+		if len == 0 {
+			return 1.0;
+		}
+		let avg = self.peer_counts.iter()
+			.fold(0, |sum: u32, &v| sum.saturating_add(v as u32)) as f64
+			/ len as f64;
+		avg.max(1.0)
+	}
+}
+
 /// This is an implementation of the light ethereum network protocol, abstracted
 /// over a `Provider` of data and a p2p network.
 ///
@@ -359,6 +405,7 @@ pub struct LightProtocol {
 	req_id: AtomicUsize,
 	sample_store: Box<SampleStore>,
 	load_distribution: LoadDistribution,
+	statistics: RwLock<Statistics>,
 }
 
 impl LightProtocol {
@@ -369,9 +416,11 @@ impl LightProtocol {
 		let genesis_hash = provider.chain_info().genesis_hash;
 		let sample_store = params.sample_store.unwrap_or_else(|| Box::new(NullStore));
 		let load_distribution = LoadDistribution::load(&*sample_store);
+		// Default load share relative to median peers
+		let load_share = MAX_LIGHTSERV_LOAD / params.config.median_peers;
 		let flow_params = FlowParams::from_request_times(
 			|kind| load_distribution.expected_time(kind),
-			params.config.load_share,
+			load_share,
 			Duration::from_secs(params.config.max_stored_seconds),
 		);
 
@@ -389,6 +438,7 @@ impl LightProtocol {
 			req_id: AtomicUsize::new(0),
 			sample_store,
 			load_distribution,
+			statistics: RwLock::new(Statistics::new()),
 		}
 	}
 
@@ -408,6 +458,16 @@ impl LightProtocol {
 		)
 	}
 
+	/// Get the number of active light peers downloading from the
+	/// node
+	pub fn leecher_count(&self) -> usize {
+		let credit_limit = *self.flow_params.read().limit();
+		// Count the number of peers that used some credit
+		self.peers.read().iter()
+			.filter(|(_, p)| p.lock().local_credits.current() < credit_limit)
+			.count()
+	}
+
 	/// Make a request to a peer.
 	///
 	/// Fails on: nonexistent peer, network error, peer not server,
@@ -772,12 +832,16 @@ impl LightProtocol {
 	fn begin_new_cost_period(&self, io: &IoContext) {
 		self.load_distribution.end_period(&*self.sample_store);
 
+		let avg_peer_count = self.statistics.read().avg_peer_count();
+		// Load share relative to average peer count +LEECHER_COUNT_FACTOR%
+		let load_share = MAX_LIGHTSERV_LOAD / (avg_peer_count * LEECHER_COUNT_FACTOR);
 		let new_params = Arc::new(FlowParams::from_request_times(
 			|kind| self.load_distribution.expected_time(kind),
-			self.config.load_share,
+			load_share,
 			Duration::from_secs(self.config.max_stored_seconds),
 		));
 		*self.flow_params.write() = new_params.clone();
+		trace!(target: "pip", "New cost period: avg_peers={} ; cost_table:{:?}", avg_peer_count, new_params.cost_table());
 
 		let peers = self.peers.read();
 		let now = Instant::now();
@@ -797,6 +861,11 @@ impl LightProtocol {
 			peer_info.awaiting_acknowledge = Some((now, new_params.clone()));
 		}
 	}
+
+	fn tick_statistics(&self) {
+		let leecher_count = self.leecher_count();
+		self.statistics.write().add_peer_count(leecher_count);
+	}
 }
 
 impl LightProtocol {
@@ -1099,6 +1168,8 @@ impl NetworkProtocolHandler for LightProtocol {
 			.expect("Error registering sync timer.");
 		io.register_timer(RECALCULATE_COSTS_TIMEOUT, RECALCULATE_COSTS_INTERVAL)
 			.expect("Error registering request timer interval token.");
+		io.register_timer(STATISTICS_TIMEOUT, STATISTICS_INTERVAL)
+			.expect("Error registering statistics timer.");
 	}
 
 	fn read(&self, io: &NetworkContext, peer: &PeerId, packet_id: u8, data: &[u8]) {
@@ -1119,6 +1190,7 @@ impl NetworkProtocolHandler for LightProtocol {
 			TICK_TIMEOUT => self.tick_handlers(&io),
 			PROPAGATE_TIMEOUT => self.propagate_transactions(&io),
 			RECALCULATE_COSTS_TIMEOUT => self.begin_new_cost_period(&io),
+			STATISTICS_TIMEOUT => self.tick_statistics(),
 			_ => warn!(target: "pip", "received timeout on unknown token {}", timer),
 		}
 	}

ethcore/light/src/net/tests/mod.rs

@@ -22,9 +22,10 @@ use ethcore::client::{EachBlockWith, TestBlockChainClient};
 use ethcore::encoded;
 use ethcore::ids::BlockId;
 use ethereum_types::{H256, U256, Address};
-use net::{LightProtocol, Params, packet, Peer};
+use net::{LightProtocol, Params, packet, Peer, Statistics};
 use net::context::IoContext;
 use net::status::{Capabilities, Status};
+use net::load_timer::MOVING_SAMPLE_SIZE;
 use network::{PeerId, NodeId};
 use provider::Provider;
 use request;
@@ -780,3 +781,34 @@ fn get_transaction_index() {
 	let expected = Expect::Respond(packet::RESPONSE, response);
 	proto.handle_packet(&expected, 1, packet::REQUEST, &request_body);
 }
+
+#[test]
+fn sync_statistics() {
+	let mut stats = Statistics::new();
+
+	// Empty set should return 1.0
+	assert_eq!(stats.avg_peer_count(), 1.0);
+
+	// Average < 1.0 should return 1.0
+	stats.add_peer_count(0);
+	assert_eq!(stats.avg_peer_count(), 1.0);
+
+	stats = Statistics::new();
+
+	const N: f64 = 50.0;
+
+	for i in 1..(N as usize + 1) {
+		stats.add_peer_count(i);
+	}
+
+	// Compute the average for the sum 1..N
+	assert_eq!(stats.avg_peer_count(), N * (N + 1.0) / 2.0 / N);
+
+	for _ in 1..(MOVING_SAMPLE_SIZE + 1) {
+		stats.add_peer_count(40);
+	}
+
+	// Test that it returns the average of the last
+	// `MOVING_SAMPLE_SIZE` values
+	assert_eq!(stats.avg_peer_count(), 40.0);
+}

ethcore/sync/src/api.rs

@@ -264,12 +264,10 @@ pub struct EthSync {
 
 fn light_params(
 	network_id: u64,
-	max_peers: u32,
+	median_peers: f64,
 	pruning_info: PruningInfo,
 	sample_store: Option<Box<SampleStore>>,
 ) -> LightParams {
-	const MAX_LIGHTSERV_LOAD: f64 = 0.5;
-
 	let mut light_params = LightParams {
 		network_id: network_id,
 		config: Default::default(),
@@ -282,9 +280,7 @@ fn light_params(
 		sample_store: sample_store,
 	};
 
-	let max_peers = ::std::cmp::max(max_peers, 1);
+	light_params.config.median_peers = median_peers;
-	light_params.config.load_share = MAX_LIGHTSERV_LOAD / max_peers as f64;
-
 	light_params
 }
 
@@ -301,9 +297,10 @@ impl EthSync {
 					.map(|mut p| { p.push("request_timings"); light_net::FileStore(p) })
 					.map(|store| Box::new(store) as Box<_>);
 
+				let median_peers = (params.network_config.min_peers + params.network_config.max_peers) as f64 / 2.0;
 				let light_params = light_params(
 					params.config.network_id,
-					params.network_config.max_peers,
+					median_peers,
 					pruning_info,
 					sample_store,
 				);
@@ -940,19 +937,3 @@ impl LightSyncProvider for LightSync {
 		Default::default() // TODO
 	}
 }
-
-#[cfg(test)]
-mod tests {
-	use super::*;
-
-	#[test]
-	fn light_params_load_share_depends_on_max_peers() {
-		let pruning_info = PruningInfo {
-			earliest_chain: 0,
-			earliest_state: 0,
-		};
-		let params1 = light_params(0, 10, pruning_info.clone(), None);
-		let params2 = light_params(0, 20, pruning_info, None);
-		assert!(params1.config.load_share > params2.config.load_share)
-	}
-}