Store voters in unsorted bags to get good stake iteration properties

frame/election-provider-support/src/onchain.rs

@@ -77,11 +77,8 @@ impl<T: Config> ElectionProvider<T::AccountId, T::BlockNumber> for OnChainSequen
 		let (desired_targets, _) =
 			Self::DataProvider::desired_targets().map_err(Error::DataProvider)?;
 
-		let mut stake_map: BTreeMap<T::AccountId, VoteWeight> = BTreeMap::new();
-
-		voters.iter().for_each(|(v, s, _)| {
-			stake_map.insert(v.clone(), *s);
-		});
+		let stake_map: BTreeMap<T::AccountId, VoteWeight> =
+			voters.iter().map(|(v, s, _)| (v.clone(), *s)).collect();
 
 		let stake_of = |w: &T::AccountId| -> VoteWeight {
 			stake_map.get(w).cloned().unwrap_or_default()

frame/staking/Cargo.toml

@@ -37,6 +37,7 @@ rand_chacha = { version = "0.2", default-features = false, optional = true }
 sp-storage = { version = "3.0.0", path = "../../primitives/storage" }
 sp-tracing = { version = "3.0.0", path = "../../primitives/tracing" }
 sp-core = { version = "3.0.0", path = "../../primitives/core" }
+sp-npos-elections = { version = "3.0.0", path = "../../primitives/npos-elections", features = ["mocks"] }
 pallet-balances = { version = "3.0.0", path = "../balances" }
 pallet-timestamp = { version = "3.0.0", path = "../timestamp" }
 pallet-staking-reward-curve = { version = "3.0.0",  path = "../staking/reward-curve" }
@@ -70,3 +71,8 @@ runtime-benchmarks = [
 	"rand_chacha",
 ]
 try-runtime = ["frame-support/try-runtime"]
+make-bags = []
+
+[[bin]]
+name = "make_bags"
+required-features = ["make-bags", "std"]

frame/staking/src/benchmarking.rs

@@ -584,9 +584,11 @@ benchmarks! {
 		(0..s).for_each(|index| {
 			add_slashing_spans::<T>(&validators[index as usize], 10);
 		});
+
+		let num_voters = (v + n) as usize;
 	}: {
-		let voters = <Staking<T>>::get_npos_voters();
-		assert_eq!(voters.len() as u32, v + n);
+		let voters = <Staking<T>>::get_npos_voters(None, num_voters);
+		assert_eq!(voters.len(), num_voters);
 	}
 
 	get_npos_targets {

frame/staking/src/bin/make_bags.rs

@@ -0,0 +1,40 @@
+// This file is part of Substrate.
+
+// Copyright (C) 2021 Parity Technologies (UK) Ltd.
+// SPDX-License-Identifier: Apache-2.0
+
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// 	http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//! Make the set of voting bag thresholds to be used in `voter_bags.rs`.
+
+use pallet_staking::voter_bags::N_BAGS;
+
+fn main() {
+	let ratio = ((u64::MAX as f64).ln() / (N_BAGS as f64)).exp();
+	println!("pub const CONSTANT_RATIO: f64 = {};", ratio);
+
+	let mut thresholds = Vec::with_capacity(N_BAGS as usize);
+
+	while thresholds.len() < N_BAGS as usize {
+		let prev_item: u64 = thresholds.last().copied().unwrap_or_default();
+		let item = (prev_item as f64 * ratio).max(prev_item as f64 + 1.0);
+		thresholds.push(item as u64);
+	}
+
+	*thresholds.last_mut().unwrap() = u64::MAX;
+
+	println!("pub const THRESHOLDS: [u64; {}] = {:#?};", N_BAGS, thresholds);
+
+	debug_assert_eq!(thresholds.len(), N_BAGS as usize);
+	debug_assert_eq!(*thresholds.last().unwrap(), u64::MAX);
+}