Store election snapshot in a more memory-friendly way. (#9275)

* Store election snapshot in a more memory-friendly way.

* fix

* re-order benchmarks

* Update frame/election-provider-multi-phase/src/lib.rs

Co-authored-by: Guillaume Thiolliere <gui.thiolliere@gmail.com>

* cargo run --release --features=runtime-benchmarks --manifest-path=bin/node/cli/Cargo.toml -- benchmark --chain=dev --steps=50 --repeat=20 --pallet=pallet_election_provider_multi_phase --extrinsic=* --execution=wasm --wasm-execution=compiled --heap-pages=4096 --output=./frame/election-provider-multi-phase/src/weights.rs --template=./.maintain/frame-weight-template.hbs

* manually fix the weights

* remove todo

Co-authored-by: Guillaume Thiolliere <gui.thiolliere@gmail.com>
Co-authored-by: Parity Bot <admin@parity.io>

substrate/frame/election-provider-multi-phase/src/benchmarking.rs

@@ -199,6 +199,18 @@ frame_benchmarking::benchmarks! {
 		assert!(<MultiPhase<T>>::current_phase().is_unsigned());
 	}
 
+	on_initialize_open_unsigned_without_snapshot {
+		// need to assume signed phase was open before
+		<MultiPhase<T>>::on_initialize_open_signed().unwrap();
+		assert!(<MultiPhase<T>>::snapshot().is_some());
+		assert!(<MultiPhase<T>>::current_phase().is_signed());
+	}: {
+		<MultiPhase<T>>::on_initialize_open_unsigned(false, true, 1u32.into()).unwrap();
+	} verify {
+		assert!(<MultiPhase<T>>::snapshot().is_some());
+		assert!(<MultiPhase<T>>::current_phase().is_unsigned());
+	}
+
 	finalize_signed_phase_accept_solution {
 		let receiver = account("receiver", 0, SEED);
 		let initial_balance = T::Currency::minimum_balance() * 10u32.into();
@@ -232,18 +244,6 @@ frame_benchmarking::benchmarks! {
 		assert_eq!(T::Currency::reserved_balance(&receiver), 0u32.into());
 	}
 
-	on_initialize_open_unsigned_without_snapshot {
-		// need to assume signed phase was open before
-		<MultiPhase<T>>::on_initialize_open_signed().unwrap();
-		assert!(<MultiPhase<T>>::snapshot().is_some());
-		assert!(<MultiPhase<T>>::current_phase().is_signed());
-	}: {
-		<MultiPhase<T>>::on_initialize_open_unsigned(false, true, 1u32.into()).unwrap();
-	} verify {
-		assert!(<MultiPhase<T>>::snapshot().is_some());
-		assert!(<MultiPhase<T>>::current_phase().is_unsigned());
-	}
-
 	// a call to `<Pallet as ElectionProvider>::elect` where we only return the queued solution.
 	elect_queued {
 		// number of votes in snapshot.

substrate/frame/election-provider-multi-phase/src/lib.rs

@@ -327,7 +327,7 @@ impl BenchmarkingConfig for () {
 }
 
 /// Current phase of the pallet.
-#[derive(PartialEq, Eq, Clone, Copy, Encode, Decode, RuntimeDebug)]
+#[derive(PartialEq, Eq, Clone, Copy, Encode, Decode, Debug)]
 pub enum Phase<Bn> {
 	/// Nothing, the election is not happening.
 	Off,
@@ -402,7 +402,7 @@ pub enum FallbackStrategy {
 }
 
 /// The type of `Computation` that provided this election data.
-#[derive(PartialEq, Eq, Clone, Copy, Encode, Decode, RuntimeDebug)]
+#[derive(PartialEq, Eq, Clone, Copy, Encode, Decode, Debug)]
 pub enum ElectionCompute {
 	/// Election was computed on-chain.
 	OnChain,
@@ -476,7 +476,7 @@ pub struct RoundSnapshot<A> {
 /// This is stored automatically on-chain, and it contains the **size of the entire snapshot**.
 /// This is also used in dispatchables as weight witness data and should **only contain the size of
 /// the presented solution**, not the entire snapshot.
-#[derive(PartialEq, Eq, Clone, Copy, Encode, Decode, RuntimeDebug, Default)]
+#[derive(PartialEq, Eq, Clone, Copy, Encode, Decode, Debug, Default)]
 pub struct SolutionOrSnapshotSize {
 	/// The length of voters.
 	#[codec(compact)]
@@ -1308,12 +1308,29 @@ impl<T: Config> Pallet<T> {
 		}
 
 		// Only write snapshot if all existed.
-		<SnapshotMetadata<T>>::put(SolutionOrSnapshotSize {
+		let metadata = SolutionOrSnapshotSize {
 			voters: voters.len() as u32,
 			targets: targets.len() as u32,
-		});
+		};
+		log!(debug, "creating a snapshot with metadata {:?}", metadata);
+
+		<SnapshotMetadata<T>>::put(metadata);
 		<DesiredTargets<T>>::put(desired_targets);
-		<Snapshot<T>>::put(RoundSnapshot { voters, targets });
+
+		// instead of using storage APIs, we do a manual encoding into a fixed-size buffer.
+		// `encoded_size` encodes it without storing it anywhere, this should not cause any allocation.
+		let snapshot = RoundSnapshot { voters, targets };
+		let size = snapshot.encoded_size();
+		log!(info, "snapshot pre-calculated size {:?}", size);
+		let mut buffer = Vec::with_capacity(size);
+		snapshot.encode_to(&mut buffer);
+
+		// do some checks.
+		debug_assert_eq!(buffer, snapshot.encode());
+		// buffer should have not re-allocated since.
+		debug_assert!(buffer.len() == size && size == buffer.capacity());
+
+		sp_io::storage::set(&<Snapshot<T>>::hashed_key(), &buffer);
 		Ok(w1.saturating_add(w2).saturating_add(w3).saturating_add(T::DbWeight::get().writes(3)))
 	}