improve weight per gas calculation (#995)

template/runtime/src/lib.rs

@@ -35,7 +35,7 @@ use frame_support::weights::constants::RocksDbWeight as RuntimeDbWeight;
 use frame_support::{
 	construct_runtime, parameter_types,
 	traits::{ConstU32, ConstU8, FindAuthor, KeyOwnerProofSystem, OnTimestampSet},
-	weights::{constants::WEIGHT_REF_TIME_PER_SECOND, ConstantMultiplier, IdentityFee, Weight},
+	weights::{constants::WEIGHT_REF_TIME_PER_MILLIS, ConstantMultiplier, IdentityFee, Weight},
 };
 use pallet_grandpa::{
 	fg_primitives, AuthorityId as GrandpaId, AuthorityList as GrandpaAuthorityList,
@@ -136,9 +136,12 @@ pub fn native_version() -> sp_version::NativeVersion {
 }
 
 const NORMAL_DISPATCH_RATIO: Perbill = Perbill::from_percent(75);
-/// We allow for 2 seconds of compute with a 6 second average block time.
-pub const MAXIMUM_BLOCK_WEIGHT: Weight =
-	Weight::from_parts(2u64 * WEIGHT_REF_TIME_PER_SECOND, u64::MAX);
+/// We allow for 2000ms of compute with a 6 second average block time.
+pub const WEIGHT_MILLISECS_PER_BLOCK: u64 = 2000;
+pub const MAXIMUM_BLOCK_WEIGHT: Weight = Weight::from_parts(
+	WEIGHT_MILLISECS_PER_BLOCK * WEIGHT_REF_TIME_PER_MILLIS,
+	u64::MAX,
+);
 pub const MAXIMUM_BLOCK_LENGTH: u32 = 5 * 1024 * 1024;
 
 parameter_types! {
@@ -312,11 +315,36 @@ impl<F: FindAuthor<u32>> FindAuthor<H160> for FindAuthorTruncated<F> {
 	}
 }
 
-const WEIGHT_PER_GAS: u64 = 20_000;
+/// `WeightPerGas` is an approximate ratio of the amount of Weight per Gas.
+/// u64 works for approximations because Weight is a very small unit compared to gas.
+///
+/// `GAS_PER_MILLIS * WEIGHT_MILLIS_PER_BLOCK * TXN_RATIO ~= BLOCK_GAS_LIMIT`
+/// `WEIGHT_PER_GAS = WEIGHT_REF_TIME_PER_MILLIS / GAS_PER_MILLIS
+///                 = WEIGHT_REF_TIME_PER_MILLIS / (BLOCK_GAS_LIMIT / TXN_RATIO / WEIGHT_MILLIS_PER_BLOCK)
+///                 = TXN_RATIO * (WEIGHT_REF_TIME_PER_MILLIS * WEIGHT_MILLIS_PER_BLOCK) / BLOCK_GAS_LIMIT`
+///
+/// For example, given the 2000ms Weight, from which 75% only are used for transactions,
+/// the total EVM execution gas limit is `GAS_PER_MILLIS * 2000 * 75% = BLOCK_GAS_LIMIT`.
+pub fn weight_per_gas(
+	block_gas_limit: u64,
+	txn_ratio: Perbill,
+	weight_millis_per_block: u64,
+) -> u64 {
+	let weight_per_block = WEIGHT_REF_TIME_PER_MILLIS.saturating_mul(weight_millis_per_block);
+	let weight_per_gas = (txn_ratio * weight_per_block).saturating_div(block_gas_limit);
+	assert!(
+		weight_per_gas >= 1,
+		"WeightPerGas must greater than or equal with 1"
+	);
+	weight_per_gas
+}
+
+const BLOCK_GAS_LIMIT: u64 = 75_000_000;
+
 parameter_types! {
-	pub BlockGasLimit: U256 = U256::from(NORMAL_DISPATCH_RATIO * MAXIMUM_BLOCK_WEIGHT.ref_time() / WEIGHT_PER_GAS);
+	pub BlockGasLimit: U256 = U256::from(BLOCK_GAS_LIMIT);
 	pub PrecompilesValue: FrontierPrecompiles<Runtime> = FrontierPrecompiles::<_>::new();
-	pub WeightPerGas: Weight = Weight::from_ref_time(WEIGHT_PER_GAS);
+	pub WeightPerGas: Weight = Weight::from_ref_time(weight_per_gas(BLOCK_GAS_LIMIT, NORMAL_DISPATCH_RATIO, WEIGHT_MILLISECS_PER_BLOCK));
 }
 
 impl pallet_evm::Config for Runtime {