Somnia Gas Differences To Ethereum

Somnia's gas costs work very differently to Ethereum's.

For background: EVM gas fees are designed to approximate the load placed on the blockchain. They are used to charge users in proportion to the system resources they consume. A transaction's gas fee (in ETH or SOMI) is calculated as number of units of gas * price per unit of gas. The number of units of gas is just the sum of the gas costs of the transaction's operations.

Because the price of a unit of gas is variable, the absolute cost of an operation in units of gas does not ultimately matter. What matters is the relative difference between operations' gas costs. In other words, if you doubled the gas costs for all operations, and halved the price per unit of gas, there would be no functional change.

In regards to determining gas costs, there are two fundamental differences between Ethereum and Somnia:

1. Somnia's architecture and implementation has drastically reduced the real-world performance cost of most EVM operations, but not all. A few operations, such as third-party library precompiles, are just as performant on Ethereum as on Somnia - both use the same code. This relative difference requires us to adjust the relative gas costs of these operations. We could either reduce the gas costs of all operations that Somnia has improved, or increase the gas costs of all operations that Somnia has not improved - the result is equivalent. Somnia does the latter, as it fits better into the Ethereum developer ecosystem.

2. Ethereum charges a flat gas cost to read any existing value from storage for the first time in a transaction. This ends up hitting a database whose latency for this read can vary by multiple orders of magnitude. This gas cost inaccuracy is a big problem when scaling these blockchains, as it requires them to leave massive amounts of unused buffer in their execution budget to account for these unpredictable latencies. Somnia has built a new database, IceDb, that is designed to fix this problem. Only the (rare) reads which actually take a long time have a high gas cost; reading from cached state is far cheaper.

These two factors mean that Somnia requires different gas semantics to Ethereum. The differences are detailed in this document.

Precompiles

These precompiles have flat multiples applied to the same gas calculation that Ethereum uses.

• ecRecover (0x01) costs 50 times the Ethereum calculated gas usage. (i.e. 150,000 gas instead of 3000 gas.)

• SHA2-256 (0x02) costs 50 times the Ethereum calculated gas usage.

• RIPEMD-160 (0x03) costs 10 times the Ethereum calculated gas usage.

• modexp (0x05) costs 10 times the Ethereum calculated gas usage.

• ecAdd (0x06) costs 50 times the Ethereum calculated gas usage.

• ecMul (0x07) costs 10 times the Ethereum calculated gas usage.

• ecPairing (0x08) costs 250 times the Ethereum calculated gas usage.

• blake2f (0x09) costs 10 times the Ethereum calculated gas usage.

• point evaluation (0x0a) costs 50 times the Ethereum calculated gas usage.

BLS operations:

• BLS12_G1ADD (0x0b) costs 220 times the Ethereum calculated gas usage.

• BLS12_G1MSM (0x0c) costs 110 times the Ethereum calculated gas usage.

• BLS12_G2ADD (0x0d) costs 330 times the Ethereum calculated gas usage.

• BLS12_G2MSM (0x0e) costs 50 times the Ethereum calculated gas usage.

• BLS12_PAIRING_CHECK (0x0f) costs 50 times the Ethereum calculated gas usage.

• BLS12_MAP_FP_TO_G1 (0x10) costs 120 times the Ethereum calculated gas usage.

• BLS12_MAP_FP2_TO_G2 (0x11) costs 50 times the Ethereum calculated gas usage.

Non-storage EVM operations

• SELFBALANCE costs 305 gas instead of 5 gas.

• KECCAK256 costs 1250 + 300 * minimum_word_size gas instead of 30 + 6 * minimum_word_size gas.

• ADDMOD costs 358 gas instead of 8 gas.

• MULMOD costs 358 gas instead of 8 gas.

Storage EVM operations

Ethereum uses an access list based gas model for all storage operations, to charge extra gas depending on if the value being accessed has already been accessed in the same transaction, and if it is creating or deleting that state.

As discussed above, Somnia instead charges extra gas based on a more accurate model of the real world latency it takes to read or write the value in question:

SLOAD

• If the storage slot key is in the set of most recently accessed 128 million contract slot keys, no extra gas is charged (on top of the static op gas fee, which is 100 gas).

• If the key does not exist, the access requires at least 1,000,000 gas remaining, but that gas is not charged, so there is no extra cost.

• Otherwise, the read costs an additional 1,000,000 gas.

SSTORE

• If the storage slot key is in the set of most recently accessed 128 million contract slot keys, no extra gas is charged (on top of the static op gas fee, which is 100 gas).

• If the key does not exist and is being written to zero, the access requires at least 1,000,000 gas remaining, but that gas is not charged, so there is no extra cost.

• If the key does not exist and is being written to a non-zero value, the access requires at least 1,000,000 gas remaining, but the caller is charged 200,000 gas.

• Otherwise, the write costs an additional 1,000,000 gas.

Account operations

The following EVM operations access account state:

• BALANCE

• EXTCODESIZE

• EXTCODECOPY

• EXTCODEHASH

• CALL

• CALLCODE

• DELEGATECALL

• STATICCALL

• CREATE

• CREATE2

• SELFDESTRUCT

These have an additional storage gas fee (in place of Ethereum's dynamic access list gas fee):

• If the account is in the set of most recently accessed 32 million accounts, no extra gas is charged.

• If the account does not exist and is being read, the access requires at least 1,000,000 gas remaining, but that gas is not charged, so there is no extra cost.

• If the account does not exist and is being created, the access requires at least 1,000,000 gas remaining, but the caller is charged 400,000 gas for the new account.

• Otherwise, the access costs 1,000,000 gas.

EVM Logs

Somnia charges an increased amount of gas for EVM logs due to the historical storage requirements.

Ethereum charges: 375 + 375 * topic_count + 8 * size + memory_expansion_cost gas

Somnia charges: 3200 + 5120 * topic_count + 160 * size + memory_expansion_cost gas

To illustrate these changes with the five different EVM log operations:

EVM Op	Ethereum gas	Somnia gas
LOG0 with 32 bytes of data	631	8,320
LOG1 with 32 bytes of data	1,006	13,440
LOG2 with 32 bytes of data	1,381	18,560
LOG3 with 32 bytes of data	1,756	23,680
LOG4 with 32 bytes of data	2,131	28,800

Contract bytecode deployment cost

Ethereum charges 200 gas per byte of deployed bytecode. Somnia charges 3125 gas per byte of deployed bytecode.

EIP-7702 delegation authorisations

Ethereum charges 25,000 gas per authorisation in a Set Code transaction. 12,500 gas is refunded if account creation isn't required.

Somnia charges 1,570,000 gas per authorisation in a Set Code transaction. 400,000 gas is refunded if account creation isn't required.