How much gas does it cost to emit an event?

Question

How much gas does it cost to emit an event?

Is there a formula for calculating how much gas the emission of an event will cost given that its parameters sum up to n bytes of data?

Any info or estimates are useful,

Thanks :)

Answer 1

First, the parameters are:

LogDataGas            uint64 = 8     // Per byte in a LOG* operation's data.
LogGas                uint64 = 375   // Per LOG* operation.
LogTopicGas           uint64 = 375   
MemoryGas             uint64 = 3

https://github.com/ethereum/go-ethereum/blob/master/params/protocol_params.go

Then, the formula is the following:

gas = static gas + dynamic gas
dynamic gas = cost of memory gas + cost of log gas

Static gas for makeLog is 375, memory gas is 3

The dynamic gas is calculated as:

func makeGasLog(n uint64) gasFunc {
    return func(evm *EVM, contract *Contract, stack *Stack, mem *Memory, memorySize uint64) (uint64, error) {
        requestedSize, overflow := stack.Back(1).Uint64WithOverflow()
        if overflow {
            return 0, ErrGasUintOverflow
        }

        gas, err := memoryGasCost(mem, memorySize)
        if err != nil {
            return 0, err
        }

        if gas, overflow = math.SafeAdd(gas, params.LogGas); overflow {
            return 0, ErrGasUintOverflow
        }
        if gas, overflow = math.SafeAdd(gas, n*params.LogTopicGas); overflow {
            return 0, ErrGasUintOverflow
        }

        var memorySizeGas uint64
        if memorySizeGas, overflow = math.SafeMul(requestedSize, params.LogDataGas); overflow {
            return 0, ErrGasUintOverflow
        }
        if gas, overflow = math.SafeAdd(gas, memorySizeGas); overflow {
            return 0, ErrGasUintOverflow
        }
        return gas, nil
    }
}

https://github.com/ethereum/go-ethereum/blob/8a24b563312a0ab0a808770e464c5598ab7e35ea/core/vm/gas_table.go#L220

For example if you have 2 topics + 200 bytes of log.Data the cost will be:

375 (static cost)
200 = 200 bytes of memory for log.Data x 3 cost of memory = 600 gas for memory gas
2 x 375 = 750 for topic gas
8 x 200 = 1600 for log.Data cost

Total cost: 375 + 600 + 750 + 1600 = 3,325 gas units

2 topics in this example means topics0 and topics1 , since the topic0 is used for index of the signature of the event, for your practical use there is only topic1 left to store something useful , like an address or hash.

Answer 2

Let's use the following sample contract to see how much gas it costs to emit an empty event:

// SPDX-License-Identifier: MIT
pragma solidity >=0.8.21;

contract Foo {
    event MyEvent();

    function emitMyEvent() external returns (uint256 gasCost) {
        uint256 initialGas = gasleft();
        emit MyEvent();
        gasCost = initialGas - gasleft();
    }
}

Configuration:

• Solidity v0.8.21
• EVM version: Shanghai

The result is 784 gas: