Block Limit

Bitcoin historically has limited the block size to 1MB

• if the block becomes too big, it makes it difficult to synchronize across the world

Ethereum does not explicitly set a byte limit to the block size. Instead it limits the amount of computations per block, or gas.

• Each computation has a gas cost.

• All the computation within a block must fall within a certain gas threshold

• Else, it will be difficult for nodes to verify the transactions quickly.

At time of writing, block limit is ~30 million gas. An eth transfer costs 21,000 gas, so theoretically, one block can hold 1,428 transfers.

• If your smart contract requires over 30 million gas to execute, it won't fit on the block.

• If there are more than 1428 people trying to make a transfer, then the highest bidder will have their transaction included on the block. This is why gas prices fluctuate.

30 million gas is not a hard limit. The gas limit changes, and is not completely static. Ethereum technically has a preferred block limit and dynamic block limit but conceptually, you can think of it as 30 million gas.

Minimum gas cost

• it costs 21,000 gas to do anything at the bare minimum: calling a fn, transfer, etc

The cost of doing nothing

contract DoNothing {

    // execution costs: 21138 gas
    function doNothing() external payable {}
}

How do we get 21138 gas?

• tx cost: 21,000

• opcodes: 65 gas

• tx.data 64 gas

  • msg.data | tx.data submitted

• memory: 9 gas

  • free memory pointer: PUSH 80 PUSH 40 MSTORE

  • you are charged for memory expansion, even if you didn't write in the intermediate slots

  • we are using slots 0x040 to 0x60

  • 0x60 / 0x20 = 3 (96/32) in dec

  • 3 slots * 3 gas = 9 gas

Summary

There are 5 places to save gas

• On deployment: The smaller the contract, the less you pay on deployment

• During computation: Using fewer/cheaper op-codes saves gas on execution

• Transaction data: The larger your tx.data, the more non-zero bytes in it, the more gas costs.

• Memory: The more memory you alocate, even if you don't use it, you pay more gas.

• Storage: More storage used, more gas used.