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Gas 成本模型

哪些操作会消耗 Gas

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Gas 成本模型 是 CoddyKit 上的免费 Web3 & DApp Development Fundamentals 课时。这是第 1 节课，共 4 节。你可以在下方免费阅读本课时的完整内容 — 然后在浏览器中使用内置代码编辑器和全天候 AI 导师进行实践。这是 Web3 & DApp Development Fundamentals 学习路径的一部分，你的进度在网页和 CoddyKit 应用中同步。 Web3 & DApp Development Fundamentals 课程共包含 4 节课。

本课时的部分内容尚未翻译，以英文显示。

Why Gas Exists

Every operation on the Ethereum Virtual Machine (EVM) costs gas. Gas is a unit that measures the computational work required to execute a transaction.

Gas exists to:

Pay validators for the resources they spend
Prevent infinite loops and denial-of-service attacks
Price scarce on-chain resources fairly

The total fee you pay equals gasUsed * gasPrice, denominated in wei.

Gas, Gas Price, and Gas Limit

Three numbers matter for every transaction:

gasUsed — how much computation actually happened
gasPrice (or maxFeePerGas under EIP-1559) — how much you pay per unit
gasLimit — the maximum gas you allow before the transaction reverts

If execution exceeds the gas limit, the transaction reverts with an out of gas error, but you still pay for the gas consumed.

// Fee calculation
// totalFee = gasUsed * effectiveGasPrice
// Example: 50000 gas * 20 gwei = 1,000,000 gwei = 0.001 ETH

Opcode Costs

Each EVM opcode has a fixed gas cost defined in the Ethereum protocol. Cheap arithmetic costs very little, while storage and external calls cost a lot.

ADD, SUB — 3 gas
MUL — 5 gas
SLOAD (read storage) — 2100 gas cold, 100 warm
SSTORE (write storage) — up to 22100 gas

Storage dominates the cost of most contracts.

The Cost of Storage

Persistent storage is the single most expensive resource on the EVM. Writing a storage slot from zero to non-zero costs 20000 gas; updating a non-zero slot costs 5000 gas.

Reading is cheaper than writing but still significant. This is why the golden rule of gas optimization is: touch storage as rarely as possible.

contract Counter {
    uint256 public count; // each increment writes a storage slot

    function increment() external {
        count += 1; // 1 SLOAD + 1 SSTORE
    }
}

Memory vs Storage vs Calldata

The EVM has three data locations, each with a different cost profile:

storage — persistent, most expensive
memory — temporary per-call, cheap but grows quadratically
calldata — read-only input data, cheapest for function arguments

Using calldata instead of memory for external function parameters avoids an unnecessary copy.

function sum(uint256[] calldata nums) external pure returns (uint256 total) {
    for (uint256 i = 0; i < nums.length; i++) {
        total += nums[i];
    }
}

Intrinsic Gas

Before any code runs, every transaction pays a base cost called intrinsic gas:

21000 gas for a base transaction
16 gas per non-zero calldata byte
4 gas per zero calldata byte

This means even an empty transfer costs 21000 gas, and larger calldata payloads make transactions more expensive before execution even begins.

Cold vs Warm Access

Since EIP-2929, the first access to an account or storage slot in a transaction is cold and expensive; subsequent accesses are warm and cheap.

Cold SLOAD: 2100 gas
Warm SLOAD: 100 gas
Cold account access: 2600 gas

Caching a storage value in a local variable converts repeated cold reads into a single read plus cheap memory access.

uint256 cached = count; // one SLOAD
for (uint256 i = 0; i < cached; i++) {
    // use cached instead of re-reading count
}

Refunds

The EVM grants a partial gas refund when you clear storage, setting a non-zero slot back to zero (via SSTORE or SELFDESTRUCT historically).

Refunds are capped at one fifth of the transaction gas used (EIP-3529). Patterns like deleting array elements or zeroing balances can recover some gas, but you should never rely on refunds as a primary optimization.

function clear(uint256 key) external {
    delete data[key]; // zeroing a slot triggers a refund
}

EIP-1559 Fee Market

EIP-1559 split the gas price into two parts:

base fee — burned, adjusts automatically per block based on demand
priority fee (tip) — paid to the validator

Users set maxFeePerGas and maxPriorityFeePerGas. The effective price never exceeds the max, and unused amounts are refunded. This makes fees more predictable.

Deployment vs Runtime Gas

Contracts have two distinct gas profiles:

Deployment gas — paid once, proportional to bytecode size (200 gas per byte stored)
Runtime gas — paid on every call to the deployed contract

Sometimes you trade a larger, more expensive deployment for cheaper runtime, or vice versa. Optimize for the dimension that matters most to your use case.

The Optimization Mindset

Effective gas optimization follows a hierarchy:

Avoid storage writes (biggest wins)
Minimize cold accesses by caching
Use calldata over memory
Reduce calldata size
Micro-optimize opcodes last

Always measure before and after. Premature micro-optimization often hurts readability for negligible gain.

Quick Check

Which operation is typically the most expensive in an EVM contract?

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Recap

You now understand the EVM gas cost model:

Fees equal gasUsed * gasPrice, bounded by a gas limit
Storage writes dominate cost; arithmetic is cheap
Cold accesses cost more than warm ones
Intrinsic gas (21000 + calldata bytes) is paid before execution
EIP-1559 splits fees into a burned base fee and a validator tip

Next we apply this knowledge to optimize contract storage layout.

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