今天在做以太坊开发时,突然想到一个问题,以太坊一个区块包含的交易个数由什么决定?

如果交易池中有足够的交易,一个区块最多可容纳多少交易?

带着这个问题,我阅读了一下go-ethereum中关于挖矿的源代码,找到了答案。

先抛出结论:

影响一个区块交易个数的因素有两个:

  1. 交易池中的交易个数。这个很好理解,交易池的交易个数直接决定了一个矿工可以打包多少交易到区块中。
  2. 区块允许的GasLimit。GasLimit又由父块GasLimit、GasFloor和GasCeil共同决定(1.8版本以前只受父块GasLimit影响),当交易的gas总计大于GasLimit时,交易将不在打包的区块中。
    其中gasFloor和gasCeil是在geth的中mine的两个配置项。详见文档:https://geth.ethereum.org/docs/interface/command-line-options

结论主要从"go-ethereum/miner/worker.go"源文件中得出。worker相当于一个挖矿工人,负责具体的挖矿工作流程。在worker对象中有一个“commitNewWork”方法,是创建一个新的区块,其中计算了该区块的GasLimit和需要处理的交易池中的交易。相关代码如下:

func (w *worker) commitNewWork(interrupt *int32, noempty bool, timestamp int64) {
    // 创建区块头
    header := &types.Header{
        ParentHash: parent.Hash(),
        Number:     num.Add(num, common.Big1),
        // 计算GasLimit
        GasLimit:   core.CalcGasLimit(parent, w.config.GasFloor, w.config.GasCeil),
        Extra:      w.extra,
        Time:       uint64(timestamp),
    }

    // 从交易池中读取交易
    pending, err := w.eth.TxPool().Pending()
    if err != nil {
        log.Error("Failed to fetch pending transactions", "err", err)
        return
    }

    // Short circuit if there is no available pending transactions
    if len(pending) == 0 {
        w.updateSnapshot()
        return
    }

    // Split the pending transactions into locals and remotes
    localTxs, remoteTxs := make(map[common.Address]types.Transactions), pending
    for _, account := range w.eth.TxPool().Locals() {
        if txs := remoteTxs[account]; len(txs) > 0 {
            delete(remoteTxs, account)
            localTxs[account] = txs
        }
    }

    if len(localTxs) > 0 {
        txs := types.NewTransactionsByPriceAndNonce(w.current.signer, localTxs)
        if w.commitTransactions(txs, w.coinbase, interrupt) {
            return
        }
    }

    if len(remoteTxs) > 0 {
        txs := types.NewTransactionsByPriceAndNonce(w.current.signer, remoteTxs)
        if w.commitTransactions(txs, w.coinbase, interrupt) {
            return
        }
    }
}

“commitNewWork”方法中获取到交易池中的交易后将交易提交给了“commitTransactions”方法对交易进行验证。

func (w *worker) commitTransactions(txs *types.TransactionsByPriceAndNonce, coinbase common.Address, interrupt *int32) bool {
    // 将区块头中的GasLimit赋值给gasPool
    if w.current.gasPool == nil {
        w.current.gasPool = new(core.GasPool).AddGas(w.current.header.GasLimit)
    }

    // 循环判断所有交易
    for {
        // gasPool中的gas小于21000是跳出循环
        if w.current.gasPool.Gas() < params.TxGas {
            log.Trace("Not enough gas for further transactions", "have", w.current.gasPool, "want", params.TxGas)
            break
        }

        // 按照交易gas从大到小的顺序获取下一个交易
        tx := txs.Peek()

        // 没有交易后跳出循环
        if tx == nil {
            break
        }
        // 提交交易
        logs, err := w.commitTransaction(tx, coinbase)
    }
}

将交易提交给“commitTransaction”方法后,回调用

receipt, _, err := core.ApplyTransaction(w.config, w.chain, &coinbase, w.current.gasPool, w.current.state, w.current.header, tx, &w.current.header.GasUsed, vm.Config{})

在ApplyTransaction方法中会将gasPool中的gas值减去该笔交易的gas,当gasPool的gas值小于21000时,剩下的交易将不在打包的该区块中。

回头在来看一下区块中的GasLimit是怎么计算的。

core.CalcGasLimit(parent, w.config.GasFloor, w.config.GasCeil)方法在“go-ethereum/core/block_validator.go”,代码如下:

// CalcGasLimit computes the gas limit of the next block after parent. It aims
// to keep the baseline gas above the provided floor, and increase it towards the
// ceil if the blocks are full. If the ceil is exceeded, it will always decrease
// the gas allowance.
func CalcGasLimit(parent *types.Block, gasFloor, gasCeil uint64) uint64 {
    // contrib = (parentGasUsed * 3 / 2) / 1024
    contrib := (parent.GasUsed() + parent.GasUsed()/2) / params.GasLimitBoundDivisor

    // decay = parentGasLimit / 1024 -1
    decay := parent.GasLimit()/params.GasLimitBoundDivisor - 1

    /*
        strategy: gasLimit of block-to-mine is set based on parent's
        gasUsed value.  if parentGasUsed > parentGasLimit * (2/3) then we
        increase it, otherwise lower it (or leave it unchanged if it's right
        at that usage) the amount increased/decreased depends on how far away
        from parentGasLimit * (2/3) parentGasUsed is.
    */
    limit := parent.GasLimit() - decay + contrib
    if limit < params.MinGasLimit {
        limit = params.MinGasLimit
    }
    // If we're outside our allowed gas range, we try to hone towards them
    if limit < gasFloor {
        limit = parent.GasLimit() + decay
        if limit > gasFloor {
            limit = gasFloor
        }
    } else if limit > gasCeil {
        limit = parent.GasLimit() - decay
        if limit < gasCeil {
            limit = gasCeil
        }
    }
    return limit
}

计算GasLimit的策略很有意思,受父块的GasLimit、挖矿的gas上限gasCeil和gas下限gasFloor三者共同决定。

当父块的交易里的gas总和大于父块GasLimit的2/3时,则增加当前块的GasLimit,反之减少当前块的GasLimit,同时,保证GasLimit的范围在gasFloor和gasCeil之间。

转载:https://www.jianshu.com/p/3588fd52ec0a