[Ethereum] How Geth and Parity knows the global gas limit

gas-limit

As said in Péter Szilágyi post DAO Wars: Your voice on the soft-fork dilemma:

Miners supporting the DAO soft-fork can do so by starting Geth 1.4.8
with –dao-soft-fork. This will cause the block gas limits to be
lowered towards Pi million until the deciding block 1800000 (approx. 6
days from now) is reached. If the gas limit of this block is below or
equal 4M, the soft-fork goes into effect and (all updating) miners
will start blocking DAO transactions that release funds.

How does the Geth (or Parity) client knows the current global network gas limit ? I can't see any explorer that shows it except https://ethstats.net/ that only shows some voluntary nodes.

Best Answer

The block gasLimit is in each block. Example:

eth.getBlock(1500000)

{
  difficulty: 34982465665323,
  extraData: "0xd783010305844765746887676f312e352e31856c696e7578",
  gasLimit: 4712388,
  ...
}

Related Solutions

Transactions – Are Gas Limit in Transaction and Block Gas Limit Different?

Yes. Each transaction has a gas limit. For example, there could be 5 unmined transactions where each has a gas limit of 10, 20, 30, 40, 50. The block gas limit determines how many transactions can fit in a block. For example, if the block gas limit is 100, then the first four transactions can fit in the block. Miners decide which transactions to include in a block. A different miner could try including the last 2 transactions in the block (50+40), and they only have space to include the first transaction (10).

Every person who creates a transaction can decide what their transaction's gas limit is: but it would not make sense for them to specify something higher than the block gas limit, so Geth and other clients would prevent them from setting too high. It is a gas limit because it is the maximum amount of gas the person creating the transaction is willing to use: they do not pay for any extra gas that is not used. But if they set the limit too low, their transaction can run out of gas and they pay for all of it to the miner.

With the block gas limit, miners are the ones who decide. The bigger it is the more they can get from transaction fees, but the more bandwidth and computing they would have to do. Miners cannot change the block gas limit too much within one block: they can only change it by a factor of 1/1024.

The transaction gas limit is the gas property in a transactionObject. The block gas limit is a parameter in the genesis file created (it is client dependent and typically means the minimum block gas limit: a block with a gas limit smaller than it will not be accepted by the nodes on that blockchain).

Geth – How to Increase Gas Limit in a Block

While it's advised to not alter a chain's gas limit once created, it may become necessary to fiddle with the gas limit of an existing private blockchain, especially during development.

On geth, this can be done by setting the --targetgaslimit flag when starting the node. For example, you may do the following for a network with id 666 running on localhost:8545:

  geth --networkid '666' --datadir 'path/to/your/chain/db' --targetgaslimit '9000000000000' --rpc --rpccorsdomain 'localhost:8545' --mine

The above will start your node and keep pushing the gas limit towards 9000000000000 as mining continues. Over time, the gas limit would have grown so large that you'll not have the Error: exceeds block gas limit issue anymore.

Of course, closing the node and restarting repeats the process from the initial gas limit of the chain all the way towards 9000000000000 again.

Basically, using --targetgaslimit as a flag is a very convenient way to increase or decrease the gas limit of a private chain while developing.

Best Answer

Related Solutions

Transactions – Are Gas Limit in Transaction and Block Gas Limit Different?

Geth – How to Increase Gas Limit in a Block

Related Topic