Block gaslimit target decays to 3141592 with custom genesis gasLimit

[aiman]

I am running tests on a private network using the example genesis block linked at the go-ethereum wiki page Private network, with the addition of a 1 ether allocation for address 0xffffffffffffffffffffffffffffffffffffffff.

{
    "nonce": "0xdeadbeefdeadbeef",
    "timestamp": "0x0",
    "parentHash": "0x0000000000000000000000000000000000000000000000000000000000000000",
    "extraData": "0x686f727365",
    "gasLimit": "0x8000000",
    "difficulty": "0x400",
    "mixhash": "0x0000000000000000000000000000000000000000000000000000000000000000",
    "coinbase": "0x3333333333333333333333333333333333333333",
    "alloc": {
        "fffffffffffffffffffffffffffffffffffffff0": {
            "balance": "1000000000000000000"
        }
    }
} 

Note I have left the gasLimit 0x8000000 (134217728) unchanged.

I am launching a single instance of geth 1.3.3 as follows:

geth \
    --mine \
    --networkid 1234 \
    --nodiscover \
    --maxpeers 0 \
    --genesis /deadbeef.json \
    --etherbase 0xffffffffffffffffffffffffffffffffffffffff

I observed that the block gasLimit decays toward the frontier gas target 3141592:

> eth.getBlock(0).gasLimit
134217728
> eth.getBlock(500).gasLimit
82347720
> eth.getBlock(1000).gasLimit
50523715
> eth.getBlock(1500).gasLimit
30998596

Updating the genesis block with a gasLimit of 0x2fefe2 (3141602), the gasLimit reaches 3141592 in the next block and remains there:

> eth.getBlock(0).gasLimit
3141602
> eth.getBlock(1).gasLimit
3141592
> eth.getBlock(50).gasLimit
3141592

But this is not expected behaviour. geth should respect the gasLimit that is set in the imported genesis block deadbeef.json.

I traced the issue to worker.commitNewWork(), which makes a call to core.CalcGasLimit(parent) to determine the gaslimit for the next block. It seems like core.CalcGasLimit() references params.GenesisGasLimit, but that variable is not updated from the imported genesis file and so remains at the geth default of 3141592. As a result the calculated gas target is adjusted downwards from the parent block gas limit towards 3141592.

[karalabe]

The gas limit specified in the genesis block is the limit of the genesis block itself, not the chain. You cannot influence the chain's target gas limit from the outside using Geth. You could always build your own version of Geth that has a modified parameter set to target a different block limit, but I don't think we aim to support such a use case any time soon via CLI parameters.

[aiman]

That makes sense now. I did not think there was such a thing as a chain gas limit separate from the genesis block. Thanks!

[narayanprusty]

@karalabe where in the source code I can change the gasLimit? You can point me to that file?

[Jacodelange]

@pnarayanprusty were you able to change the gasLimit of the chain?

[karalabe]

Just set your target gas limit in your miner to whatever you want --targetgaslimit and mine till it gets there.