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Fix expected surplus in sequencer #4038

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Fix expected surplus in sequencer #4038

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6cb625f
Fix expected surplus in sequencer
alexandrosfilios Nov 20, 2025
d332244
Merge branch 'master' into afilios/fix-expected-surplus
pmikolajczyk41 Dec 19, 2025
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1 change: 1 addition & 0 deletions execution/gethexec/sequencer.go
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Original file line number Diff line number Diff line change
Expand Up @@ -1565,6 +1565,7 @@ func (s *Sequencer) updateExpectedSurplus(ctx context.Context) (int64, error) {
} else {
// l1GasPrice can be zero because of roundings, hence backlogCost is calculated separately
backlogFee := big.NewInt(backlogCallDataUnits)
backlogFee.Mul(backlogFee, blobFeePerByte)
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@MishkaRogachev MishkaRogachev Nov 20, 2025

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I don't have enough context to understand this line change. Could you please add a brief description?

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@tsahee tsahee Nov 26, 2025

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now I look at the entire computation and not sure I get it.

can you add a comment with the full computation for computing backlog cost here?

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@alexandrosfilios alexandrosfilios Dec 1, 2025

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Sure! We want to capture two values: l1GasPrice and backlogCost. This is the code we had initially:

			blobFeePerByte, _ := s.parentChain.BlobFeePerByte(ctx, header)
			blobFeePerByte.Mul(blobFeePerByte, blobTxBlobGasPerBlob)
			blobFeePerByte.Div(blobFeePerByte, usableBytesInBlob)
			l1GasPrice = blobFeePerByte.Int64() / 16
			backlogCost = (backlogCallDataUnits * blobFeePerByte.Int64()) / 16

This means:

l1GasPrice = (blobFeePerByte * blobTxBlobGasPerBlob) / (usableBytesInBlob * 16)
backlogCost = backlogCallDataUnits * (blobFeePerByte * blobTxBlobGasPerBlob) / (usableBytesInBlob * 16)

The problem here was that we divide with usableBytesInBlob too early and because of rounding, the value of blobFeePerByte becomes zero. Then even if we multiply with backlogCallDataUnits, it will still stay zero. The solution here was to multiply with backlogCallDataUnits before we divide. The problem here was that I forgot a line that multiplied with blobFeePerByte (the intention was to use it there as, well, this is why I had even extracted it outside of the if else).

backlogFee.Mul(backlogFee, blobTxBlobGasPerBlob)
backlogFee.Div(backlogFee, usableBytesInBlob)
backlogCost = backlogFee.Int64() / 16
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