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README.md
README.md
 
 
expend_gas.sol
expend_gas.sol
 
 
gas-refunds.js
gas-refunds.js
 
 
package.json
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README.md

Throttling through partial excess gas refunds

The way that gas refunds work when excess gas is sent to a transaction work in a different manner on Hedera compared to other EVM-compatible networks.

  • In Ethereum, gasUsed = gasConsumed; therefore gasLimit is not used as an input in determining the value of gasConsumed.
  • In Hedera, on the other hand, gasUsed = max(gasConsumed, gasLimit * 0.8); therefore the gasLimit is used as an input in determining the value of gasConsumed.

This is done to throttle over-reservation of gas, by penalising that behaviour. The full explanation for this is in HIP-185.

Therefore, this incentivises the users submitting transactions to specify the "correct" amount of gas. Since the exact value of gas consumed cannot be known ahead of time, the recommended/ usual way to accomplish this is by making an eth_estimateGas RPC request.

  • When doing this on Hedera, however, the value returned is way too large
  • In fact, it turns out that the RPC relay returns static values instead of performing calculations
  • This shortcoming has been detailed in HIP-584, and has an upcoming implementation

Code

In the expend_gas.sol file in this directory, we have the smart contract used for demo purposes by the script. This has been deployed at Hedera Testnet at 0x9C58D0159495F7a8853A24574f2B8F348a72424c, and this is what the script interacts with.

In the gas-refunds.js file in this directory, we have a script that uses ethers.js to interact with the smart contract. It invoked the same function 3 times in a row, with the exact same arguments. In fact, the only difference is the gasLimit value.

  • 1st time: (equal to) gasLimit is 100% of the value of gasEstimate.
  • 2nd time: (small excess) gasLimit is 110% of the value of gasEstimate.
  • 2nd time: (tiny fraction) gasLimit is 6.4% of the value of gasEstimate.

The difference in the gasUsed values between the 1st time and the 2nd time (small excess scenario) demonstrates the implementation of HIP-185 in play.

The difference in the gasUsed values between the 1st time and the 3rd time (tiny fraction scenario) demonstrates the current lack of implementation for HIP-584.

References