v2.5.4-beta.2 - Gas Exactimation 📏
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David Murdoch
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David Murdoch
Highlights
How to Upgrade
Changelog
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A long-standing issue with Ganache has been the fact that we haven't returned EIP-114 compliant gas estimations, AKA the "1/64ths rule". This caused our gas estimates to be too low in cases where a transaction executed opcodes that are subject to this EIP. This in turn was the cause of many frustrations, especially in situations where tools would use gas estimations in transactions without allowing the user to intercept or change the transaction's supplied gas.
Highlights
EIP-114 mandates that certain stackdepth-creating opcodes withhold 1/64th of remaining gas from the stack they create. This has two non-obvious effects:
- The gas required for a successful transaction can be greater than the actual gas spent. This is similar to how gas refunds behave.
- The extra gas required for a successful transaction varies depending on the transaction's initial
gasamount!
Let's say we want to run the following example transaction (note: this transaction is nonsensical and is intended only to illustrate the 1/64ths rule):
| DEPTH | OPCODE | OP FEE |
|---|---|---|
| 0 | PUSH1 | 3 |
| 0 | CALL | 700 |
| 1 | ADD | 3 |
| 1 | EXPENSIVE | 10000 |
| 1 | RETURN | 0 |
| 0 | POP | 2 |
| 0 | STOP | 0 |
| Total Gas: | 10708 |
This transaction would cost exactly 10708 gas. However, if you attempted to run this transaction by supplying only 10708 gas it would fail with an "out of gas" exception when it gets to the EXPENSIVE opcode!
The reason for this failure is that the opcode CALL is subject to EIP-114. Here's a breakdown of how this gas is used:
| DEPTH | OPCODE | GAS AVAILABLE | OP FEE | 1⁄64 WITHHELD |
|---|---|---|---|---|
| 0 | PUSH1 | 10708 | 3 | |
| 0 | CALL | 10705 | 700 | 156 |
| 1 | ADD | 9849 | 3 | |
| 1 | EXPENSIVE | 9846 | 10000 | |
| REVERT | OOG |
Instead of 10005 gas available (10705 gas available − 700 op fee) at the ADD opcode,
only 9849 is actually available due to the "withheld" 1/64th: 10005 − FLOOR( 10005 ⁄ 64 ) = 9849; we are now 154 gas short!
All we need to do now is increase our supplied gas by 154, right? Let's see what happens now:
| DEPTH | OPCODE | GAS AVAILABLE | OP FEE | 1⁄64 WITHHELD |
|---|---|---|---|---|
| 0 | PUSH1 | 10862 | 3 | |
| 0 | CALL | 10859 | 700 | 158 |
| 1 | ADD | 10001 | 3 | |
| 1 | EXPENSIVE | 9998 | 10000 | |
| REVERT | OOG |
We still don't have enough gas! The amount withheld increased by 2 gas because the gas supplied to the transaction increased as well!
Because the 1/64ths rule gets applied to the gas remaining, which has increased since our first estimate, 2 more gas gets withheld, resulting in an OOG error!
Now, let's run the same transaction one more time, this time using the result of our new gas estimation algorithm:
| DEPTH | OPCODE | GAS AVAILABLE | OP FEE | 1⁄64 WITHHELD |
|---|---|---|---|---|
| 0 | PUSH1 | 10864 | 3 | |
| 0 | CALL | 10861 | 700 | 158 |
| 1 | ADD | 10003 | 3 | |
| 1 | EXPENSIVE | 10000 | 10000 | |
| 1 | RETURN | 0 | 0 | −158 |
| 0 | POP | 158 | 2 | |
| 0 | STOP | 156 | 0 | |
| 156 |
It works! Notice that 156 gas was unspent at the end of the transaction, but had we provided even 1 less gas the transaction would have resulted in an "out of gas" exception at EXPENSIVE opcode.
This series of simplified examples attempts to provide a high-level overview of the 1/64th gas estimation calculation. To correctly estimate gas on any contract, we must consider how the gas withheld at any nested stack depth/frame affects the gas needed outside of its execution context.
It's a tricky problem to solve. Some implementations of gas estimation after EIP-114 use interval halving (binary search) by running the transaction through the EVM until the gas estimation converges. This seemed like an unnecessarily CPU-intensive approach to the problem, and we set out to find a (theoretically) more performant, and perfectly accurate, way of estimating gas (which we now refer to as "gas exactimation" here at Truffle).
Our new gas estimation algorithm is being released as a beta in order to get real-world metrics on its accuracy as well as to prove some assumptions we've made about the EVM behavior. It's possible the algorithm still has crash-inducing bugs or could even return estimates that are too low.
If you find that any of your transactions are running in to unexpected OOG errors please let us know by filing an issue.
We are especially proud of this algorithm, and we are even more proud of the brilliant minds that went into its creation. Thank you Nick Paterno (@nicholasjpaterno), Amal Sudama (@cds-amal), Chris Cowell (@ccowell), Mike Seese (@seesemichaelj), Benjamin Burns (@benjamincburns), and David Murdoch (@davidmurdoch) for all the work you've put in to this release!
We'll follow up with a detailed blog post on the algorithm itself in the coming weeks.
How to Upgrade
Upgrade to the beta version of ganache-core by running:
npm
npm uninstall ganache-core
npm install ganache-core@betayarn
yarn remove ganache-core
yarn add ganache-core@betaChangelog
Bug Fixes
- Gas Estimation (#292) – @nicholasjpaterno
Documentation
- Reformatted and rewrote our documentation (#348) – @nicholasjpaterno
Related Releases
💖 The Truffle Team