# EIP 2200: 结构化定义 净消耗Gas测算
作者 | 讨论 | 状态 | 类型 | 分类 | 创建时间 |
---|---|---|---|---|---|
Wei Tang | issues | Draft | Standards Track | Core | 2019-07-18 |
# Simple Summary
This is an EIP that implements net gas metering. It's a combined version of EIP-1283 and EIP-1706, with a structured definition so as to make it interoperable with other gas changes such as EIP-1884.
# Abstract
This EIP provides a structured definition of net gas metering changes
for SSTORE
opcode, enabling new usages for contract storage, and
reducing excessive gas costs where it doesn’t match how most
implementation works.
This is a combination of EIP-1283 and EIP-1706.
# Motivation
This EIP proposes a way for gas metering on SSTORE
, using information
that is more universally available to most implementations, and
require as little change in implementation structures as possible.
- Storage slot’s original value.
- Storage slot’s current value.
- Refund counter.
Usages that benefits from this EIP’s gas reduction scheme includes:
- Subsequent storage write operations within the same call frame. This includes reentry locks, same-contract multi-send, etc.
- Exchange storage information between sub call frame and parent call frame, where this information does not need to be persistent outside of a transaction. This includes sub-frame error codes and message passing, etc.
The original definition of EIP-1283 created a danger of a new kind of
reentrancy attacks on existing contracts as Solidity by default grants
a "stipend" of 2300 gas to simple transfer calls. This danger is
easily mitigated if SSTORE
is not allowed in low gasleft state,
without breaking the backward compatibility and the original intention
of EIP-1283.
This EIP also replaces the original EIP-1283 value definitions of gas by parameters, so that it's more structured, and easier to define changes in the future.
# Specification
Define variables SLOAD_GAS
, SSTORE_SET_GAS
, SSTORE_RESET_GAS
and
SSTORE_CLEARS_SCHEDULE
. The old and new values for those variables
are:
SLOAD_GAS
: changed from200
to800
.SSTORE_SET_GAS
:20000
, not changed.SSTORE_RESET_GAS
:5000
, not changed.SSTORE_CLEARS_SCHEDULE
:15000
, not changed.
Change the definition of EIP-1283 using those variables. The new specification, combining EIP-1283 and EIP-1706, will look like below. The terms original value, current value and new value are defined in EIP-1283.
Replace SSTORE
opcode gas cost calculation (including refunds) with
the following logic:
- If gasleft is less than or equal to gas stipend, fail the current call frame with 'out of gas' exception.
- If current value equals new value (this is a no-op),
SLOAD_GAS
is deducted. - If current value does not equal new value
- If original value equals current value (this storage slot has
not been changed by the current execution context)
- If original value is 0,
SSTORE_SET_GAS
is deducted. - Otherwise,
SSTORE_RESET_GAS
gas is deducted. If new value is 0, addSSTORE_CLEARS_SCHEDULE
gas to refund counter.
- If original value is 0,
- If original value does not equal current value (this storage
slot is dirty),
SLOAD_GAS
gas is deducted. Apply both of the following clauses.- If original value is not 0
- If current value is 0 (also means that new value is not
0), remove
SSTORE_CLEARS_SCHEDULE
gas from refund counter. - If new value is 0 (also means that current value is not
0), add
SSTORE_CLEARS_SCHEDULE
gas to refund counter.
- If current value is 0 (also means that new value is not
0), remove
- If original value equals new value (this storage slot is
reset)
- If original value is 0, add
SSTORE_SET_GAS - SLOAD_GAS
to refund counter. - Otherwise, add
SSTORE_RESET_GAS - SLOAD_GAS
gas to refund counter.
- If original value is 0, add
- If original value is not 0
- If original value equals current value (this storage slot has
not been changed by the current execution context)
An implementation should also note that with the above definition, if the implementation uses call-frame refund counter, the counter can go negative. If the implementation uses transaction-wise refund counter, the counter always stays positive.
# Rationale
This EIP mostly achieves what a transient storage tries to do (EIP-1087 and EIP-1153), but without the complexity of introducing the concept of "dirty maps", or an extra storage struct.
- We don't suffer from the optimization limitation of EIP-1087. EIP-1087 requires keeping a dirty map for storage changes, and implicitly makes the assumption that a transaction's storage changes are committed to the storage trie at the end of a transaction. This works well for some implementations, but not for others. After EIP-658, an efficient storage cache implementation would probably use an in-memory trie (without RLP encoding/decoding) or other immutable data structures to keep track of storage changes, and only commit changes at the end of a block. For them, it is possible to know a storage's original value and current value, but it is not possible to iterate over all storage changes without incurring additional memory or processing costs.
- It never costs more gas compared with the current scheme.
- It covers all usages for a transient storage. Clients that are easy to implement EIP-1087 will also be easy to implement this specification. Some other clients might require a little bit extra refactoring on this. Nonetheless, no extra memory or processing cost is needed on runtime.
Regarding SSTORE
gas cost and refunds, see Appendix for proofs of
properties that this EIP satisfies.
- For absolute gas used (that is, actual gas used minus refund), this EIP is equivalent to EIP-1087 for all cases.
- For one particular case, where a storage slot is changed, reset to its original value, and then changed again, EIP-1283 would move more gases to refund counter compared with EIP-1087.
Examine examples provided in EIP-1087's Motivation (with SLOAD_GAS
being
200
):
- If a contract with empty storage sets slot 0 to 1, then back to 0,
it will be charged
20000 + 200 - 19800 = 400
gas. - A contract with empty storage that increments slot 0 5 times will be
charged
20000 + 5 * 200 = 21000
gas. - A balance transfer from account A to account B followed by a
transfer from B to C, with all accounts having nonzero starting and
ending balances, it will cost
5000 * 3 + 200 - 4800 = 10400
gas.
In order to keep in place the implicit reentrancy protection of existing contracts, transactions should not be allowed to modify state if the remaining gas is lower then the gas stipend given to "transfer"/"send" in Solidity. These are other proposed remediations and objections to implementing them:
- Drop EIP-1283 and abstain from modifying
SSTORE
cost- EIP-1283 is an important update
- It was accepted and implemented on test networks and in clients.
- Add a new call context that permits LOG opcodes but not changes to state.
- Adds another call type beyond existing regular/staticcall
- Raise the cost of
SSTORE
to dirty slots to >=2300 gas- Makes net gas metering much less useful.
- Reduce the gas stipend
- Makes the stipend almost useless.
- Increase the cost of writes to dirty slots back to 5000 gas, but add
4800 gas to the refund counter
- Still doesn’t make the invariant explicit.
- Requires callers to supply more gas, just to have it refunded
- Add contract metadata specifying per-contract EVM version, and only
apply
SSTORE
changes to contracts deployed with the new version.
# Backwards Compatibility
This EIP requires a hard fork to implement. No gas cost increase is anticipated, and many contracts will see gas reduction.
Performing SSTORE
has never been possible with less than 5000 gas, so
it does not introduce incompatibility to the Ethereum mainnet. Gas
estimation should account for this requirement.
# Test Cases
Code | Used Gas | Refund | Original | 1st | 2nd | 3rd |
---|---|---|---|---|---|---|
0x60006000556000600055 | 1612 | 0 | 0 | 0 | 0 | |
0x60006000556001600055 | 20812 | 0 | 0 | 0 | 1 | |
0x60016000556000600055 | 20812 | 19200 | 0 | 1 | 0 | |
0x60016000556002600055 | 20812 | 0 | 0 | 1 | 2 | |
0x60016000556001600055 | 20812 | 0 | 0 | 1 | 1 | |
0x60006000556000600055 | 5812 | 15000 | 1 | 0 | 0 | |
0x60006000556001600055 | 5812 | 4200 | 1 | 0 | 1 | |
0x60006000556002600055 | 5812 | 0 | 1 | 0 | 2 | |
0x60026000556000600055 | 5812 | 15000 | 1 | 2 | 0 | |
0x60026000556003600055 | 5812 | 0 | 1 | 2 | 3 | |
0x60026000556001600055 | 5812 | 4200 | 1 | 2 | 1 | |
0x60026000556002600055 | 5812 | 0 | 1 | 2 | 2 | |
0x60016000556000600055 | 5812 | 15000 | 1 | 1 | 0 | |
0x60016000556002600055 | 5812 | 0 | 1 | 1 | 2 | |
0x60016000556001600055 | 1612 | 0 | 1 | 1 | 1 | |
0x600160005560006000556001600055 | 40818 | 19200 | 0 | 1 | 0 | 1 |
0x600060005560016000556000600055 | 10818 | 19200 | 1 | 0 | 1 | 0 |
# Implementation
To be added.
# Appendix: Proof
Because the storage slot's original value is defined as the value
when a reversion happens on the current transaction, it's easy to
see that call frames won't interfere SSTORE
gas calculation. So
although the below proof is discussed without call frames, it applies
to all situations with call frames. We will discuss the case
separately for original value being zero and not zero, and use
induction to prove some properties of SSTORE
gas cost.
Final value is the value of a particular storage slot at the end of
a transaction. Absolute gas used is the absolute value of gas used
minus refund. We use N
to represent the total number of SSTORE
operations on a storage slot. For states discussed below, refer to
State Transition in Explanation section.
Below we do the proof under the assumption that all parameters are
unchanged, meaning SLOAD_GAS
is 200
. However, note that the proof
still applies no matter how SLOAD_GAS
is changed.
# Original Value Being Zero
When original value is 0, we want to prove that:
- Case I: If the final value ends up still being 0, we want to charge
200 * N
gases, because no disk write is needed. - Case II: If the final value ends up being a non-zero value, we want to
charge
20000 + 200 * (N-1)
gas, because it requires writing this slot to disk.
# Base Case
We always start at state A. The first SSTORE
can:
- Go to state A: 200 gas is deducted. We satisfy Case I because
200 * N == 200 * 1
. - Go to state B: 20000 gas is deducted. We satisfy Case II because
20000 + 200 * (N-1) == 20000 + 200 * 0
.
# Inductive Step
- From A to A. The previous gas cost is
200 * (N-1)
. The current gas cost is200 + 200 * (N-1)
. It satisfy Case I. - From A to B. The previous gas cost is
200 * (N-1)
. The current gas cost is20000 + 200 * (N-1)
. It satisfy Case II. - From B to B. The previous gas cost is
20000 + 200 * (N-2)
. The current gas cost is200 + 20000 + 200 * (N-2)
. It satisfy Case II. - From B to A. The previous gas cost is
20000 + 200 * (N-2)
. The current gas cost is200 - 19800 + 20000 + 200 * (N-2)
. It satisfy Case I.
# Original Value Not Being Zero
When original value is not 0, we want to prove that:
- Case I: If the final value ends up unchanged, we want to
charge
200 * N
gases, because no disk write is needed. - Case II: If the final value ends up being zero, we want to
charge
5000 - 15000 + 200 * (N-1)
gas. Note that15000
is the refund in actual definition. - Case III: If the final value ends up being a changed non-zero
value, we want to charge
5000 + 200 * (N-1)
gas.
# Base Case
We always start at state X. The first SSTORE
can:
- Go to state X: 200 gas is deducted. We satisfy Case I because
200 * N == 200 * 1
. - Go to state Y: 5000 gas is deducted. We satisfy Case III because
5000 + 200 * (N-1) == 5000 + 200 * 0
. - Go to state Z: The absolute gas used is
5000 - 15000
where 15000 is the refund. We satisfy Case II because5000 - 15000 + 200 * (N-1) == 5000 - 15000 + 200 * 0
.
# Inductive Step
- From X to X. The previous gas cost is
200 * (N-1)
. The current gas cost is200 + 200 * (N-1)
. It satisfy Case I. - From X to Y. The previous gas cost is
200 * (N-1)
. The current gas cost is5000 + 200 * (N-1)
. It satisfy Case III. - From X to Z. The previous gas cost is
200 * (N-1)
. The current absolute gas cost is5000 - 15000 + 200 * (N-1)
. It satisfy Case II. - From Y to X. The previous gas cost is
5000 + 200 * (N-2)
. The absolute current gas cost is200 - 4800 + 5000 + 200 * (N-2)
. It satisfy Case I. - From Y to Y. The previous gas cost is
5000 + 200 * (N-2)
. The current gas cost is200 + 5000 + 200 * (N-2)
. It satisfy Case III. - From Y to Z. The previous gas cost is
5000 + 200 * (N-2)
. The current absolute gas cost is200 - 15000 + 5000 + 200 * (N-2)
. It satisfy Case II. - From Z to X. The previous gas cost is
5000 - 15000 + 200 * (N-2)
. The current absolute gas cost is200 + 10200 + 5000 - 15000 + 200 * (N-2)
. It satisfy Case I. - From Z to Y. The previous gas cost is
5000 - 15000 + 200 * (N-2)
. The current absolute gas cost is200 + 15000 + 5000 - 15000 + 200 * (N-2)
. It satisfy Case III. - From Z to Z. The previous gas cost is
5000 - 15000 + 200 * (N-2)
. The current absolute gas cost is200 + 5000 - 15000 + 200 * (N-2)
. It satisfy Case II.
# Copyright
Copyright and related rights waived via CC0.