FIP-0032: Gas model adjustment for non-programmable FVM

FIPS/fip-0032.md

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+---
+fip: "0032"
+title: Gas model adjustment for non-programmable FVM
+author: Raúl Kripalani (@raulkr), Steven Allen (@stebalien)
+discussions-to: https://github.com/filecoin-project/FIPs/issues/204
+status: Draft
+type: Technical
+category: Core
+created: 2022-03-08
+spec-sections:
+  - TBD
+requires:
+  - FIP-0030
+  - FIP-0031
+replaces: N/A
+---
+
+# FIP-0032: Gas model adjustment for non-programmable FVM
+
+## Simple Summary
+
+This FIP is part of a two-FIP series that adjust the gas model of the Filecoin
+blockchain in preparation of user-programmable actors. This FIP focuses on
+attaining high-fidelity gas accounting under the Wasm runtime that built-in
+system actors will migrate to as part of FIP-0030 and FIP-0031. It does not
+cover upcoming system features like actor deployment, actor loading, memory
+management, and more. Those will be addressed in a subsequent FIP that focuses
+on gas model changes to support user-programmability.
+
+## Abstract
+
+With the arrival of the Filecoin Virtual Machine, the Filecoin network will gain
+the capability to run untrusted code in the form of user-defined actors.
+However, the current gas model is designed strictly for predefined, trusted
+code. In order to maintain network security guarantees, the gas model needs to
+evolve to accurately charge for computational instructions and resource usage of
+actor logic.
+
+This FIP expands the current gas model by introducing a new gas category to
+account for the real compute cost of the logic performed by every actor in the
+system. It also rearchitects IPLD state management gas, and adjusts the actor
+call fee to account for Wasm invocation container mechanics.
+
+This is the first FIP out of a two-FIP series to revise the gas model as a
+result of the introduction of the Filecoin Virtual Machine in FIP-0030 and
+FIP-0031. This FIP should accompany those FIPs under the same network upgrade.
+
+## Change Motivation
+
+Gas is an essential mechanism to (a) compensate the network for the computation
+performed during transaction execution, and to (b) restrict the compute capacity
+of blocks to achieve a designated epoch time (30 seconds).
+
+Today, Filecoin determines the amount of gas a transaction consumes by
+intercepting specific operations and actions, and applying gas explicit gas
+charges on them. Concretely:
+
+- On including a chain message, variable on the length of the message.
+- On returning a value, variable on the length of returned payload.
+- On performing a call, dependent on the type of call (simple value transfer, or
+  other).
+- On performing an IPLD store get, fixed amount.
+- On performing an IPLD store put, variable on block payload size.
+- On actor creation, fixed amount.
+- On actor deletion, fixed amount.
+- On cryptographic operations:
+    - On signature verification, variable on signature type and plaintext
+      length.
+    - On hashing, fixed price.
+    - On computing the CID of an unsealed sector, variable on proof type and
+      piece characteristics.
+    - On verifying a seal, variable on the seal characteristics.
+    - On verifying a window PoSt, variable on the PoSt characteristics.
+    - On verifying a consensus fault.
+
+None of these gas charges accounts for the costs of the execution of the actor’s
+compute logic itself. This results in an incomplete resource accounting model.
+So far, the network has coped without it because:
+
+1. The cost of built-in actor logic is negligible when compared to the cost of
+   the above operations, most of which are triggered through syscalls.
+2. Only pre-determined, trusted logic runs on chain in the form of built-in
+   actors.
+3. Technical limitations. Prior to the FVM (FIP-0030, FIP-0031), there was no
+   portable actor bytecode whose execution could be metered identically across
+   all implementations.
+
+With the ability to deploy user-defined actors on the FVM, these assumptions no
+longer stand and it becomes important to start tracking real execution costs
+with the highest fidelity possible, to preserve security guarantees, and to
+compensate the network for the logic it executes.
+
+## Specification
+
+Here’s an overview of proposed changes to the existing gas model. We explain
+each one in detail in the following sections.
+
+- Introduce Wasm bytecode-metered execution gas.
+- Adjust the *actor call* fee, to account for new call dispatch overhead.
+- Rearchitect the IPLD state IO fees.
+
+### Wasm bytecode-metered execution gas
+
+We introduce a new category of gas. Execution gas is charged per Wasm
+instruction. Every Wasm instruction has a predefined amount of execution units
+associated with it.
+
+Every Wasm instruction expends a flat 1 execution unit, except the following
+structural Wasm instructions: `nop`, `drop`, `block`, `loop`, `unreachable`,
+`return`, `else`  and `end` . These instructions expend 0 execution units
+because either they do not translate into machine instructions by themselves, or
+they are control flow instructions with no logic to evaluate.
+
+**Pricing formula**
+
+The conversion of Wasm execution units to Filecoin gas units is at a predefined
+ratio of `<<TODO: currently benchmarking>>`. This equivalence has been
+calculated by performing extensive benchmarks, with the goal of honouring the
+invariant baseline target of 10 gas units/ns of wall clock time[^1].
+
+Both the pricing policy for Wasm instructions and the conversion rate need to be
+assessed and revised continuously as technology improves (e.g. Wasm compilers
+and processors). The ongoing goal is to sustain high execution fidelity.
+
+### Call gas fee
+
+In Filecoin network version 15 and earlier, the call gas fee is higher for value
+transfers than it is for actor code invocations. This may sound
+counterintuitive, but it follows the fact that the call fee has to cover the
+full validation and state update cost of the transaction, because these
+transactions don’t invoke actor code. Conversely, the call fee for an actor
+invocation represents only the dispatch effort, hence why it’s lower.
+
+With FVM actors, the call cost structure changes. For actor code invocations, a
+Wasm-based invocation container needs to be instantiated, and parameters need to
+be serialized and inserted into the IPLD blockstore. In other words, there is
+more work to do that than previously, and therefore the fee is adjusted upward.
+
+**Pricing formula**
+
+```
+TODO: benchmarking
+```
+
+### IPLD state management fees
+
+Currently, IPLD block read incur a fixed fee, and IPLD block writes incur a
+variable fee, dependent on the number of bytes written.
+
+With the FVM, IPLD state I/O is managed through five syscalls, with the
+specified overhead and pricing:
+
+- `block_open`: looks up the supplied CID, loads the block from the state
+  blockstore into the FVM kernel’s block cache, reports its size and codec. It
+  returns a numeric block handle.
+    - Overhead: This syscall traverses the FVM<>client boundary via an extern in
+      order to query the state blockstore. This leads to disk IO with associated
+      read amplification. The bytes returned through the extern are copied into
+      the FVM’s memory so that the buffer can be reclaimed by the client side;
+      thus there is an alloc and memcpy cost. The bytes are retained in memory
+      until the actor call finishes and the invocation container is dropped, and
+      thus the actor is charged for this memory usage.
+    - Cost: `<<TODO: benchmarking>>`.
+- `block_read`: takes a block handle and an output buffer, and copies the
+  block’s data from the FVM kernel’s block cache into the actor’s memory.
+    - Overhead: This syscall resolves inside the FVM and doesn’t traverse the
+      FVM<>node boundary. The overhead is that of a memcpy operation.
+    - Cost: `<<TODO: benchmarking>>`.
+- `block_write`: takes some block data and a codec, and merely copies it to the
+  FVM kernel’s block cache. It returns a numeric block handle.
+    - Overhead: This syscall resolves inside the FVM and doesn’t traverse the
+      FVM<>node boundary. It involves a memcpy to a buffer that is retained
+      until the actor call finishes. The bytes are retained in memory until the
+      actor call finishes and the invocation container is dropped, and thus the
+      actor is charged for this memory usage.
+    - Cost: `<<TODO: benchmarking>>`.
+- `block_link`: computes the CID of a block, and commits the block to the state
+  blockstore.
+    - Overhead: This syscall traverses the FVM<>client boundary via an extern in
+      order to write to the state blockstore (although this is deferred until
+      the machine flush). This leads to disk IO with associated write
+      amplification. The bytes are copied over to the client’s memory, thus
+      there is an alloc and memcpy cost. Computing the CID involves a hashing
+      operation (akin to the hash syscall, but without the syscall overhead).
+      Because the CID is returned to the actor via an output buffer, there is
+      another memcpy involved.
+    - Cost: `<<TODO: benchmarking>>`.
+- `block_stat`: returns the length and the codec of a block.
+    - Overhead: This syscall resolves inside the FVM and doesn’t traverse the
+      FVM<>node boundary. It performs a map lookup and returns an integer tuple.
+    - Cost: `<<TODO: benchmarking>>`.
+
+## Design Rationale
+
+To simplify community discussion and align with the FVM deployment roadmap, all
+changes related to new system features related to user-programmable actors are
+kept out of scope of this FIP. This set of changes covers only the technological
+transition to a Wasm runtime for built-in actors. This FIP will be followed by
+another FIP introducing the former.
+
+Concerning the design of each individual change, the specification section
+elaborates on overhead that is being accounted for and the variables that make
+up every pricing formula.
+
+## Backwards Compatibility
+
+The data model by which gas allowance and usage is represented in external
+interfaces (gas limit, gas used, etc.) remains unchanged. However, the same
+transaction made before and after this FIP will, with very high likelihood,
+result in different values for *gas used*. Transactions sitting in the mempool
+at the activation epoch of this FIP will need to be re-estimated and replaced.
+
+## Test Cases
+
+Testing for this FIP involves two efforts:
+
+1. An updated corpus of FVM-compatible [test
+   vectors](https://github.com/filecoin-project/fvm-test-vectors) with the new
+   gas model.
+2. The testnet plan laid out in FIP-0031.
+
+## Security Considerations
+
+TODO draft.
+
+## Incentive Considerations
+
+With the introduction of execution gas to account for real execution cost, actor
+compute logic will no longer be exempted from gas charges, thus resulting in a
+net increase in gas expenditure in the network.
+
+## Product Considerations
+
+No product considerations apply.
+
+## Implementation
+
+For clients relying on the reference FVM implementation
+([filecoin-project/ref-fvm](https://github.com/filecoin-project/ref-fvm)), the
+implementation of this FIP will be transparent, as it is self-contained inside
+the FVM.
+
+## Copyright
+
+Copyright and related rights waived via
+[CC0](https://creativecommons.org/publicdomain/zero/1.0/).
+
+## Footnotes
+
+[^1]: The baseline target itself determined by the need to sustain a 30 second
+    epoch frequency, with a 6 second block propagation time, under these
+    circumstances, which represent the absolute worst possible scenario:
+      - Block gas limit of 15B gas units.
+      - Attaining the maximum winrate at an epoch (11).
+      - No duplicated messages across blocks in a tipset.
+      - 100% saturation of every block with compute.
+
+    The choice of 10 gas units/ns as a baseline implies a block validation time
+    of 1.5s and a fully saturated tipset validation time of 16.5s (given that
+    message and block execution cannot be parallelized).