Conditional CU metering

proposals/simd-0182-conditional-cu-metering.md

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+---
+simd: '0182'
+title: Consume requested CUs for sBPF failures
+authors:
+  - Tao Zhu (Anza)
+category: Standard
+type: Core
+status: Review 
+created: 2024-10-03
+feature:
+supersedes:
+superseded-by:
+extends:
+---
+
+## Summary
+
+Adjusting how CU consumption is measured based on the conditions of transaction
+execution: successful completion will consume actual CUs, but certain irregular
+failures in the sBPF VM will result in the transaction automatically consuming
+all requested CUs.
+
+## Motivation
+
+In the Solana protocol, tracking transaction Compute Unit (CU) consumption is a
+critical aspect of maintaining consensus. Block costs are part of this
+consensus, meaning that all clients must agree on the execution cost of each
+transaction, including those that error out during execution. Ensuring
+consistency in CU tracking across clients is essential for maintaining protocol
+integrity.
+
+To improve performance, Solana programs are often compiled with a JIT that works
+at the level of Basic Blocks — linear sequences of sBPF instructions with a
+single entry and exit point, and no loops or branches. Basic Blocks allow for
+efficient execution by reducing the overhead associated with tracking CU
+consumption for each individual sBPF instruction.
+
+Other than in less common situations discussed below, the total CU
+consumption for a Basic Block is deterministic and CU accounting can be
+done once per basic block instead of at each instruction. A transaction
+completing successfully or with most errors implies that execution exited each
+basic block at its single exit point, and thus that the total CU consumption of
+the execution is equal to the sum of the CU cost of each Basic Block executed.
+
+However, when an exception is thrown during the execution of a Basic Block
+(e.g., a null memory dereference or other faults), determining the exact number
+of CUs consumed up to the point of failure requires additional effort. For
+instance, the Agave client implements a mechanism that tracks the Instruction
+Pointer (IP) or Program Counter (PC) to backtrack and estimate the CUs consumed
+when an exception occurs. More details on this mechanism can be found
+[here](https://github.com/solana-labs/rbpf/blob/57139e9e1fca4f01155f7d99bc55cdcc25b0bc04/src/jit.rs#L267).
+
+While this approach is effective, it introduces additional work and complexity.
+These mechanisms are often implementation-specific, and requiring all clients to
+track the exact number of executed sBPF instructions for consensus is costly and
+unnecessary. Such precision is not essential for protocol-level consensus,
+especially since these cases are infrequent.
+
+Instead of mandating implementation-specific work to handle exceptions, we
+propose the following clarification in the protocol:
+
+- For successful execution of a Basic Block (i.e., the block exits at the last
+ sBPF instruction), the deterministic CU cost of the block will be charged to
+the transaction’s CU meter. This ensures that CU consumption for successful
+transactions is accurately accounted for.
+- In the event of irregular failure, where execution aborts from the middle of
+basic block, the requested CUs for the transaction will be charged to the CU
+meter. This allows for a simple and efficient fallback mechanism that avoids the
+need for tracking the exact number of executed instructions up to the point of
+failure.
+
+By adopting this approach, the protocol avoids the overhead of requiring precise
+instruction-level CU tracking for transactions that fail. Instead, the requested
+CU limit of the transaction will be used, simplifying the handling of
+irregularly failed transactions while still maintaining consensus.
+
+## Alternatives Considered
+
+None
+
+## New Terminology
+
+- [Basic Block](https://en.wikipedia.org/wiki/Basic_block): In the context of
+  JIT execution and BPF processing, a Basic Block is a sequence of BPF
+instructions that forms a single, linear flow of control with no loops or
+conditional branches except for the entry and exit points. It represents a
+segment of code where execution starts at the first instruction and proceeds
+sequentially through to the last instruction without deviation. The Basic Block
+is characterized by its predictable execution path, allowing for efficient
+budget checks and optimizations, as its Compute Unit (CU) cost can be determined
+before execution and verified at the end of the block.
+
+- Irregular transaction failure: A rare case that a Transaction execution aborts
+in the middle of executing basic block, results in consuming all requested CUs.
+
+## Detailed Design
+
+If VM execution returns any error except `SyscallError`, transaction's CU meter
+should be fully depleted, in another words, all requested CUs are consumed;
+otherwise consumes the actual executed CUs.
+
+## Impact
+
+None
+
+## Security Considerations
+
+None