Fastpath

Fastpath is a term used in computing to describe a specialized sequence of instructions designed to execute a common operation as quickly as possible. It denotes a fastpath alongside a slow path; the slow path handles less frequent cases, errors, or complex validations. The fastpath aims to minimize latency and resource usage by reducing checks, branching, or overhead when input or environmental conditions meet predefined preconditions.

Implementation typically relies on techniques such as inlining, branchless code, precomputed values, caching, or hardware acceleration. When the fastpath preconditions are not met, execution is diverted to the slower but more robust code paths. This separation helps systems deliver low-latency results for routine tasks while preserving correctness for more complex scenarios.

Contexts where fastpath concepts are applied include database engines, where a fastpath may handle simple lookups or prepared statements; networking software, where fastpath mechanisms process packets using hardware offloads or dedicated data planes; and libraries or APIs, which may expose fastpath shortcuts for common operations. In operating systems and drivers, fastpath ideas appear in optimized I/O and interprocess communication paths to reduce overhead in the common case.

Considerations when designing a fastpath include balancing speed with correctness, ensuring the preconditions are well-defined and testable, and monitoring performance to avoid regressions. Maintainability can be impacted by having separate code paths, so clear documentation and thorough testing are important.