Design Notes#

Performance Impact of Boxing Futures#

future_form uses BoxFuture and LocalBoxFuture under the hood. This section discusses the performance implications of that choice.

Costs#

Allocation

• One heap allocation per boxed future (typically small, but adds up in hot paths)
• Deallocation when the future completes

Indirection

• Virtual dispatch through the dyn Future trait object on each poll
• Pointer chasing can hurt CPU cache locality

In Practice#

The impact is often negligible because:

1. Futures are usually polled relatively few times compared to the work they represent
2. A single network call or disk read dwarfs the cost of one allocation
3. The allocator is highly optimized for small, short-lived allocations

When It Matters#

• Tight loops spawning thousands of tiny futures
• Latency-critical code paths (microsecond-level)
• Embedded/no_std environments with constrained allocators

Alternatives#

Recommendation#

For most async code doing I/O, the boxing cost is noise. If profiling shows it's a bottleneck, you'd typically avoid the abstraction entirely in that hot path rather than try to optimize the boxing.