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48 lines
2.1 KiB
Markdown
48 lines
2.1 KiB
Markdown
# FFI wire-format codec benchmark
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`bench_codec.nim` is a single-process Nim microbenchmark comparing the two FFI
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wire-format codecs head-to-head on identical payloads:
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- **cbor** — `cborEncode` / `cborDecode`, self-describing bytes over
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`seq[byte]`. The codec the `cbor` ABI uses on every boundary crossing.
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- **c (cwire)** — `cwirePack` / `cwireUnpack` / `cwireFree`, flat C-struct
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shared-memory packing. The codec the `c` ABI uses, emitted for every
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`{.ffi: "abi = c".}` type as its `<T>_CWire` companion.
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Both paths run in the same process on the same values, so the numbers isolate
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**codec cost only** — no thread hop, no callback dispatch, no chronos work. The
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full FFI round-trip (thread channel + callback) is identical for both ABIs, so
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the codec is where the ABI difference actually lives.
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## Running
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```sh
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nimble bench_codec
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# or directly, with the size sweep extended to 1 MiB:
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nim c -r --mm:orc -d:danger tests/bench/bench_codec.nim --include-1mib
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```
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Build with `-d:danger` (the nimble task does) so the figures reflect optimized
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codegen rather than a debug build.
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## Payload shapes covered
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| Type | Shape |
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|------------------|----------------------------------------------------|
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| `EchoRequest` | 1 string + 1 int (small struct) |
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| `EchoResponse` | 2 strings |
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| `ComplexRequest` | `seq[EchoRequest]`, `seq[string]`, `Option[...]` |
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| `BytesPayload` | `seq[byte]`, swept 100 B → 150 KiB (`--include-1mib` adds 1 MiB) |
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## Interpreting
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Small structs are dominated by CBOR's per-field tag/length framing, so cwire
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wins by a large factor. As payloads grow into big `seq[byte]` blobs, both codecs
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become `memcpy`-bound and the ratio converges toward ~1×. The byte-blob sweep
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also reports throughput (MiB/s) for each codec.
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> Note: this benchmark exercises the `c` ABI **codec** (the cwire companions on
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> the Nim side). Wiring the `c` ABI through the full proc-dispatch path and the
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> foreign (C++/Rust) generators is tracked separately; only `cbor` currently
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> generates working end-to-end bindings.
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