Summary:
The outer HNSW search loop called `vt->advance()` a second time. `HNSW::search`
already ends with `vt.advance()` at `impl/HNSW.cpp:1861`, so the caller in
`IndexHNSW.cpp` advanced the visited table once more per query. This diff
removes that call and adds a comment that names the owner of the advance.
The sibling call in `search_level_0` stays, because `search_level_0_impl` does
not advance.
**Microbenchmark.** The host holds 72 Cooper Lake cores. The build is
`mode/opt`. One core is pinned, and the two binaries interleave over 5 rounds.
The table reports the median of the medians. `IndexHNSWFlat`, `d=64`, `M=16`,
`use_visited_hashset=false` to select the versioned array, one query for each
iteration.
| ntotal | before | after | ratio |
|---|---|---|---|
| 100k | 44.2 us | 44.2 us | 1.00x |
| 400k | 63.8 us | 69.1 us | 0.92x |
**The change gives no measurable speed gain.** Both rows sit inside a 10% to 27%
spread. The arithmetic agrees: `visno` is a `uint8_t` that cycles from 1 to 254,
so the code clears the table once for every 250 advances. Two advances for each
query halve that interval, which costs about 800 extra bytes of memset for each
query against a search of 44 us to 69 us.
The value of the diff is the removal of a redundant call, not a speed gain.
Pull Request resolved: https://github.com/facebookresearch/faiss/pull/5596
Test Plan:
`buck2 test fbcode//faiss/tests:test_ivf_index fbcode//faiss/tests:test_custom_result_handler fbcode//faiss/tests:test_hnsw fbcode//faiss/tests:test_rabitq_simd_neon fbcode//faiss/tests:test_params_override fbcode//faiss/tests:test_hnsw_rabitq fbcode//faiss/tests:test_rabitq fbcode//faiss/tests:test_search_params fbcode//faiss/tests:test_io_corrupted fbcode//faiss/tests:test_rabitq_utils`, run at the top of the stack on aarch64: 173 pass, 0 fail, 3 skip.
`buck2 build fbcode//faiss:faiss` succeeds in the static aarch64 build and with `-c faiss.dynamic_dispatch=true`.
Reviewed By: alibeklfc
Differential Revision: D121846632
Pulled By: mnorris11
fbshipit-source-id: 8928c4d4648ce061e63c084ded9c74fa17ef66bd
lyang24
6c51305e64