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blake2 simd benches produce unexpected results

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Assessment

Difficulty
3/5
Estimated time
1-2 days
Newbie friendliness
42/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
rust
Domain
cryptography, performance

Research direction

Start by running the two cargo +nightly bench commands shown for the default and simd configurations, then inspect the Blake2 benchmark entry points and how throughput is calculated. Done means the reported MB/s values accurately reflect the measured benchmark times and remain consistent between the two configurations.

Written by the indexing model from the issue text.

Description

I ran blake2 tests with and without simd feature enabled and found that behavior is strange and doesn't make a lot of sense.

Default (no simd):

$ cargo +nightly bench
...
running 8 tests
test blake2b512_10    ... bench:           8 ns/iter (+/- 0) = 1250 MB/s
test blake2b512_100   ... bench:          74 ns/iter (+/- 4) = 1351 MB/s
test blake2b512_1000  ... bench:         708 ns/iter (+/- 85) = 1412 MB/s
test blake2b512_10000 ... bench:       7,005 ns/iter (+/- 531) = 1427 MB/s
test blake2s256_10    ... bench:          13 ns/iter (+/- 0) = 769 MB/s
test blake2s256_100   ... bench:         121 ns/iter (+/- 7) = 826 MB/s
test blake2s256_1000  ... bench:       1,155 ns/iter (+/- 62) = 865 MB/s
test blake2s256_10000 ... bench:      11,791 ns/iter (+/- 1,005) = 848 MB/s

test result: ok. 0 passed; 0 failed; 0 ignored; 8 measured; 0 filtered out; finished in 28.22s

Simd:

$ cargo +nightly bench --features simd
...
running 8 tests
test blake2b512_10    ... bench:          11 ns/iter (+/- 0) = 909 MB/s
test blake2b512_100   ... bench:         102 ns/iter (+/- 0) = 980 MB/s
test blake2b512_1000  ... bench:       1,010 ns/iter (+/- 11) = 990 MB/s
test blake2b512_10000 ... bench:      10,076 ns/iter (+/- 68) = 992 MB/s
test blake2s256_10    ... bench:          16 ns/iter (+/- 0) = 625 MB/s
test blake2s256_100   ... bench:         152 ns/iter (+/- 1) = 657 MB/s
test blake2s256_1000  ... bench:       1,483 ns/iter (+/- 12) = 674 MB/s
test blake2s256_10000 ... bench:      14,750 ns/iter (+/- 344) = 677 MB/s

test result: ok. 0 passed; 0 failed; 0 ignored; 8 measured; 0 filtered out; finished in 2.43s

I expected simd version to produce better results and judging by the total execution time it is the case, but throughput reported by the benchmark says the opposite.

I suspect something is wrong with benchmarking code.

Using latest master (cc523733e5ba84b5a366825a7d1150bbbfea368b right now) and rustc 1.65.0-nightly (20ffea693 2022-08-11).

Dominant language
Rust
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Merged PRs (30d)
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