Comments (4)
Const generics are now stable in 1.51.
from lzma-rs.
I had a brief look, the min_const_generics
feature was indeed stabilized in 1.51, but we still need the more complex const_evaluatable_checked
feature due to the 1 << num_bits
expression here.
In the meantime, it might be worth experimenting with it on nightly to see if benchmarks would improve with const generics.
from lzma-rs.
I've been doing some experiments in https://github.com/chyyran/lzma-rs/commits/feature-perf-experiments and found that const generics in BitTree
and RangeCoder
doesn't actually contribute that much performance benefit, except for large dictionaries.
Without const generics (but with some additional optimizations) SnowflakePowered@ecc71a0
running 8 tests
test compress_65536 ... bench: 1,372,700 ns/iter (+/- 32,501)
test compress_empty ... bench: 713 ns/iter (+/- 35)
test compress_hello ... bench: 1,021 ns/iter (+/- 50)
test decompress_after_compress_65536 ... bench: 2,463,569 ns/iter (+/- 77,148)
test decompress_after_compress_empty ... bench: 1,720 ns/iter (+/- 146)
test decompress_after_compress_hello ... bench: 2,219 ns/iter (+/- 50)
test decompress_big_file ... bench: 3,959,745 ns/iter (+/- 3,155,824)
test decompress_huge_dict ... bench: 2,221 ns/iter (+/- 136)
With const generics SnowflakePowered@ad7d096
running 8 tests
test compress_65536 ... bench: 1,379,341 ns/iter (+/- 160,163)
test compress_empty ... bench: 711 ns/iter (+/- 57)
test compress_hello ... bench: 1,016 ns/iter (+/- 30)
test decompress_after_compress_65536 ... bench: 2,442,160 ns/iter (+/- 125,797)
test decompress_after_compress_empty ... bench: 544 ns/iter (+/- 110)
test decompress_after_compress_hello ... bench: 1,003 ns/iter (+/- 47)
test decompress_big_file ... bench: 3,939,209 ns/iter (+/- 177,650)
test decompress_huge_dict ... bench: 1,065 ns/iter (+/- 40)
Probably worth pursuing in the future because of that benefit but SnowflakePowered@ad7d096 uses kind of a nasty workaround to get around the lack of #[feature(generic_const_exprs)]
by putting 1 << N
into a const fn and passing it in at the constructor site.
from lzma-rs.
Vec2D
from #77 lets us easily experiment with different backing stores for the literal_probs
array which is the main allocation.
I used arrayvec which gives us a nice API over a raw array.
Since DecoderState
needs to be able to update lclppb at runtime, we can't just parameterize it with const generics unlike BitTree
, so we have to account for the theoretical maximum parameters which is sizeof(u16) * 0x300 * { 1 << (4 + 8) }
= 3145728 * 2. An array of 6 megabytes is way too large to fit on the stack and I was unable to actually decompress anything without a stack overflow.
For LZMA2 streams, the theoretical maximum parameters for the literal_probs
array is sizeof(u16) * 0x300 * { 1 << 4 }
= 12288 * 2. While this isn't great, 24KB is way smaller than 6MB so I was able to actually run some benchmarks, which are not great.
These benchmarks are with an allocating BitTree
, i.e. without const-generics.
ArrayVec<u16, 0x300 * { 1 << 4 }>
running 8 tests
test compress_65536 ... bench: 1,323,468 ns/iter (+/- 174,348)
test compress_empty ... bench: 695 ns/iter (+/- 20)
test compress_hello ... bench: 1,035 ns/iter (+/- 90)
test decompress_after_compress_65536 ... bench: 1,336,165 ns/iter (+/- 57,379)
test decompress_after_compress_empty ... bench: 6,166 ns/iter (+/- 359)
test decompress_after_compress_hello ... bench: 6,531 ns/iter (+/- 224)
test decompress_big_file ... bench: 3,716,763 ns/iter (+/- 121,865)
test decompress_huge_dict ... bench: 6,572 ns/iter (+/- 315)
test result: ok. 0 passed; 0 failed; 0 ignored; 8 measured; 0 filtered out; finished in 13.64s
Box<[u16]>
running 8 tests
test compress_65536 ... bench: 1,334,964 ns/iter (+/- 1,371,984)
test compress_empty ... bench: 709 ns/iter (+/- 436)
test compress_hello ... bench: 1,033 ns/iter (+/- 64)
test decompress_after_compress_65536 ... bench: 1,145,697 ns/iter (+/- 82,369)
test decompress_after_compress_empty ... bench: 1,890 ns/iter (+/- 92)
test decompress_after_compress_hello ... bench: 2,231 ns/iter (+/- 295)
test decompress_big_file ... bench: 3,620,380 ns/iter (+/- 152,660)
test decompress_huge_dict ... bench: 2,271 ns/iter (+/- 75)
test result: ok. 0 passed; 0 failed; 0 ignored; 8 measured; 0 filtered out; finished in 7.76s
The huge array on the stack probably causes a lot of cache misses and substantially reduces performance. It seems the only benefit to using const generics here is with BitTree
, where it does show an improvement in the benchmarks as above.
from lzma-rs.
Related Issues (20)
- Publish some benchmarks on `README.md`
- add `xz_decompress_with_options` API
- Panic on decoding xz file with sha256 checksum
- Test against the XZ test suite HOT 4
- Behavior mismatches between liblzma and lzma-rs HOT 1
- Provide features to import parts of the crate HOT 3
- Enable no_std support HOT 4
- Add configurable memory limit
- Support multi-stream files
- .lz (lzip) HOT 2
- Expose decoder/encoder primitives as public HOT 4
- Add unit tests
- Report code coverage
- Reach 100% coverage.
- Problems when porting lzma code Java -> Rust HOT 1
- New release
- XZ: Unknown filter id 7 HOT 2
- API Suggestion: Add encoder and decoder structs
- Extraction fails with `Unknown filter id 8` HOT 2
- lzma2_compress and zx_compress do not compress input HOT 2
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