7-Zip is legendary for compression ratio and speed. It's also legendary for
eating RAM: compressing a 500 MB log file at 16 threads peaks at ~1.8 GB.
I wondered: how much of that is the algorithm, and how much is the
implementation? So I wrote r7z, a Rust clone of the core a/x commands,
and measured.
The numbers
| Metric | r7z | 7z |
|---|---|---|
| Time (500 MB text, 1 thread) | 68.4 s | 19.4 s |
| Peak RSS (same) | 85 MB | 376 MB |
| Peak RSS (16 threads) | 85 MB | 1.8 GB |
| Ratio | 0.02% | 0.02% |
Takeaways
- Ratios are essentially identical — the algorithm dominates.
- Memory is an implementation choice. 7z's solid-block arenas and per-thread buffers scale with thread count. r7z's bounded buffers don't.
- Speed is where C++ still wins — SIMD range coder, tuned match finder, and a custom work-stealing pool. Pure Rust can't match that yet without
target-cpu=native, SIMD intrinsics, or FFI to liblzma.
What's next
- FFI path via
xz2for parity speed - SIMD CRC-32 / match finder
- Real multi-threading (currently single-threaded encoder)
- Solid-block size knob
Repo: https://github.com/WhAnci/r7z (English/Korean/Chinese README)
