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Sihwan Lee
Sihwan Lee

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I rewrote 7-Zip in Rust — here's what I learned about memory

#rust #performance #compression #cli


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

  1. Ratios are essentially identical — the algorithm dominates.
  2. Memory is an implementation choice. 7z's solid-block arenas and per-thread buffers scale with thread count. r7z's bounded buffers don't.
  3. 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 xz2 for 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)

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