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Report the C++20 rewrite: allocation, size, throughput and memory
docs/performance.md compares master (C) with the C++ engine on macOS arm64: zero heap allocations per connection or datagram (from 27-89), TCP throughput within noise, and about 20 MiB more RSS at 1,000 open connections from the inline per-connection buffers. docs/cpp-core.md and CLAUDE.md now describe the finished layout.
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Max Lv
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# Modernization measurements
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## C++20 rewrite (2026-10-14)
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Local macOS arm64 (Apple clang), Release, bundled dependencies. Baseline:
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`master` at `553cdc7` (C). Comparison: the C++20 engine branch stack. Raw
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JSON is in [measurements](measurements/) (`cpp20-*`). These are short
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loopback runs on one machine, not capacity guarantees.
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### Heap allocations per relayed connection
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`tests/alloc_audit/measure_alloc.py --target 127.0.0.1`: 200 TCP connections
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with a small payload each, after warm-up. Each value is allocations per
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connection as ss-server / ss-local.
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| Method | C | C++ |
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|---|---:|---:|
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| aes-256-gcm | 39.0 / 39.0 | 0 / 0 |
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| chacha20-ietf-poly1305 | 33.1 / 33.0 | 0 / 0 |
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| 2022-blake3-aes-256-gcm | 34.0 / 32.9 | 0 / 0 |
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| aes-256-cfb | 27.1 / 26.4 | 0 / 0 |
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With a host-name target (`localhost`, resolved by ss-server) the C++ programs
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are still at 0, because c-ares allocates from its fixed heap. The C ss-server
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rises to 89.1 per connection. UDP relaying is also 0 per datagram, where the C relay allocated a
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64 KiB buffer for every one. ctest enforces all of this (`alloc_steady_*`, including
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`alloc_steady_programs_*`).
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### Size
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Unstripped Release executables, in bytes:
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| Program | C | C++ |
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|---|---:|---:|
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| ss-local | 1,086,888 | 1,129,960 |
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| ss-server | 1,086,952 | 1,130,408 |
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| ss-tunnel | 542,200 | 789,144 |
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ss-tunnel grows because it now shares the full relay engine with the other
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programs, including the resolver, the UDP relay and every codec.
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### TCP throughput and memory
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`tests/stress_test.py --size 100 --repeat 3`: median of three runs, server RSS
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after the transfer.
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| Cipher | C Mbps | C++ Mbps | C server RSS, KiB | C++ server RSS, KiB |
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|---|---:|---:|---:|---:|
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| aes-128-gcm | 6,353 | 6,285 | 3,472 | 3,648 |
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| aes-256-gcm | 6,829 | 6,637 | 3,440 | 3,616 |
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| chacha20-ietf-poly1305 | 3,037 | 2,891 | 3,456 | 3,664 |
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The throughput differences are within the run-to-run spread, which is about
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±20% for these 0.1–0.3 s transfers.
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RSS with 1,000 established, idle connections through ss-local and ss-server
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(aes-256-gcm):
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| | C server | C++ server | C local | C++ local |
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|---|---:|---:|---:|---:|
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| idle | 2,624 KiB | 2,992 KiB | 2,272 KiB | 2,576 KiB |
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| 1,000 connections | 60,256 KiB | 80,912 KiB | 54,416 KiB | 74,752 KiB |
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Each connection slot carries about 104 KiB of inline buffers, against roughly
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58 KiB that the C relay allocated on demand. The pool is reserved virtually at
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startup and only touched slots become resident. A shared buffer pool with read
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backpressure would cut this; it is the main open trade-off of the rewrite.
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### UDP
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`tests/measure_udp.py`: median payload Mbps over three runs of 2,000 echoed
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1,200-byte packets.
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| Cipher | C | C++ |
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|---|---:|---:|
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| aes-128-gcm | 80.9 | 81.4 |
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| aes-256-gcm | 81.1 | 55.1 |
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| chacha20-ietf-poly1305 | 56.2 | 65.6 |
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This benchmark is dominated by Python and SOCKS round trips. Single runs vary
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by more than 30% in both builds (for example, C aes-256-gcm ranged from 64 to
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81), so no UDP throughput change is claimed either way.
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### Reproduce
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```sh
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python3 tests/alloc_audit/measure_alloc.py --bin build/bin --shim build/lib/libss_alloc_audit.dylib \
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--method aes-256-gcm --target 127.0.0.1
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python3 tests/stress_test.py --bin build/bin --size 100 --repeat 3 --json tcp.json
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python3 tests/measure_udp.py --bin build/bin --output udp.json
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```
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## Bundled dependencies (2026-09-10)
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Local macOS arm64 results, 2026-09-10. Base commit: `8fe386b`; comparison:
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`feature/self-contained-portable-c`. Raw JSON lives in [measurements](measurements/).
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These are short loopback measurements on one machine, not capacity guarantees.
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