knot
View SourceFast non-cryptographic PRNG NIF for Erlang. One function: knot:uniform/1.
wyrand (1 multiply + 1 xor + 1 add per draw; passes BigCrush),
bounded via biased multiply-and-shift (bias < 1e-7 for the small bounds
shackle uses). State is per OS thread via __thread — no locks on the
hot path. Lazy seeding from getrandom/arc4random_buf on first use.
A drop-in replacement for granderl:uniform/1 that builds cleanly on
modern OTP and scales linearly across schedulers.
API
-spec knot:uniform(pos_integer()) -> pos_integer().uniform(N) returns a uniformly random integer in [1, N]. N must
fit in a u32 (1..=4_294_967_295).
Install
{deps, [{knot, "0.1.0"}]}.Requires a C compiler (cc) on the build host — universally available
on systems that already run Erlang. No Rust toolchain, no cargo, no
extra deps.
Build
rebar3 compile runs c_src/build.sh, which:
- Resolves
ERTS_INCLUDE_DIRviaerl -noshell -eval ... -s init stop(option order is correct for OTP 27+ — the bug that affectedgranderl 0.1.5is fixed here). - Compiles
c_src/knot.cwith-O3 -march=native -mtune=native. - Outputs
priv/knot.so.
Env vars honored:
| Var | Effect |
|---|---|
ERTS_INCLUDE_DIR | Skip the erl probe; use this path for erl_nif.h. |
CC | Compiler (default cc). |
CFLAGS | Extra flags appended after defaults. |
KNOT_NO_NATIVE | If set, omit -march=native/-mtune=native (use for portable cross-platform builds). |
Benchmark
Apple Silicon (M-series), OTP 29, 10M iterations of uniform(254),
median of 5 runs:
| concurrency | rand:uniform/1 | granderl:uniform/1 | knot:uniform/1 |
|---|---|---|---|
| 1 | 34 ns/op | 13 ns/op | 12 ns/op |
| 8 | 8 ns/op | 7 ns/op | 3.3 ns/op |
| 32 | 6 ns/op | 8 ns/op | 3.2 ns/op |
| 128 | 6 ns/op | 8.5 ns/op | 3.1 ns/op |
Single-process: dispatch-bound (~12 ns is the NIF boundary floor for both knot and granderl).
Concurrent: knot scales linearly across schedulers because state is
strictly per-OS-thread (__thread uint64_t), no atomics or locks
anywhere on the hot path.
Reproduce: make bench.
License
MIT.