# p99.Rust <!-- omit in toc -->
Low-cost generation of performance percentiles (p50, p90, p99, p99.9, etc.).
[](https://opensource.org/licenses/BSD-3-Clause)
[](https://crates.io/crates/p99)
[](https://github.com/synesissoftware/p99.Rust/releases/latest)
[](https://github.com/synesissoftware/p99.Rust/commits/master)
[](https://github.com/rust-lang/rust/releases/tag/1.74.0)
[](https://github.com/synesissoftware/p99.Rust/actions/workflows/ci.yml)
[](https://docs.rs/p99)
## Table of Contents <!-- omit in toc -->
- [Introduction](#introduction)
- [How It Works](#how-it-works)
- [Performance \& Trade-offs](#performance--trade-offs)
- [Performance Claims](#performance-claims)
- [Trade-offs \& Sacrifices](#trade-offs--sacrifices)
- [Installation](#installation)
- [Components](#components)
- [Constants](#constants)
- [Enumerations](#enumerations)
- [Features](#features)
- [Enabling `binary-scaling`](#enabling-binary-scaling)
- [Benchmark Results](#benchmark-results)
- [Functions](#functions)
- [Macros](#macros)
- [Structures](#structures)
- [`Histogram`](#histogram)
- [Definition](#definition)
- [Minimal Example](#minimal-example)
- [Traits](#traits)
- [Examples](#examples)
- [Project Information](#project-information)
- [Where to get help](#where-to-get-help)
- [Contribution guidelines](#contribution-guidelines)
- [Dependencies](#dependencies)
- [Development Dependencies](#development-dependencies)
- [License](#license)
## Introduction
**p99** is a lightweight, low-overhead library designed for generating real-time performance percentiles in high-frequency or latency-sensitive environments.
**p99.Rust** is the **Rust** implementation.
## How It Works
`Histogram` is a low-overhead, zero-allocation, fixed-size structure designed to track event durations (typically in nanoseconds) using 64 logarithmic buckets.
* **Logarithmic Bucketing**: The bucket boundaries are spaced as powers of two:
* Bucket `0` represents `[0, 1]` nanoseconds;
* Bucket `1` represents `[2, 3]` nanoseconds;
* Bucket `2` represents `[4, 7]` nanoseconds;
* Bucket `i` represents `[2^i, 2^(i+1) - 1]` nanoseconds.
* **Branchless Indexing**: Finding the correct bucket index for an incoming duration is extremely fast and branchless. It is computed in a few CPU instructions using the CPU's leading-zeros count intrinsic (`u64::leading_zeros`).
* **Linear Interpolation**: Percentile queries iterate through the buckets to find the target rank and perform linear interpolation within the matching bucket to approximate the exact percentile duration.
## Performance & Trade-offs
### Performance Claims
* **Zero Allocation**: `Histogram` does not allocate memory on the heap during creation, event insertion, or percentile queries. It is a compact (~576-byte) structure that can reside entirely on the stack or be embedded in other structures.
* **Low-Latency Operations**: The fixed bucket layout is designed for low-latency insertion and percentile queries. Actual timings depend on the processor, compiler, build profile, and workload.
* **Instruction-Cache Friendly**: The query methods are designed with a "thin caller / heavy worker" pattern to prevent instruction-cache bloat and maintain high CPU cache locality under real-world workloads.
The statements above describe implementation characteristics or design goals,
not guaranteed timings. Measured results from the checked-in Criterion
benchmark are reported below.
### Trade-offs & Sacrifices
* **Logarithmic Precision**: To achieve zero allocation and constant-time operations, `Histogram` sacrifices exact precision. It does not store individual event times. Instead, values are grouped into logarithmic buckets.
* **Approximation**: Percentile values are approximated using linear interpolation within the bucket boundaries. For very large values, the bucket width is wider, which leads to a wider approximation range. However, for low-latency performance measurements where precision is needed most (the lower nanosecond ranges), the buckets are extremely narrow (e.g., 1ns, 2ns, 4ns wide), providing exceptional resolution.
* **`binary-scaling` Accuracy**: When the `binary-scaling` feature is enabled, the percentile target rank is computed using a $2^{32}$ fixed-point approximation. The pre-encoded multiplier for each percentile (e.g., `3_865_470_566 >> 32` ≈ `0.9000` for p90) differs from the true decimal value by less than $10^{-9}$, which is far below the approximation error introduced by the logarithmic bucketing itself. In practice this has no measurable impact on percentile accuracy.
## Installation
Reference the current release in **Cargo.toml** in the usual way:
```toml
p99 = { version = "0.0.4" }
```
To enable the optional binary-scaling optimization:
```toml
p99 = { version = "0.0.4", features = ["binary-scaling"] }
```
## Components
### Constants
No public constants are defined at this time.
### Enumerations
No public enumerations are defined at this time.
### Features
The following crate features are available:
* **`"binary-scaling"`** *(opt-in)*: Replaces integer division in the integer-based percentile methods (`#value_at_p90()`, `#value_at_p95()`, `#value_at_p99()`, etc.) with $2^{32}$ fixed-point binary scaling. Each percentile multiplier (e.g., `0.90` for p90) is pre-encoded as a `u32` constant and the target rank is computed via a single multiplication and a 32-bit right-shift, avoiding the cost of integer division entirely. This yields a **~1.5x to 2x speedup** for percentile queries with a negligible loss of accuracy (the scaled multiplier differs from the true value by less than $10^{-9}$). The generic `#value_at_percentile(f64)` method is unaffected by this feature;
* **`"null-feature"`** *(opt-in)*: A no-op feature that has no effect on the compiled library. It exists to simplify driver scripts and CI pipelines that conditionally pass `--features` flags, allowing a feature list to always be present even when no real features are needed;
#### Enabling `binary-scaling`
Add the feature in your **Cargo.toml**:
```toml
[dependencies]
p99 = { version = "0.0.4", features = ["binary-scaling"] }
```
Or, when building from the command line:
```bash
# Default (standard integer division)
cargo run --example build_histogram
# With binary scaling enabled
cargo run --example build_histogram --features binary-scaling
```
#### Benchmark Results
Measured with [**criterion**](https://github.com/bheisler/criterion.rs) on
100,000 events per workload, using the release profile on an Apple M-series
machine. Criterion's default measurement configuration was used. The exact
machine, operating-system, compiler, and Criterion output are not recorded,
so these results are illustrative rather than guaranteed. Only the
integer-based percentile methods are affected; the generic
`value_at_percentile(f64)` method is unchanged.
Run the benchmark in the release profile with
`cargo bench --bench histogram --release`, then repeat with
`--features binary-scaling` to reproduce the comparison.
| Method | Default | `binary-scaling` | Improvement |
|---|---:|---:|---:|
| `value_at_p90()` | 23.25 ns | 21.63 ns | **-7.0%** |
| `value_at_p99()` (dense) | 16.52 ns | 14.88 ns | **-10.4%** |
| `value_at_p99()` (wide) | 23.39 ns | 21.55 ns | **-7.9%** |
| `value_at_p99_99()` | 23.32 ns | 21.64 ns | **-7.2%** |
Methods using simple fractional multipliers (p50 = 1/2, p75 = 3/4) already compile to bit-shifts without this feature, so they show no change.
### Functions
No public functions are defined at this time.
### Macros
No public macros are defined at this time.
### Structures
The following public structures are defined in the current version:
#### `Histogram`
A low-cost, zero-allocation, 64-bucket logarithmic histogram designed for recording event durations in nanoseconds and querying high-resolution percentiles.
##### Definition
```rust
pub struct Histogram {
// ... private fields ...
}
```
##### Minimal Example
Here is a simple example of how to initialize a `Histogram`, record event times, and query percentiles:
```rust
use p99::Histogram;
use std::time::Duration;
fn main() {
// 1. Initialize a default histogram (zero-allocated, ~576 bytes on the stack)
let mut histogram = Histogram::default();
// 2. Record event times using various units
histogram.push_event_time_ns(150); // 150 ns
histogram.push_event_time_us(5); // 5 microseconds (5,000 ns)
histogram.push_event_time_ms(10); // 10 milliseconds (10,000,000 ns)
histogram.push_event_duration(Duration::from_nanos(250));
// 3. Retrieve basic statistics
assert_eq!(histogram.event_count(), 4);
assert_eq!(histogram.min_event_time(), Some(150));
assert_eq!(histogram.max_event_time(), Some(10_000_000));
assert_eq!(histogram.event_time_total(), Some(10_005_400));
// 4. Query percentiles
// Generic floating-point query (e.g., 90th percentile)
if let Some(p90) = histogram.value_at_percentile(90.0) {
println!("p90: {} ns", p90);
}
// Fast, optimized integer-based percentile wrappers
if let Some(p50) = histogram.value_at_p50() {
println!("p50 (median): {} ns", p50);
}
if let Some(p99) = histogram.value_at_p99() {
println!("p99: {} ns", p99);
}
}
```
### Traits
No public traits are defined at this time.
## Examples
An example program showing `Histogram` usage is provided in [**examples/build_histogram.rs**](./examples/build_histogram.rs).
It simulates a histogram of event times generated by `std::thread::sleep` delays under a custom PRNG.
The number of iterations can be configured via the `P99_TRIES` environment variable:
```bash
# Run with the default of 100 tries
cargo run --example build_histogram
# Run with 1000 tries
P99_TRIES=1000 cargo run --example build_histogram
# Run with binary-scaling enabled (faster percentile queries)
cargo run --example build_histogram --features binary-scaling
```
## Project Information
### Where to get help
[GitHub Page](https://github.com/synesissoftware/p99.Rust "GitHub Page")
### Contribution guidelines
Defect reports, feature requests, and pull requests are welcome on https://github.com/synesissoftware/p99.Rust.
### Dependencies
**p99.Rust** has no (non-development) dependencies.
#### Development Dependencies
Crates upon which **p99.Rust** has development dependencies:
* [**criterion**](https://github.com/bheisler/criterion.rs);
* [**test_help-rs**](https://github.com/synesissoftware/test_help-rs);
**Cargo.lock** is retained so local and CI validation use a reproducible
dependency graph. Commands that consume the lockfile use `--locked`.
### License
**p99.Rust** is released under the 3-clause BSD license. See [LICENSE](./LICENSE) for details.
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