Armchair

Armchair is a load test binary that can be used to benchmark Rime's TTS service with concurrent requests.

Primary use cases:

• To find the time-to-first-byte (TTFB) and real-time factor (RTF) for a given concurrency level.
• To find the maximum concurrency that satisfy the given performance targets.
• To find an optimal client-side buffer size to avoid underrun issues.

For audio streaming with concurrent sessions, TTFB and RTF are the key performance indicators. To achieve real-time streaming, it is imperative that RTF is under 1 and a maximum concurrency is typically imposed to ensure that RTF is under 1.

Supported features:

• Bisection to find maximum concurrency based on configurable performance target (success, TTFB, RTF)
• Time-to-first byte and RTF metrics for a given concurrency
• Session start staggering via exponential distribution
• Intra-session delays via truncated normal distribution with playback-aware waiting
• Client-side buffer simulation and underrun detection

Methodology

This tool simulates many concurrent streaming sessions and evaluates performance against a configurable target.

• Session model:

  • At a given concurrency C, C sessions are launched.
  • Session starts are staggered by an exponential inter-arrival process with rate λ (--session-rate, starts/second).
  • Each session performs -n/--requests-per-session sequential requests.

• Per-request timing and metrics:

  • TTFB is measured from request send to the first received byte.
  • Elapsed is the total time to stream the entire response.
  • Audio duration is parsed from the WAV headers; if parsing fails the request is treated as non-audio for RTF purposes.
  • RTF is computed as (elapsed − TTFB) / audio_duration. RTF values requiring missing/invalid audio duration are excluded from RTF percentiles.

• Intra-session delay model (traffic shaping):

  • After each request completes, the tool waits any remaining playback time if the audio was synthesized faster than real time, i.e. max(0, audio_duration − (elapsed − TTFB)).
  • Then it sleeps an additional delay sampled from a Normal distribution with parameters --intra-session-delay-mu and --intra-session-delay-sigma, truncated to [--intra-session-delay-min, --intra-session-delay-max].
  • The first request in a session has no intra-session delay; session start staggering is controlled by the exponential process above.

• Buffer underrun detection:

  • Simulates a client-side buffer of size --client-buffer (default 0ms).
  • Playback starts once the buffer is full.
  • An underrun occurs if the buffer empties before playback completes.
  • Requires valid WAV headers to determine the byte rate.

• Aggregation and statistics:

  • Success is counted when HTTP status is 2xx and the body is non-empty.
  • Percentiles (p50/p90/p95/p99) are computed via linear interpolation over sorted samples; NaN/invalid values are excluded from the relevant metric’s distribution.
  • A startup config dump prints all key parameters for reproducibility.

• Performance target evaluation (--target):

  • The target is a conjunction: all configured clauses must pass.
  • Supported clauses: success:<fraction>, ttfb:pXX@<duration>, rtf:pXX@<value>, underrun:<fraction>.
  • If a metric cannot be computed (e.g., no valid audio for RTF), that clause fails.
  • Results show OK/FAIL per clause, with color when the terminal supports it.

• Maximum concurrency search (when --concurrency 0):

  • Exponential growth: repeatedly doubles concurrency (1, 2, 4, …) until the performance target fails; waits 10s between trials.
  • Binary search: bisection between last known-good and first failing to find the largest concurrency that still satisfies the target.
  • After discovery, a final run at the chosen concurrency prints a full summary.

Note: The traffic and delay processes are stochastic; repeated runs will vary. Randomness is seeded from system entropy.

Usage

Installation

cargo install armchair

Maximum concurrency

To find the maximum concurrency where each session sends 5 requests with:

• session starts following an exponential process (lambda=5 starts/sec)
• intra-session delays sampled from a truncated normal N(mu=10s, sigma=5s), clamped to [0s, 20s]
• performance targets success:1.00,ttfb:p99@1s,rtf:p99@1.00,underrun:0.00 (default)

armchair --url '<RIME_SERVICE>' --token '<RIME_API_KEY>'

The tool should then report metrics like:

=== MAXIMUM CONCURRENCY FOUND: 16 ===

...

----- Summary -----
total: 80 success: 80 (100.0%)
Buffer underrun: 0 (0.0%)
TTFB ms: mean=104.4 p50=100.8 p90=117.6 p95=126.2 p99=141.0
Elapsed ms: mean=13924.0 p50=13772.5 p90=16412.8 p95=17065.3 p99=18527.3
RTF: mean=1.067 p50=1.061 p90=1.170 p95=1.208 p99=1.254

Fixed concurrency

By specifying the flag --concurrency, the tool skips the bisection and simply produces the latency metrics.

Request customization

• -n: Number of requests in each session, e.g. 5
• --session-rate: Session starts per second following a Poisson distribution for staggered starts, e.g. 5
• --intra-session-delay-mu: Intra-session delay mean, e.g. 10s
• --intra-session-delay-sigma: Intra-session delay standard deviation, e.g. 5s
• --intra-session-delay-min: Intra-session delay minimum clamp, e.g. 0s
• --intra-session-delay-max: Intra-session delay maximum clamp, e.g. 20s
• --client-buffer: Client-side initial playback buffer, e.g. 100ms
• --target: Performance target specification, e.g. success:1.00,ttfb:p90@500ms,rtf:p90@1.00,underrun:0.00
• --percentiles: List of percentiles to report, e.g. 1,25,50,90,99

Duration value syntax

Flags that accept durations (e.g., --intra-session-delay-mu) take values with units:

500ms, 1.5s, 10s

Performance target flag

--target accepts a comma-separated list:

success:<fraction>,ttfb:pXX@<duration>,rtf:pXX@<value>,underrun:<fraction>

Examples:

--target success:0.99,ttfb:p95@800ms,rtf:p90@1.20,underrun:0.01
--target success:1.00,ttfb:p90@1s,rtf:p90@1.00,underrun:0.00