Stream Density Testing#

This article explains how to run density tests in ViPPET and interpret the results. A density test finds the maximum number of streams that can run while keeping the target minimum FPS per stream. Compared to a standard performance test (fixed stream count), density testing increases the load and searches for the highest stable stream count that still meets your FPS requirement. Therefore, it answers the question: How many concurrent streams can this platform sustain at my required FPS floor?

Density Testing Algorithm#

The density testing algorithm is designed to find the maximum number of concurrent video streams that can be processed while maintaining a minimum performance threshold (FPS floor). The algorithm uses a two-phase approach:

Phase 1: Exponential Growth#

  • Start with 1 stream and run the pipeline.

  • Double the stream count after each successful run that meets the FPS threshold.

  • Continue exponentially (1 -> 2 -> 4 -> 8 -> 16...) until the per-stream FPS drops below the specified fps_floor.

  • Track the best configuration that still meets the performance requirements.

Phase 2: Binary Search Refinement#

  • Switch to binary search once performance drops below the threshold.

  • Set bounds:

    • Lower bound = last successful stream count (N/2).

    • Upper bound = current failing stream count (N).

  • Bisect the range and test the midpoint.

  • Adjust bounds based on results:

    • If FPS >= threshold: update best config, move lower bound up.

    • If FPS < threshold: move upper bound down.

  • Continue until bounds converge.

Stream Distribution#

  • Multiple pipelines can be tested simultaneously.

  • Stream allocation is proportional based on stream_rate ratios (must sum to 100%).

  • Rounding handling: the last pipeline gets remaining streams to account for rounding errors.

Algorithm result#

The algorithm returns the optimal configuration with:

  • Maximum number of streams that meet the FPS requirement.

  • Distribution of streams across pipelines.

  • Achieved per-stream FPS.

  • Output file paths for video results (from the best-passing iteration).

  • Latency statistics (avg/min/max) when latency metrics are enabled.

Running Density Testing#

Density testing helps you find the maximum number of concurrent streams that still meet a required FPS floor.

Step 1: Test Configuration#

Before running the test, configure the workload in the Density tab:

  1. Open the Density tab.

  2. Set FPS Floor — minimum acceptable per-stream FPS (for example, 30).

  3. Add one or more pipelines.

  4. For each pipeline, set Stream Rate so all pipelines sum to 100%.

  5. Set Max runtime — duration of each iteration in seconds (for example, 10).

Parameter

Description

Example

FPS Floor

Minimum acceptable per-stream FPS. The algorithm stops when it cannot maintain this threshold.

30

Stream Rate

Percentage of total streams allocated to this pipeline. All rates must sum to 100%.

Pipeline A: 60%, Pipeline B: 40%

Max runtime

How long each iteration runs before measuring FPS (seconds). Longer iterations provide more stable measurements.

10

Output mode

disabled or file. Live streaming is not supported for density tests.

disabled

Enable latency metrics

When enabled, measures end-to-end pipeline latency (avg/min/max) per reporting interval.

disabled

Note: USB cameras are not supported in density testing because the algorithm needs to spawn multiple copies of the same pipeline, which is not possible with a single physical camera device.

Density Testing

Step 2: Running the Test#

After configuration, click Run density test.

What happens during execution:

  • ViPPET starts with 1 stream and runs the pipeline for max_runtime seconds.

  • If per-stream FPS ≥ fps_floor, the stream count doubles (exponential growth phase).

  • When FPS drops below fps_floor, ViPPET switches to binary search to refine the exact maximum.

  • The process ends when the algorithm converges on the best stable configuration.

  • Progress is reported in real time via the job status details.

Important: If you cancel a density test while it is running, the job is marked as FAILED. Partial density results are not reported because the algorithm has not yet converged on a reliable answer.

Density Run

Step 3: Interpreting Test Results#

When the job completes, ViPPET reports the best configuration that met the FPS floor:

Result

Description

Per Stream FPS

Achieved FPS per stream in the best configuration (≥ fps_floor)

Total Streams

Maximum number of concurrent streams that sustained the required FPS

Streams per Pipeline

Distribution of streams across pipelines according to stream_rate ratios

Output videos

Paths to output files from the best iteration (if output mode was file)

Latency (avg / min / max)

End-to-end pipeline latency in milliseconds, reported when latency metrics are enabled

Density Results

Comparing results across platforms#

Use density results to compare hardware capabilities:

  • Higher total streams at the same FPS floor indicates better density — the platform can handle more workloads.

  • Per-stream FPS should stay at or above the configured floor.

  • For stable comparison between platforms, keep the same FPS floor, input video, and pipeline configuration.

  • Compare results across devices (CPU vs GPU vs NPU) using the same test profile.

Example: reading density results#

If you set fps_floor = 30 and the test reports total_streams = 12 with per_stream_fps = 31.5:

  • The platform can run 12 concurrent streams of this pipeline while maintaining at least 30 FPS per stream.

  • At 13 streams, per-stream FPS dropped below 30, so the algorithm reported 12 as the maximum.

  • The actual measured FPS (31.5) slightly exceeds the floor — this is the result from the best-passing iteration.