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killown edited this page Mar 16, 2026 · 1 revision

Sync Variance (SVA) is a statistical measure of the stability of a compositor's frame delivery timing. While the Sync Score (SYN) tells you how well a single frame aligned with a VSync pulse, SVA measures whether that alignment is consistent over time or oscillating wildly.

Technical Definition

SVA is calculated as the Standard Deviation ($\sigma$) of the Sync Scores recorded over a rolling 500ms window:

$$SVA = \sqrt{\frac{1}{N} \sum_{i=1}^{N} (SYN_i - \mu_{SYN})^2}$$

By measuring the variance rather than just the average, SVA can distinguish between a compositor that is intentionally offset (stable but delayed) and one that is struggling (jittery and unpredictable).

Interpretation Benchmarks

SVA Value Status Interpretation
< 2.0 Rock Solid Perfect phase-lock. The compositor is hitting its timing target with sub-millisecond precision.
2.0 - 5.0 Stable Normal OS jitter. Minor scheduling fluctuations that are generally imperceptible to the eye.
5.0 - 15.0 Jittery The scheduler is "hunting" for the pulse. Motion may appear slightly nervous or "micro-stuttery."
> 15.0 Unstable Serious timer drift or CPU contention. The compositor cannot maintain a consistent cadence.

SVA vs. SYN: The Safety Offset Case

A common point of confusion is seeing a Sync Score of 50 with a very Low SVA (e.g., 0.5).

  • Low SYN (~50): Means the frames are consistently landing in the dead-center of the refresh cycle (the "Safety Offset" strategy).
  • Low SVA (< 1.0): Means that 50% offset is being hit with perfect precision.

In this scenario, the rendering is perfectly stable. The compositor is intentionally trading half a frame of latency for a massive safety buffer to ensure no frames are ever dropped. If the SVA were high (e.g., 20.0), it would indicate that the 50% score is just a lucky average of a broken, drifting clock.

Diagnostic Use

If you are experiencing "judder" despite a high average FPS:

  1. Check SYN: If it is low, the compositor is poorly aligned or using a safety offset.
  2. Check SVA: If it is above 10.0, your OS scheduler or compositor timer is being interrupted by background tasks, thermal throttling, or driver-level power management.

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