[BUG] Unhelpful Plan Parallel Skew warning at DOP 2

[WinterIssue]

Component

Desktop App (Windows)

SQL Performance Studio Version

0.9.0

Operating System

Windows

Describe the Bug

For me at least the skewed worker workload detection seems to be over zealous when DOP 2 is used.

Is the problem that the node.PerThreadStats.Count equals three because of the coordinating thread?

var skewThreshold = node.PerThreadStats.Count == 2 ? 0.75 : 0.50;

Steps to Reproduce

1. Set DOP to 2
2. Run a parallel plan
3. View in application

Expected Behavior

In this case it would be expected that there would be no warning about skew when a DOP 2 plan runs and one thread does 53% of the work.

Actual Behavior

It produces a warning one thread doing 53% of the work and the other thread does 47%.

Plan File

No response

Screenshots

[erikdarlingdata]

Thanks for the great bug report — you nailed the root cause. The coordinator thread (thread 0) was inflating PerThreadStats.Count, so at DOP 2 we had 3 entries instead of 2, which dropped the threshold to 50% instead of the intended 75%.

Fixed in PR #74:

• Filter out thread 0 (coordinator) from the skew calculation
• Raised the DOP 2 threshold to 80% — a 53/47 split with 2 workers is completely normal

DOP 2 is truly the worst possible DOP. Not parallel enough to be useful, just parallel enough to be confusing. It's the tandem bicycle of query execution — twice the coordination overhead, and someone's always doing more work than the other. 🚲

Fix is also synced to the Dashboard and Lite copies in PerformanceMonitor.