Worst negative slack (WNS) is the smallest slack across all paths. Total negative slack (TNS) is the sum of the negative slacks only.
Positive slacks contribute nothing to TNS. WNS is the minimum over all paths, so once nothing violates, WNS is the smallest positive slack.
Metric | Definition | Answers |
|---|---|---|
WNS | Minimum slack over all paths | How bad is the single worst path. |
TNS | Sum of all negative slacks | How much failure exists across the design. |
Run 1 has three failing paths out of five. Run 2 fixes path A only, and the calc block recomputes both totals.
run 1 path slacks in delay units: A -0.4, B -0.2, C -0.1, D +0.3, E +0.5
WNS = minimum = -0.4
TNS = -0.4 + -0.2 + -0.1 = -0.7
run 2 after path A is fixed: A +0.1, B -0.2, C -0.1, D +0.3, E +0.5
WNS = minimum = -0.2
TNS = -0.2 + -0.1 = -0.3
WNS gain = -0.2 - -0.4 = +0.2; TNS gain = -0.3 - -0.7 = +0.4WNS and TNS move for different reasons, so read them as a pair. The table below covers the four common patterns.
WNS | TNS | Reading |
|---|---|---|
Improving | Improving | Effort works across the failing paths. |
Improving | Flat | One path is fixed while many still fail; broaden the effort. |
Flat | Improving | Shared logic helps many paths but the worst one resists; attack it directly. |
Flat | Flat | Effort is spent out; restructure or stop. |
Paths A, B, and C fail. Their total shortfall is 0.7 delay units.
The worst path improved but other failures remain or grew. Effort must broaden beyond that one path.