allocator: add adjustRangeCountForScoring

This commit adds `adjustRangeCountForScoring` to the `ScorerOptions` interface. The
method takes a provided range count, adjusts it according to the scorer’s
heuristic, and returns the result. It is similar to other scoring metrics such
as the convergence score, and will later be used in candidate.compare as an
attribute for breaking ties. The compare method evaluates the relative
difference between two stores and favors the one with fewer ranges as a better
rebalance target. Currently and by default, `adjustRangeCountForScoring` simply
returns the provided range count. Future scorer options will override it with 0
since the heuristics is to not consider range count at all.

Author: wenyihu6

pkg/kv/kvserver/allocator/allocatorimpl/allocator_scorer.go

@@ -292,6 +292,15 @@ type ScorerOptions interface {
 	// the store represented by `sc` classifies as underfull, aroundTheMean, or
 	// overfull relative to all the stores in `sl`.
 	balanceScore(sl storepool.StoreList, sc roachpb.StoreCapacity) balanceStatus
+	// adjustRangeCountForScoring returns the adjusted range count for scoring.
+	// Scorer options (IOOverloadOnlyScorerOptions) that do not score based on
+	// range count convergence returns 0. Otherwise, the returned range count here
+	// should usually be the same as the range count passed in
+	// (RangeCountScorerOptions, LoadScorerOptions, ScatterScorerOptions). This
+	// will later be used to calculate the relative difference when comparing the
+	// range count of the candidate stores. Lower range count means a better
+	// candidate to move the replica to.
+	adjustRangeCountForScoring(rangeCount int) int
 	// rebalanceFromConvergenceScore assigns a convergence score to the store
 	// referred to by `eqClass.existing` based on whether moving a replica away
 	// from this store would converge its stats towards the mean (relative to the
@@ -385,6 +394,13 @@ func (bo BaseScorerOptions) maybeJitterStoreStats(
 	return sl
 }
 
+// adjustRangeCountForScoring returns the provided range count since that is the
+// default behavior. BaseScorerOptionsNoConvergence is the only scorer option
+// that does not want to consider range count in its scoring, so it returns 0.
+func (bo BaseScorerOptions) adjustRangeCountForScoring(rangeCount int) int {
+	return rangeCount
+}
+
 // RangeCountScorerOptions is used by the replicateQueue to tell the Allocator's
 // rebalancing machinery to base its balance/convergence scores on range counts.
 // This means that the resulting rebalancing decisions will further the goal of
@@ -1155,14 +1171,15 @@ func rankedCandidateListForAllocation(
 			}
 			hasNonVoter = StoreHasReplica(s.StoreID, nonVoterReplTargets)
 		}
+		rangeCountScore := options.adjustRangeCountForScoring(int(s.Capacity.RangeCount))
 		candidates = append(candidates, candidate{
 			store:          s,
 			necessary:      necessary,
 			valid:          constraintsOK,
 			diversityScore: diversityScore,
 			balanceScore:   balanceScore,
 			hasNonVoter:    hasNonVoter,
-			rangeCount:     int(s.Capacity.RangeCount),
+			rangeCount:     rangeCountScore,
 		})
 	}
 	if options.deterministicForTesting() {
@@ -1270,7 +1287,7 @@ func candidateListForRemoval(
 		candidates[i].balanceScore = options.balanceScore(
 			removalCandidateStoreList, candidates[i].store.Capacity,
 		)
-		candidates[i].rangeCount = int(candidates[i].store.Capacity.RangeCount)
+		candidates[i].rangeCount = options.adjustRangeCountForScoring(int(candidates[i].store.Capacity.RangeCount))
 	}
 	// Re-sort to account for the ordering changes resulting from the addition of
 	// convergesScore, balanceScore, etc.
@@ -1735,7 +1752,7 @@ func rankedCandidateListForRebalancing(
 			balanceScore := options.balanceScore(comparable.candidateSL, existing.store.Capacity)
 			existing.convergesScore = convergesScore
 			existing.balanceScore = balanceScore
-			existing.rangeCount = int(existing.store.Capacity.RangeCount)
+			existing.rangeCount = options.adjustRangeCountForScoring(int(existing.store.Capacity.RangeCount))
 		}
 
 		var candidates candidateList
@@ -1761,7 +1778,7 @@ func rankedCandidateListForRebalancing(
 			)
 			cand.balanceScore = options.balanceScore(comparable.candidateSL, s.Capacity)
 			cand.convergesScore = options.rebalanceToConvergesScore(comparable, s)
-			cand.rangeCount = int(s.Capacity.RangeCount)
+			cand.rangeCount = options.adjustRangeCountForScoring(int(s.Capacity.RangeCount))
 			candidates = append(candidates, cand)
 		}