compact: make TombstoneDenseCompactionThreshold dynamically reconfigurable

Previously, TombstoneDenseCompactionThreshold was a static float64 value
that could only be set at initialization time. This made it impossible to
dynamically adjust the threshold based on changing workload characteristics
or operational requirements.

This commit changes TombstoneDenseCompactionThreshold from a float64 to
a function returning float64, following the pattern used by other dynamic
options like CompactionGarbageFractionForMaxConcurrency and
VirtualTableRewriteUnreferencedFraction. The default value remains 0.10.

Benefits:
- Allows runtime adjustment of tombstone compaction behavior
- Enables workload-specific tuning without restarting the database
- Makes it easier to turn down or disable tombstone compactions when they
  lead to increased resource usage with no visible latency benefits

Changes:
- Updated Options.Experimental.TombstoneDenseCompactionThreshold to func() float64
- Modified EnsureDefaults to use function wrapper
- Updated String() and Parse() methods to handle function type
- Modified pickTombstoneDensityCompaction to call the function
- Updated all test files to use function syntax

Fixes #5457.

Author: ever0de

compaction_picker.go

@@ -1827,7 +1827,8 @@ func (p *compactionPickerByScore) pickBlobFileRewriteCandidate(
 func (p *compactionPickerByScore) pickTombstoneDensityCompaction(
 	env compactionEnv,
 ) (pc *pickedTableCompaction) {
-	if p.opts.Experimental.TombstoneDenseCompactionThreshold <= 0 {
+	threshold := p.opts.Experimental.TombstoneDenseCompactionThreshold()
+	if threshold <= 0 {
 		// Tombstone density compactions are disabled.
 		return nil
 	}
@@ -1855,7 +1856,7 @@ func (p *compactionPickerByScore) pickTombstoneDensityCompaction(
 			if !propsValid {
 				continue
 			}
-			if props.TombstoneDenseBlocksRatio < p.opts.Experimental.TombstoneDenseCompactionThreshold {
+			if props.TombstoneDenseBlocksRatio < threshold {
 				continue
 			}
 			overlaps := p.vers.Overlaps(lastNonEmptyLevel, f.UserKeyBounds())

compaction_test.go

@@ -3507,7 +3507,7 @@ func TestTombstoneDensityCompactionMoveOptimization(t *testing.T) {
 	)
 
 	opts := DefaultOptions()
-	opts.Experimental.TombstoneDenseCompactionThreshold = 0.5 // Lower for test
+	opts.Experimental.TombstoneDenseCompactionThreshold = func() float64 { return 0.5 } // Lower for test
 	opts.Experimental.NumDeletionsThreshold = 1
 	opts.Experimental.CompactionScheduler = func() CompactionScheduler {
 		return NewConcurrencyLimitSchedulerWithNoPeriodicGrantingForTest()
@@ -3609,7 +3609,7 @@ func TestTombstoneDensityCompactionMoveOptimization_NoMoveWithOverlap(t *testing
 	)
 
 	opts := DefaultOptions()
-	opts.Experimental.TombstoneDenseCompactionThreshold = 0.5 // Lower for test
+	opts.Experimental.TombstoneDenseCompactionThreshold = func() float64 { return 0.5 } // Lower for test
 	opts.Experimental.NumDeletionsThreshold = 1
 	opts.Experimental.CompactionScheduler = func() CompactionScheduler {
 		return NewConcurrencyLimitSchedulerWithNoPeriodicGrantingForTest()
@@ -3693,7 +3693,7 @@ func TestTombstoneDensityCompactionMoveOptimization_GrandparentOverlapTooLarge(t
 	)
 
 	opts := DefaultOptions()
-	opts.Experimental.TombstoneDenseCompactionThreshold = 0.5
+	opts.Experimental.TombstoneDenseCompactionThreshold = func() float64 { return 0.5 }
 	opts.Experimental.NumDeletionsThreshold = 1
 	opts.Experimental.CompactionScheduler = func() CompactionScheduler {
 		return NewConcurrencyLimitSchedulerWithNoPeriodicGrantingForTest()
@@ -3761,7 +3761,7 @@ func TestTombstoneDensityCompactionMoveOptimization_BelowDensityThreshold(t *tes
 	)
 
 	opts := DefaultOptions()
-	opts.Experimental.TombstoneDenseCompactionThreshold = 0.9 // Set high threshold
+	opts.Experimental.TombstoneDenseCompactionThreshold = func() float64 { return 0.9 } // Set high threshold
 	opts.Experimental.NumDeletionsThreshold = 1
 	opts.Experimental.CompactionScheduler = func() CompactionScheduler {
 		return NewConcurrencyLimitSchedulerWithNoPeriodicGrantingForTest()
@@ -3813,7 +3813,7 @@ func TestTombstoneDensityCompactionMoveOptimization_InvalidStats(t *testing.T) {
 	)
 
 	opts := DefaultOptions()
-	opts.Experimental.TombstoneDenseCompactionThreshold = 0.5
+	opts.Experimental.TombstoneDenseCompactionThreshold = func() float64 { return 0.5 }
 	opts.Experimental.NumDeletionsThreshold = 1
 	opts.Experimental.CompactionScheduler = func() CompactionScheduler {
 		return NewConcurrencyLimitSchedulerWithNoPeriodicGrantingForTest()

options.go

@@ -695,17 +695,19 @@ type Options struct {
 		// sstable writers. The default value is 0.5.
 		DeletionSizeRatioThreshold float32
 
-		// TombstoneDenseCompactionThreshold is the minimum percent of data
-		// blocks in a table that must be tombstone-dense for that table to be
-		// eligible for a tombstone density compaction. It should be defined as a
-		// ratio out of 1. The default value is 0.10.
+		// TombstoneDenseCompactionThreshold is a function that returns the minimum
+		// percent of data blocks in a table that must be tombstone-dense for that
+		// table to be eligible for a tombstone density compaction. The value should
+		// be defined as a ratio out of 1. The default value is 0.10.
 		//
 		// If multiple tables are eligible for a tombstone density compaction, then
 		// tables with a higher percent of tombstone-dense blocks are still
 		// prioritized for compaction.
 		//
-		// A zero or negative value disables tombstone density compactions.
-		TombstoneDenseCompactionThreshold float64
+		// Using a function allows for dynamic reconfiguration of the threshold based
+		// on workload characteristics. A zero or negative value disables tombstone
+		// density compactions.
+		TombstoneDenseCompactionThreshold func() float64
 
 		// FileCacheShards is the number of shards per file cache.
 		// Reducing the value can reduce the number of idle goroutines per DB
@@ -1672,8 +1674,8 @@ func (o *Options) EnsureDefaults() {
 	if o.Experimental.DeletionSizeRatioThreshold == 0 {
 		o.Experimental.DeletionSizeRatioThreshold = sstable.DefaultDeletionSizeRatioThreshold
 	}
-	if o.Experimental.TombstoneDenseCompactionThreshold == 0 {
-		o.Experimental.TombstoneDenseCompactionThreshold = 0.10
+	if o.Experimental.TombstoneDenseCompactionThreshold == nil {
+		o.Experimental.TombstoneDenseCompactionThreshold = func() float64 { return 0.10 }
 	}
 	if o.Experimental.FileCacheShards <= 0 {
 		o.Experimental.FileCacheShards = runtime.GOMAXPROCS(0)
@@ -1821,7 +1823,7 @@ func (o *Options) String() string {
 	fmt.Fprintf(&buf, "  read_sampling_multiplier=%d\n", o.Experimental.ReadSamplingMultiplier)
 	fmt.Fprintf(&buf, "  num_deletions_threshold=%d\n", o.Experimental.NumDeletionsThreshold)
 	fmt.Fprintf(&buf, "  deletion_size_ratio_threshold=%f\n", o.Experimental.DeletionSizeRatioThreshold)
-	fmt.Fprintf(&buf, "  tombstone_dense_compaction_threshold=%f\n", o.Experimental.TombstoneDenseCompactionThreshold)
+	fmt.Fprintf(&buf, "  tombstone_dense_compaction_threshold=%f\n", o.Experimental.TombstoneDenseCompactionThreshold())
 	// We no longer care about strict_wal_tail, but set it to true in case an
 	// older version reads the options.
 	fmt.Fprintf(&buf, "  strict_wal_tail=%t\n", true)
@@ -2257,7 +2259,11 @@ func (o *Options) Parse(s string, hooks *ParseHooks) error {
 				o.Experimental.DeletionSizeRatioThreshold = float32(val)
 				err = parseErr
 			case "tombstone_dense_compaction_threshold":
-				o.Experimental.TombstoneDenseCompactionThreshold, err = strconv.ParseFloat(value, 64)
+				var threshold float64
+				threshold, err = strconv.ParseFloat(value, 64)
+				if err == nil {
+					o.Experimental.TombstoneDenseCompactionThreshold = func() float64 { return threshold }
+				}
 			case "table_cache_shards":
 				o.Experimental.FileCacheShards, err = strconv.Atoi(value)
 			case "table_format":

options_test.go

@@ -406,7 +406,7 @@ func TestOptionsParse(t *testing.T) {
 			opts.Experimental.ReadSamplingMultiplier = 400
 			opts.Experimental.NumDeletionsThreshold = 500
 			opts.Experimental.DeletionSizeRatioThreshold = 0.7
-			opts.Experimental.TombstoneDenseCompactionThreshold = 0.2
+			opts.Experimental.TombstoneDenseCompactionThreshold = func() float64 { return 0.2 }
 			opts.Experimental.FileCacheShards = 500
 			opts.Experimental.SecondaryCacheSizeBytes = 1024
 			opts.Experimental.ValueSeparationPolicy = func() ValueSeparationPolicy {