db: improve pprof labels for compactions

We add a label for the compaction output level.

We also remove the "table-cache" label which is not very useful - you
can just focus on `load() / releaseLoop()` to get the same
information. This label "erases" the top-level labels because the
context is not plumbed correctly.

compaction.go

@@ -36,8 +36,15 @@ var errEmptyTable = errors.New("pebble: empty table")
 // concurrent excise or ingest-split operation.
 var ErrCancelledCompaction = errors.New("pebble: compaction cancelled by a concurrent operation, will retry compaction")
 
-var compactLabels = pprof.Labels("pebble", "compact")
-var flushLabels = pprof.Labels("pebble", "flush")
+var compactLabelsNoLevel = pprof.Labels("pebble", "compact")
+var compactLabelsPerLevel = func() [manifest.NumLevels]pprof.LabelSet {
+	var labels [manifest.NumLevels]pprof.LabelSet
+	for i := 0; i < manifest.NumLevels; i++ {
+		labels[i] = pprof.Labels("pebble", "compact", "output-level", fmt.Sprintf("L%d", i))
+	}
+	return labels
+}()
+var flushLabels = pprof.Labels("pebble", "flush", "output-level", "L0")
 var gcLabels = pprof.Labels("pebble", "gc")
 
 // expandedCompactionByteSizeLimit is the maximum number of bytes in all
@@ -2175,7 +2182,14 @@ func checkDeleteCompactionHints(
 
 // compact runs one compaction and maybe schedules another call to compact.
 func (d *DB) compact(c *compaction, errChannel chan error) {
-	pprof.Do(context.Background(), compactLabels, func(context.Context) {
+	var labels pprof.LabelSet
+	if c.outputLevel == nil {
+		// Delete-only compactions don't have an output level.
+		labels = compactLabelsNoLevel
+	} else {
+		labels = compactLabelsPerLevel[c.outputLevel.level]
+	}
+	pprof.Do(context.Background(), labels, func(context.Context) {
 		d.mu.Lock()
 		defer d.mu.Unlock()
 		if err := d.compact1(c, errChannel); err != nil {

file_cache.go

@@ -10,7 +10,6 @@ import (
 	"fmt"
 	"io"
 	"runtime/debug"
-	"runtime/pprof"
 	"sync"
 	"sync/atomic"
 	"unsafe"
@@ -85,8 +84,6 @@ func tableNewRangeKeyIter(newIters tableNewIters) keyspanimpl.TableNewSpanIter {
 	}
 }
 
-var fileCacheLabels = pprof.Labels("pebble", "table-cache")
-
 // fileCacheOpts contains the db specific fields of a file cache. This is stored
 // in the fileCacheContainer along with the file cache.
 //
@@ -396,12 +393,10 @@ func (c *fileCacheShard) init(size int) {
 }
 
 func (c *fileCacheShard) releaseLoop() {
-	pprof.Do(context.Background(), fileCacheLabels, func(context.Context) {
-		defer c.releaseLoopExit.Done()
-		for v := range c.releasingCh {
-			v.release(c)
-		}
-	})
+	defer c.releaseLoopExit.Done()
+	for v := range c.releasingCh {
+		v.release(c)
+	}
 }
 
 // checkAndIntersectFilters checks the specific table and block property filters
@@ -915,9 +910,7 @@ func (c *fileCacheShard) findNodeInternal(
 
 	// Note adding to the cache lists must complete before we begin loading the
 	// table as a failure during load will result in the node being unlinked.
-	pprof.Do(context.Background(), fileCacheLabels, func(context.Context) {
-		v.load(ctx, backingFileNum, c, dbOpts)
-	})
+	v.load(ctx, backingFileNum, c, dbOpts)
 	return v
 }