fix: export prometheus metrics for cache GC triggers (#9815)

Bonus change to use channel to serialize triggers,
instead of using atomic variables. More efficient
mechanism for synchronization.

Co-authored-by: Nitish Tiwari <nitish@minio.io>

cmd/disk-cache-backend.go

@@ -125,12 +125,14 @@ func (m *cacheMeta) ToObjectInfo(bucket, object string) (o ObjectInfo) {
 
 // represents disk cache struct
 type diskCache struct {
-	gcCounter uint64 // ref: https://golang.org/pkg/sync/atomic/#pkg-note-BUG
 	// is set to 0 if drive is offline
-	online uint32
+	online       uint32 // ref: https://golang.org/pkg/sync/atomic/#pkg-note-BUG
+	purgeRunning int32
 
-	dir           string // caching directory
-	quotaPct      int    // max usage in %
+	triggerGC     chan struct{}
+	dir           string         // caching directory
+	stats         CacheDiskStats // disk cache stats for prometheus
+	quotaPct      int            // max usage in %
 	pool          sync.Pool
 	after         int // minimum accesses before an object is cached.
 	lowWatermark  int
@@ -142,12 +144,14 @@ type diskCache struct {
 }
 
 // Inits the disk cache dir if it is not initialized already.
-func newDiskCache(dir string, quotaPct, after, lowWatermark, highWatermark int) (*diskCache, error) {
+func newDiskCache(ctx context.Context, dir string, quotaPct, after, lowWatermark, highWatermark int) (*diskCache, error) {
 	if err := os.MkdirAll(dir, 0777); err != nil {
 		return nil, fmt.Errorf("Unable to initialize '%s' dir, %w", dir, err)
 	}
 	cache := diskCache{
 		dir:           dir,
+		triggerGC:     make(chan struct{}),
+		stats:         CacheDiskStats{Dir: dir},
 		quotaPct:      quotaPct,
 		after:         after,
 		lowWatermark:  lowWatermark,
@@ -161,6 +165,8 @@ func newDiskCache(dir string, quotaPct, after, lowWatermark, highWatermark int)
 		},
 		nsMutex: newNSLock(false),
 	}
+	go cache.purgeWait(ctx)
+	cache.diskUsageHigh() // update if cache usage is already high.
 	cache.NewNSLockFn = func(ctx context.Context, cachePath string) RWLocker {
 		return cache.nsMutex.NewNSLock(ctx, nil, cachePath, "")
 	}
@@ -181,7 +187,12 @@ func (c *diskCache) diskUsageLow() bool {
 		return false
 	}
 	usedPercent := (di.Total - di.Free) * 100 / di.Total
-	return int(usedPercent) < gcStopPct
+	low := int(usedPercent) < gcStopPct
+	atomic.StoreUint64(&c.stats.UsagePercent, usedPercent)
+	if low {
+		atomic.StoreInt32(&c.stats.UsageState, 0)
+	}
+	return low
 }
 
 // Returns if the disk usage reaches high water mark w.r.t the configured cache quota.
@@ -196,7 +207,12 @@ func (c *diskCache) diskUsageHigh() bool {
 		return false
 	}
 	usedPercent := (di.Total - di.Free) * 100 / di.Total
-	return int(usedPercent) >= gcTriggerPct
+	high := int(usedPercent) >= gcTriggerPct
+	atomic.StoreUint64(&c.stats.UsagePercent, usedPercent)
+	if high {
+		atomic.StoreInt32(&c.stats.UsageState, 1)
+	}
+	return high
 }
 
 // Returns if size space can be allocated without exceeding
@@ -230,24 +246,36 @@ var (
 	errDoneForNow = errors.New("done for now")
 )
 
+func (c *diskCache) purgeWait(ctx context.Context) {
+	for {
+		select {
+		case <-ctx.Done():
+		case <-c.triggerGC: // wait here until someone triggers.
+			c.purge(ctx)
+		}
+	}
+}
+
 // Purge cache entries that were not accessed.
 func (c *diskCache) purge(ctx context.Context) {
-	if c.diskUsageLow() {
+	if atomic.LoadInt32(&c.purgeRunning) == 1 || c.diskUsageLow() {
 		return
 	}
+
 	toFree := c.toClear()
 	if toFree == 0 {
 		return
 	}
+
+	atomic.StoreInt32(&c.purgeRunning, 1) // do not run concurrent purge()
+	defer atomic.StoreInt32(&c.purgeRunning, 0)
+
 	// expiry for cleaning up old cache.json files that
 	// need to be cleaned up.
 	expiry := UTCNow().Add(-cacheExpiryDays)
 	// defaulting max hits count to 100
-	scorer, err := newFileScorer(toFree, time.Now().Unix(), 100)
-	if err != nil {
-		logger.LogIf(ctx, err)
-		return
-	}
+	// ignore error we know what value we are passing.
+	scorer, _ := newFileScorer(toFree, time.Now().Unix(), 100)
 
 	// this function returns FileInfo for cached range files and cache data file.
 	fiStatFn := func(ranges map[string]string, dataFile, pathPrefix string) map[string]os.FileInfo {
@@ -326,27 +354,20 @@ func (c *diskCache) purge(ctx context.Context) {
 		return
 	}
 
-	for _, path := range scorer.fileNames() {
-		removeAll(path)
-		slashIdx := strings.LastIndex(path, SlashSeparator)
-		pathPrefix := path[0:slashIdx]
-		fname := path[slashIdx+1:]
-		if fname == cacheDataFile {
-			removeAll(pathPrefix)
+	scorer.purgeFunc(func(qfile queuedFile) {
+		fileName := qfile.name
+		removeAll(fileName)
+		slashIdx := strings.LastIndex(fileName, SlashSeparator)
+		if slashIdx >= 0 {
+			fileNamePrefix := fileName[0:slashIdx]
+			fname := fileName[slashIdx+1:]
+			if fname == cacheDataFile {
+				removeAll(fileNamePrefix)
+			}
 		}
-	}
-}
+	})
 
-func (c *diskCache) incGCCounter() {
-	atomic.AddUint64(&c.gcCounter, 1)
-}
-
-func (c *diskCache) resetGCCounter() {
-	atomic.StoreUint64(&c.gcCounter, 0)
-}
-
-func (c *diskCache) gcCount() uint64 {
-	return atomic.LoadUint64(&c.gcCounter)
+	scorer.reset()
 }
 
 // sets cache drive status
@@ -630,7 +651,7 @@ func newCacheEncryptMetadata(bucket, object string, metadata map[string]string)
 // Caches the object to disk
 func (c *diskCache) Put(ctx context.Context, bucket, object string, data io.Reader, size int64, rs *HTTPRangeSpec, opts ObjectOptions, incHitsOnly bool) error {
 	if c.diskUsageHigh() {
-		c.incGCCounter()
+		c.triggerGC <- struct{}{}
 		io.Copy(ioutil.Discard, data)
 		return errDiskFull
 	}