Further performance optimization

thumb.go

@@ -12,6 +12,7 @@ import (
 	"os"
 	"path"
 	"path/filepath"
+	"runtime"
 	"strconv"
 	"strings"
 	"sync"
@@ -48,8 +49,13 @@ var (
 	thumbLocks   = map[string]*sync.Mutex{}
 	thumbLocksMu sync.Mutex
 
-	digestCache  = map[string]digestEntry{}
+	digestCache   = map[string]digestEntry{}
 	digestCacheMu sync.Mutex
+
+	// thumbGenSlots bounds concurrent thumbnail generation. A grid of uncached
+	// photos otherwise decodes one full-size image per browser connection at
+	// once, which on the NAS means memory pressure and CPU thrash.
+	thumbGenSlots = make(chan struct{}, runtime.NumCPU())
 )
 
 // digestEntry remembers the digest of a source file so repeated requests do
@@ -179,7 +185,10 @@ func (h *handler) serveThumbnail(w http.ResponseWriter, r *http.Request, srcFS s
 		return
 	}
 
-	if err := generateThumb(cacheFS, write); err != nil {
+	thumbGenSlots <- struct{}{}
+	err = generateThumb(cacheFS, write)
+	<-thumbGenSlots
+	if err != nil {
 		log.Printf("thumb %s: %v", rel, err)
 		http.Error(w, "thumbnail failed", http.StatusInternalServerError)
 		return