adjust group limit based on download speed

server/download.go

@@ -152,6 +152,36 @@ func (b *blobDownload) Run(ctx context.Context, requestURL *url.URL, opts *regis
 	_ = file.Truncate(b.Total)
 
 	g, inner := NewLimitGroup(ctx, numDownloadParts)
+
+	go func() {
+		ticker := time.NewTicker(time.Second)
+		var n int64 = 1
+		var maxDelta float64
+		var buckets []int64
+		for {
+			select {
+			case <-ticker.C:
+				buckets = append(buckets, b.Completed.Load())
+				if len(buckets) < 2 {
+					continue
+				} else if len(buckets) > 10 {
+					buckets = buckets[1:]
+				}
+
+				delta := float64((buckets[len(buckets)-1] - buckets[0])) / float64(len(buckets))
+				slog.Debug(fmt.Sprintf("delta: %s/s max_delta: %s/s", format.HumanBytes(int64(delta)), format.HumanBytes(int64(maxDelta))))
+				if delta > maxDelta*1.5 {
+					maxDelta = delta
+					g.SetLimit(n)
+					n++
+				}
+
+			case <-ctx.Done():
+				return
+			}
+		}
+	}()
+
 	for i := range b.Parts {
 		part := b.Parts[i]
 		if part.Completed == part.Size {
@@ -413,6 +443,7 @@ func (g *LimitGroup) Go(fn func() error) {
 
 func (g *LimitGroup) SetLimit(n int64) {
 	if n > 0 {
+		slog.Debug(fmt.Sprintf("setting limit to %d", n))
 		g.weight = g.max_weight / n
 	}
 }