Reduce memory usage during Alter execution (#9301)

We currently use y.Throttle type to ensure that we do not concurrently
build indexes for many predicates at the same time to avoid opening too
many files on disk. This type allocates large memory initially to
achieve this. By default, on my machine we allocates 2GB of memory
every time Alter is called. Now, Go is slow to release this memory back
to OS increasing the memory usage.

This PR sets the max to 1024 predicates at a time. It would be lower if
Alter request involves fewer predicates in the request

worker/mutation.go

@@ -182,18 +182,15 @@ func runSchemaMutation(ctx context.Context, updates []*pb.SchemaUpdate, startTs
 	var wg sync.WaitGroup
 	wg.Add(1)
 	defer wg.Done()
-	// This throttle allows is used to limit the number of files which are opened simultaneously
-	// by badger while building indexes for predicates in background.
-	maxOpenFileLimit, err := x.QueryMaxOpenFiles()
-	if err != nil {
-		// Setting to default value on unix systems
-		maxOpenFileLimit = 1024
-	}
-	glog.Infof("Max open files limit: %d", maxOpenFileLimit)
+
 	// Badger opens around 8 files for indexing per predicate.
-	// The throttle limit is set to maxOpenFileLimit/8 to ensure that indexing does not throw
-	// "Too many open files" error.
-	throttle := y.NewThrottle(maxOpenFileLimit / 8)
+	// The throttle limit is set to 1024 to ensure that indexing
+	// does not throw "Too many open files" error.
+	maxPredicateAtATime := 1024
+	if len(updates) < maxPredicateAtATime {
+		maxPredicateAtATime = len(updates)
+	}
+	throttle := y.NewThrottle(maxPredicateAtATime)
 
 	buildIndexes := func(update *pb.SchemaUpdate, rebuild posting.IndexRebuild, c *z.Closer) {
 		// In case background indexing is running, we should call it here again.
@@ -237,6 +234,7 @@ func runSchemaMutation(ctx context.Context, updates []*pb.SchemaUpdate, startTs
 
 		// Start opIndexing task only if schema update needs to build the indexes.
 		if shouldRebuild && !gr.Node.isRunningTask(opIndexing) {
+			var err error
 			closer, err = gr.Node.startTaskAtTs(opIndexing, startTs)
 			if err != nil {
 				return err