Fix(OOM): Avoid too many splits

In edge cases, we end up with too many splits during compactions, which
make compactions take up too much RAM. Avoid that by limiting splits to
max 5. Also, avoid running more compactions when the memory usage is
going above 16GB.

Author: manishrjain

Diff for: levels.go

@@ -426,9 +426,6 @@ func (s *levelsController) runCompactor(id int, lc *z.Closer) {
 		return
 	}
 
-	ticker := time.NewTicker(50 * time.Millisecond)
-	defer ticker.Stop()
-
 	moveL0toFront := func(prios []compactionPriority) []compactionPriority {
 		idx := -1
 		for i, p := range prios {
@@ -474,10 +471,21 @@ func (s *levelsController) runCompactor(id int, lc *z.Closer) {
 		return false
 	}
 
+	ticker := time.NewTicker(50 * time.Millisecond)
+	defer ticker.Stop()
+	var backOff int
 	for {
 		select {
 		// Can add a done channel or other stuff.
 		case <-ticker.C:
+			if z.NumAllocBytes() > 16<<30 {
+				// Back off. We're already using a lot of memory.
+				backOff++
+				if backOff%1000 == 0 {
+					s.kv.opt.Infof("Compaction backed off %d times\n", backOff)
+				}
+				break
+			}
 			runOnce()
 		case <-lc.HasBeenClosed():
 			return
@@ -982,8 +990,16 @@ type compactDef struct {
 func (s *levelsController) addSplits(cd *compactDef) {
 	cd.splits = cd.splits[:0]
 
-	// Pick one every 3 tables.
-	const N = 3
+	// Let's say we have 10 tables in cd.bot and min width = 3. Then, we'll pick
+	// 0, 1, 2 (pick), 3, 4, 5 (pick), 6, 7, 8 (pick), 9 (pick, because last table).
+	// This gives us 4 picks for 10 tables.
+	// In an edge case, 142 tables in bottom led to 48 splits. That's too many splits, because it
+	// then uses up a lot of memory for table builder.
+	// We should keep it so we have at max 5 splits.
+	width := int(math.Ceil(float64(len(cd.bot) / 5.0)))
+	if width < 3 {
+		width = 3
+	}
 	skr := cd.thisRange
 	skr.extend(cd.nextRange)
 
@@ -1000,7 +1016,7 @@ func (s *levelsController) addSplits(cd *compactDef) {
 			addRange([]byte{})
 			return
 		}
-		if i%N == N-1 {
+		if i%width == width-1 {
 			// Right should always have ts=maxUint64 otherwise we'll lose keys
 			// in subcompaction. Consider the following.
 			// Top table is [A1...C3(deleted)]