statistics: stats cache set default quota as 20% (pingcap#58013)

close pingcap#58014

pkg/statistics/handle/cache/internal/lfu/lfu_cache.go

@@ -75,14 +75,14 @@ func NewLFU(totalMemCost int64) (*LFU, error) {
 }
 
 // adjustMemCost adjusts the memory cost according to the total memory cost.
-// When the total memory cost is 0, the memory cost is set to half of the total memory.
+// When the total memory cost is 0, the memory cost is set to 20% of the total memory.
 func adjustMemCost(totalMemCost int64) (result int64, err error) {
 	if totalMemCost == 0 {
 		memTotal, err := memory.MemTotal()
 		if err != nil {
 			return 0, err
 		}
-		return int64(memTotal / 2), nil
+		return int64(memTotal * 20 / 100), nil
 	}
 	return totalMemCost, nil
 }