hasParentQuery search slow down fielddata _parent memory clear

Elastic Stack Elasticsearch
1

es2.1.1
principle of Parent Child
When I build a index, and real-time update data ,_parent memory has been cleared. then hasParentQuery search a very long time.
how to change the _parent memory cleanup strategy?

es v2.1.1
"indices": {
        "fielddata": {
          "memory_size": "0b",
          "memory_size_in_bytes": 0,
          "evictions": 0,
          "fields": {
            "_parent": {
              "memory_size": "0b",
              "memory_size_in_bytes": 0
            }
          }
        }
      }

es v1.1.1
      "indices": {
        "fielddata": {
          "memory_size": "606.3mb",
          "memory_size_in_bytes": 635833336,
          "evictions": 0,
            "_parent": {
              "memory_size": "188.1mb",
              "memory_size_in_bytes": 197273307
            }
       }
    }