我在mongodb命令行运行db.currentOp()，返回结果中出现"op" : "getmore"，，请问一下getmore是一个什么操作？而且特别耗时：

>db.currentOp()
{
    "inprog" : [
            {
                    "opid" : 821006778,
                    "active" : true,
                    "secs_running" : 0,
                    "microsecs_running" : NumberLong(252939),
                    "op" : "getmore",
                    "ns" : "local.oplog.$main",
                    "query" : {

                    },
                    "client" : "10.171.53.62:34632",
                    "desc" : "conn9912",
                    "threadId" : "0x7f9665e39700",
                    "connectionId" : 9912,
                    "waitingForLock" : false,
                    "numYields" : 0,
                    "lockStats" : {
                            "timeLockedMicros" : {
                                    "r" : NumberLong(14),
                                    "w" : NumberLong(0)
                            },
                            "timeAcquiringMicros" : {
                                    "r" : NumberLong(1),
                                    "w" : NumberLong(0)
                            }
                    }
            }
    ]
}