MongoDB getmore on a collection is very slow -


i trying debug high cpu issue on mongodb instance. have 2 shard r3.large aws instances. there not many page faults compared ops count.

system profile shows ton of getmore entries below. please me in finding out causing getmore slow.

    {         "op" : "getmore",         "ns" : "mydb.mycollection",         "cursorid" : 74493486271,         "ntoreturn" : 0,         "keyupdates" : 0,         "numyield" : 7,         "lockstats" : {             "timelockedmicros" : {                 "r" : numberlong(16140),                 "w" : numberlong(0)             },             "timeacquiringmicros" : {                 "r" : numberlong(6458801),                 "w" : numberlong(294321)             }         },         "nreturned" : 120,         "responselength" : 13100,         "millis" : 6304,         "execstats" : {          },         "ts" : isodate("2015-06-16t14:20:39.886z"),         "client" : "1.5.1.3",         "allusers" : [ ],         "user" : ""     }

answering own question may 1 else.

  • enabled more logging when cpu high db.admincommand( { setparameter: 1, loglevel: 1 } ) , after did reset loglevel: 0 (default).
  • then log showed aggregate query 0ms, right after that, getmore entry 5 6 secs.

  • aggregate query has cursor: { batchsize: 0 } initial batch size zero. so, query returns quickly. when application started iterating thru cursor, getmore logged , entry doesn't have query details.

    fixing aggregating query $match use indexes solved problem


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