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  1. Core Server
  2. SERVER-9415

MongoDB only uses small amount of RAM

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    Details

    • Type: Bug
    • Status: Closed
    • Priority: Blocker - P1
    • Resolution: Works as Designed
    • Affects Version/s: None
    • Fix Version/s: None
    • Component/s: Performance
    • Labels:
      None
    • Environment:
      4-CPU 16GB RAM CenOS 6.2 VM
    • Operating System:
      ALL
    • Case:

      Description

      I've a single Mongo instance running on a 4-CPU 16GB RAM CenOS 6.2 VM. It's running very slowly mostly of the time. Here is the mongostat output:

      insert  query update delete getmore command flushes mapped  vsize    res        faults       locked db         idx miss %     qr|qw   ar|aw  netIn netOut  conn repl       time
          *0     62     12        *0           0             9|0               0          82g   174g       2.61g     10       gowmain:13.2%          0            52|0     1|5      12k    64k     315  PRI   14:41:27
          *0     90      26       *0           0            21|0               0         82g   174g       2.61g     12       gowmain:56.0%          0            63|0     0|1      23k   145k    315  PRI   14:41:29
          *0    157     38       *0           0            19|0              1         82g   174g        2.6g        44       gowmain:56.4%         0            61|0     1|0      36k   330k    315  PRI   14:41:31
          *0    372     95       *0           0            62|0              0         82g   174g       2.61g      141      gowmain:38.1%         0            53|0     1|0     208k    1m    315  PRI   14:41:32
          *0    303     58       *0           0            31|0              0         82g   174g       2.62g       43       gowmain:33.5%         0            52|0     1|0      68k    429k    315  PRI   14:41:33
          *0    277     43       *0           0            17|0              0         82g   174g       2.61g       53       gowmain:21.8%          0            55|0     0|1    103k   1m      315  PRI   14:41:34
          *0    176     24       *0           0             9|0               0         82g   174g       2.61g       17       gowmain:63.8%          0            58|0     1|0     48k     581k   315  PRI   14:41:35
          *0    506     79       *0           0            38|0              0         82g   174g        2.6g        91       gowmain:47.6%          0            62|0     1|0     143k   852k   316  PRI   14:41:37
          *0    236     39       *0           0            15|0              0         82g   174g       2.61g       95       gowmain:21.2%          0            55|1     0|1     86k    1m      316  PRI   14:41:38
          *0    195     41       *0           0            15|0              0         82g   174g        2.6g       86       gowmain:28.3%          0            50|1     2|0       89k   1m      316  PRI   14:41:40
       

      The command line options are: --rest --fork --master --oplogSize=22770 --port 27017 --dbpath /data/mongodb/dbdata/ --logpath /data/logs/mongodb.log --auth

      Here is the top output:

      top - 15:05:58 up 18 days, 21:03,  2 users,  load average: 2.37, 2.79, 2.83
      Tasks: 121 total,   1 running, 120 sleeping,   0 stopped,   0 zombie
      Cpu(s):  0.5%us,  0.5%sy,  0.0%ni, 74.9%id, 24.1%wa,  0.0%hi,  0.0%si,  0.0%st
      Mem:  16139308k total, 15988616k used,   150692k free,    11336k buffers
      Swap:  4194296k total,      536k used,  4193760k free, 14952988k cached
       
        PID   USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+   COMMAND
      21253  root        20    0   173g  2.6g   2.4g  S    7.0        17.0   468:05.07  mongod

      The weird thing is MongoDB consistently only consumes only 2.6G of RAM! However, the output from 'top' shows all 16GB RAM is used up.

      A couple of other observations:

      • The CPU load from 'top' is usually between 2.00 to 3.00
      • The lock% range between 40 to 80.
      • When the read queue goes up to 150 to 200, the system is unusable.
      • The number of faults is not too bad, but iostat shows 100% IO itilization on the mongo partition. Also, sometimes the await time in iostat is more than 1000ms!!!

      The DB has one large collection having about 50 million documents; all other collections are small. The DB serves about 2000 users frontended by Tomcat (which has almost no load).

      I cannot figure out why it uses less than 3GB of RAM while the system has 16GB.
      Are we doing something obviously wrong here?

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                  Updated:
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