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  1. Core Server
  2. SERVER-16593

mongod memory usage much larger than cache_size for wiredtiger b-tree

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    • Type: Icon: Bug Bug
    • Resolution: Duplicate
    • Priority: Icon: Major - P3 Major - P3
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    • Affects Version/s: None
    • Component/s: Storage
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      Start WT b-tree via:

      mongod --config /data/mysql/mongo.28/mongo.conf --storageEngine wiredTiger --wiredTigerEngineConfig="cache_size=8G,direct_io=[data]"

      With WT LSM this does not reproduce:

      mongod --config /data/mysql/mongo.28/mongo.conf --storageEngine wiredTiger --wiredTigerEngineConfig="cache_size=8G,direct_io=[data]" --wiredTigerIndexConfig="type=lsm"

      mongo.conf is:

      processManagement:
  fork: true
systemLog:
  destination: file
  path: /data/mysql/mongo.28/log
  logAppend: true
storage:
  dbPath: /data/mysql/mongo.28/data
  journal:
    enabled: true
  mmapv1:
    syncPeriodSecs: 60
    journal:
      commitIntervalMs: 100

      The test client is iibench with 10 concurrent load threads as explained in https://jira.mongodb.org/browse/SERVER-16575

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      Start WT b-tree via: mongod --config /data/mysql/mongo.28/mongo.conf --storageEngine wiredTiger --wiredTigerEngineConfig= "cache_size=8G,direct_io=[data]" With WT LSM this does not reproduce: mongod --config /data/mysql/mongo.28/mongo.conf --storageEngine wiredTiger --wiredTigerEngineConfig= "cache_size=8G,direct_io=[data]" --wiredTigerIndexConfig= "type=lsm" mongo.conf is: processManagement: fork: true systemLog: destination: file path: /data/mysql/mongo.28/log logAppend: true storage: dbPath: /data/mysql/mongo.28/data journal: enabled: true mmapv1: syncPeriodSecs: 60 journal: commitIntervalMs: 100 The test client is iibench with 10 concurrent load threads as explained in https://jira.mongodb.org/browse/SERVER-16575
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      Memory usage by mongod is much larger than the cache_size setting for the WT b-tree. This is also a problem for the WT LSM. When using cache_size=8G and then running iibench I see that mongod vsz and rss are larger than 60G for the WT b-tree:
      root 1919 111 44.3 66988456 65824412 ? Sl Dec17 1378:17 bin.tcm/mongod --config /data/mysql/mongo.28/mongo.conf --storageEngine wiredTiger --wiredTigerEngineConfig=cache_size=8G,direct_io=[data]

      And larger than 30G for the WT LSM:
      root 20861 225 22.0 40058892 32678208 ? Sl Dec17 2783:38 bin.tcm/mongod --config /data/mysql/mongo.28/mongo.conf --storageEngine wiredTiger --wiredTigerEngineConfig=cache_size=8G,direct_io=[data] --wiredTigerIndexConfig=type=lsm

      From /proc/$pid/smaps for the WT b-tree there is one huge allocation for the heap:
      02c10000-febde6000 rw-p 00000000 00:00 0 [heap]
      Size: 66733912 kB
      Rss: 65805752 kB
      Pss: 65805752 kB
      Shared_Clean: 0 kB
      Shared_Dirty: 0 kB
      Private_Clean: 0 kB
      Private_Dirty: 65805752 kB
      Referenced: 65805752 kB
      Anonymous: 65805752 kB
      AnonHugePages: 0 kB
      Swap: 0 kB
      KernelPageSize: 4 kB
      MMUPageSize: 4 kB
      Locked: 0 kB

      And for the WT LSM the heap allocation isn't as huge (less than 8G):

      02c10000-1c46fe000 rw-p 00000000 00:00 0 [heap]
      Size: 7367608 kB
      Rss: 5496284 kB
      Pss: 5496284 kB
      Shared_Clean: 0 kB
      Shared_Dirty: 0 kB
      Private_Clean: 0 kB
      Private_Dirty: 5496284 kB
      Referenced: 5496284 kB
      Anonymous: 5496284 kB
      AnonHugePages: 0 kB
      Swap: 0 kB
      KernelPageSize: 4 kB
      MMUPageSize: 4 kB
      Locked: 0 kB

            Assignee:
            michael.cahill@mongodb.com Michael Cahill (Inactive)
            Reporter:
            mdcallag Mark Callaghan
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              Created:
              Updated:
              Resolved: