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  1. Core Server
  2. SERVER-51340

MongoDB $accumulator: MongoError: Out of memory

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Details

    • Bug
    • Status: Closed
    • Major - P3
    • Resolution: Works as Designed
    • 4.4.1
    • None
    • Aggregation Framework
    • ALL
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      Create 35 documents, 3 MB each. Run an aggregation query, including $accumulator.   I also opened a question in stackoverflow:  https://stackoverflow.com/questions/64193318/mongodb-mongoerror-out-of-memory

    Description

      I have a newly installed MongoDB server (v4.4.1) running on an AWS Ubuntu EC2. The server has nothing else installed on it. The DB currently contains 35 documents of 3 MB each, meaning less than 110 MB of data.

      I'm running an aggregation query that contains 2 stages: a simple $match stage that makes sure that only 31 documents are aggregated, and a $group stage with $accumulator inside (that stage doesn't allocate much new space, and even if it did, I expect that when handling with 3 MB documents it won't be much more). The query's result should be a single 3 MB document that is a merger of all of the above.

      Running the query when only 3 documents exist - finishes without a problem. But with 35 - I consistently get the following error: 

      MongoError: Out of memory
      at MessageStream.messageHandler (/app/node_modules/mongodb/lib/cmap/connection.js:268:20)
      at MessageStream.emit (events.js:314:20)
      at processIncomingData (/app/node_modules/mongodb/lib/cmap/message_stream.js:144:12)
      at MessageStream._write (/app/node_modules/mongodb/lib/cmap/message_stream.js:42:5)
      at writeOrBuffer (_stream_writable.js:352:12)
      at MessageStream.Writable.write (_stream_writable.js:303:10)
      at Socket.ondata (_stream_readable.js:717:22)
      at Socket.emit (events.js:314:20)
      at addChunk (_stream_readable.js:307:12)
      at readableAddChunk (_stream_readable.js:282:9)
       
      

      I've read a lot online and couldn't solve it, I've tried:

      1. Upgrading the server's hardware to t3.large (8 GB RAM).
      2. Adding allowDiskUse to the query:

       

      const options = { allowDiskUse: true, // explain: true, };
      mongoConn.db(dbConfig.database).collection(collection).aggregate(aggregationPipeline, options).toArray((err: MongoError, result: any) => {
      

       

      1. With htop, I can see the total memory usage of 210M/7.68G after restarting MongoDB, and during the query it climbs to a peak of 691M/7.68G, fails, and remains on 627M/7.68G afterward. 
      1. With db.enableFreeMonitoring() I can see a constant 2 GB virtual memory usage, with peaks to 2.1 GB: 
      1. The result of db.serverStatus().tcmalloc.tcmalloc.formattedString
      1. The result of db.serverStatus().mem

      Summary: I know that MongoDB has a 100MB memory limit, but I guess that it shouldn't reach it with 3 MB documents, and allowDiskUse. What am I missing here?

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              dmitry.agranat@mongodb.com Dmitry Agranat
              asafshifer@gmail.com Asaf Shifer
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