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?