MongoDB MongoError: Out of memory

I have a newly installed MongoDB server 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 documents 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) 

/etc/mongod.conf:

# mongod.conf # for documentation of all options, see: # # Where and how to store data. storage: dbPath: /var/lib/mongodb journal: enabled: true # engine: # mmapv1: # wiredTiger: # where to write logging data. systemLog: destination: file logAppend: true path: /var/log/mongodb/mongod.log # network interfaces net: port: 27017 bindIp: 127.0.0.1,<IP> # how the process runs processManagement: timeZoneInfo: /usr/share/zoneinfo #security: security: authorization: "enabled" #operationProfiling: #replication: #sharding: ## Enterprise-Only Options: #auditLog: #snmp: 

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

1. Upgrading the server's hardware to t2.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) => { 

3. 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.

4. With db.enableFreeMonitoring() I can see a constant 2 GB virtual memory usage, with peaks to 2.1 GB:

5. The result of db.serverStatus().tcmalloc.tcmalloc.formattedString:

6. 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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