Hi @Kim_Hakseon,
It’s a great question – thanks for asking! I didn’t find the reference in a quick search, either, but I do know the default stack size per thread has been 1MB since MongoDB 2.0.
The documentation section I was thinking of still exists in the 4.0 and earlier manual versions: How do I calculate how much RAM I need for my application?.
There’s an open issue in the docs backlog which you can upvote & comment on: DOCS-12986: Add information about Connection Memory Consumption.
It looks like this section was removed when the documentation was being revised for the MongoDB 4.2 release. The docs concern appears to be that the existing description didn’t provide enough guidance for the FAQ topic of “How do I calculate how much RAM I need for my application?” (which is potentially a full standalone tutorial/course!).
I’ll bring this to the attention of the docs team, as we should have some mention of the connection-per-thread memory usage. Thanks again for asking the question!
Regards,
Stennie