MongoDB - How to increase create index performance?

Question

I did some tests to create index on MongoDB. My goal is to understand how to shorten the build index time. I was thinking to change "maxIndexBuildMemoryUsageMegabytes" value to allow mongod to use more RAM to build index.
https://docs.mongodb.com/manual/reference/parameters/#param.maxIndexBuildMemoryUsageMegabytes

I did the following tests and got some interesting results:
Test #1
VM memory: 2GB
Document size: 100 million docs (7.7 GB data size)
maxIndexBuildMemoryUsageMegabytes: 500 MB
Took 2269 sec to build index.

Test #2
Changed maxIndexBuildMemoryUsageMegabytes to 800 MB
Took 1865 sec to build index.
This is what I expected.

Test #3
Increase the document size to 150 million (11.642 GB db size)
maxIndexBuildMemoryUsageMegabytes: 500 MB
Took 6085 (1.69 hrs) to build index

Test #4
Same as test #3 but changed maxIndexBuildMemoryUsageMegabytes to 800 MB
Took 26315 (7.3 hrs) to build index.
This is NOT what I'm expected. After the index built, used swap is 477 MB.

Then I tried on another VM will more memory and larger document size.

Test #5
VM memory: 4GB
Document size: 200 million docs (15 GB data size)
maxIndexBuildMemoryUsageMegabytes: 500 MB
Took 15032 sec to build index.

Test #6
Same as test #5 but changed maxIndexBuildMemoryUsageMegabytes to 1 GB.
Took 15053 sec to build index.
It didn't shorten the build index time.

Then I increase the document size to 400 million docs (37 GB data size).
I have tried both 500 MB and 1 GB on maxIndexBuildMemoryUsageMegabytes value.
The build index time are exactly the same.

My question is:
1. What should I set for maxIndexBuildMemoryUsageMegabytes? I know it is depended on memory size vs data size and other factors.
What is the good ratio that I can maximize the memory usage on building index but not fall into using swap. (swappiness is set to 1 already).

2. Any other ideas on how to increase build index performance?

Thanks!

Answer 1

If you are version lesser than 4.2, this parameter will only work on foreground process.

From 4.2, it will work on all index builds.(foreground and background). Maybe you can check on the version you are trying.

https://docs.mongodb.com/manual/reference/parameters/#param.maxIndexBuildMemoryUsageMegabytes