Each index build is allowed to use up to 200MB of memory outside of the WT cache by default. This is controlled by maxIndexBuildMemoryUsageMegabytes. In an attempt to bound memory usage and reduce WT cache pressure, we limit the number of concurrent index builds started by a user on a primary node.
Any index builds started over the limit will block until the number of concurrent index builds drops below the limit. The log message "Too many index builds running simultaneously, waiting until the number of active index builds is below the threshold" (ID 4715500) will be logged when the limit is reached and an index build has blocked.
This default limit can be raised with the maxNumActiveUserIndexBuilds setParameter. This can be changed at startup or runtime.
Both the primary and secondary nodes will have an unlimited thread pool size. This is done to allow secondary nodes to startup as many index builders as necessary in order to prevent scheduling deadlocks during initial sync or oplog application.
When commands are run from user connections that need to create indexes, those commands will hang until there are less than 3 running index builder threads, or until the operation is interrupted.