This was easily reproducible with M/R.
The easy workaround is to disable JS scope caching.
In scripting/engine.cpp, line 266:

if ( l.size() > 10 )

for

if ( true )
delete s;
}

It could be due to:

• us not doing cleanup of certain JS objects, but then we would hit an OOM from JS
• JS objects that are very small but point to large BSON data that is not reclaimed. Still should be cleared when GCing.
• leak / fragmentation in SM heap.