Testing QA-175 for the case where working set size exceeds available RAM, but I can't seem to come up with results that make sense, not sure if this is a bug or flaw in my procedure here.
On a linux machine with these mem characteristics:

             total       used       free     shared    buffers     cached
Mem:        435132     355152      79980          0      12440     185944
-/+ buffers/cache:     156768     278364
Swap:       262140      19200     242940

I loaded up the enron data set, size is:

"count" : 501513,
"size" : 1527603312,
"avgObjSize" : 3045.9894598943597,
"storageSize" : 1605427200,

Tried to get db to access the entire data set, about 1.4gb which should exceed ram size, and thus consider the entire collection as the "working set".
To do this I queried for the whole collection repeatedly (using both true and false for snapshot, which should force the _id index to be paged in):

function query_data(snapshot){
    var counter = 0;
    if(snapshot){
        x = db.messages.find()
    }else{
        x = db.messages.find().snapshot()
    }
 
    while(x.hasNext()){
    counter++;
    var y = x.next();
    if(counter % 10000 == 0){
        print(counter)
    }
    }   
    print(counter, "documents read.")
}

Afterwards when I run workingSet, the result is 264134 pagesInMemory, which works out to 1081892864 bytes = about 1 gb, way under the actual data size (about 1.4gb).
I can't seem to get the pagesInMemory to exceed that value, tried other queries as well as .count() and db.runCommand(

) but none seem to have an effect.