Cleaning/Releasing large arrays from memory

[Clinton via Digitalmars-d-learn]

Hi guys, I have a question on how to free large arrays in D after 
they're no longer needed.

Let's say I have this:

SomeKey[] getKeys() {
  SomeKey[] n;

  foreach(batching stuff...) {
   SomeKey newstuff = db.select!(SomeKey[])(...); // gets around 
6M of these from db

   redis.send("newItems", newStuff.length);

   n ~= newStuff;

   destroy(newStuff);
  }

  return n;
}

void main() {
  SomeKey[] myarray = getKeys();
  ulong[string] mappedValues;

  foreach(i, item; myArray) {
   mappedValues[item.name] = i;
  }

  // No longer need myarray, and we need the memory back
  destory(myarray);
  GC.collect;

  // Another memory intensive operation below
}


This seems to work for me to a point, but I notice my program 
still holds a lot in memory even after certain memory intensive 
arrays should have been collected.

Is this just a hit and miss with the GC or am I doing something 
wrong? Uses around 40% less memory forcing the GC and cleaning 
arrays but seems like a lot is left over, especially for threads 
that have already finished(I've expect a thread to free all it's 
memory). I'm not sharing anything across threads besides a config 
struct.

This is a simplified version of my app. At the moment when at 
around 9GB at it's peak instead of 19GB like before.