title: Immutable Map implementation for huge maps
body: <p>If I have an <strong>immutable</strong> Map which I might expect (over a very short period of time - like a few seconds) to be adding/removing <em>hundreds of thousands</em> of items from, is the standard <code>HashMap</code> a bad idea? Let's say I want to pass 1Gb of data through the Map in &lt;10 seconds in such a way that the maximum size of the Map at any once instant is only 256Mb.</p>
  
  <p>I get the impression that the map keeps some kind of "history" but I will <em>always</em> be accessing the last-updated table (i.e. I do not pass the map around) because it is a private member variable of an <code>Actor</code> which is updated/accessed only from within reactions.</p>
  
  <p>Basically I suspect that this data structure may be (partly) at fault <a href="https://stackoverflow.com/questions/2297920/jvm-outofmemory-error-death-spiral-not-memory-leak">for issues I am seeing around JVMs going out of memory</a> when reading in large amounts of data in a short time.</p>
  
  <p>Would I be better off with a different map implementation and, if so, what is it?</p>