Tutorial :Immutable Map implementation for huge maps



Question:

If I have an immutable Map which I might expect (over a very short period of time - like a few seconds) to be adding/removing hundreds of thousands of items from, is the standard HashMap a bad idea? Let's say I want to pass 1Gb of data through the Map in <10 seconds in such a way that the maximum size of the Map at any once instant is only 256Mb.

I get the impression that the map keeps some kind of "history" but I will always be accessing the last-updated table (i.e. I do not pass the map around) because it is a private member variable of an Actor which is updated/accessed only from within reactions.

Basically I suspect that this data structure may be (partly) at fault for issues I am seeing around JVMs going out of memory when reading in large amounts of data in a short time.

Would I be better off with a different map implementation and, if so, what is it?


Solution:1

Ouch. Why do you have to use an immutable map? Poor garbage collector! Immutable maps generally require (log n) new objects per operation in addition to (log n) time, or they really just wrap mutable hash maps and layer changesets on top (which slows things down and can increase the number of object creations).

Immutability is great, but this does not seem to me like the time to use it. If I were you, I'd stick with scala.collection.mutable.HashMap. If you need concurrent access, wrap the Java util.concurrent one instead.

You also might want to increase the size of the young generation in the JVM: -Xmn1G or more (assuming you're running with -Xmx3G). Also, use the throughput (parallel) garbage collector.


Solution:2

That would be awful. You say you always want to access the last-updated table, that means you only need an ephemeral data structure, there is no need to pay the cost for a persistent data structure - it's like trading time and memory to gain completely arguable "style points". You are not building your karma by using blindly persistent structures when they are not called for.

Also, a hashtable is a particularly difficult structure to make persistent. In other words, "very, very slow" (basically it is usable when reads greatly outnumber writes - and you seem to talk about many writes).

By the way, a ConcurrentHashMap wouldn't make sense in this design, given that the map is accessed from a single actor (that's what I understand from the description).


Solution:3

Scala's so-called(*) immutable Map is broken beyond basic usage up to Scala 2.7. Don't trust me, just look up the number of open tickets for it. And the solution is just "it will be replaced with something else on Scala 2.8" (which it did).

So, if you want an immutable map for Scala 2.7.x, I'd advise looking for it in something other than Scala. Or just use TreeHashMap instead.

(*) Scala's immutable Map isn't really immutable. It is a mutable data structure internally, which requires lot of synchronization.


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