Scala: Veränderliche vs. unveränderliches Objekt Leistung - OutOfMemoryError

Question

Ich wollte die Leistungsmerkmale von immutable.Map und mutable.Map in Scala für einen ähnlichen Betrieb (nämlich vergleichen, viele Karten zu einem einzigen verschmelzen. Siehe diese Frage ). Ich habe, was für ähnliche Implementierungen zu sein scheinen beide wandelbar und unveränderlich Karten (siehe unten).

Als Test erzeugte ich eine Liste mit 1.000.000 Einzelpl Map [Int, Int] und übergeben Sie diese Liste in die Funktionen, die ich testete. Bei ausreichenden Speichern, waren die Ergebnisse überraschend,: ~ 1200ms für mutable.Map, ~ 1800ms für immutable.Map und ~ 750 ms für einen Imperativ Implementierung mutable.Map mit - nicht sicher, was für den großen Unterschied ausmacht dort, aber das Gefühl frei zu Kommentar auf das auch.

Was mir ein wenig überrascht hat, vielleicht, weil ich ein bisschen dick zu sein habe, ist, dass mit der Standard-Laufzeitkonfiguration in IntelliJ 8.1, beide wandelbar Implementierungen eine OutOfMemoryError getroffen, aber das unveränderlich Sammlung nicht. Der unveränderliche Test bis zur Fertigstellung lief, aber es tat so sehr langsam - es dauert etwa 28 Sekunden. Wenn ich den maximalen JVM-Speicher erhöht (auf etwa 200 MB, nicht sicher, wo die Schwelle ist), ich habe über die Ergebnisse.

Wie auch immer, hier ist was ich wirklich wissen will:

Warum haben die wandelbar Implementierungen über genügend Arbeitsspeicher ausgeführt, aber die unveränderliche Implementierung nicht Ich vermute, dass die unveränderliche Version der Garbage Collector Speicher zu laufen und frei nach oben ermöglicht, bevor die wandelbar Implementierungen tun? - und alle diese Garbage Collection erklären die Langsamkeit des unveränderlichen Low-Memory-Lauf -. aber ich würde gerne eine ausführlichere Erklärung als die

Implementations unten. (Anmerkung:.. Ich behaupte nicht, dass dies die besten Implementierungen möglich Fühlen Sie sich frei Verbesserungen vorzuschlagen)

  def mergeMaps[A,B](func: (B,B) => B)(listOfMaps: List[Map[A,B]]): Map[A,B] =
    (Map[A,B]() /: (for (m <- listOfMaps; kv <-m) yield kv)) { (acc, kv) =>
      acc + (if (acc.contains(kv._1)) kv._1 -> func(acc(kv._1), kv._2) else kv)
    }

  def mergeMutableMaps[A,B](func: (B,B) => B)(listOfMaps: List[mutable.Map[A,B]]): mutable.Map[A,B] =
    (mutable.Map[A,B]() /: (for (m <- listOfMaps; kv <- m) yield kv)) { (acc, kv) =>
      acc + (if (acc.contains(kv._1)) kv._1 -> func(acc(kv._1), kv._2) else kv)
    }

  def mergeMutableImperative[A,B](func: (B,B) => B)(listOfMaps: List[mutable.Map[A,B]]): mutable.Map[A,B] = {
    val toReturn = mutable.Map[A,B]()
    for (m <- listOfMaps; kv <- m) {
      if (toReturn contains kv._1) {
        toReturn(kv._1) = func(toReturn(kv._1), kv._2)
      } else {
        toReturn(kv._1) = kv._2
      }
    }
    toReturn
  }

Answer 1

Well, it really depends on what the actual type of Map you are using. Probably HashMap. Now, mutable structures like that gain performance by pre-allocating memory it expects to use. You are joining one million maps, so the final map is bound to be somewhat big. Let's see how these key/values get added:

protected def addEntry(e: Entry) { 
  val h = index(elemHashCode(e.key)) 
  e.next = table(h).asInstanceOf[Entry] 
  table(h) = e 
  tableSize = tableSize + 1 
  if (tableSize > threshold) 
    resize(2 * table.length) 
} 

See the 2 * in the resize line? The mutable HashMap grows by doubling each time it runs out of space, while the immutable one is pretty conservative in memory usage (though existing keys will usually occupy twice the space when updated).

Now, as for other performance problems, you are creating a list of keys and values in the first two versions. That means that, before you join any maps, you already have each Tuple2 (the key/value pairs) in memory twice! Plus the overhead of List, which is small, but we are talking about more than one million elements times the overhead.

You may want to use a projection, which avoids that. Unfortunately, projection is based on Stream, which isn't very reliable for our purposes on Scala 2.7.x. Still, try this instead:

for (m <- listOfMaps.projection; kv <- m) yield kv

A Stream doesn't compute a value until it is needed. The garbage collector ought to collect the unused elements as well, as long as you don't keep a reference to the Stream's head, which seems to be the case in your algorithm.

EDIT

Complementing, a for/yield comprehension takes one or more collections and return a new collection. As often as it makes sense, the returning collection is of the same type as the original collection. So, for example, in the following code, the for-comprehension creates a new list, which is then stored inside l2. It is not val l2 = which creates the new list, but the for-comprehension.

val l = List(1,2,3)
val l2 = for (e <- l) yield e*2

Now, let's look at the code being used in the first two algorithms (minus the mutable keyword):

(Map[A,B]() /: (for (m <- listOfMaps; kv <-m) yield kv)) 

The foldLeft operator, here written with its /: synonymous, will be invoked on the object returned by the for-comprehension. Remember that a : at the end of an operator inverts the order of the object and the parameters.

Now, let's consider what object is this, on which foldLeft is being called. The first generator in this for-comprehension is m <- listOfMaps. We know that listOfMaps is a collection of type List[X], where X isn't really relevant here. The result of a for-comprehension on a List is always another List. The other generators aren't relevant.

So, you take this List, get all the key/values inside each Map which is a component of this List, and make a new List with all of that. That's why you are duplicating everything you have.

(in fact, it's even worse than that, because each generator creates a new collection; the collections created by the second generator are just the size of each element of listOfMaps though, and are immediately discarded after use)

The next question -- actually, the first one, but it was easier to invert the answer -- is how the use of projection helps.

When you call projection on a List, it returns new object, of type Stream (on Scala 2.7.x). At first you may think this will only make things worse, because you'll now have three copies of the List, instead of a single one. But a Stream is not pre-computed. It is lazily computed.

What that means is that the resulting object, the Stream, isn't a copy of the List, but, rather, a function that can be used to compute the Stream when required. Once computed, the result will be kept so that it doesn't need to be computed again.

Also, map, flatMap and filter of a Stream all return a new Stream, which means you can chain them all together without making a single copy of the List which created them. Since for-comprehensions with yield use these very functions, the use of Stream inside the prevent unnecessary copies of data.

Now, suppose you wrote something like this:

val kvs = for (m <- listOfMaps.projection; kv <-m) yield kv
(Map[A,B]() /: kvs) { ... }

In this case you aren't gaining anything. After assigning the Stream to kvs, the data hasn't been copied yet. Once the second line is executed, though, kvs will have computed each of its elements, and, therefore, will hold a complete copy of the data.

Now consider the original form::

(Map[A,B]() /: (for (m <- listOfMaps.projection; kv <-m) yield kv)) 

In this case, the Stream is used at the same time it is computed. Let's briefly look at how foldLeft for a Stream is defined:

override final def foldLeft[B](z: B)(f: (B, A) => B): B = { 
  if (isEmpty) z 
  else tail.foldLeft(f(z, head))(f) 
} 

If the Stream is empty, just return the accumulator. Otherwise, compute a new accumulator (f(z, head)) and then pass it and the function to the tail of the Stream.

Once f(z, head) has executed, though, there will be no remaining reference to the head. Or, in other words, nothing anywhere in the program will be pointing to the head of the Stream, and that means the garbage collector can collect it, thus freeing memory.

The end result is that each element produced by the for-comprehension will exist just briefly, while you use it to compute the accumulator. And this is how you save keeping a copy of your whole data.

Finally, there is the question of why the third algorithm does not benefit from it. Well, the third algorithm does not use yield, so no copy of any data whatsoever is being made. In this case, using projection only adds an indirection layer.