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Comment by jillesvangurp | Hacker News Reader

I think the real issue is that you probably should worry about this overhead only in the context of really tight loops where you are basically wasting a lot of memory allocation and garbage collection for no good reason. But otherwise, Java is actually pretty good dealing with short lived small objects. The garbage collector is pretty good for that.

Normal usage of this stuff is not going to cause any more issues than trigger the OCD of people that obsess about this stuff. And in the rare case that you do have an issue, you can do something more optimal indeed.

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GC performance isn't the only thing you need to care about here. Iterating over an ArrayList<T> in Java already makes you chase pointers all over the place due to Java's lack of value types, which wrecks cache locality. ArrayList<Optional<T>> just makes you chase two pointers for each object instead.

List<Optional<T>> is pure insanity fwiw. Filter out your empties before making the list.

Sure, but Optional<T> is what we were already discussing, and List<T> is the collection type that most obviously requires no indirection. Feel free to replace Optional<T> with any other sort of sum type and the point still stands — you always have one extra object between the list and the "actual value".

> List<T> is the collection type that most obviously requires no indirection.

Collections in Java as of now (before project Valhalla) are actually collections of pointers to objects, so at least one level of indirection. Another level of indirection is List itself, which is an interface, so the compiler can’t ahead of time devirtualise method calls, only after JIT detects which List implementation it’s dealing with at runtime.

Virtual calls are trivial optimizations and doesn’t really matter here either way as it only has to happen once, no matter the list size.

Sorry, I wasn't clear: Tree maps have built-in indirection, hash maps can use open addressing but they can also use chaining, which has indirection. An array-backed list is the only collection type that obviously requires no indirection in concept. In a world where Java has value types, you could build ArrayList<T> with no indirection (other than polymorphism/dispatch).

Nothing insane about it, e.g. you might be using some sort of SoA structure and that specific component is optional.

If you're using an SoA pattern you're already optimizing for performance and, imo, should be using null to begin with.

That's the point, though. Java's Optional<T> always incurs that extra cost, but e.g. Kotlin's T? doesn't have that (because ? just allows null), and Rust has an optimisation for Option<Box<T>> that reduces the representation to a single pointer (and the empty case is a null pointer). You absolutely can have that level of optimisation while still having the optional type.

What if you have an in-memory DataFrame? Not hard to end up with something that looks like List<Optional<T>> (say, NullableSeries<T>) as one of the columns.

Java uses thread local allocation buffers such that objects allocated one after another are typically contiguous in RAM. Most modern Java gcs are also compacting, meaning that the heap ends up approximately breadth first ordered after GC.

What this means is that in practice, pointer chasing is less of an issue than you’d expect. Even a linked list will end up with decent cache locality.

Obviously this won’t always work, but it generally works a lot better than the same structure in a systems language.

https://shipilev.net/jvm/anatomy-quarks/11-moving-gc-localit...

If you're writing a database engine, and you accidentally wrap each table cell in a Java object, you'll very quickly hit a GC performance wall, particularly when using a low pause GC. So we try to write Java without objects. Guess how easy that is. ;)

Yeah I deal with that a lot with my search engine index too. Honestly it's not that bad once you get used to it.

You can get away with referencing the data through (mutable and reusable) pointer objects that reference memory mapped areas yet provide a relatively comfortable higher level interface. This gets rid of object churn while keeping a relatively sane interface.

Can you give an example how to do that? I've only seen IO and parsing libraries that copy around byte arrays, but you can't cast byte arrays to arbitrary objects, which means you usually have to manipulate the bytes directly (unless you're lucky to work with only strings). At that point I imagine it would be far easier to just use C++ or Rust.

Yeah this sort of work isn't Java's strong suite. A lot of it's sort of like programming old-school C with oven mitts. It'll get a bit better with the Foreign Memory API which is in the JEP pipe.

But a very bare bones example might look something like

  class MdrLayout {
     static final int FOO_OFFSET = 0;
     static final int BAR_OFFSET = 4;
     static final int BAZ_OFFSET = 12;
     static final int ENTRY_SIZE = 13;
  }

  class MyDataRecord {

    int idx = Integer.MIN_VALUE;
    ByteBuffer buffer = null;

    MyDataRecord() { }

    void movePointer(ByteBuffer buffer, int idx) { 
      this.buffer = buffer;
      this.idx = idx; 
    }

    int foo() { return buffer.getInt(ENTRY_SIZE * idx + FOO_OFFSET); }
    long bar() { return buffer.getLong(ENTRY_SIZE * idx + BAR_OFFSET); }
    byte baz() { return buffer.get(ENTRY_SIZE * idx + BAZ_OFFSET); }

    // may have operators like reset(), next() or skip() as well
  }

  class MyDataRecord2 {

    final ByteBuffer buffer;

    MyDataRecord2(ByteBuffer buffer) { this.buffer = buffer; }

    int foo(int idx) { return buffer.getInt(ENTRY_SIZE * idx + FOO_OFFSET); }
    long bar(int idx) { return buffer.getLong(ENTRY_SIZE * idx + BAR_OFFSET); }
    byte baz(int idx) { return buffer.get(ENTRY_SIZE * idx + BAZ_OFFSET); }
  }

Yes, this is the way. Surprising how high level you can make this work - I’ve seen Collection implementations and all sorts.

If you do stuff like that, use a profiler and identify and fix your real performance bottlenecks. As opposed to applying premature optimization blindly. Same with GC tuning. This has gotten easier over the years. But there are still lots of tradeoffs here.

There are plenty of fast performing databases and other middleware written in Java. The JVM is a popular platform for that kind of thing for a good reason. Writing good software of course is a bit of a skill. Benchmarks like this are kind of pointless. Doing an expensive thing in a loop is slow. Well duh. Don't do that.

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