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Comment by lucian1900 | Hacker News Reader

lucian1900

Actually, Dalvik's GC just plain sucks, too.

Clojure has a similar problem: works fine on desktop JVMs, but even initialising it takes many seconds on Dalvik.

It's been a while since I looked at the Clojure source, but from what I recall, the initialization problem is almost wholly related to loading all of clojure.core, not garbage collection.

If you're hitting slowdowns after initialization, there's a good chance it's reflection, not garbage collection. Type hinting can help if that is the case.

Because of the way that Clojure reuses immutable data structures, it doesn't do as much garbage collection as one might think.

Your laptop has 10x memory bandwidth of top-end Android phones (and 20x of midrange phones). Double the numbers for desktop.

In short, mobile devices are different than laptops/desktops. If you threat them same, your application will suck.

Closer to 5x than 10x. Don't do the math based on the clock speed, look at the cycle times (which are mostly the same between DDR3 2000 and LPDDR2 533). A desktop or laptop CPU (unless by "laptop" you mean "netbook" -- laptops based on mainstream CPU cores have the same memory interconnect as desktops) will have two 64 bit DRAM channels, a typical phone will have a single 32 bit channel.

I use completely different threats for mobile devices, laptops, and desktops.

If you don't stop misbehaving and start running my code correctly I'll ((forget to take you out of my pants pocket on laundry day)|(see just how far that hinge will bend)|(take you to the landfill))

None of these work BTW.

Edit: Sorry

Works without problems in Scala. Maybe Clojure isn't using the available resources efficiently enough?

Maybe the Scala examples you think of indulge in quite a bit of destructive updates?

Functional programming style tends to generate lots and lots of objects which die very young. Garbage collectors must be tuned for that. At least they should be generational. Mayhaps Dalvik's current GC is not?

Aye, seems Dalvik's GC is a pretty simple mark-and-sweep.

> Functional programming style tends to generate lots and lots of objects which die very young.

Well, there is a lot of Scala code out there doing exactly that.

HotSpot has no problems with short-lived objects in pretty much all its garbage collectors while Dalvik's GC is just a lot less sophisticated and mature.

So I mainly agree with you, but the proposition of

      Language allows producing lots of short-lived objects
    + Dalvik sucks
      ------------
      Programs in that language have to be slow

doesn't hold in my opinion, as seen in the differences between Scala and Clojure.

Clojure still generates a lot more garbage, especially during bootstrap. This can be seen easily with VisualVM on the desktop, comparing hello world in the two languages.

> Well, there is a lot of Scala code out there doing exactly that.

I talked about specific examples that I don't even know of. I don't know Scala enough to know whether non-destructive updates are the default. Are they? What about the core libraries?

I agree your proposition in fixed width font does not hold. I was merely asserting this one:

    Program P produces lots of short-lived objects
  + Dalvik sucks (or is tuned for long lived-objects)
    -------------------------------------------------
    Program P have to be slow

Scala generates a lot less garbage, by construction. A lot of that has to do with its compilation model, which is a lot closer to Java's than Clojure's is.

Clojure just assumes the JVM can deal with it, and desktop ones do just fine, hence why no one noticed.

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