Performance characteristics of mutable static primitives? - JVM Languages

Message from discussion Performance characteristics of mutable static primitives?

Marcelo Fukushima

More options Apr 3, 7:37 pm

From: "Marcelo Fukushima" <takesh...@gmail.com>

Date: Thu, 3 Apr 2008 20:37:06 -0300

Local: Thurs, Apr 3 2008 7:37 pm

Subject: Re: [jvm-l] Re: Performance characteristics of mutable static primitives?

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On 4/3/08, John Cowan <johnwco...@gmail.com> wrote:

> On Thu, Apr 3, 2008 at 7:01 PM, John Rose <John.R...@sun.com> wrote:

> > Yes. You can then cast it to the subclass and go on from there.

> That's what I had hoped, but the JLS doesn't actually seem to guarantee this.

> > > If so, that provides a different approach
> > > to thread-local state, where the state is held in the instance
> > > variables of MyThread objects, and the JVM is used to thread them
> > > through :-) the code until the point where they're needed.

> > That's how Java thread locals are implemented in the JVM.

> I don't understand how that can be, as I can create as many
> ThreadLocals as I want. They can't all fit in the Thread object,
> surely. And how can reference to them be fast if they have to
> indirect through a Thread subclass object?

the source for ThreadLocal explains the first bit:

public void set(T value) {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
}

apparently, every thread has a map that holds all ThreadLocal's registered

> > You can only use this approach if (a) you control the creation of the
> > thread and can specify your own subclass for it,

> That's what I had in mind.

> > You might be interested to know that Hotspot intrinsifies
> > Thread.currentThread to a couple of instructions; it's cheap. We did
> > that to make things like thread locals efficient.

> Good.

> --
> GMail doesn't have rotating .sigs, but you can see mine at
> http://www.ccil.org/~cowan/signatures

--
[]'s
Marcelo Takeshi Fukushima

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