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Re: Why "lock" functionality is introduced for all the objects?

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On 6/30/2011 6:04 PM, Tom Anderson wrote:
> On Tue, 28 Jun 2011, Alex J wrote:
>
>> I'm curious why Java designers once decided to allow every object to
>> be lockable (i.e. allow using lock on those). I know, that out of such
>> a design decision every Java object contain lock index, i.e. new
>> Object() results in allocation of at least 8 bytes where 4 bytes is
>> object index and 4 bytes is lock index on 32- bit JVM.
>
> That's not quite right. In the olden days, it's true that every object
> header contained room for a lock pointer - but back then, that meant
> that every header was *three* words (12 bytes), not two. Two words were
> needed for the header (one for a vtable pointer, one for various other
> things), and the third was for the lock.
>
> What happened then was that a very clever chap called David Bacon, who
> worked for IBM, invented a thing called a thin lock:
>
> http://www.research.ibm.com/people/d/dfb/papers.html#Bacon98Thin
>
> Which was subsequently improved by another clever chap called Tamiya
> Onodera into a thing called a tasuki lock, which you don't hear so much
> about.
>
> The details are described quite clearly in the papers, but the upshot is
> that an object is created with neither a lock nor a slot for a lock
> pointer (and so only a two-word header), and the lock is allocated only
> when needed, and then wired in. Some fancy footwork means that the
> object doesn't need to grow a pointer when this happens; the header
> remains two words, at the expense of some slight awkwardness elsewhere.
> Some even fancier footwork means that if only one thread locks the
> object at a time (a very common pattern), then a lock doesn't even need
> to be allocated.

Remember to distinguish between Java (as in JLS and JVM spec) and
a specific Java implementation.

Arne

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Re: Why "lock" functionality is introduced for all the objects? Arne Vajhøj <arne@vajhoej.dk> - 2011-07-21 20:27 -0400

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