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

=?ISO-8859-1?Q?Arne_Vajh=F8j?= <>
Thu, 21 Jul 2011 20:27:27 -0400
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:

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

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.


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