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

Tom Anderson <>
Fri, 1 Jul 2011 21:22:58 +0100
On Thu, 30 Jun 2011, KitKat wrote:

On 30/06/2011 8:05 PM, Patricia Shanahan wrote:

On 6/30/2011 3:29 PM, KitKat wrote:

On 30/06/2011 6:04 PM, Tom Anderson wrote:


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.

Such as? I can think of only one possibility that could be even close to
efficient: maintain an IdentityHashMap<Object,Lock> somewhere under the

The obvious alternative is the make one of the existing words dual
purpose, either directly containing its data or containing an index to a
structure containing both the lock and the original use of the word.
That does require, in effect, a spare bit to indicate which mode the
object is in.

Yeah, that could work if you can spare a bit from the non-lock stuff in the
other original two words of object header.

The above assumed that all the bits in the other two words were already
spoken for. But if not, your suggestion fits well the phrase "thin lock"
since the lock is essentially only 1 bit wide for most objects.

That is indeed pretty much exactly what a thin lock is.


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