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

Tom Anderson <>
Fri, 1 Jul 2011 21:40:25 +0100
On Thu, 30 Jun 2011, KitKat wrote:

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

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

Are you sure that last one was a "chap"? "Tamiya" sounds rather feminine to

Perhaps - and a quick google reveals that it is a girl's name in Hebrew.
However, in Japanese, i believe it's a family name, and that Tamiya
Onodera is Dr Tamiya's name written in the normal Japanese order, putting
his family name first. Although i could be wrong.

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?

The object's identity hash is shuffled between the object and its lock
according to whether it has an expanded lock or not.

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

That might be memory-efficient, but it would not be at all time-efficient,
as it would require a map lookup to lock an object. Resizing the hash
would be an interesting exercise, too. Actually, i think early JVMs (1.1
era, IIRC, perhaps even 1.0) used something a bit like this; they didn't
use the identity hash, but back then the garbage collector was non-moving,
so they could use addresses as keys, and there was a global lock table
somewhere. I don't know how it handled resizing. Badly, i expect.


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