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

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On 01/07/2011 4:40 PM, Tom Anderson wrote:
> On Thu, 30 Jun 2011, KitKat wrote:
>
>> Are you sure that last one was a "chap"? "Tamiya" sounds rather
>> feminine to me.
>
> 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.

???

Regardless of which, "Onodera" also sounds feminine.

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

That would work, if that's what the second word in the object header 
normally is. Assuming it's the heap address at time of creation, and 
objects are aligned on word boundaries, the two least order bits of the 
identity hash are going to be zero, so you can use those bits for 
something else and mask them off to get the hash.

On the other hand, that suggests a way to make object headers of only 
*one* word.

Consider: how likely are we to have four billion vtables in a running 
32-bit JVM? Let alone Long.MAX_VALUE - Long.MIN_VALUE + 1 in a 64-bit one?

Reserve a low chunk of the address space (and call it part of permgen?) 
for vtables and your vtable pointers get quite short. The vtable pointer 
plus a few bits of the object's initial address would still make a 
pretty decent identity hash for collections with heterogeneous keys; 
homogeneous keys, in my experience, are usually value objects with 
overridden hashCode such as Strings and you can make the 
initial-address-bits (and the thin lock bit) the low order bits. Shift 
right one bit to lose the lock bit and have the hash; shift right n bits 
for some fairly small n to get the vtable pointer. Vtable lookup is a 
tiny bit slower due to a test of the lock bit plus one added shift 
instruction on each lookup, but the critical performance points tend to 
get JITted into direct calls or branch-predictable is-it-a-Foo? 
jump-or-normal-vtable-lookup choices. And the vast majority of 
production Java code is I/O bound anyway.

Well, except when the object needs a fat lock. Then the whole word 
becomes a pointer to a structure that points to the vtable and contains 
the lock and identity hash. Now vtable lookup has an added indirection. 
But the bottleneck with such objects will usually be contention for the 
lock itself, not CPU cycles.

> 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.

Map lookups are O(1) and a low level implementation in C built into the 
JVM would boil down to masking and shifting the hash and then a pointer 
addition and dereference, only needed when you wanted to lock or unlock 
an object -- and again, the time spent on this will be dwarfed by the 
time spent in contention for the lock anyway, fairly often. Branch 
prediction and pipelining might help in the case of high-CPU areas that 
lock an object with low contention, in that some of the work might 
proceed in parallel with lock acquisition in the absence of contention 
(though, only as far as work that can be done in cache or registers, 
since the lock must be held prior to any memory reads or writes in the 
guarded object, and on initial acquisition failure that work may have to 
be repeated later on acquisitoon).

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Why "lock" functionality is introduced for all the objects? Alex J <vstrength@gmail.com> - 2011-06-28 02:29 -0700
  Re: Why "lock" functionality is introduced for all the objects? Lew <noone@lewscanon.com> - 2011-06-28 07:33 -0400
    OT "sic" (was Re: Why "lock" functionality is introduced for all the objects? blmblm@myrealbox.com <blmblm.myrealbox@gmail.com> - 2011-06-28 15:56 +0000
      Re: OT "sic" (was Re: Why "lock" functionality is introduced for all the objects? Lew <noone@lewscanon.com> - 2011-06-28 12:19 -0400
    Re: Why "lock" functionality is introduced for all the objects? Michal Kleczek <kleku75@gmail.com> - 2011-06-28 18:41 +0200
      Re: Why "lock" functionality is introduced for all the objects? Lew <noone@lewscanon.com> - 2011-06-28 13:10 -0400
        Re: Why "lock" functionality is introduced for all the objects? Michal Kleczek <kleku75@gmail.com> - 2011-06-28 19:53 +0200
          Re: Why "lock" functionality is introduced for all the objects? Lew <noone@lewscanon.com> - 2011-06-28 14:13 -0400
            Re: Why "lock" functionality is introduced for all the objects? supercalifragilisticexpialadiamaticonormalizeringelimatisticantations <supercalifragilisticexpialadiamaticonormalizeringelimatisticantations@averylongandannoyingdomainname.com> - 2011-06-28 14:23 -0400
              Re: Why "lock" functionality is introduced for all the objects? Lew <noone@lewscanon.com> - 2011-06-28 14:33 -0400
                Re: Why "lock" functionality is introduced for all the objects? supercalifragilisticexpialadiamaticonormalizeringelimatisticantations <supercalifragilisticexpialadiamaticonormalizeringelimatisticantations@averylongandannoyingdomainname.com> - 2011-06-28 14:52 -0400
                Re: Why "lock" functionality is introduced for all the objects? Lew <noone@lewscanon.com> - 2011-06-28 16:20 -0400
                Re: Why "lock" functionality is introduced for all the objects? supercalifragilisticexpialadiamaticonormalizeringelimatisticantations <supercalifragilisticexpialadiamaticonormalizeringelimatisticantations@averylongandannoyingdomainname.com> - 2011-06-29 00:53 -0400
                Re: Why "lock" functionality is introduced for all the objects? Lew <noone@lewscanon.com> - 2011-06-29 01:04 -0400
                Re: Why "lock" functionality is introduced for all the objects? supercalifragilisticexpialadiamaticonormalizeringelimatisticantations <supercalifragilisticexpialadiamaticonormalizeringelimatisticantations@averylongandannoyingdomainname.com> - 2011-06-29 01:43 -0400
              Re: Why "lock" functionality is introduced for all the objects? Patricia Shanahan <pats@acm.org> - 2011-06-28 11:42 -0700
                Re: Why "lock" functionality is introduced for all the objects? supercalifragilisticexpialadiamaticonormalizeringelimatisticantations <supercalifragilisticexpialadiamaticonormalizeringelimatisticantations@averylongandannoyingdomainname.com> - 2011-06-28 14:54 -0400
                Re: Why "lock" functionality is introduced for all the objects? Patricia Shanahan <pats@acm.org> - 2011-06-28 12:34 -0700
                Re: Why "lock" functionality is introduced for all the objects? markspace <-@.> - 2011-06-28 13:20 -0700
                Re: Why "lock" functionality is introduced for all the objects? Patricia Shanahan <pats@acm.org> - 2011-06-28 13:44 -0700
                Re: Why "lock" functionality is introduced for all the objects? supercalifragilisticexpialadiamaticonormalizeringelimatisticantations <supercalifragilisticexpialadiamaticonormalizeringelimatisticantations@averylongandannoyingdomainname.com> - 2011-06-29 01:05 -0400
                Re: Why "lock" functionality is introduced for all the objects? Lew <noone@lewscanon.com> - 2011-06-28 16:21 -0400
                Re: Why "lock" functionality is introduced for all the objects? supercalifragilisticexpialadiamaticonormalizeringelimatisticantations <supercalifragilisticexpialadiamaticonormalizeringelimatisticantations@averylongandannoyingdomainname.com> - 2011-06-29 01:06 -0400
                Re: Why "lock" functionality is introduced for all the objects? BGB <cr88192@hotmail.com> - 2011-06-28 14:30 -0700
          Re: Why "lock" functionality is introduced for all the objects? Robert Klemme <shortcutter@googlemail.com> - 2011-06-29 18:56 +0200
      Re: Why "lock" functionality is introduced for all the objects? BGB <cr88192@hotmail.com> - 2011-06-28 13:43 -0700
        Re: Why "lock" functionality is introduced for all the objects? Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-06-28 20:43 -0400
          Re: Why "lock" functionality is introduced for all the objects? BGB <cr88192@hotmail.com> - 2011-06-28 21:14 -0700
        Re: Why "lock" functionality is introduced for all the objects? supercalifragilisticexpialadiamaticonormalizeringelimatisticantations <supercalifragilisticexpialadiamaticonormalizeringelimatisticantations@averylongandannoyingdomainname.com> - 2011-06-29 01:12 -0400
          Re: Why "lock" functionality is introduced for all the objects? Joshua Maurice <joshuamaurice@gmail.com> - 2011-07-01 18:28 -0700
            Re: Why "lock" functionality is introduced for all the objects? supercalifragilisticexpialadiamaticonormalizeringelimatisticantations <supercalifragilisticexpialadiamaticonormalizeringelimatisticantations@averylongandannoyingdomainname.com> - 2011-07-02 00:19 -0400
          Re: Why "lock" functionality is introduced for all the objects? Joshua Cranmer <Pidgeot18@verizon.invalid> - 2011-07-01 19:05 -0700
            Re: Why "lock" functionality is introduced for all the objects? supercalifragilisticexpialadiamaticonormalizeringelimatisticantations <supercalifragilisticexpialadiamaticonormalizeringelimatisticantations@averylongandannoyingdomainname.com> - 2011-07-02 00:26 -0400
            Re: Why "lock" functionality is introduced for all the objects? BGB <cr88192@hotmail.com> - 2011-07-04 09:39 -0700
              Re: Why "lock" functionality is introduced for all the objects? supercalifragilisticexpialadiamaticonormalizeringelimatisticantations <supercalifragilisticexpialadiamaticonormalizeringelimatisticantations@averylongandannoyingdomainname.com> - 2011-07-05 02:11 -0400
    Re: Why "lock" functionality is introduced for all the objects? Alex J <vstrength@gmail.com> - 2011-07-05 16:56 -0700
      Re: Why "lock" functionality is introduced for all the objects? "John B. Matthews" <nospam@nospam.invalid> - 2011-07-06 00:57 -0400
      Re: Why "lock" functionality is introduced for all the objects? supercalifragilisticexpialadiamaticonormalizeringelimatisticantations <supercalifragilisticexpialadiamaticonormalizeringelimatisticantations@averylongandannoyingdomainname.com> - 2011-07-06 05:55 -0400
  Re: Why "lock" functionality is introduced for all the objects? Lew <noone@lewscanon.com> - 2011-06-28 14:40 -0400
  Re: Why "lock" functionality is introduced for all the objects? Robert Klemme <shortcutter@googlemail.com> - 2011-06-29 19:15 +0200
  Re: Why "lock" functionality is introduced for all the objects? Tom Anderson <twic@urchin.earth.li> - 2011-06-30 23:04 +0100
    Re: Why "lock" functionality is introduced for all the objects? KitKat <kitkat_11697@gmail.example.com> - 2011-06-30 18:29 -0400
      Re: Why "lock" functionality is introduced for all the objects? Patricia Shanahan <pats@acm.org> - 2011-06-30 17:05 -0700
        Re: Why "lock" functionality is introduced for all the objects? KitKat <kitkat_11697@gmail.example.com> - 2011-06-30 20:17 -0400
          Re: Why "lock" functionality is introduced for all the objects? Tom Anderson <twic@urchin.earth.li> - 2011-07-01 21:22 +0100
      Re: Why "lock" functionality is introduced for all the objects? Tom Anderson <twic@urchin.earth.li> - 2011-07-01 21:40 +0100
        Re: Why "lock" functionality is introduced for all the objects? KitKat <kitkat_11697@gmail.example.com> - 2011-07-01 18:08 -0400
          Re: Why "lock" functionality is introduced for all the objects? BGB <cr88192@hotmail.com> - 2011-07-05 12:15 -0700
            Re: Why "lock" functionality is introduced for all the objects? KitKat <kitkat_11697@gmail.example.com> - 2011-07-05 15:30 -0400
              Re: Why "lock" functionality is introduced for all the objects? blmblm@myrealbox.com <blmblm.myrealbox@gmail.com> - 2011-07-05 21:10 +0000
                Re: Why "lock" functionality is introduced for all the objects? BGB <cr88192@hotmail.com> - 2011-07-05 22:08 -0700
                Re: Why "lock" functionality is introduced for all the objects? KitKat <kitkat_11697@gmail.example.com> - 2011-07-06 05:57 -0400
                Re: Why "lock" functionality is introduced for all the objects? blmblm@myrealbox.com <blmblm.myrealbox@gmail.com> - 2011-07-06 17:07 +0000
                Re: Why "lock" functionality is introduced for all the objects? Steve Erwin <trollHunter@Usenet.4.usenetizens.org.invalid> - 2011-07-07 04:08 +1000
                Re: Why "lock" functionality is introduced for all the objects? blmblm@myrealbox.com <blmblm.myrealbox@gmail.com> - 2011-07-06 19:09 +0000
                Re: Why "lock" functionality is introduced for all the objects? Steve Erwin <trollHunter@Usenet.4.usenetizens.org.invalid> - 2011-07-07 09:26 +1000
                Re: Why "lock" functionality is introduced for all the objects? KitKat <kitkat_11697@gmail.example.com> - 2011-07-06 20:25 -0400
                Re: Why "lock" functionality is introduced for all the objects? blmblm@myrealbox.com <blmblm.myrealbox@gmail.com> - 2011-07-07 19:37 +0000
                Re: Why "lock" functionality is introduced for all the objects? blmblm@myrealbox.com <blmblm.myrealbox@gmail.com> - 2011-07-07 19:35 +0000
                Re: Why "lock" functionality is introduced for all the objects? Steve Erwin <trollHunter@Usenet.4.usenetizens.org.invalid> - 2011-07-07 14:34 -0700
                OT names/nyms/etc. (was Re: Why "lock" functionality is introduced for all the objects?) blmblm@myrealbox.com <blmblm.myrealbox@gmail.com> - 2011-07-08 17:19 +0000
                Re: OT names/nyms/etc. (was Re: Why "lock" functionality is introduced for all the objects?) Steve Erwin <trollHunter@Usenet.4.usenetizens.org.invalid> - 2011-07-09 05:41 +1000
                Re: OT names/nyms/etc. (was Re: Why "lock" functionality is introduced for all the objects?) blmblm@myrealbox.com <blmblm.myrealbox@gmail.com> - 2011-07-08 19:58 +0000
                Re: OT names/nyms/etc. (was Re: Why "lock" functionality is introduced for all the objects?) lewbloch <lewbloch@gmail.com> - 2011-07-08 13:45 -0700
                Re: OT names/nyms/etc. (was Re: Why "lock" functionality is introduced for all the objects?) Steve Erwin <trollHunter@Usenet.4.usenetizens.org.invalid> - 2011-07-10 01:50 -0400
                Re: OT names/nyms/etc. (was Re: Why "lock" functionality is introduced for all the objects?) blmblm@myrealbox.com <blmblm.myrealbox@gmail.com> - 2011-07-10 19:15 +0000
                Re: OT names/nyms/etc. (was Re: Why "lock" functionality is introduced for all the objects?) KitKat <kitkat_11697@gmail.example.com> - 2011-07-10 18:38 -0400
                Re: OT names/nyms/etc. (was Re: Why "lock" functionality is introduced for all the objects?) KitKat <kitkat_11697@gmail.example.com> - 2011-07-09 00:29 -0400
                Re: Why "lock" functionality is introduced for all the objects? KitKat <kitkat_11697@gmail.example.com> - 2011-07-09 00:26 -0400
                Re: Why "lock" functionality is introduced for all the objects? KitKat <kitkat_11697@gmail.example.com> - 2011-07-06 20:05 -0400
                Re: Why "lock" functionality is introduced for all the objects? Steve Erwin <trollHunter@Usenet.4.usenetizens.org.invalid> - 2011-07-07 10:24 +1000
                Re: Why "lock" functionality is introduced for all the objects? KitKat <kitkat_11697@gmail.example.com> - 2011-07-06 21:52 -0400
                Re: Why "lock" functionality is introduced for all the objects? Steve Erwin <trollHunter@Usenet.4.usenetizens.org.invalid> - 2011-07-07 12:43 +1000
                Re: Why "lock" functionality is introduced for all the objects? KitKat <kitkat_11697@gmail.example.com> - 2011-07-06 23:00 -0400

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