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Re: where's your Vector, victor

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On 31.05.2014 00:23, Eric Sosman wrote:
> On 5/30/2014 5:30 PM, Jukka Lahtinen wrote:

>> And about the synchronization: in many cases, when you need
>> synchronization, you will need to synchronize something more than just
>> the Collection manipulation. And if the Collection handling is already
>> within a synchronized block, why use one more synchronization block
>> inside another one?
>
>      In a multi-threaded setting you need to pay close attention to
> exactly which manipulations need to be atomic.  Vector or the Lists
> produced by Collections.synchronizedList() will guarantee atomicity
> for the individual method calls, but (as has been pointed out a few
> times already) the program may require some combinations of those to
> be atomic.  The classic example is iterating over the collection:
> Just because .add() and .remove() and so on (and the Iterator methods
> themselves) are atomic doesn't imply that the entire iteration is so.

>      Finally, I question the wisdom of a program design that requires
> iterating over a collection shared by multiple threads.

There is another effect of synchronization of all Vector methods which 
has not yet been quoted as far as I can see: because of the Java memory 
model every entry into and exit from a mutex forms a memory barrier 
which requires some synchronization (sorry for the term here) between 
thread local and global memory; it also permits some optimizations of 
the JRE with regard to reordering operations.  So even if each Vector 
instance is used only inside at most one thread and the overhead of 
synchronized in the usual simple tests where only a single instance is 
manipulated in a real world application the synchronization between 
local and global memory may actually have an adverse effect if many 
threads are doing a lot of state changes.  I would guess that the effect 
is even worse on NUMA architectures but I do not have enough insight to 
explore this further.

Kind regards

	robert

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where's your Vector, victor bob smith <bob@coolfone.net63.net> - 2014-05-30 01:11 -0700
  Re: where's your Vector, victor Josip Almasi <joe@vrspace.org> - 2014-05-30 12:45 +0200
    Re: where's your Vector, victor Eric Sosman <esosman@comcast-dot-net.invalid> - 2014-05-30 08:06 -0400
    Re: where's your Vector, victor Robert Klemme <shortcutter@googlemail.com> - 2014-05-30 19:28 +0200
  Re: where's your Vector, victor Joerg Meier <joergmmeier@arcor.de> - 2014-05-30 12:58 +0200
  Re: where's your Vector, victor Knute Johnson <eternal@knutejohnson.com> - 2014-05-30 07:32 -0700
    Re: where's your Vector, victor Eric Sosman <esosman@comcast-dot-net.invalid> - 2014-05-30 11:31 -0400
      Re: where's your Vector, victor Robert Klemme <shortcutter@googlemail.com> - 2014-05-30 19:30 +0200
      Re: where's your Vector, victor Jukka Lahtinen <jtfjdehf@hotmail.com.invalid> - 2014-05-31 00:30 +0300
        Re: where's your Vector, victor Eric Sosman <esosman@comcast-dot-net.invalid> - 2014-05-30 18:23 -0400
          Re: where's your Vector, victor Robert Klemme <shortcutter@googlemail.com> - 2014-05-31 12:17 +0200
      Re: where's your Vector, victor leif.roar@invalid.com (Leif Roar Moldskred) - 2014-06-01 07:37 -0500
  Re: where's your Vector, victor Roedy Green <see_website@mindprod.com.invalid> - 2014-06-06 01:00 -0700

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