Groups > comp.lang.java.programmer > #8288

Re: Bulk Array Element Allocation, is it faster?

From	Robert Klemme <shortcutter@googlemail.com>
Newsgroups	comp.lang.java.programmer
Subject	Re: Bulk Array Element Allocation, is it faster?
Date	2011-09-25 12:57 +0200
Message-ID	<9e8fplF19bU1@mid.individual.net> (permalink)
References	<j5lvf0$bhl$1@news.albasani.net> <Ypydnd8C3rt7G-PTnZ2dnUVZ_iydnZ2d@earthlink.com>

Show all headers | View raw

On 09/25/2011 03:41 AM, Patricia Shanahan wrote:
> On 9/24/2011 6:17 PM, Jan Burse wrote:
>> Dear All,
>>
>> I just wonder wether modern JIT do optimize
>> Code of the following form:
>>
>> Bla[] bla = new Bla[n];
>> for (int i=0; i<n; i++) {
>> bla[i] = new Bla();
>> }
>>
>> When I use the above in my code, my application
>> spends 8800 ms in the benchmark I have.
>>
>> When I then change the code to:
>>
>> Bla[] bla = new Bla[n];
>>
>> ...
>>
>> if (bla[i]==null)
>> bla[i] = new Bla();
>>
>> ..
>>
>> So instead of allocating all elements at once,
>> I will allocate them on demand in the subsequent
>> flow of my application.
>>
>> When I use the above in my code, my application
>> now suddently sends 9600 ms in the benchmark
>> I have.
>>
>> So I wonder whether eventually the JIT has
>> a way to detect the bulk allocate of the first
>> version and transform it into something more
>> efficient than my lazy allocation.
>>
>> Any clues?
>
> You also need to consider the general optimization of processors in
> favor of doing efficiently those things they have done in the recent past.
>
> When you do the allocation all at once, the code and data for "new
> Bla()" is in cache on the second and subsequent calls. There may be
> cache conflicts between "new Bla()" and the actual work, leading to
> many more cache misses when you interleave them.
>
> Doing the initialization on demand may be adding an unpredictable
> conditional branch to the subsequent flow.

This and the fact that lazy initialization has concurrency issues when 
used in a multi threading context (which e.g. final members do not have) 
has made me use this approach less frequently.  Also, for short lived 
objects in total it might be much more efficient to just allocate the 
object even if it is not used because it won't survive new generation 
anyway.  I think the lazy init idiom only really pays off if 
construction is expensive (e.g. because it involves IO or time consuming 
calculations).  In all other cases it's better to just unconditionally 
create and let GC work.  And because of improvements in JVM technology 
the balance has moved a lot away from the lazy side because allocation 
and deallocation overhead became smaller than in earlier Java versions.

Kind regards

	robert

Back to comp.lang.java.programmer | Previous | Next — Previous in thread | Next in thread | Find similar

Thread

Bulk Array Element Allocation, is it faster? Jan Burse <janburse@fastmail.fm> - 2011-09-25 03:17 +0200
  Re: Bulk Array Element Allocation, is it faster? Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-09-24 21:37 -0400
    Re: Bulk Array Element Allocation, is it faster? Jan Burse <janburse@fastmail.fm> - 2011-09-25 10:39 +0200
  Re: Bulk Array Element Allocation, is it faster? Patricia Shanahan <pats@acm.org> - 2011-09-24 18:41 -0700
    Re: Bulk Array Element Allocation, is it faster? Robert Klemme <shortcutter@googlemail.com> - 2011-09-25 12:57 +0200
      Re: Bulk Array Element Allocation, is it faster? Jan Burse <janburse@fastmail.fm> - 2011-09-25 13:38 +0200
        Re: Bulk Array Element Allocation, is it faster? Robert Klemme <shortcutter@googlemail.com> - 2011-09-25 14:16 +0200
          Re: Bulk Array Element Allocation, is it faster? Jan Burse <janburse@fastmail.fm> - 2011-09-25 16:04 +0200
            Re: Bulk Array Element Allocation, is it faster? Jan Burse <janburse@fastmail.fm> - 2011-09-25 16:11 +0200
              Re: Bulk Array Element Allocation, is it faster? Patricia Shanahan <pats@acm.org> - 2011-09-25 09:30 -0700
            Re: Bulk Array Element Allocation, is it faster? Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-09-25 10:59 -0400
              Re: Bulk Array Element Allocation, is it faster? Jan Burse <janburse@fastmail.fm> - 2011-09-25 17:14 +0200
                Re: Bulk Array Element Allocation, is it faster? Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-09-25 15:26 +0000
                Re: Bulk Array Element Allocation, is it faster? Lew <lewbloch@gmail.com> - 2011-09-25 11:02 -0700
                Re: Bulk Array Element Allocation, is it faster? Jan Burse <janburse@fastmail.fm> - 2011-09-25 20:19 +0200
                Re: Bulk Array Element Allocation, is it faster? Jan Burse <janburse@fastmail.fm> - 2011-09-25 20:21 +0200
                Re: Bulk Array Element Allocation, is it faster? Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-09-25 17:48 -0400
                Re: Bulk Array Element Allocation, is it faster? Jan Burse <janburse@fastmail.fm> - 2011-09-26 00:10 +0200
  Re: Bulk Array Element Allocation, is it faster? Roedy Green <see_website@mindprod.com.invalid> - 2011-09-24 19:05 -0700
    Re: Bulk Array Element Allocation, is it faster? Lew <lewbloch@gmail.com> - 2011-09-24 23:22 -0700
      Re: Bulk Array Element Allocation, is it faster? Roedy Green <see_website@mindprod.com.invalid> - 2011-09-25 00:23 -0700
        Re: Bulk Array Element Allocation, is it faster? Lew <lewbloch@gmail.com> - 2011-09-25 11:17 -0700
        Re: Bulk Array Element Allocation, is it faster? Joshua Cranmer <Pidgeot18@verizon.invalid> - 2011-09-25 14:00 -0500
    Re: Bulk Array Element Allocation, is it faster? Jan Burse <janburse@fastmail.fm> - 2011-09-25 10:47 +0200
      Re: Bulk Array Element Allocation, is it faster? Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-09-25 08:07 -0400
        Re: Bulk Array Element Allocation, is it faster? Jan Burse <janburse@fastmail.fm> - 2011-09-25 19:21 +0200
          Re: Bulk Array Element Allocation, is it faster? Jan Burse <janburse@fastmail.fm> - 2011-09-25 19:41 +0200
          Re: Bulk Array Element Allocation, is it faster? Lew <lewbloch@gmail.com> - 2011-09-25 11:28 -0700
            Re: Bulk Array Element Allocation, is it faster? Jan Burse <janburse@fastmail.fm> - 2011-09-25 21:25 +0200
              Re: Bulk Array Element Allocation, is it faster? Jan Burse <janburse@fastmail.fm> - 2011-09-25 23:50 +0200
              Re: Bulk Array Element Allocation, is it faster? Lew <lewbloch@gmail.com> - 2011-09-25 15:43 -0700
                Re: Bulk Array Element Allocation, is it faster? Jan Burse <janburse@fastmail.fm> - 2011-09-26 01:06 +0200
                Re: Bulk Array Element Allocation, is it faster? Jan Burse <janburse@fastmail.fm> - 2011-09-26 01:21 +0200
                Re: Bulk Array Element Allocation, is it faster? Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-09-26 06:00 +0000
                Re: Bulk Array Element Allocation, is it faster? Jan Burse <janburse@fastmail.fm> - 2011-09-26 14:52 +0200
                Re: Bulk Array Element Allocation, is it faster? Jan Burse <janburse@fastmail.fm> - 2011-09-26 15:02 +0200

csiph-web