Robert Klemme schrieb:
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
Yes, really seems so. Looks that the infinite heap idea
works (notion borrowed from a talk by Cliff Click found
on the net, should indicate that we just do this, allocate
and don't care much).
I made some additional benchmarks, in the present case the lazy
does not so much good in that it saves allocates. I got the
following figures for the application at hand:
Bulk: 84'393'262 allocates
Lazy: 81'662'843 allocates
So the application does not have many exit points in the flow
following the array creation, except for the last exit point
when anyway all objects were needed.
But still I have some doubts! The array I allocate are only
4 elements long or so? Why is there still such a big
difference between allocating in bulk only 4 elements and
doing them one after the other?
Overhead by the lazy detection itself? I doubt this also
since the array is anyway accessed, and the lazy check
is a small null pointer check.