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Re: Why is java consumer/producer so much faster than C++

Cross-posted to 2 groups.

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On Mon, 23 Jul 2012 11:46:37 +0000 (UTC)
Juha Nieminen <nospam@thanks.invalid> wrote:

> In comp.lang.c++ Melzzzzz <mel@zzzzz.com> wrote:
> > I think that pressure on condition variable is greater with non
> > dynamic allocation, therefore it is slower.
> 
> That sentence doesn't make any kind of sense.
I has sense in this context. I will provide example.

> 
> > I don't think it's
> >> physically possible for 'new int' to be faster than using ints by
> >> value under any possible circumstance, even if we assumed a highly
> >> optimized version of 'new' that does magic under the hood to be 10
> >> times faster than the regular 'new'.
> > 
> > That's because new is very fast and don;t take much time of program
> > in this example.
> 
> 'new' is a very slow operation in C++, and even if it weren't, it just
> can't be faster than using an integer by value. It's physically
> impossible. (The pointer that points to the allocated int is also an
> integral value at the low level, so by allocating something
> dynamically you are only adding to the amount of values being
> handled, and the overall complexity of the program.)

Yes, but overall complexity can be masked with something else like in
this case.

> 
> Can you give any scenario where handling pointers would be faster than
> handling ints?

This scenario is exactly example. My queue is limited to capacity
nodes. When capacity is reached, producer waits on condition
variable until it is emptied.
In first example queue was 100000 nodes so slow down with plain ints
isn't just that noticabe.
But if I put just 1000 nodes here is what happens:
(dynamic ints , unique_ptr, random function and array assignement)
bmaxa@maxa:~/examples$ time ./consprod

real	0m23.523s
user	0m32.390s
sys	0m10.581s

but look at this:
(no dynamic ints, no random function, no assignment)
:
bmaxa@maxa:~/examples$ time ./consprod1

real	0m35.654s
user	0m30.882s
sys	0m34.202s

it's more than 10 seconds slower! just ints!


> 
> >> 'new' is slow, especially when compared to java's.
> > 
> > In this example new takes very little time.
> 
> You are executing tens of thousands of 'new's. That is slow.

It is slow but waiting on condition is much slower so,
faster producer increases number of times condition waits,
so actually that makes program slower...

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Thread

Why is java consumer/producer so much faster than C++ Melzzzzz <mel@zzzzz.com> - 2012-07-22 23:59 +0200
  Re: Why is java consumer/producer so much faster than C++ Joshua Maurice <joshuamaurice@gmail.com> - 2012-07-22 15:42 -0700
    Re: Why is java consumer/producer so much faster than C++ Melzzzzz <mel@zzzzz.com> - 2012-07-23 01:28 +0200
      Re: Why is java consumer/producer so much faster than C++ Luca Risolia <luca.risolia@studio.unibo.it> - 2012-07-23 02:03 +0200
        Re: Why is java consumer/producer so much faster than C++ Melzzzzz <mel@zzzzz.com> - 2012-07-23 12:17 +0200
          Re: Why is java consumer/producer so much faster than C++ Luca Risolia <luca.risolia@studio.unibo.it> - 2012-07-24 00:33 +0200
  Re: Why is java consumer/producer so much faster than C++ Juha Nieminen <nospam@thanks.invalid> - 2012-07-23 06:37 +0000
    Re: Why is java consumer/producer so much faster than C++ Melzzzzz <mel@zzzzz.com> - 2012-07-23 12:33 +0200
      Re: Why is java consumer/producer so much faster than C++ Juha Nieminen <nospam@thanks.invalid> - 2012-07-23 11:46 +0000
        Re: Why is java consumer/producer so much faster than C++ Melzzzzz <mel@zzzzz.com> - 2012-07-23 15:33 +0200
  Re: Why is java consumer/producer so much faster than C++ Dombo <dombo@disposable.invalid> - 2012-07-23 22:57 +0200
    Re: Why is java consumer/producer so much faster than C++ Joshua Maurice <joshuamaurice@gmail.com> - 2012-07-23 21:54 -0700
      Re: Why is java consumer/producer so much faster than C++ Jorgen Grahn <grahn+nntp@snipabacken.se> - 2012-07-24 12:50 +0000
  Re: Why is java consumer/producer so much faster than C++ Ricardo Nabinger Sanchez <rnsanchez@wait4.org> - 2012-08-14 22:48 +0000

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