llvm dev - Jun 2012 - [LLVMdev] no differnce in the execution time between seq. and parallel programs

Hi all
please can any one help me to find the reason??

i am using LLVM to execute two programs both of them was exactly  
similar but the difference was the first one is a sequential program  
has three functions. In the second program i am trying to execute it  
by giving each function to thread (three thread work in parallel way)  
,but when i am trying to calculate the execution time i found the  
execution time for parallel one larger than sequential one ????

note
I execute both of these program in gcc compiler and i found there is a  
difference in the execution time between these programs.The parallel  
one was faster than the sequential one.

i will be thankful if any one can help me
Regards
esraa

Hi esraa,
> i am using LLVM to execute two programs both of them was exactly
> similar but the difference was the first one is a sequential program
> has three functions. In the second program i am trying to execute it
> by giving each function to thread (three thread work in parallel way)
> ,but when i am trying to calculate the execution time i found the
> execution time for parallel one larger than sequential one ????
this is a very vague description and it's hard to understand what you mean
here.  Maybe you mean that you used clang to compile a program containing
openmp macros.  If so you have discovered that clang does not support openmp:
use the dragonegg LLVM front-end instead.

Ciao, Duncan.
>
> note
> I execute both of these program in gcc compiler and i found there is a
> difference in the execution time between these programs.The parallel
> one was faster than the sequential one.
>
> i will be thankful if any one can help me
> Regards
> esraa
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

Duncan Sands <baldrick <at> free.fr> writes:
> 
> Hi esraa,
> 
> > i am using LLVM to execute two programs both of them was exactly
> > similar but the difference was the first one is a sequential program
> > has three functions. In the second program i am trying to execute it
> > by giving each function to thread (three thread work in parallel way)
> > ,but when i am trying to calculate the execution time i found the
> > execution time for parallel one larger than sequential one ????
> 
> this is a very vague description and it's hard to understand what you
mean
> here.  Maybe you mean that you used clang to compile a program containing
> openmp macros.  If so you have discovered that clang does not support
openmp:
> use the dragonegg LLVM front-end instead.
> 
> Ciao, Duncan.
> 
> >
> > note
> > I execute both of these program in gcc compiler and i found there is a
> > difference in the execution time between these programs.The parallel
> > one was faster than the sequential one.
> >
> > i will be thankful if any one can help me
> > Regards
> > esraa
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > LLVMdev <at> cs.uiuc.edu         http://llvm.cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> 
Hi Ciao
Thank you very much for the quick reply and sorry for misunderstanding me. 
I am working with some optimizing techniques to transform the sequential program
to parallel program  and after I finish this transformation  I need to  make
comparison between the execution time for seq. and optimized one(parallel ).
Before I automated  this method using the llvm , I did this work manually.
 
In the first time I used the gcc to compile two versions of program seq. and
parallel( three threads work together  using  POSIX Pthread libraries).   I
found that there was difference in the execution time between these two programs
(parallel one was faster than the seq. program).

Now I have repeating the same comparison (same two version of program that I
wrote before using c language )  by using llvm-gcc to get llvm byte code and
then llvm-ld to get executable program and made the  comparison between them.

I was surprised when I found the result  for my gcc experiments was different
from the result of llvm experiment.

I would be happy if you give me an insight into what could have caused the
difference.
Many thanks
Esraa