This is not an MPI problem.
Without looking at your code in detail, I'm guessing that you're
accessing memory without any regard to memory layout and/or caching.
Such an access pattern will therefore thrash your L1 and L2 caches
and access memory in a truly horrible pattern that guarantees abysmal
performance.
Google around for cache effects or check out an operating systems
textbook; there's lots of material around about this kind of effect.
Good luck.
On Oct 26, 2007, at 5:10 AM, 42af...@niit.edu.pk wrote:
Thanks,
The array bounds are the same on all the nodes and also the
compute nodes
are identical i.e. SunFire V890 nodes. And I have also changed the
root
process to be on different nodes, but the problem remains the same. I
still dont understand the problem very well and my progress is in
stand
still situation.
regards aftab hussain
Hi,
Please ensure if following things are correct
1) The array bounds are equal. Means "my_x" and "size_y" has the same
value on all nodes.
2) Nodes are homogenous. To check that, you could decide root to be
some
different node and run the program
-Neeraj
On Fri, October 26, 2007 10:13 am, 42af...@niit.edu.pk wrote:
Thanks for your reply,
I used MPI_Wtime for my application but even then process 0 took
longer
time executing the mentioned code segment. I might be worng, but
what I see
is process 0 takes more time to access the array elements than other
processes. Now I dont see what to do because the mentioned code
segment is
creating a bottleneck for the timing of my application.
Can any one suggest somthing in this regard. I will be very thankful
regards
Aftab Hussain
On Thu, October 25, 2007 9:38 pm, jody wrote:
HI
I'm not sure if that is a problem,
but in MPI applications you shoud use MPI_WTime() for time-
measurements
Jody
On 10/25/07, 42af...@niit.edu.pk <42af...@niit.edu.pk> wrote:
Hi all,
I am a research assistant (RA) at NUST Pakistan in High Performance
Scientific Computing Lab. I am working on the parallel
implementation of the Finitie Difference Time Domain (FDTD) method
using MPI. I am using the OpenMPI environment on a cluster of 4
SunFire v890 cluster connected through Myrinet. I am having problem
that when I run my code with let say 4 processes. Process 0 takes
about 3 times more time than other three processes, executing a for
loop which is the main cause of load imbalance in my code. I am
writing the code that is causing the problem. The code is run by
all
the processes simultaneously and independently and I have timed it
independent of segments of code.
start = gethrtime(); for (m = 1; m < my_x ; m++){ for (n = 1; n <
size_y-1; n++) { Ez(m,n) = Ez(m,n) + cezh*((Hy(m,n) - Hy(m-1,n)) -
(Hx(m,n) - Hx(m,n-1)));
}
}
stop = gethrtime(); time = (stop-start);
In my implementation I used 1-D array to realize 2-D arrays.I have
used the following macros for accesing the array elements.
#define Hx(I,J) hx[(I)*(size_y) + (J)]
#define Hy(I,J) hy[(I)*(size_y) + (J)]
#define Ez(I,J) ez[(I)*(size_y) + (J)]
Can any one tell me what am I doing wrong here, or macros are
creating the problems or it can be related to any OS issue. I
will be
looking forward for help because this problem has stopped my
progress
for the last two weeks
regards aftab hussain
RA High Performance Scientific Computing Lab
NUST Institue of Information Technology
National University of Sciences and Technology Pakistan
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