using core i7 cpu for parallel solving

murx · September 4, 2011, 05:57

Of course, you are right. But a deviation of lets say 2 seconds caused by taking the time manually at measured times between 2 and 3 minutes, only means an error of 1%.
So I guess this is not the reason for the poor performance improvement. Any other idea what the reason could be?

ghorrocks · September 4, 2011, 20:11

I do not trust your measurement yet. Please recalculate based on CFD Solver wall clock seconds reported immediately after the iterations are complete.

murx · September 5, 2011, 07:39

Serial: CFD Solver wall clock seconds: 2.2300E+02

HP MPI Local Parallel, 4 Processes: CFD Solver wall clock seconds: 1.2000E+02 (factor 1.86)

HP MPI Local Parallel, 7 Processes: CFD Solver wall clock seconds: 1.1000E+02 (factor 2.03)

This time, i used a different case with a smaller mesh (~ 600 000 elements).

ghorrocks · September 5, 2011, 18:47

I see. Forget about the 7 processes model, you are always going to get terrible speedups with hyperthreading. A speedup of 1.86 at 4 processes is not very good. You should be over 3 for modern processors. Have you run the benchmark simulation? That is the reference I use to benchmark solver speed.

But I think you can be pretty sure something is wrong with your setup and is robbing you of multi processor speed.

murx · September 8, 2011, 03:43

First of all, thanks for your help!

Here are the results for the Benchmark run:
Serial: CFD Solver wall clock seconds: 3.0000E+01
HP MPI Local Parallel, 4: CFD Solver wall clock seconds: 1.6000E+01 (--> factor 1.88)

I checked the memory usage during my last runs and it was never fully used.
Also, I tested another machine. It's a Core i-5 2500 (the one I usually use is a Core i-7 2600). The speedup factor on this machine was only 1.9, too.

I dont know too much about CPUs, but maybe there is some kind of automatic down-clocking when several cores are used ...

ghorrocks · September 8, 2011, 07:51

If the benchmark problem runs at a similar speed then you definitely have a problem with your set up, it is not the simulation.

Recent Intel CPUs do run a higher clock speed when running single processor, this could explain it. To check this run it 1, 2, 3 and 4 processor. If the 1 processor result is clearly different then this probably explains it.

The benchmark in 30s and 4 processes in 16s is very fast - must be quite a new machine.

So I would recommend trying other multi processor implementations such as MPI, HPMPI, PVM, Intel etc. You may be able to get better speedups from them.

oj.bulmer · June 25, 2012, 08:55

I have i7-2860QM (4 cores) and 8 GB RAM.
Popular opinion in this thread is that 4 cores give optimum performance.

Question is, would keeping simulation on for hours harmful for life of system? I can feel the system heating up as I see the 4 processors loaded at ~100%.

oj.bulmer · June 25, 2012, 09:01

I have i7-2860QM (4 cores) and 8 GB RAM.
Popular opinion in this thread is that 4 cores give optimum performance.

Question is, would keeping simulation on for hours harmful for life of system? I can feel the system heating up as I see the 4 processors loaded at ~100%.

Quote:

Originally Posted by ghorrocks

... So I would recommend trying other multi processor implementations such as MPI, HPMPI, PVM, Intel etc. You may be able to get better speedups from them.

Glenn, given my processor build, which of these (MPI, HPMPI, PVM, Intel ) would you recommend for best performance?

Thanks
OJ

ghorrocks · June 25, 2012, 19:01

When CPUs run hard they run hot. Well designed systems can handle the load and should still function fine. Poorly designed systems will overheat and cause problems - I think the i7 chips sense their own temeprature and if they are too hot they slow down to stop overheating. This saves the CPU but means you are running at reduced speed.

As for which multi processor implementation is best, the simple answer is to benchmark them all on your system.

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September 4, 2011, 05:57		#21
murx Member Max Join Date: May 2011 Location: old europe Posts: 88 Rep Power: 14	Of course, you are right. But a deviation of lets say 2 seconds caused by taking the time manually at measured times between 2 and 3 minutes, only means an error of 1%. So I guess this is not the reason for the poor performance improvement. Any other idea what the reason could be?

September 4, 2011, 20:11		#22
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	I do not trust your measurement yet. Please recalculate based on CFD Solver wall clock seconds reported immediately after the iterations are complete.

September 5, 2011, 07:39		#23
murx Member Max Join Date: May 2011 Location: old europe Posts: 88 Rep Power: 14	Serial: CFD Solver wall clock seconds: 2.2300E+02 HP MPI Local Parallel, 4 Processes: CFD Solver wall clock seconds: 1.2000E+02 (factor 1.86) HP MPI Local Parallel, 7 Processes: CFD Solver wall clock seconds: 1.1000E+02 (factor 2.03) This time, i used a different case with a smaller mesh (~ 600 000 elements).

September 5, 2011, 18:47		#24
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	I see. Forget about the 7 processes model, you are always going to get terrible speedups with hyperthreading. A speedup of 1.86 at 4 processes is not very good. You should be over 3 for modern processors. Have you run the benchmark simulation? That is the reference I use to benchmark solver speed. But I think you can be pretty sure something is wrong with your setup and is robbing you of multi processor speed.

September 8, 2011, 03:43		#25
murx Member Max Join Date: May 2011 Location: old europe Posts: 88 Rep Power: 14	First of all, thanks for your help! Here are the results for the Benchmark run: Serial: CFD Solver wall clock seconds: 3.0000E+01 HP MPI Local Parallel, 4: CFD Solver wall clock seconds: 1.6000E+01 (--> factor 1.88) I checked the memory usage during my last runs and it was never fully used. Also, I tested another machine. It's a Core i-5 2500 (the one I usually use is a Core i-7 2600). The speedup factor on this machine was only 1.9, too. I dont know too much about CPUs, but maybe there is some kind of automatic down-clocking when several cores are used ...

September 8, 2011, 07:51		#26
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	If the benchmark problem runs at a similar speed then you definitely have a problem with your set up, it is not the simulation. Recent Intel CPUs do run a higher clock speed when running single processor, this could explain it. To check this run it 1, 2, 3 and 4 processor. If the 1 processor result is clearly different then this probably explains it. The benchmark in 30s and 4 processes in 16s is very fast - must be quite a new machine. So I would recommend trying other multi processor implementations such as MPI, HPMPI, PVM, Intel etc. You may be able to get better speedups from them.

June 25, 2012, 08:55		#27
oj.bulmer Senior Member OJ Join Date: Apr 2012 Location: United Kindom Posts: 473 Rep Power: 20	I have i7-2860QM (4 cores) and 8 GB RAM. Popular opinion in this thread is that 4 cores give optimum performance. Question is, would keeping simulation on for hours harmful for life of system? I can feel the system heating up as I see the 4 processors loaded at ~100%.

June 25, 2012, 19:01		#29
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	When CPUs run hard they run hot. Well designed systems can handle the load and should still function fine. Poorly designed systems will overheat and cause problems - I think the i7 chips sense their own temeprature and if they are too hot they slow down to stop overheating. This saves the CPU but means you are running at reduced speed. As for which multi processor implementation is best, the simple answer is to benchmark them all on your system.