[petsc-users] Very poor speed up performance

[Barry Smith]

On Dec 22, 2010, at 9:55 AM, Yongjun Chen wrote:

> 
> Satish,
> 
> I have reconfigured the PETSC with –download-mpich=1 and –with-device=ch3:sock. The results show that the speed up can now remain increasing when computing cores increase from 1 to 16. However, the maximum speed up is still only around 6.0 with 16 cores. The new log files can be found in the attachment.
> 
>  
> (1)
> 
> I checked the configuration of the first server again. This server is a shared-memory computer, with
> 
> Processors: 4 CPUS * 4Cores/CPU, with each core 2500MHz
> 
> Memories: 16 *2 GB DDR2 333 MHz, dual channel, data width 64 bit, so the memory Bandwidth for 2 memories is 64/8*166*2*2=5.4GB/s.

   Wait a minute. You have 16 cores that share 5.4 GB/s???? This is not enough for iterative solvers, in fact this is absolutely terrible for iterative solvers. You really want 5.4 GB/s PER core! This machine is absolutely inappropriate for iterative solvers. No package can give you good speedups on this machine.

   Barry

> 
> It seems that each core can get 2.7GB/s memory bandwidth which can fulfill the basic requirement for sparse iterative solvers.
> 
> Is this correct? Does the shared-memory type of computer have no benefit for PETSC when the memory bandwidth is limited?
> 
>  
> (2)
> 
> Beside, we would like to continue our work by employing a matrix partitioning / reordering algorithm, such as Metis or ParMetis, to improve the speed up performance of the program. (The current program works without any matrix decomposition.)
> 
>  
> Matt, as you said in http://lists.mcs.anl.gov/pipermail/petsc-users/2007-January/001017.html ,“Reordering a matrix can result in fewer iterations for an iterative solver“.
> 
> Do you think the matrix partitioning/reordering will work for this program? Or any further suggestions?
> 
>  
> Any comments are very welcome! Thank you!
> 
>  
> 
> 
> 
> 
> 
> 
> On Mon, Dec 20, 2010 at 11:04 PM, Satish Balay <balay at mcs.anl.gov> wrote:
> On Mon, 20 Dec 2010, Yongjun Chen wrote:
> 
> > Matt, Barry, thanks a lot for your reply! I will try mpich hydra firstly and
> > see what I can get.
> 
> hydra is just the process manager.
> 
> Also --download-mpich uses a slightly older version - with
> device=ch3:sock for portability and valgrind reasons [development]
> 
> You might want to install latest mpich manually with the defaut
> device=ch3:nemsis and recheck..
> 
> satish
> 
> 
> 
> <log_ch3sock_jacobi_bicg_4cpus.txt><log_ch3sock_jacobi_bicg_8cpus.txt><log_ch3sock_jacobi_bicg_12cpus.txt><log_ch3sock_jacobi_bicg_16cpus.txt>