[mpich-discuss] Latency or the B/W, which is the bottleneck for most MPI codes?

[Si Hammond]

Hi Junli,

I'm afraid that this depends on lots of aspects of the job being run:

- Algorithm or code being used (message decomposition etc)
- Underlying architecture (what is the latency of the network etc?)
- Code configuration (e.g. are problems too small to achieve good parallel efficiency etc).

Something else to consider is the time spent in the MPI stack, this can be quite a significant part of the cost in a message transfer.

In my experience with some large codes, lots of the users will try to target a processor and application configuration so that the factors above are "optimised" for the run.



S.



On 5 Mar 2010, at 20:55, junli gu wrote:

> Hi everyone:
>  
>   I am wondering to most MPI codes, which is the performance bottleneck, the latency or the B/W?
> 
>   I am doing research to optimize the cache-coherent shared memory architecture to run MPI codes more efficiently. Although I am trying to reduce the latency for message data transferring, then I realized that maybe the B/W is the real bottleneck. I have heard the comments that most well-written MPI codes  are not latency-sensitive but B/W sensitive. So instead of making single message transfer faster, increasing the B/W to transfer the chunk of data faster is the right way to improve performance.
> 
>   Can you give us some comments about this?
> 
>   Thank you in advance!
> 
> -- 
> ************************************************
> Junli Gu--谷俊丽
> Coordinate Science Lab
> University of Illinois at Urbana-Champaign
> ************************************************
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Si Hammond

Research & Knowledge Transfer Associate
Performance Modelling, Analysis and Optimisation Team
High Performance Systems Group
Department of Computer Science
University of Warwick, CV4 7AL, UK
http://go.warwick.ac.uk/hpsg
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