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Forum: Multithreading
Re: Question What's the best choice for number of processes and threads? (Huan Yao)
Re: Feedback Re: What's the best choice for number of processes and threads? (Andrea Dotti)
Re: None Re: What's the best choice for number of processes and threads? (Tom Roberts)
Date: 13 Jan, 2017
From: Andrea Dotti <Andrea Dotti>

The user is using a VM. I assume the two cores are the one seen by the VM. So HT is not extremely relevant in this case (not even sure the user has an Intel based machine or if HT is enabled). It is true that in a VM a 10% over-scheduling could be helpful.

In any case over-scheduling for a factor x46 is not very effective in all cases... In all MT applications that I've seen I've never seen getting benefits from over-scheduling beyond 1 or 2 extra threads. If this extreme over-scheduling is indeed necessary I would suggest a detailed analysis of the workload of the application, since this is not normal for a G4 applications. With a 2-core VM, I would doubt that the configuration NP*NT>3 gives benefits. Said this for a single box, MT is usually better than MPI (for memory considerations and complexity of the system). Thus in this case I would scale over the nodes via MPI and over the cores with MT, with: NP=#nodes NT=#cores(+10% to be experimentally verified)

Beside performance considerations, if the test fails at maximum scale it is an indication of an error in the application level or maybe the memory resources of the system have been exhausted and there is a crash.

Hope this clarifies, Andrea

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1 None: Re: What's the best choice for number of processes and threads?   (Huan Yao - 13 Jan, 2017)
(_ More: Re: What's the best choice for number of processes and threads?   (Andrea Dotti - 14 Jan, 2017)
(_ None: Re: What's the best choice for number of processes and threads?   (Huan Yao - 17 Jan, 2017)
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