MPI and Multicore: Random Ramblings

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MPI and MulticoreRandom Ramblings

• Brian Barrett
• Scalable Computer Architectures
• Sandia National Laboratories
• June 4, 2008

Sandia is a multiprogram laboratory operated by
Sandia Corporation, a Lockheed Martin
Company,for the United States Department of
Energys National Nuclear Security
Administration under contract DE-AC04-94AL85000.
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Development Model
Mary Shaw, Prospects for an Engineering
Discipline of Software, Carnegie Mellon,1990
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Why MPI worked
NX Shmem P4, PVM Sockets
Distributed Memory Hardware
Message passing woks
Legacy MPI codes
MPICH LAM/MPI Open MPI
MPI
Mary Shaw, Prospects for an Engineering
Discipline of Software, Carnegie Mellon,1990
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Multicore Panic
MPI OpenMP HPCS PGAS
Multicore Architctures
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Mary Shaw, Prospects for an Engineering
Discipline of Software, Carnegie Mellon,1990
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What is MPI?

• MPI is an interface, not an implementation
• Implementations have different goals
• research, stability, performance, portability
• TCP, InfiniBand, or Portals
• Threads / independent progress or lowest latency
• Latency vs. zero copy eager
• MPI implementations do adapt and change
• SMP optimizations
• RDMA interfaces
• But what are we adapting to?

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Multicore MPI

• MPIs path forward dependent on hardware trends
• Yes, we know multicore is coming, but...
• Will caches be coherent?
• Will it have hardware multithreading?
• Will it have lightweight synchronization
  primitives?
• Will it have homogeneous cores?
• Will there be better hardware offload for
  messaging?
• HPC will have little influence over the answers
  to these questions
• The answers will influence MPI implementations

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Multicore MPI

• MPI point-to-point performance limited by
  matching costs
• NIC offload makes this pretty easy, of course
• Software approaches have cache collision problems
• Threading limited cache coherency?
• MPI Threads pushes towards multi-threaded MPI
• Currently quite expensive (locks in critical
  path)
• Lightweight synchronization limits cost
• Independent progress finally arrive?
• Needs hardware support or threads

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