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Open MPI

Overview and Current Status. Open MPI. Tim Woodall - LANL Galen Shipman - LANL/UNM. Overview. Point-to-Point Architecture OpenIB Implementation Results Future Work. Point-to-Point Architecture. Component Architecture: “Plug-ins” for different capabilities (e.g. different networks)

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Open MPI

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  1. Overview and Current Status Open MPI Tim Woodall - LANL Galen Shipman - LANL/UNM

  2. Overview Point-to-Point Architecture OpenIB Implementation Results Future Work

  3. Point-to-Point Architecture Component Architecture: “Plug-ins” for different capabilities (e.g. different networks) Tunable run-time parameters Three component frameworks: Point-to-point messaging layer (PML) implements MPI semantics Byte Transfer Layer (BTL) abstracts network interfaces Memory Pool (mpool) provides for memory management/registration

  4. PML Framework Single PML manages multiple BTL modules Maintains set of BTLs on a per-peer basis Message fragmentation and scheduling Implements MPI semantics Synchronous / buffered / ready / normal sends Persistent requests / Request completion Eager/Rendezvous protocol Eager send of short messages Configurable threshold (short vs. long) Multiple long protocols

  5. PML Protocols Send / receive pipeline to / from pre-registered buffers (non-contiguous data) MPI_Alloc_mem support Red/black tree of memory registrations BTL associated with registration is used by scheduler Xfer of contiguous data with 1 RDMA (after match) “Leave pinned” run-time parameter Registration on first-use MRU cache (configurable size) of registrations Bandwidth equivalent to pre-registered buffers (MPI_Alloc_mem)

  6. PML Protocols (Continued) Dynamic memory registration/deregistration Fragment message and build pipeline of RDMA requests Overlap [de-]registration with RDMA Bandwidth 97% of pre-registered memory at large message sizes (8Mbytes) Performance impacted by bus type/bandwidth

  7. BTL Framework MPI agnostic Provides simple API to upper layers Tagged send/receive primitives One-sided put/get operations Access to data type engine for zero copy data transfer BTL modules natively support commodity networks: Current (self, shared memory, myrinet GM/MX, Infiniband mvapi/OpenIB, Portals, TCP) Planned (LAPI, Quadrics Elan4)

  8. OpenIB BTL BTL module initialization Resources allocation Connection management Small message Xfer Large message Xfer OpenIB Issues Future Work

  9. BTL module initialization A separate BTL module is initialized for each port on each HCA The PML schedules across these BTL modules just as any other interconnect When multiple BTL modules exist peers establish QP connections by matching subnets

  10. Resource Allocation

  11. SRQ Scalability Nodes Frag size- Kbytes #posted RQ per QP- Mbytes K* SRQ- Mbytes 128 8 64 64 2 1 256 8 64 128 4 2 512 8 64 256 8 4 1024 8 64 512 10 5 K- multiplier based on number of nodes

  12. Connection management Addressing information is exchanged dynamically via an OOB channel This greatly improves scalability but at the cost of increased first message latency Connections are established with peers in the same subnet (local subnet routing only)

  13. Small Message Xfer Maintain list of pre-registered fragments for send and recv List grows dynamically in chunks (more efficient to register) Small messages are copied to/from pre-registered buffers Recv descriptors are posted as needed based on min/max thresholds

  14. Small Message Performance * Send/Recv based protocol

  15. Large Message Xfer RDMA Write and RDMA Read are both supported RDMA Read provides better performance than RDMA Write - control messages are reduced RDMA pipeline protocol performance highly dependent on I/O Bus performance

  16. Results OpenMPI/OpenIB - All

  17. Results OpenMPI/OpenIB - All - Log

  18. Results OpenMPI/OpenIB - Eager limit

  19. Results Combined Results

  20. Results Combined Results - Log

  21. OpenIB Opportunities User level notification of VM activity Caching of memory registrations can be dangerous Need the ability to detect VM changes that effect memory registrations (such as sbrk and munmap) Reliable Multicast for collectives SRQ performance, 2/10 usec penalty, but who’s counting?

  22. Future Work Small message RDMA (using working set of peers) - optional Dynamic connection management using Unreliable Datagrams Dynamic connection teardown - optional

  23. Source Code Access • Subversion repository • Download client from: • http://subversion.tigris.org/ • v1.2.1 or later • Check out with: • svn co http://svn.open-mpi.org/svn/ompi/trunk ompi • Anonymous, read-only access

  24. Questions? Tim Woodall Email: twoodall@lanl.gov Phone: 505-665-5224 Galen Shipman Email: gshipman@lanl.gov

  25. Hardware Specs • Dual Intel Xeon 3.2 GHz • 1024 KB Cache • 2 Gbytes memory • Bus: Intel Corp. E7525/E7520/E7320 PCI Express • Mellanox Technologies MT25208 InfiniHost III Ex • 288 Port Voltaire switch

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