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Presented by Dealing with the Scale Problem Innovative Computing Laboratory MPI Team

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2 Bosilca_OpenMPI_SC07 2 Dongarra_KOJAK_SC07

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3 Bosilca_OpenMPI_SC07 3 Dongarra_KOJAK_SC07 Runtime scalability Undirected graph G:=(V, E), |V|=n (any size) Node i={0,1,2,…,n-1} has links to a set of nodes U U={i±1, i±2,…, i±2 k | 2 k ≤ n} in a circular space U={ (i+1)mod n, (i+2)mod n,…, (i+2 k )mod n | 2 k ≤ n } and { (n+i-1)mod n, (n+i-2)mod n,…, (n+i-2 k )mod n | 2 k ≤ n } Merging all links create binomial graph from each node of the graph Broadcast from any node in Log 2 (n) steps Binomial graph

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4 Bosilca_OpenMPI_SC07 4 Dongarra_KOJAK_SC07 Degree = number of neighbors = number of connections = resource consumption Node-Connectivity (κ) Link-Connectivity (λ) Optimally Connected κ = λ =δ min Diameter Average Distance Binomial graph properties Runtime scalability

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5 Bosilca_OpenMPI_SC07 5 Dongarra_KOJAK_SC07 Broadcast: optimal in number of steps log 2 (n) (binomial tree from each node) Allgather: Bruck algorithm log 2 (n) steps At step s: Node i send data to node i-2s Node i receive data from node i+2s Routing cost Runtime scalability Because of the counter- clockwise links the routing problem is NP- complete. Good approximations exist, with a max overhead of 1 hop.

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6 Bosilca_OpenMPI_SC07 6 Dongarra_KOJAK_SC07 Self-Healing BMG # of nodes # of added nodes # of new connections Runtime scalability Dynamic environments

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7 Bosilca_OpenMPI_SC07 7 Dongarra_KOJAK_SC07 Optimization process Run-time selection process We use performance models, graphical encoding, and statistical learning techniques to build platform-specific, efficient, and fast runtime decision functions. Collective communications

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8 Bosilca_OpenMPI_SC07 8 Dongarra_KOJAK_SC07 Collective communications Model prediction (A)(B) (C)(D)

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9 Bosilca_OpenMPI_SC07 9 Dongarra_KOJAK_SC07 Model prediction Collective communications (A)(B) (C)(D)

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10 Bosilca_OpenMPI_SC07 10 Dongarra_KOJAK_SC07 64 Opterons with1Gb/s TCP Tuning broadcast Collective communications

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11 Bosilca_OpenMPI_SC07 11 Dongarra_KOJAK_SC07 Tuning broadcast Collective communications

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12 Bosilca_OpenMPI_SC07 12 Dongarra_KOJAK_SC07 Application tuning Parallel Ocean Program (POP) on a Cray XT4. Dominated by MPI_Allreduce of 3 doubles. Default Open MPI select recursive doubling Similar with Cray MPI (based on MPICH). Cray MPI has better latency. I.e., POP using Open MPI is 10% slower on 256 processes. Profile the system for this specific collective and determine that “reduce + bcast” is faster. Replace the decision function. New POP performance is about 5% faster than Cray MPI. Collective communications

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13 Bosilca_OpenMPI_SC07 13 Dongarra_KOJAK_SC07 Fault tolerance P1P2P3P4 4 available processors P1P2P3P4 Add a fifth and perform a checkpoint(Allreduce) Pc + P4 ++= P1P2P3P4 Ready to continue PcP4.... P1P2P3P4 Failure PcP4 P1P3P4 Ready for recovery PcP4 P1P3P4 Recover the processor/data PcP2 ---= Diskless checkpointing

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14 Bosilca_OpenMPI_SC07 14 Dongarra_KOJAK_SC07 Diskless checkpointing How to checkpoint? Either floating-point arithmetic or binary arithmetic will work. If checkpoints are performed in floating ‐ point arithmetic then we can exploit the linearity of the mathematical relations on the object to maintain the checksums. How to support multiple failures? Reed-Salomon algorithm. Support p failures require p additional processors (resources). Fault tolerance

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15 Bosilca_OpenMPI_SC07 15 Dongarra_KOJAK_SC07 Performance of PCG with different MPI libraries For ckpt we generate one ckpt every 2000 iterations Fault tolerance PCG Fault tolerant CG 64x2 AMD 64 connected using GigE

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16 Bosilca_OpenMPI_SC07 16 Dongarra_KOJAK_SC07 Checkpoint overhead in seconds PCG Fault tolerance

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17 Bosilca_OpenMPI_SC07 17 Dongarra_KOJAK_SC07 Fault tolerance Detailing event types to avoid intrusiveness

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18 Bosilca_OpenMPI_SC07 18 Dongarra_KOJAK_SC07 Fault tolerance Interposition in Open MPI We want to avoid tainting the base code with #ifdef FT_BLALA. Vampire PML loads a new class of MCA components. Vprotocols provide the entire FT protocol (only pessimistic for now). You can use the ability to define subframeworks in your components! Keep using the optimized low level and zero-copy devices (BTL) for communication. Unchanged message scheduling logic.

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19 Bosilca_OpenMPI_SC07 19 Dongarra_KOJAK_SC07 Fault tolerance Performance overhead Myrinet 2G (mx 1.1.5)—Opteron 146x2—2GB RAM—Linux 2.6.18 —gcc/gfortran 4.1.2—NPB3.2— NetPIPE. Only two application kernels show non-deterministic events (MG, LU).

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20 Bosilca_OpenMPI_SC07 20 Dongarra_KOJAK_SC07 Interactive Debugging Testing Release Design and code Fault tolerance Debugging applications Usual scenario involves Programmer design testing suite Testing suite shows a bug Programmer runs the application in a debugger (such as gdb) up to understand the bug and fix it Programmer runs the testing suite again Cyclic debugging

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21 Bosilca_OpenMPI_SC07 21 Dongarra_KOJAK_SC07 Fault tolerance Interposition Open MPI Events are stored (asynchronously on disk) during initial run. Keep using the optimized low level and zero-copy devices (BTL) for communication. Unchanged message scheduling logic. We expect low impact on application behavior.

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22 Bosilca_OpenMPI_SC07 22 Dongarra_KOJAK_SC07 Fault tolerance Performance overhead Myrinet 2G (mx 1.0.3)—Opteron 146x2—2GB RAM—Linux 2.6.18— gcc/gfortran 4.1.2—NPB3.2—NetPIPE 2% overhead on bare latency, no overhead on bandwidth. Only two application kernels show non-deterministic events. Receiver-based has more overhead; moreover, it incurs large amount of logged data—350 MB on simple ping-pong (this is beneficial; it is enabled on-demand).

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23 Bosilca_OpenMPI_SC07 23 Dongarra_KOJAK_SC07 Log size per process on NAS Parallel Benchmarks (kB) Fault tolerance Log size on NAS parallel benchmarks Among the 7 NAS kernels, only 2 NPB generates nondeterministic events. Log size does not correlate with number of processes. The more scalable is the application, the more scalable is the log mechanism. Only 287KB of log/process for LU.C.1024 (200MB memory footprint/process).

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24 Bosilca_OpenMPI_SC07 24 Dongarra_KOJAK_SC07 Contact Dr. George Bosilca Innovative Computing Laboratory Electrical Engineering and Computer Science Department University of Tennessee bosilca@eecs.utk.edu

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