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The Sensitivity of Communication Mechanisms to Bandwidth and Latency

The Sensitivity of Communication Mechanisms to Bandwidth and Latency. Frederic T. Chong, Rajeev Barua, Fredrik Dahlgrenz, John D. Kubiatowicz, and Anant Agarwal. presented by: Scott Beamer. Overview.

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The Sensitivity of Communication Mechanisms to Bandwidth and Latency

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  1. The Sensitivity of Communication Mechanisms to Bandwidth and Latency • Frederic T. Chong, Rajeev Barua, • Fredrik Dahlgrenz, John D. Kubiatowicz, and • Anant Agarwal presented by: Scott Beamer

  2. Overview • Comparison of message passing and shared memory with respect to the latency and bandwidth of the network • Done on hardware (Alewife) with real apps • Also considers optimizations such as prefetching, polling, and bulk transfers

  3. Supported options • Message Passing • with interrupts - active message style • with polling - app checks queue • with bulk transfer - DMA to network • Shared Memory • without prefetching - normal • with prefetching - done in background

  4. Performance Models

  5. EM3D • Models properties of electromagnetic waves through 3d objects

  6. UNSTRUC • Simulates fluid flows over unstructured meshes in 3D

  7. ICCG • General iterative sparse matrix solver using conjugate gradients with preconditioning

  8. MOLDYN • Computes interactions between molecules within a cut-off distance

  9. Communication Volume

  10. Bandwidth Experiment • Controlled amount of I/O traffic across 2D mesh to reduce available bandwidth • I/O message size didn’t affect MP, bigger messages improved SM’s performance

  11. Latency Experiment • To decrease latency, slowed down processor clock • To increase latency, context switches to a delay loop

  12. Conclusion • Shared memory required less coding effort • Message passing less sensitive to increased latency or reduced bandwidth • Shared memory can provide good performance if there is enough bandwidth

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