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CS 179: Lecture 2 Lab Review 1

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The Problem Add two arrays A[] + B[] -> C[]

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GPU Computing: Step by Step Setup inputs on the host (CPU-accessible memory) Allocate memory for inputs on the GPU Copy inputs from host to GPU Allocate memory for outputs on the host Allocate memory for outputs on the GPU Start GPU kernel Copy output from GPU to host (Copying can be asynchronous)

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The Kernel Determine a thread index from block ID and thread ID within a block:

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Calling the Kernel …

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CUDA implementation (2)

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Fixing the Kernel For large arrays, our kernel doesn’t work! Bounds-checking – be on the lookout! Also, need a way for kernel to handle a few more elements…

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Fixing the Kernel – Part 1

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Fixing the Kernel – Part 2

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Fixing our Call

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Lab 1! Sum of polynomials – Fun, parallelizable example! Suppose we have a polynomial P(r) with coefficients c 0, …, c n-1, given by: We want, for r 0, …, r N-1, the sum: Output condenses to one number!

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Calculating P(r) once Pseudocode (one possible method): Given r, coefficients[] result <- 0.0 power <- 1.0 for all coefficient indecies i from 0 to n-1: result += (coefficients[i] * power) power *= r

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Accumulation atomicAdd() function Important for safe operations!

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Accumulation

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Shared Memory Faster than global memory Per-block One block

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Linear Accumulation atomicAdd() has a choke point! What if we reduced our results in parallel?

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Linear Accumulation …

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Linear Accumulation (2)

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Can we do better?

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Last notes minuteman.cms.caltech.edu – the easiest option CMS accounts! Office hours Kevin: Monday, 8-10 PM Connor: Tuesday, 8-10 PM

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