nvidia cuda - ohjelmistotekniikan laboratoriotlilja/multicore/slides/cuda.pdf · nvidia cuda...

NVIDIA CUDASeminar on Multi-core Programming

Feb 26, 2009

Hello, in parallel!

__global__voidfunctinttid=threadI__shared__floatif(tid<n){inttmp=tid

Outline

Introductionto CUDA

ProgrammingBasics

Planningfor CUDA

IntroductionGPGPU = General Purpose computing on GPU

“ (Gordon Moore)3 ”

2003 2004 2005 2006 2007 2008

GT200933 Gflops

G80

G703.0 GHz

Core2 Duo

3.2 GHzHarpertown

Memory Bandwidth100 GB/s = 12.5 Gfloat read-writes/s

100

02003 2004 2005 2006 2007

GB/s

80

60

40

20

First, there was just a graphics pipeline

What could they do with itby reading from textures and writing to others

?

Stream mapping

OP

Parallel reduction

OP OP

Gather input from textures

OP

Scatter output as vertices

OP

Map Reduce

Gather Scatter

Back to present…

What to use GPU for?


Physics Simulations


Linear Algebra

Finance Pattern Recognition…


Biomedical Imaging

Do you have a casewhere CUDA could be used?

CUDA Architecture

Yet Another...

S ingle I nstruction M ultiple T hreads

Warp = 32 threads, lock-step, masked

Chapter 2. Programming Model

!

10 CUDA Programming Guide Version 2.1!

Figure 2-1. Grid of Thread Blocks

2.3 Memory Hierarchy

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C')!7319*3B!/16,&*6&B!*6+!&)8&2()!-)-1(.!,5*/),!*()!5)(,4,&)6&!*/(1,,!@)(6)3!3*26/'),!9.!&')!,*-)!*5534/*&416=!

Grid

Block (1, 1)

Thread (0, 0) Thread (1, 0) Thread (2, 0) Thread (3, 0)



Block (2, 1) Block (1, 1) Block (0, 1)

Block (2, 0) Block (1, 0) Block (0, 0)

Abstraction

1) You won’t know which core or when.

2) You don’t care how many cores.

3) Forget synchronization if you can.

GeForce, Quadro or Tesla?

Compute Capability?

1.0 – 1st generation (Nov. 2006)

1.1 – Atomics & asynchronous memory transfers

1.2 – Relaxed alignment requirements, voting intrinsics

1.3 – Double support (on 1/8th float speed)

CUDA code can be compiled todifferent architectures,

including multi-core CPU’s.

PTX is an intermediate language between CUDA and CUBIN

IntroductionGPGPU = General Purpose computing on GPU


ProgrammingWhat’s needed to run GPGPU with CUDA?

CUDA is close to C or C++01enum{max_coeff=128;};02__constant__floatcoeff[max_coeff];0304__device__floateval(floatx,intorder){05floatr=0;06for(inti=order;i>=0;‐‐i)07r=(r+x)*coeff[i];08returnr;09}10__global__voidpolynomial(floatconst*x,float*y,11intn,intorder)//order<max_coeff12{13inti=threadIdx.x+blockIdx.x*blockDim.x;14if(i<n)15y[i]=eval(x[i],order);16}

But GPU ≠ CPU...

GPU ≠ CPUGPU isn’t independent. CPU takes the initiative:01voidcalculate_polynomial(floatconst*a,floatconst*x,02float*y,intn,intorder)03{04float*z=0;05cudaMalloc((void**)&z,n*sizeof(float));06cudaMemcpy(z,x,n*sizeof(float),cudaMemcpyHostToDevice);07cudaMemcpyToSymbol(coeff,a,order*sizeof(float),0,08cudaMemcpyHostToDevice);09unsignedblock=256;10unsignedgrid=(n+block‐1)/block;11polynomial<<<grid,block>>>(z,z,n,order);//kernellaunch12cudaMemcpy(y,z,n*sizeof(float),cudaMemcpyDeviceToHost);13cudaFree(z);14}

GPU ≠ CPU

GPU cannot access CPU memoryData must be explicitly transferred.

There’s no...

stack

recursion

function pointers or calls

dynamic memory allocation (from device)

GPU ≠ CPU

thread: runs the kernel with given thread index

warp: 32 threads in lock-step

block: max. 512 threads with shared cache, block-level synchronization: __syncthreads()

grid: 100’s or 1000’s of blocks; no synchronization

device: kernel-level synchronizationhost: enqueues kernel calls for device

GPU ≠ CPU

There’s hardly any silicon spent on a GPU cache

GPU ≠ CPU

constant memorysame address for threads

textures2D, read-only

shared memoryread-write, within block

! Chapter 1. Introduction

!

CUDA Programming Guide Version 2.1 3!

!

!

Figure 1-2. The GPU Devotes More Transistors to Data Processing

!

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

1.2 CUDA™: a General-Purpose Parallel Computing Architecture

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Cache

ALU Control

ALU

ALU

ALU

DRAM

CPU

DRAM

GPU

NVCC separates,

compiles &

embeds GPU code

nvccmy_program.cu‐omy_program

NVCC separates,

compiles &

embeds GPU code


host code(C or C++)

GPU functions(cu)

NVCC separates,

compiles &

embeds GPU code


host code(C or C++)

GPU functions(cu)

GPU kernels(ptx)

GPU kernels(cubin)


host code(C or C++)

GPU functions(cu)

GPU kernels(ptx)

GPU kernels(cubin)

Runtime API vs. Driver API?

Where to feed input from CPU?CPU GPU

RAM

Global

Texture

Constant

KERNEL

uncached read/writealignment (coalescing)

2D spatial cache, read-only1D buffer texturing

read same address at a time

Where to feed input from CPU?GPU

Global

KERNEL

uncached read/writealignment (coalescing)

cudaMalloc(&dptr,n);cudaFree(dptr);cudaMemcpy(dptr,p,n,cudaMemcpyHostToDevice);

Threads have specialized memory

Global

Texture

Constant

KERNEL

registersthread-private,

usually only ~10’sper thread

local memorythread-privateglobal memory,

spilled-over registers

__shared__between block threads,limited size < 16 KB/MP,

16 banks (half-warp),broadcast function


ProgrammingWhat’s needed to run GPGPU with CUDA?

Four steps to CUDA performance

Planning

Four steps to performance

1. REDESIGNyour algorithm

2. RESTRUCTUREdata

3. COOPERATEwith block threads

4. SQUEEZEthe last juice out

1. Redesign

• redesign algorithm for GPU and big datamost C code won’t copy-paste to CUDA

• maximize parallel executiongo data parallel, with 10000’s threads

• find arithmetic intensity (flops / transfers)cache less, compute more; 1 global load ≈ 100 flops

• don’t leave the MP’s unemployed

2. Restructure

• strive for coherent global memory accessesit’s a matter of 100 GB/s vs. 10 GB/s

• access locality? could textures help?1D buffer textures read directly from global memory

• prevent CPU roundtripsGPU–CPU: ~5 GB/s; group transfers if possible

3. Cooperate• block threads talk through shared memory

save global memory loads when gathering

• use __syncthreads() if neededbut go lock-step within warps

• prevent cache bank conflictshalf-warp threads read different banks, or broadcast

• warp voting?

4. Squeeze

• parameterize your applicationauto-tuning algorithms?

• minimize registers and shared memory

• loop unrolling and template tricksthey help as long as GPU architecture differs from CPU’s

Four steps to performance

1. REDESIGNyour algorithm

2. RESTRUCTUREdata

3. COOPERATEwith block threads

4. SQUEEZEthe last juice out

Examples

Questions

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