1ITCS 6/8010 CUDA Programming, UNC-Charlotte, B. Wilkinson, Feb 14, 2011Streams.pptx

CUDA StreamsThese notes will introduce the use of multiple CUDA streams to overlap memory transfers with kernel computations.

Also introduced is paged-locked memory

These materials come from Chapter 10 of “CUDA by Example” by Jason Sanders and Edwards Kandrot.

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Page-locked host memory(also called pinned host memory)

Page-locked memory is not paged in and out main memory by the OS through paging but will remain resident.

Allows:

• Concurrent host/device memory transfers with kernel operations (Compute capability 2.x) – see next

• Host memory can be mapped to device address space (Compute capability > 1.0)

• Memory bandwidth is higher• Uses real addresses rather than virtual addresses• Does not need to intermediate copy buffering

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Note on using page-locked memory

Using page-locked memory will reduce memory available to the OS for paging and so need to be careful in allocating it

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Allocating page locked memory

cudaMallocHost ( void ** ptr, size_t size ) Allocates page-locked host memory that is accessible to device

cudaHostAlloc ( void ** ptr, size_t size, unsigned int flags)

Allocates page-locked host memory that is accessible to device – seems to have more options

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//Pinned memory test written by Barry Wilkinson, UNC-Charlotte. Feb 10, 2011.

#include <stdio.h>#include <cuda.h>#include <stdlib.h>

#define SIZE (10*1024*1024) // number of bytes in arrays 10 MBytes

int main(int argc, char *argv[]) {

int i; // loop counterint *a;int *dev_a;

cudaEvent_t start, stop; // using cuda events to measure timecudaEventCreate(&start); // create eventscudaEventCreate(&stop);

float elapsed_time_ms1, elapsed_time_ms3;

/* --------------------ENTER INPUT PARAMETERS AND DATA -----------------------*/

cudaMalloc((void**)&dev_a, SIZE); // allocate memory on device

/* ---------------- COPY USING PINNED MEMORY -------------------- */

cudaHostAlloc((void**)&a, SIZE ,cudaHostAllocDefault); // allocate page-locked memory on host

cudaEventRecord(start, 0);

for(i = 0; i < 100; i++) {

cudaMemcpy(dev_a, a , SIZE ,cudaMemcpyHostToDevice); //copy to device

cudaMemcpy(a,dev_a, SIZE ,cudaMemcpyDeviceToHost); //copy back to host}

cudaEventRecord(stop, 0); // instrument code to measue end time

cudaEventSynchronize(stop);cudaEventElapsedTime(&elapsed_time_ms1, start, stop );

printf("Time to copy %d bytes of data 100 times on GPU, pinned memory: %f ms\n", SIZE, elapsed_time_ms1); // exec. time

Test of Pinned Memory

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/* ---------------- COPY USING REGULAR MEMORY-------------------- */

a = (int*) malloc(SIZE); // allocate regular memory on host

cudaEventRecord(start, 0);

for(i = 0; i < 100; i++) {

cudaMemcpy(dev_a, a , SIZE ,cudaMemcpyHostToDevice); //copy to device

cudaMemcpy(a,dev_a, SIZE ,cudaMemcpyDeviceToHost); //copy back to host}

cudaEventRecord(stop, 0); // instrument code to measue end time

cudaEventSynchronize(stop);cudaEventElapsedTime(&elapsed_time_ms3, start, stop );

printf("Time to copy %d bytes of data 100 times on GPU: %f ms\n", SIZE, elapsed_time_ms3); // exec. time

/*--------------------------SPEEDUP ---------------------------------*/

printf("Speedup of using pinned memory = %f\n", (float) elapsed_time_ms3 / (float) elapsed_time_ms1);

/* -------------- clean up ---------------------------------------*/

free(a);cudaFree(dev_a);cudaEventDestroy(start);cudaEventDestroy(stop);

return 0;}

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My code

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./bandwidthTest Starting...                                                             

Running on...

 Device 0: Tesla C2050 Quick Mode

 Host to Device Bandwidth, 1 Device(s), Paged memory   Transfer Size (Bytes)        Bandwidth(MB/s)   33554432                     1026.7

 Device to Host Bandwidth, 1 Device(s), Paged memory   Transfer Size (Bytes)        Bandwidth(MB/s)   33554432                     1108.1

 Device to Device Bandwidth, 1 Device(s)   Transfer Size (Bytes)        Bandwidth(MB/s)   33554432                     84097.6

[bandwidthTest] - Test results:PASSED

Press <Enter> to Quit...-----------------------------------------------------------

Using NVIDIA sample code for bandwidth on coit-grid06

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CUDA Streams

A CUDA Stream is a sequence of operations (commands) that are executed in order.

CUDA streams can be created and executed together and interleaved although the “program order” is always maintained within each stream.

Streams proved a mechanism to overlap memory transfer and computations operations in different stream for increased performance if sufficient resources are available.

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Creating a stream

Done by creating a stream object and associated it with a series of CUDA commands that then becomes the stream. CUDA commands have a stream pointer as an argument:

cudaStream_t stream1;cudaStreamCreate(&stream1);

cudaMemcpyAsync(…, stream1);MyKernel<<< grid, block, stream1>>>(…);cudaMemcpyAsync(… , stream1);

Cannot use regular cudaMemcpy with streams, need asynchronous commands for concurrent operation see nextStream

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cudaMemcpyAsync( …, stream)

Asynchronous version of cudaMemcpy that copies date to/from host and the device

May return before copy complete

A stream argument specified.

Needs “page-locked” memory

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#define SIZE (N*20)…int main(void) { int *a, *b, *c; int *dev_a, *dev_b, *dev_c;

cudaMalloc( (void**)&dev_a, N * sizeof(int) ); cudaMalloc( (void**)&dev_b, N * sizeof(int) ); cudaMalloc( (void**)&dev_c, N * sizeof(int) );

cudaHostAlloc((void**)&a,SIZE*sizeof(int),cudaHostAllocDefault); // paged-locked

cudaHostAlloc((void**)&b,SIZE*sizeof(int),cudaHostAllocDefault); cudaHostAlloc((void**)&c,SIZE*sizeof(int),cudaHostAllocDefault);

for(int i=0;i<SIZE;i++) { // load dataa[i] = rand();b[i] = rand();

}

for(int i=0;I < SIZE;i+= N { // loop over data in chunks

cudaMemcpyAsync(dev_a,a+i,N*sizeof(int),cudaMemcpyHostToDevice,stream); cudaMemcpyAsync(dev_b,a+i,N*sizeof(int),cudaMemcpyHostToDevice,stream); kernel<<<N/256,256,0,stream>>>(dev_a,dev-b,dev_c); cudaMemcpyAsync(c+1,dev_c,N*sizeof(int),cudaMemcpyDeviceToHost,stream); } cudaStreamSynchronise(stream); // wait for stream to finish return 0;}

Code ExamplePage 194-95 CUDA by Example, without error

detection macros

One stream

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Multiple streams

Assuming device can support it (can check in code if needed), create two streams with:

cudaStream_t stream1, stream2;

cudaStreamCreate(&stream1);

cudaStreamCreate(&stream2);

and then duplicate stream code for each stream

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int *dev_a1, *dev_b1, *dev_c1; // stream 1 mem ptrsint *dev_a2, *dev_b2, *dev_c2; // stream 2 mem ptrs//stream 1cudaMalloc( (void**)&dev_a1, N * sizeof(int) );cudaMalloc( (void**)&dev_b1, N * sizeof(int) );cudaMalloc( (void**)&dev_c1, N * sizeof(int) );//stream 2cudaMalloc( (void**)&dev_a2, N * sizeof(int) );cudaMalloc( (void**)&dev_b2, N * sizeof(int) );cudaMalloc( (void**)&dev_c2, N * sizeof(int) );…for(int i=0;I < SIZE;i+= N*2 { // loop over data in chunks// stream 1 cudaMemcpyAsync(dev_a1,a+i,N*sizeof(int),cudaMemcpyHostToDevice,stream1); cudaMemcpyAsync(dev_b1,a+i,N*sizeof(int),cudaMemcpyHostToDevice,stream1); kernel<<<N/256,256,0,stream1>>>(dev_a,dev-b,dev_c); cudaMemcpyAsync(c+1,dev_c1,N*sizeof(int),cudaMemcpyDeviceToHost,stream1);//stream 2 cudaMemcpyAsync(dev_a2,a+i,N*sizeof(int),cudaMemcpyHostToDevice,stream2); cudaMemcpyAsync(dev_b2,a+i,N*sizeof(int),cudaMemcpyHostToDevice,stream2); kernel<<<N/256,256,0,stream2>>>(dev_a,dev-b,dev_c); cudaMemcpyAsync(c+1,dev_c2,N*sizeof(int),cudaMemcpyDeviceToHost,stream2);}cudaStreamSynchronise(stream1); // wait for stream to finishcudaStreamSynchronise(stream2); // wait for stream to finish

First attempt described in book

concatenate statements of each

stream

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Simply concatenating statements does not work well because of the way the GPU schedules work

Page 206 CUDA by Example,

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Page 207 CUDA by Example,

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Page 208 CUDA by Example

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for(int i=0;I < SIZE;i+= N*2 { // loop over data in chunks// interleave stream 1 and stream 2 cudaMemcpyAsync(dev_a1,a+i,N*sizeof(int),cudaMemcpyHostToDevice,stream1); cudaMemcpyAsync(dev_a2,a+i,N*sizeof(int),cudaMemcpyHostToDevice,stream2); cudaMemcpyAsync(dev_b1,a+i,N*sizeof(int),cudaMemcpyHostToDevice,stream1); cudaMemcpyAsync(dev_b2,a+i,N*sizeof(int),cudaMemcpyHostToDevice,stream2);

kernel<<<N/256,256,0,stream1>>>(dev_a,dev-b,dev_c); kernel<<<N/256,256,0,stream2>>>(dev_a,dev-b,dev_c);

cudaMemcpyAsync(c+1,dev_c1,N*sizeof(int),cudaMemcpyDeviceToHost,stream1); cudaMemcpyAsync(c+1,dev_c2,N*sizeof(int),cudaMemcpyDeviceToHost,stream2);}

Second attempt described in bookInterleave statements of each stream

19Page 210 CUDA by Example

Questions