Changes between Initial Version and Version 1 of Workshops/cypress/OffloadingWithOpenMP

Timestamp:

08/16/25 14:14:21 (13 hours ago)

Author:

fuji

Comment:

—

Workshops/cypress/OffloadingWithOpenMP

@@ +1 @@
+[[PageOutline]]
+= Explicit Offloading with OpenMP =
+Note that "host" is the CPU, and "device" is MIC/GPU.
+
+This is a simple OpenMP code:
+{{{
+#include <iostream>
+#include <omp.h>
+
+int main( void ) {
+        int totalProcs;
+        totalProcs = omp_get_num_procs();
+        std::cout << "Number of Threads = " << totalProcs << std::endl;
+        return 0;
+}
+}}}
+If run on Cypress computing node, the "Number of Threads" will be 20.
+
+Add a one-line directive #pragma that offloads to the device a line of executable code.
+{{{
+#include <iostream>
+#include <omp.h>
+
+int main( void ) {
+        int totalProcs;
+#pragma omp target device(0)
+        totalProcs = omp_get_num_procs();
+        std::cout << "Number of Threads = " << totalProcs << std::endl;
+        return 0;
+}
+}}}
+codes now return "240"
+Note that the host pauses until the device code is finished.
+This code offloads only one line of
+{{{
+         totalProcs = omp_get_num_procs();
+}}}
+to the device. Use { } to offload a block of codes.
+ 
+What happens to 'totalProcs'?
+
+Primitive variables are automatically transferred to/from the device.
+
+[[Image(https://docs.google.com/drawings/d/e/2PACX-1vRSu_BN8fhGC6vUHyrWPmwFgM60MjQdt8xOJt3gLruenwkjfMtleZR7m7n8Zy6uSy2F9DFUAp03gdxN/pub?w=533&h=285)]]
+
+== Parallel Loop ==
+OpenMP region is defined by an omp directive. This for-loop runs on device.
+{{{
+int main( void ) {
+        double a[500000];
+        // static arrays are allocated on the stack; literal here is important
+        int i;
+#pragma omp target device(0) 
+#pragma omp parallel for
+        for ( i=0; i<500000; i++ ) { 
+                a[i] = (double)i;
+        } 
+        ...
+}}}
+What happens to “a”?
+1. Detect a device
+2. Allocate 'a' on the device memory.
+3. The static array “a” is transferred to the device memory.
+4. Execute the device-side code
+5. Values in “a” in the device memory are transferred back to the host memory.
+ 
+== Controlling the Offload ==
+Get the number of devices
+{{{
+        const int num_dev = omp_get_num_devices();
+        std::cout << "number of devices : " << num_dev << std::endl;
+}}}
+Control data transfer
+
+Transfer data from the device at the end of the offload section
+{{{
+int main( void ) {
+        double a[500000];
+        // static arrays are allocated on the stack; literal here is important
+        int i;
+#pragma omp target device(0) map(from:a)
+#pragma omp parallel for
+        for ( i=0; i<500000; i++ ) {
+                a[i] = (double)i;
+        }
+}
+}}}
+
+Transfer data to the device at the beginning of the offload section
+{{{
+#pragma omp target device(0) map(to:a) 
+}}}
+If not specified, do both.
+
+
+
+
+=== Transfer dynamic arrays ===
+
+You have to specify the range in the array.
+{{{
+#pragma omp target device(0)    map(from:phi[0:num * num]) 
+}}}
+
+=== Keeping Data on Device Memory ===
+
+This will allocate a space for the array a on Device memory. 
+{{{
+#pragma omp target if (dev != num_dev) device(dev) map(to:a) map(from:a[dev:dev+1])
+}}}
+ The memory block for a on Device will be freed when the offload section ends.
+
+To keep data on Device memory, we have to allocate array on Device memory explicitly. 
+{{{
+        void *data;
+#pragma omp target device(0) map(from:data)
+        {
+                double *vdata = new double[100];
+#pragma omp parallel
+                for (int i = 0 ; i < 100 ; i++) vdata[i]= i;
+                data = (void *)vdata;
+        }
+
+#pragma omp target device(0) map(to:data)
+        {
+                double *vdata = (double *)data;
+                for (int i = 0 ; i < 100 ; i++){
+                        std::cout << vdata[i] << std::endl;
+                }
+        }
+}}}
+ Use void * pointer variable to store the address of array on Device memory.
+
+=== Controlling data transfer ===
+{{{
+#pragma omp target data map(to:aArray[0:num], bArray[0:num]) map(alloc:cArray[0:num])
+  {  // aArray, bArray, cArray are allocated on Device memory, and the elements of aArray & bArray are transferred from CPU to Device                   
+#pragma omp target   // Use aArray,bArray,cArray on Device memory
+#pragma omp parallel for  // Runs on Device
+    for (int i = 0 ; i < num ; i++){
+      double sum = 0.0;
+      for (int j = 0 ; j < num ; j++){
+       sum += aArray[i] * bArray[j];
+      }
+      cArray[i] = sum;
+    }
+
+
+    //Compute ||C|| . Host gets the results.
+    double cNorm = 0.0;
+#pragma omp target  // Use aArray,bArray,cArray on Device memory
+#pragma omp parallel for reduction(+:cNorm) // Runs on Device
+    for (int i = 0 ; i < num ; i++){
+      cNorm += cArray[i] * cArray[i];
+    }
+    cNorm = std::sqrt(cNorm); // Runs on CPU
+    std::cout << "||C||=" << cNorm << std::endl;// Runs on CPU
+
+
+    // do the same on CPU 
+    cNorm = 0.0;
+#pragma omp target update from(cArray[0:num]) // Transfer cArray from Device to CPU
+#pragma omp parallel for reduction(+:cNorm)  // Runs on CPU
+    for (int i = 0 ; i < num ; i++){
+      cNorm += cArray[i] * cArray[i];
+    }
+
+
+    cNorm = std::sqrt(cNorm);
+    std::cout << "||C||=" << cNorm << std::endl;    
+  }// aArray, bArray, cArray on Device memory are freed
+ }}}
+
+=== Host-Device Parallelism  ===
+{{{
+#include <iostream>
+#include <cmath>
+#include <omp.h>
+
+int main(const int argc, const char** argv) {
+        omp_set_nested(1);
+        int num_dev = omp_get_num_devices();
+        std::cout << "number of devices " << num_dev << std::endl;
+        int a[10] = { 0 };
+
+#pragma omp parallel firstprivate(num_dev) num_threads(num_dev + 1)
+#pragma omp single
+        {
+                for (int dev = 0; dev < num_dev + 1; dev++) {
+#pragma omp task firstprivate(dev)
+                        {
+#pragma omp target if (dev != num_dev) device(dev) map(to:a) map(from:a[dev:dev+1])
+                                {
+#pragma omp parallel
+                                        {
+#pragma omp master
+                                                a[dev] = omp_get_num_threads();
+                                        }
+                                }
+                        }
+                }
+        }
+        for (int i = 0; i < num_dev + 1; i++) {
+                std::cout << a[i] << std::endl;
+        }
+        return 0;
+}
+}}}
+ 'if' in pragma directive 
+{{{
+#pragma omp target if (dev != num_dev) device(dev) map(to:a) map(from:a[dev:dev+1])
+}}}
+In this case, when dev is equal to num_dev, this directive is ignored. So next scope of code will run on Host (CPU).