复旦大学软件学院计算机系统课件 Memory Hierarchy (IV).pptVIP

* * * * * * * * * * * * * * * Memory mountain test function /* The test function */ void test (int elems, int stride) { int i ; double result = 0.0 ; volatile double sink; for (i = 0; i elems; i += stride) result += data[i]; sink = result; /* So compiler doesnt optimize away the loop */ } * Memory mountain test function /* Run test (elems, stride) and return read throughput (MB/s) */ double run (int size, int stride, double Mhz) { double cycles; int elems = size / sizeof(double); test (elems, stride); /* warm up the cache */ cycles = fcyc2(test, elems, stride, 0); /* call test (elems,stride) */ return (size / stride) / (cycles / Mhz); /* convert cycles to MB/s */ } * The Memory Mountain Data Size MAXBYTES(64M) bytes or MAXELEMS(8M) doubles Partially accessed Working set: from 64MB to 2KB Stride: from 1 to 64 * The Memory Mountain * Ridges of temporal locality Slice through the memory mountain with stride=16 illuminates read throughputs of different caches and memory * Ridges of temporal locality * A slope of spatial locality Slice through memory mountain with size=4M shows cache block size. * A slope of spatial locality * * * * * * * * * * * * * * * * * * * * * * * * Compilers Autumn 2002 Compilers Autumn 2002 * Memory Hierarchy (Ⅳ) * Outline Write cache friendly codes Matrix multiplication Memory mountain Suggested Reading: 6.5, 6.6 * Writing Cache-Friendly Code Principles Programs with better locality will tend to have lower miss rates Programs with lower miss rates will tend to run faster than programs with higher miss rates * Writing Cache-Friendly Code Basic approach Make the common case go fast Programs often spend most of their time in a few core functions. These functions often spend most of their time in a few loops Minimize the number of cache misses in each inner loop All things being equal * Writing Cache-Friendly Code 8[h] 7[h] 6[h] 5[m] 4[h] 3[h] 2[h] 1[m] Access order, [h]it or