The cache characteristics of a processor can have a dramatic impact on the performance of memory-intensive algorithms. It's often helpful to align key data structures on cache line boundaries. However, cache characteristics vary from processor to processor, and it would be nice to be able to select behavior at run time based on the characteristics of the current processor.
Since 2008, IBM processors have included the "EXTRACT CPU ATTRIBUTE (ECAG)" non-privileged instruction (originally called "EXTRACT CACHE ATTRIBUTE") to provide this information. You can easily get this information by using a small inline assembler routine; the following C program illustrates how. Here's it's output (from a Z13):
level 0: private data: line size=256, set associativity=8, total size=128K instruction: line size=256, set associativity=6, total size=96Klevel 1: private data: line size=256, set associativity=8, total size=2048K instruction: line size=256, set associativity=8, total size=2048Klevel 2: shared unified: line size=256, set associativity=16, total size=65536Klevel 3: shared unified: line size=256, set associativity=30, total size=491520K
And here's the code. It was compiled with this command:
xlc -g -qasm -q64 -o attributes attributes.c
#include <stdio.h>static unsigned long int extractCPUAttribute(unsigned long int arg){ unsigned long int result = 0; __asm(" LG 1,%1 \n" " ECAG 2,0,0(1) \n" " STG 2,%0 \n" : "=m"(result) : "m"(arg) : "r1 r2"); return result;}typedef union{ unsigned long int value; struct{ unsigned long reserved:54; unsigned attributeSetIndication:2; unsigned attributeIndication:4; unsigned levelIndication:3; unsigned typeIndication:1; } args;} ExtractArguments;typedef struct { unsigned reserved:4; unsigned scope:2; unsigned cacheType:2;} LevelInfo ;typedef union{ unsigned long int value; LevelInfo level[8];} ExtractResults;typedef enum { Summary = 0, LineSize = 1, TotalSize = 2, SetAssociativity = 3} AttributeIndication;typedef struct{ unsigned long int lineSize; unsigned long int totalSize; /* in K */ unsigned long int associativity;} CacheSizes;staticCacheSizes getCacheSizes(unsigned int level, unsigned which){ ExtractArguments arg; CacheSizes result; arg.value = 0; arg.args.levelIndication = level; arg.args.typeIndication = which; arg.args.attributeIndication = LineSize; result.lineSize = extractCPUAttribute(arg.value); arg.args.attributeIndication = TotalSize; result.totalSize = extractCPUAttribute(arg.value)/1024; arg.args.attributeIndication = SetAssociativity; result.associativity = extractCPUAttribute(arg.value); return result;}staticvoid printTypeAndSizes(unsigned int level, LevelInfo info){ CacheSizes values; switch (info.cacheType){ case 0: /* separate */ values = getCacheSizes(level, 0); printf(" data: line size=%lu, set associativity=%lu, total size=%luK\n", values.lineSize, values.associativity, values.totalSize); /* fall through! */ case 1: /* I only */ values = getCacheSizes(level, 1); printf(" instruction: line size=%lu, set associativity=%lu, total size=%luK\n", values.lineSize, values.associativity, values.totalSize); break; case 2: /* D only */ values = getCacheSizes(level, 0); printf(" data: line size=%lu, set associativity=%lu, total size=%luK\n", values.lineSize, values.associativity, values.totalSize); break; case 3: /* unified */ values = getCacheSizes(level, 0); printf(" unified: line size=%lu, set associativity=%lu, total size=%luK\n", values.lineSize, values.associativity, values.totalSize); break; }}staticvoid produceReport(){ ExtractArguments arg; ExtractResults result; arg.value = 0; result.value = extractCPUAttribute(arg.value); for (unsigned level = 0; level < 8; ++level){ const char* scope = "undefined"; if (result.level[level].scope == 0){ break; } switch (result.level[level].scope){ case 1: scope = "private"; break; case 2: scope = "shared"; break; case 3: default: break; } printf("level %u: %s\n", level, scope); printTypeAndSizes(level, result.level[level]); printf("\n"); }}int main (int argc, char** argv){ if (argc == 1){ produceReport(); } else if (argc != 2){ printf("usage: %s <arg to EXTRACT CPU ATTRIBUTE>\n", argv[0]); return (1); } else { /* driving by hand */ unsigned int arg = 0; if (1 != sscanf(argv[1], "%i", &arg)) { printf("%s is not a valid number\n", argv[1]); return (1); } unsigned long int result = extractCPUAttribute(arg); printf("0x%x produced 0x%lx (%lu, %dk)\n", arg, result, result, result/1024); } return 0;}