c - What is the Cost of an L1 Cache Miss?

Edit: For reference purposes (if anyone stumbles across this question), Igor Ostrovsky wrote a great post about cache misses. It discusses several different issues and shows example numbers. End Edit

I did some testing and am wondering if a performance difference is due to memory cache misses. The following code demonstrates the issue and boils it down to the critical timing portion. The following code has a couple of loops that visit memory in random order and then in ascending address order.

I ran it on an XP machine (compiled with VS2005: cl /O2) and on a Linux box (gcc –Os). Both produced similar times. These times are in milliseconds. I believe all loops are running and are not optimized out (otherwise it would run “instantly”).

*** Testing 20000 nodes
Total Ordered Time: 888.822899
Total Random Time: 2155.846268

Do these numbers make sense? Is the difference primarily due to L1 cache misses or is something else going on as well? There are 20,000^2 memory accesses and if every one were a cache miss, that is about 3.2 nanoseconds per miss. The XP (P4) machine I tested on is 3.2GHz and I suspect (but don’t know) has a 32KB L1 cache and 512KB L2. With 20,000 entries (80KB), I assume there is not a significant number of L2 misses. So this would be (3.2*10^9 cycles/second) * 3.2*10^-9 seconds/miss) = 10.1 cycles/miss. That seems high to me. Maybe it’s not, or maybe my math is bad. I tried measuring cache misses with VTune, but I got a BSOD. And now I can’t get it to connect to the license server (grrrr).

typedef struct stItem

{
   long     lData;
   //char     acPad[20];
} LIST_NODE;



#if defined( WIN32 )
void StartTimer( LONGLONG *pt1 )
{

   QueryPerformanceCounter( (LARGE_INTEGER*)pt1 );
}

void StopTimer( LONGLONG t1, double *pdMS )
{
   LONGLONG t2, llFreq;

   QueryPerformanceCounter( (LARGE_INTEGER*)&t2 );
   QueryPerformanceFrequency( (LARGE_INTEGER*)&llFreq );
   *pdMS = ((double)( t2 - t1 ) / (double)llFreq) * 1000.0;

}
#else
// doesn't need 64-bit integer in this case
void StartTimer( LONGLONG *pt1 )
{
   // Just use clock(), this test doesn't need higher resolution
   *pt1 = clock();
}

void StopTimer( LONGLONG t1, double *pdMS )

{
   LONGLONG t2 = clock();
   *pdMS = (double)( t2 - t1 ) / ( CLOCKS_PER_SEC / 1000 );
}
#endif



long longrand()
{

   #if defined( WIN32 )
   // Stupid cheesy way to make sure it is not just a 16-bit rand value
   return ( rand() << 16 ) | rand();
   #else
   return rand();
   #endif
}

// get random value in the given range
int randint( int m, int n )

{
   int ret = longrand() % ( n - m + 1 );
   return ret + m;
}

// I think I got this out of Programming Pearls (Bentley).
void ShuffleArray
(
   long *plShuffle,  // (O) return array of "randomly" ordered integers
   long lNumItems    // (I) length of array

)
{
   long i;
   long j;
   long t;

   for ( i = 0; i < lNumItems; i++ )
      plShuffle[i] = i;

   for ( i = 0; i < lNumItems; i++ )

      {
      j = randint( i, lNumItems - 1 );

      t = plShuffle[i];
      plShuffle[i] = plShuffle[j];
      plShuffle[j] = t;
      }
}




int main( int argc, char* argv[] )
{
   long          *plDataValues;
   LIST_NODE     *pstNodes;
   long          lNumItems = 20000;
   long          i, j;
   LONGLONG      t1;  // for timing
   double dms;


   if ( argc > 1 && atoi(argv[1]) > 0 )
      lNumItems = atoi( argv[1] );

   printf( "\n\n*** Testing %u nodes\n", lNumItems );

   srand( (unsigned int)time( 0 ));

   // allocate the nodes as one single chunk of memory
   pstNodes = (LIST_NODE*)malloc( lNumItems * sizeof( LIST_NODE ));
   assert( pstNodes != NULL );


   // Create an array that gives the access order for the nodes
   plDataValues = (long*)malloc( lNumItems * sizeof( long ));
   assert( plDataValues != NULL );

   // Access the data in order
   for ( i = 0; i < lNumItems; i++ )
      plDataValues[i] = i;

   StartTimer( &t1 );


   // Loop through and access the memory a bunch of times
   for ( j = 0; j < lNumItems; j++ )
      {
      for ( i = 0; i < lNumItems; i++ )
         {
         pstNodes[plDataValues[i]].lData = i * j;
         }
      }


   StopTimer( t1, &dms );
   printf( "Total Ordered Time: %f\n", dms );

   // now access the array positions in a "random" order
   ShuffleArray( plDataValues, lNumItems );

   StartTimer( &t1 );

   for ( j = 0; j < lNumItems; j++ )
      {

      for ( i = 0; i < lNumItems; i++ )
         {
         pstNodes[plDataValues[i]].lData = i * j;
         }
      }

   StopTimer( t1, &dms );
   printf( "Total Random Time: %f\n", dms );

}

Answer

While I can't offer an answer to whether or not the numbers make sense (I'm not well versed in the cache latencies, but for the record ~10 cycle L1 cache misses sounds about right), I can offer you Cachegrind as a tool to help you actually see the differences in cache performance between your 2 tests.

Cachegrind is a Valgrind tool (the framework that powers the always-lovely memcheck) which profiles cache and branch hits/misses. It will give you an idea of how many cache hits/misses you are actually getting in your program.