Hi,
I was exploring the affect of cache on program
performance/optimisation.Is it the compilers responsibility only to
consider this kind of optimisation or the programmer can do his bit in
this case ?
Reading through the "Expert C Programming" text,it mentions how the
below program can be efficient taking the cache details into accont.
The below program can be executed using the two versions of copy
alternatively and running the time command on the executable on Unix,to
see the difference.As obvious,the slowdown happens in DUMBCOPY.
#include<stdio.h>
#include<string.h>
#define DUMBCOPY for (i = 0; i < 65536; i++) \
destination[i] = source[i]
#define SMARTCOPY memcpy(destination, source, 65536)
int main()
{
char source[65536], destination[65536];
int i, j;
for (j = 0; j < 100; j++)
SMARTCOPY;
/* DUMBCOPY; */
return 0;
}
Below are the reasonings :
The slowdown happens because the source and destination are an exact
multiple of the cache size apart.The particular algorithm used happens
to fill the same line for main memory addresses that are exact multiples
of the cache size apart.
In this particular case both the source and destination use the same
cache line, causing every memory reference to miss the cache and stall
the processor while it waited for regular memory to deliver. The library
memcpy() routine is especially tuned for high performance.
It unrolls the loop to read for one cache line and then write, which
avoids the problem.Using the smart copy, we were able to get a huge
performance improvement. This also shows the folly of drawing
conclusions from simple-minded benchmark programs.
I dont fully understand the above 2 paragraphs,so if someone could give
a better explanation.Would also appreciate any helpful pointers.
This might not be something directly related to C,but I thought I would
get better answers in this newsgroup and hence the posting.
-TIA
