Byte counters reset at ~4GB

[Brooks Davis]

On Mon, Mar 15, 2004 at 07:48:21PM -0500, Mike Jakubik wrote:
> Max Laier said:
> 
> > Sure, you measure it ;) ... no, of course it is more expensive to update a
> > 64bit counter on a 32bit arch, but the key (once again) is descision:
> > While
> > (almost) all of the pf counters are 64bit types you can configure it not
> > to
> > use the loginterface or whatsoever more. So it's up to you: You need 64bit
> > counters? You shall have them! You need *fast* 64bit counters? AMD sells
> > nice processors (they say)! ... you get the idea.
> 
> Got it. In just curious though... realistically, how big of an impact on
> performance is this on a modern CPU? Is it not simply the original 32bit
> calculation x 2?

No, you have to do overflow handling so that adds some to the cost.

I was curious what the actual overhead was so I ran the following
program with both uint32_t and uint64_t counters.  With 64-bit counters,
it was a bit over four times slower on a the dual 2.2GHz Xeon (~2sec vs
~8.4sec).  On a dual opteron, the 32-bit math had a slight edge, but
not much.  Intestingly, runtime was longer then on the Xeon (~3.1s for
32-bit and ~3.8 for 64-bit.)

If you do this test, be sure not to use any optimizer flags or the whole
loop gets optimized out.

-- Brooks

#include <stdio.h>
#include <stdint.h>

int
main (int argc, char **argv)
{
	uint32_t j = 0;

	for (j = 0; j < 1000000000; j++) {}
	printf("%d\n", j);
}




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