> The following program prints:
>
> Average msecs per MB 4.914158; Average copy rate: 0.000005 msecs/byte
>
> if run without root permission (i.e. mlockall() fails), and
>
> Average msecs per MB 7.417227; Average copy rate: 0.000007 msecs/byte
>
> if run with root permission.
>
> Is there a simple explanation ?
mlockall() is a privileged call. Performance could improve if all pages
are locked. The non-root version doesn't perform as well because
mlockall() fails, subjecting the pages to the system's evil memory
management schemes (paging/etc).
I'm almost positive you know this though, so your question seems strange.
-MB
> #include <stdlib.h>
> #include <sys/mman.h>
> #include <asm/msr.h>
>
> main ()
>
> {
> char buf[1048576];
> char obuf[1048576];
> int i;
> float total;
> unsigned long now, then;
>
> #define N 1000
> #define CYCLES_PER_MSEC 450000.0f
>
> total = 0;
>
> mlockall (MCL_CURRENT);
>
> for (i = 0; i < N; i++) {
> rdtscl (then);
> memcpy (buf, obuf, sizeof (obuf));
> rdtscl (now);
> total += now - then;
> }
>
> printf ("Average msecs per MB %.6f; "
> "Average copy rate: %.6f msecs/byte\n",
> total / (N * CYCLES_PER_MSEC),
> total / (N * 1048576.0 * CYCLES_PER_MSEC));
> }
> --------------------------------------------------------------------------
>
> -
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