> 2GB memset is very slow

[Jason Nymble]

I was using memset on a reserved area of memory (64bit x86 kernel and system), and noticed that as soon as I exceed a size of 2GB, the function becomes extremely slow, e.g just below 2GB it takes typically about 0.3s, and just above 2GB is takes about 39s to complete...

I tried tracing the eventual function that is called in the kernel, and I think it resolves to the below (even on x86_64 if I'm not mistaken):
static inline void *__memset_generic(void *s, char c, size_t count)
{
        int d0, d1;
        asm volatile("rep\n\t"
                     "stosb"
                     : "=&c" (d0), "=&D" (d1)
                     : "a" (c), "1" (s), "0" (count)
                     : "memory");
        return s;
}

size_t is defined as (unsigned long) on my platform, but I suspect the d0 and d1 variables above cause problems because they are int... Is this a kernel bug, or known limitation, or what?