> 2GB memset is very slow

[vinit dhatrak]

On Tue, Feb 1, 2011 at 3:53 PM, Jason Nymble <jason.nymble at gmail.com> wrote:

> 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?
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Hi Jason,

How did you allocate the memory? What is the size of physical memory that is
attached to your machine ?

-Vinit
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