makedumpfile: optimize is_zero_page

[Atsushi Kumagai]

>There are local complaints that  filtering out only zero pages is slow.  I
>found that is_zero_page was inefficient.  It checks if the page contains any
>non-zero bytes - one byte at a time.
>
>Improve performance by checking for non-zero data 64 bits at a time.
>
>Did testing in x86_64 mode on an Intel Xeon x5560 system with 18GB RAM.
>Executed:
>  time makedumpfile -d 1 /proc/vmcore <destination>
>
>The amount of time taken in User space was reduced by 64%.  The total time to
>dump memory was reduced by 27%.
>
>Change Log:
>
>v1 => v2)
>
>o Eliminate loop unrolling as it is of minimal benefit based on CPU.

Thank you for fixing, I'll merge this into v1.5.6.

Atsushi Kumagai

>is_zero_page
>Signed-off-by: Marc Milgram <mmilgram at redhat.com>
>---
>diff --git a/makedumpfile.h b/makedumpfile.h
>index 3d270c6..1751e3a 100644
>--- a/makedumpfile.h
>+++ b/makedumpfile.h
>@@ -1634,9 +1634,11 @@ static inline int
> is_zero_page(unsigned char *buf, long page_size)
> {
> 	size_t i;
>+	unsigned long long *vect = (unsigned long long *) buf;
>+	long page_len = page_size / sizeof(unsigned long long);
>
>-	for (i = 0; i < page_size; i++)
>-		if (buf[i])
>+	for (i = 0; i < page_len; i++)
>+		if (vect[i])
> 			return FALSE;
> 	return TRUE;
> }
>
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