makedumpfile: optimize is_zero_page

[Marc Milgram]

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.

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;
 }