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Subject[PATCH 24/35] ocfs2: One more hamming code optimization.
Date
From: Joel Becker <joel.becker@oracle.com>

The previous optimization used a fast find-highest-bit-set operation to
give us a good starting point in calc_code_bit(). This version lets the
caller cache the previous code buffer bit offset. Thus, the next call
always starts where the last one left off.

This reduces the calculation another 39%, for a total 80% reduction from
the original, naive implementation. At least, on my machine. This also
brings the parity calculation to within an order of magnitude of the
crc32 calculation.

Signed-off-by: Joel Becker <joel.becker@oracle.com>
Signed-off-by: Mark Fasheh <mfasheh@suse.com>
---
fs/ocfs2/blockcheck.c | 61 +++++++++++++++---------------------------------
1 files changed, 19 insertions(+), 42 deletions(-)
diff --git a/fs/ocfs2/blockcheck.c b/fs/ocfs2/blockcheck.c
index f102ec9..2a947c4 100644
--- a/fs/ocfs2/blockcheck.c
+++ b/fs/ocfs2/blockcheck.c
@@ -41,34 +41,6 @@


/*
- * Find the log base 2 of 32-bit v.
- *
- * Algorithm found on http://graphics.stanford.edu/~seander/bithacks.html,
- * by Sean Eron Anderson. Code on the page is in the public domain unless
- * otherwise noted.
- *
- * This particular algorithm is credited to Eric Cole.
- */
-static int find_highest_bit_set(unsigned int v)
-{
-
- static const int MultiplyDeBruijnBitPosition[32] =
- {
- 0, 1, 28, 2, 29, 14, 24, 3, 30, 22, 20, 15, 25, 17, 4, 8,
- 31, 27, 13, 23, 21, 19, 16, 7, 26, 12, 18, 6, 11, 5, 10, 9
- };
-
- v |= v >> 1; /* first round down to power of 2 */
- v |= v >> 2;
- v |= v >> 4;
- v |= v >> 8;
- v |= v >> 16;
- v = (v >> 1) + 1;
-
- return MultiplyDeBruijnBitPosition[(u32)(v * 0x077CB531UL) >> 27];
-}
-
-/*
* Calculate the bit offset in the hamming code buffer based on the bit's
* offset in the data buffer. Since the hamming code reserves all
* power-of-two bits for parity, the data bit number and the code bit
@@ -81,10 +53,14 @@ static int find_highest_bit_set(unsigned int v)
* so it's a parity bit. 2 is a power of two (2^1), so it's a parity bit.
* 3 is not a power of two. So bit 1 of the data buffer ends up as bit 3
* in the code buffer.
+ *
+ * The caller can pass in *p if it wants to keep track of the most recent
+ * number of parity bits added. This allows the function to start the
+ * calculation at the last place.
*/
-static unsigned int calc_code_bit(unsigned int i)
+static unsigned int calc_code_bit(unsigned int i, unsigned int *p_cache)
{
- unsigned int b, p;
+ unsigned int b, p = 0;

/*
* Data bits are 0-based, but we're talking code bits, which
@@ -92,24 +68,25 @@ static unsigned int calc_code_bit(unsigned int i)
*/
b = i + 1;

- /*
- * As a cheat, we know that all bits below b's highest bit must be
- * parity bits, so we can start there.
- */
- p = find_highest_bit_set(b);
+ /* Use the cache if it is there */
+ if (p_cache)
+ p = *p_cache;
b += p;

/*
* For every power of two below our bit number, bump our bit.
*
- * We compare with (b + 1) becuase we have to compare with what b
+ * We compare with (b + 1) because we have to compare with what b
* would be _if_ it were bumped up by the parity bit. Capice?
*
- * We start p at 2^p because of the cheat above.
+ * p is set above.
*/
- for (p = (1 << p); p < (b + 1); p <<= 1)
+ for (; (1 << p) < (b + 1); p++)
b++;

+ if (p_cache)
+ *p_cache = p;
+
return b;
}

@@ -126,7 +103,7 @@ static unsigned int calc_code_bit(unsigned int i)
*/
u32 ocfs2_hamming_encode(u32 parity, void *data, unsigned int d, unsigned int nr)
{
- unsigned int i, b;
+ unsigned int i, b, p = 0;

BUG_ON(!d);

@@ -145,7 +122,7 @@ u32 ocfs2_hamming_encode(u32 parity, void *data, unsigned int d, unsigned int nr
* i is the offset in this hunk, nr + i is the total bit
* offset.
*/
- b = calc_code_bit(nr + i);
+ b = calc_code_bit(nr + i, &p);

/*
* Data bits in the resultant code are checked by
@@ -201,7 +178,7 @@ void ocfs2_hamming_fix(void *data, unsigned int d, unsigned int nr,
* nr + d is the bit right past the data hunk we're looking at.
* If fix after that, nothing to do
*/
- if (fix >= calc_code_bit(nr + d))
+ if (fix >= calc_code_bit(nr + d, NULL))
return;

/*
@@ -209,7 +186,7 @@ void ocfs2_hamming_fix(void *data, unsigned int d, unsigned int nr,
* start b at the offset in the code buffer. See hamming_encode()
* for a more detailed description of 'b'.
*/
- b = calc_code_bit(nr);
+ b = calc_code_bit(nr, NULL);
/* If the fix is before this hunk, nothing to do */
if (fix < b)
return;
--
1.5.6


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