Messages in this thread Patch in this message | | | From | Mark Fasheh <> | Subject | [PATCH 24/35] ocfs2: One more hamming code optimization. | Date | Thu, 25 Dec 2008 10:04:39 -0800 |
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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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