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Use a rolling checksum based sketch value for a rabin chunk instead of a CRC64 checksum.

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Avoids additional table-lookup memory access.
Reduce Rabin window size to avoid overflows in sketch value.
No need to maintain rolling checksum in Rabin context.
A few comment cleanups.
This commit is contained in:
Moinak Ghosh 2012-07-13 22:06:55 +05:30
parent 0091a0da02
commit 1da2c40888
2 changed files with 19 additions and 41 deletions
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53
rabin/rabin_polynomial.c
View file

@ -62,18 +62,6 @@
#include <allocator.h>
#include <utils.h>
// CRC64 pieces from LZMA's implementation -----------------
#include <crc_macros.h>
#ifdef WORDS_BIGENDIAN
# define A1(x) ((x) >> 56)
#else
# define A1 A
#endif
extern const uint64_t lzma_crc64_table[4][256];
// ---------------------------------------------------------
#include "rabin_polynomial.h"
extern int lzma_init(void **data, int *level, ssize_t chunksize);
@ -175,8 +163,6 @@ reset_rabin_context(rabin_context_t *ctx)
{
memset(ctx->current_window_data, 0, RAB_POLYNOMIAL_WIN_SIZE);
ctx->window_pos = 0;
ctx->cur_roll_checksum = 0;
ctx->cur_checksum = 0;
}
void
@ -218,12 +204,14 @@ rabin_dedup(rabin_context_t *ctx, uchar_t *buf, ssize_t *size, ssize_t offset, s
uint32_t blknum;
char *buf1 = (char *)buf;
uint32_t length;
uint64_t cur_roll_checksum[2];
length = offset;
last_offset = 0;
blknum = 0;
ctx->valid = 0;
ctx->cur_checksum = 0;
cur_roll_checksum[0] = 0;
cur_roll_checksum[1] = 0;
j = 0;
/*
@ -243,40 +231,40 @@ rabin_dedup(rabin_context_t *ctx, uchar_t *buf, ssize_t *size, ssize_t offset, s
* We want to do:
* cur_roll_checksum = cur_roll_checksum * RAB_POLYNOMIAL_CONST + cur_byte;
* cur_roll_checksum -= pushed_out * polynomial_pow;
* cur_checksum = cur_checksum * RAB_POLYNOMIAL_CONST + cur_byte;
*
* However since RAB_POLYNOMIAL_CONST == 2, we use shifts.
*/
ctx->cur_roll_checksum = (ctx->cur_roll_checksum << 1) + cur_byte;
ctx->cur_roll_checksum -= (pushed_out << RAB_POLYNOMIAL_WIN_SIZE);
cur_roll_checksum[1] = (cur_roll_checksum[1] << 1) + cur_byte;
cur_roll_checksum[1] -= (pushed_out << RAB_POLYNOMIAL_WIN_SIZE);
// CRC64 Calculation swiped from LZMA
ctx->cur_checksum = lzma_crc64_table[0][cur_byte ^ A1(ctx->cur_checksum)] ^ S8(ctx->cur_checksum);
// Compute Sum 0 mod 25 Sketch. We are avoiding a branch here.
// See: http://www.armedia.com/wp/SimilarityIndex.pdf
j += cur_roll_checksum[(cur_roll_checksum[1] % 25 == 0)];
// Count how many bytes have msb set. Needed to detect 7-bit text data.
j += (cur_byte >> 7);
// Window pos has to rotate from 0 .. RAB_POLYNOMIAL_WIN_SIZE-1
// This requires RAB_POLYNOMIAL_WIN_SIZE to be power of 2
/*
* Window pos has to rotate from 0 .. RAB_POLYNOMIAL_WIN_SIZE-1
* We avoid a branch here by masking. This requires RAB_POLYNOMIAL_WIN_SIZE
* to be power of 2
*/
ctx->window_pos = (ctx->window_pos + 1) & (RAB_POLYNOMIAL_WIN_SIZE-1);
length++;
if (length < ctx->rabin_poly_min_block_size) continue;
// If we hit our special value or reached the max block size update block offset
if ((ctx->cur_roll_checksum & ctx->rabin_avg_block_mask) == ctx->rabin_break_patt ||
if ((cur_roll_checksum[1] & ctx->rabin_avg_block_mask) == ctx->rabin_break_patt ||
length >= rabin_polynomial_max_block_size) {
if (rabin_pos == NULL) {
ctx->blocks[blknum].offset = last_offset;
ctx->blocks[blknum].index = blknum; // Need to store for sorting
ctx->blocks[blknum].cksum_n_offset = ctx->cur_checksum;
ctx->blocks[blknum].cksum_n_offset = j;
ctx->blocks[blknum].length = length;
ctx->blocks[blknum].refcount = 0;
blknum++;
}
ctx->cur_checksum = 0;
last_offset = i+1;
length = 0;
j = 0;
}
}
@ -284,10 +272,6 @@ rabin_dedup(rabin_context_t *ctx, uchar_t *buf, ssize_t *size, ssize_t offset, s
*rabin_pos = last_offset;
return (0);
}
if (j > *size * 0.40)
ctx->data_type = DATA_BINARY;
else
ctx->data_type = DATA_TEXT;
// If we found at least a few chunks, perform dedup.
if (blknum > 2) {
@ -303,11 +287,10 @@ rabin_dedup(rabin_context_t *ctx, uchar_t *buf, ssize_t *size, ssize_t offset, s
if (last_offset < *size) {
ctx->blocks[blknum].offset = last_offset;
ctx->blocks[blknum].index = blknum;
ctx->blocks[blknum].cksum_n_offset = ctx->cur_checksum;
ctx->blocks[blknum].cksum_n_offset = j;
ctx->blocks[blknum].length = *size - last_offset;
ctx->blocks[blknum].refcount = 0;
blknum++;
ctx->cur_checksum = 0;
last_offset = *size;
}
@ -355,14 +338,12 @@ rabin_dedup(rabin_context_t *ctx, uchar_t *buf, ssize_t *size, ssize_t offset, s
matchlen += prev_length;
continue;
}
prev_offset = buf1 + ctx->blocks[blk].offset;
prev_cksum = ctx->blocks[blk].cksum_n_offset;
prev_length = ctx->blocks[blk].length;
prev_index = ctx->blocks[blk].index;
prev_blk = blk;
}
if (matchlen < rabin_index_sz) {
ctx->valid = 0;
return;

5
rabin/rabin_polynomial.h
View file

@ -80,7 +80,7 @@
#define RAB_POLYNOMIAL_MIN_BLOCK_SIZE2 2048
#define LZMA_WINDOW_MAX (128L * 1024L * 1024L)
#define RAB_POLYNOMIAL_WIN_SIZE 32
#define RAB_POLYNOMIAL_WIN_SIZE 16
#define RAB_POLYNOMIAL_MIN_WIN_SIZE 8
#define RAB_POLYNOMIAL_MAX_WIN_SIZE 64
@ -126,8 +126,6 @@ typedef struct {
rabin_blockentry_t *blocks;
unsigned char *cbuf;
int window_pos;
uint64_t cur_roll_checksum;
uint64_t cur_checksum;
uint32_t rabin_poly_max_block_size;
uint32_t rabin_poly_min_block_size;
uint32_t rabin_poly_avg_block_size;
@ -135,7 +133,6 @@ typedef struct {
uint32_t rabin_break_patt;
uint64_t real_chunksize;
short valid;
short data_type;
void *lzma_data;
int level;
} rabin_context_t;