1/* 2-*- linux-c -*- 3 drbd_receiver.c 4 This file is part of DRBD by Philipp Reisner and Lars Ellenberg. 5 6 Copyright (C) 2001-2008, LINBIT Information Technologies GmbH. 7 Copyright (C) 1999-2008, Philipp Reisner <philipp.reisner@linbit.com>. 8 Copyright (C) 2002-2008, Lars Ellenberg <lars.ellenberg@linbit.com>. 9 10 drbd is free software; you can redistribute it and/or modify 11 it under the terms of the GNU General Public License as published by 12 the Free Software Foundation; either version 2, or (at your option) 13 any later version. 14 15 drbd is distributed in the hope that it will be useful, 16 but WITHOUT ANY WARRANTY; without even the implied warranty of 17 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 18 GNU General Public License for more details. 19 20 You should have received a copy of the GNU General Public License 21 along with drbd; see the file COPYING. If not, write to 22 the Free Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA. 23 */ 24 25#ifndef _DRBD_VLI_H 26#define _DRBD_VLI_H 27 28/* 29 * At a granularity of 4KiB storage represented per bit, 30 * and stroage sizes of several TiB, 31 * and possibly small-bandwidth replication, 32 * the bitmap transfer time can take much too long, 33 * if transmitted in plain text. 34 * 35 * We try to reduce the transfered bitmap information 36 * by encoding runlengths of bit polarity. 37 * 38 * We never actually need to encode a "zero" (runlengths are positive). 39 * But then we have to store the value of the first bit. 40 * The first bit of information thus shall encode if the first runlength 41 * gives the number of set or unset bits. 42 * 43 * We assume that large areas are either completely set or unset, 44 * which gives good compression with any runlength method, 45 * even when encoding the runlength as fixed size 32bit/64bit integers. 46 * 47 * Still, there may be areas where the polarity flips every few bits, 48 * and encoding the runlength sequence of those areas with fix size 49 * integers would be much worse than plaintext. 50 * 51 * We want to encode small runlength values with minimum code length, 52 * while still being able to encode a Huge run of all zeros. 53 * 54 * Thus we need a Variable Length Integer encoding, VLI. 55 * 56 * For some cases, we produce more code bits than plaintext input. 57 * We need to send incompressible chunks as plaintext, skip over them 58 * and then see if the next chunk compresses better. 59 * 60 * We don't care too much about "excellent" compression ratio for large 61 * runlengths (all set/all clear): whether we achieve a factor of 100 62 * or 1000 is not that much of an issue. 63 * We do not want to waste too much on short runlengths in the "noisy" 64 * parts of the bitmap, though. 65 * 66 * There are endless variants of VLI, we experimented with: 67 * * simple byte-based 68 * * various bit based with different code word length. 69 * 70 * To avoid yet an other configuration parameter (choice of bitmap compression 71 * algorithm) which was difficult to explain and tune, we just chose the one 72 * variant that turned out best in all test cases. 73 * Based on real world usage patterns, with device sizes ranging from a few GiB 74 * to several TiB, file server/mailserver/webserver/mysql/postgress, 75 * mostly idle to really busy, the all time winner (though sometimes only 76 * marginally better) is: 77 */ 78 79 80/* compression "table": 81 transmitted x 0.29 82 as plaintext x ........................ 83 x ........................ 84 x ........................ 85 x 0.59 0.21........................ 86 x ........................................................ 87 x .. c ................................................... 88 x 0.44.. o ................................................... 89 x .......... d ................................................... 90 x .......... e ................................................... 91 X............. ................................................... 92 x.............. b ................................................... 932.0x............... i ................................................... 94 #X................ t ................................................... 95 #................. s ........................... plain bits .......... 96-+----------------------------------------------------------------------- 97 1 16 32 64 98*/ 99 100/* LEVEL: (total bits, prefix bits, prefix value), 101 * sorted ascending by number of total bits. 102 * The rest of the code table is calculated at compiletime from this. */ 103 104/* fibonacci data 1, 1, ... */ 105#define VLI_L_1_1() do { \ 106 LEVEL( 2, 1, 0x00); \ 107 LEVEL( 3, 2, 0x01); \ 108 LEVEL( 5, 3, 0x03); \ 109 LEVEL( 7, 4, 0x07); \ 110 LEVEL(10, 5, 0x0f); \ 111 LEVEL(14, 6, 0x1f); \ 112 LEVEL(21, 8, 0x3f); \ 113 LEVEL(29, 8, 0x7f); \ 114 LEVEL(42, 8, 0xbf); \ 115 LEVEL(64, 8, 0xff); \ 116 } while (0) 117 118/* finds a suitable level to decode the least significant part of in. 119 * returns number of bits consumed. 120 * 121 * BUG() for bad input, as that would mean a buggy code table. */ 122static inline int vli_decode_bits(u64 *out, const u64 in) 123{ 124 u64 adj = 1; 125 126#define LEVEL(t,b,v) \ 127 do { \ 128 if ((in & ((1 << b) -1)) == v) { \ 129 *out = ((in & ((~0ULL) >> (64-t))) >> b) + adj; \ 130 return t; \ 131 } \ 132 adj += 1ULL << (t - b); \ 133 } while (0) 134 135 VLI_L_1_1(); 136 137 /* NOT REACHED, if VLI_LEVELS code table is defined properly */ 138 BUG(); 139#undef LEVEL 140} 141 142/* return number of code bits needed, 143 * or negative error number */ 144static inline int __vli_encode_bits(u64 *out, const u64 in) 145{ 146 u64 max = 0; 147 u64 adj = 1; 148 149 if (in == 0) 150 return -EINVAL; 151 152#define LEVEL(t,b,v) do { \ 153 max += 1ULL << (t - b); \ 154 if (in <= max) { \ 155 if (out) \ 156 *out = ((in - adj) << b) | v; \ 157 return t; \ 158 } \ 159 adj = max + 1; \ 160 } while (0) 161 162 VLI_L_1_1(); 163 164 return -EOVERFLOW; 165#undef LEVEL 166} 167 168#undef VLI_L_1_1 169 170/* code from here down is independend of actually used bit code */ 171 172/* 173 * Code length is determined by some unique (e.g. unary) prefix. 174 * This encodes arbitrary bit length, not whole bytes: we have a bit-stream, 175 * not a byte stream. 176 */ 177 178/* for the bitstream, we need a cursor */ 179struct bitstream_cursor { 180 /* the current byte */ 181 u8 *b; 182 /* the current bit within *b, nomalized: 0..7 */ 183 unsigned int bit; 184}; 185 186/* initialize cursor to point to first bit of stream */ 187static inline void bitstream_cursor_reset(struct bitstream_cursor *cur, void *s) 188{ 189 cur->b = s; 190 cur->bit = 0; 191} 192 193/* advance cursor by that many bits; maximum expected input value: 64, 194 * but depending on VLI implementation, it may be more. */ 195static inline void bitstream_cursor_advance(struct bitstream_cursor *cur, unsigned int bits) 196{ 197 bits += cur->bit; 198 cur->b = cur->b + (bits >> 3); 199 cur->bit = bits & 7; 200} 201 202/* the bitstream itself knows its length */ 203struct bitstream { 204 struct bitstream_cursor cur; 205 unsigned char *buf; 206 size_t buf_len; /* in bytes */ 207 208 /* for input stream: 209 * number of trailing 0 bits for padding 210 * total number of valid bits in stream: buf_len * 8 - pad_bits */ 211 unsigned int pad_bits; 212}; 213 214static inline void bitstream_init(struct bitstream *bs, void *s, size_t len, unsigned int pad_bits) 215{ 216 bs->buf = s; 217 bs->buf_len = len; 218 bs->pad_bits = pad_bits; 219 bitstream_cursor_reset(&bs->cur, bs->buf); 220} 221 222static inline void bitstream_rewind(struct bitstream *bs) 223{ 224 bitstream_cursor_reset(&bs->cur, bs->buf); 225 memset(bs->buf, 0, bs->buf_len); 226} 227 228/* Put (at most 64) least significant bits of val into bitstream, and advance cursor. 229 * Ignores "pad_bits". 230 * Returns zero if bits == 0 (nothing to do). 231 * Returns number of bits used if successful. 232 * 233 * If there is not enough room left in bitstream, 234 * leaves bitstream unchanged and returns -ENOBUFS. 235 */ 236static inline int bitstream_put_bits(struct bitstream *bs, u64 val, const unsigned int bits) 237{ 238 unsigned char *b = bs->cur.b; 239 unsigned int tmp; 240 241 if (bits == 0) 242 return 0; 243 244 if ((bs->cur.b + ((bs->cur.bit + bits -1) >> 3)) - bs->buf >= bs->buf_len) 245 return -ENOBUFS; 246 247 /* paranoia: strip off hi bits; they should not be set anyways. */ 248 if (bits < 64) 249 val &= ~0ULL >> (64 - bits); 250 251 *b++ |= (val & 0xff) << bs->cur.bit; 252 253 for (tmp = 8 - bs->cur.bit; tmp < bits; tmp += 8) 254 *b++ |= (val >> tmp) & 0xff; 255 256 bitstream_cursor_advance(&bs->cur, bits); 257 return bits; 258} 259 260/* Fetch (at most 64) bits from bitstream into *out, and advance cursor. 261 * 262 * If more than 64 bits are requested, returns -EINVAL and leave *out unchanged. 263 * 264 * If there are less than the requested number of valid bits left in the 265 * bitstream, still fetches all available bits. 266 * 267 * Returns number of actually fetched bits. 268 */ 269static inline int bitstream_get_bits(struct bitstream *bs, u64 *out, int bits) 270{ 271 u64 val; 272 unsigned int n; 273 274 if (bits > 64) 275 return -EINVAL; 276 277 if (bs->cur.b + ((bs->cur.bit + bs->pad_bits + bits -1) >> 3) - bs->buf >= bs->buf_len) 278 bits = ((bs->buf_len - (bs->cur.b - bs->buf)) << 3) 279 - bs->cur.bit - bs->pad_bits; 280 281 if (bits == 0) { 282 *out = 0; 283 return 0; 284 } 285 286 /* get the high bits */ 287 val = 0; 288 n = (bs->cur.bit + bits + 7) >> 3; 289 /* n may be at most 9, if cur.bit + bits > 64 */ 290 /* which means this copies at most 8 byte */ 291 if (n) { 292 memcpy(&val, bs->cur.b+1, n - 1); 293 val = le64_to_cpu(val) << (8 - bs->cur.bit); 294 } 295 296 /* we still need the low bits */ 297 val |= bs->cur.b[0] >> bs->cur.bit; 298 299 /* and mask out bits we don't want */ 300 val &= ~0ULL >> (64 - bits); 301 302 bitstream_cursor_advance(&bs->cur, bits); 303 *out = val; 304 305 return bits; 306} 307 308/* encodes @in as vli into @bs; 309 310 * return values 311 * > 0: number of bits successfully stored in bitstream 312 * -ENOBUFS @bs is full 313 * -EINVAL input zero (invalid) 314 * -EOVERFLOW input too large for this vli code (invalid) 315 */ 316static inline int vli_encode_bits(struct bitstream *bs, u64 in) 317{ 318 u64 code = code; 319 int bits = __vli_encode_bits(&code, in); 320 321 if (bits <= 0) 322 return bits; 323 324 return bitstream_put_bits(bs, code, bits); 325} 326 327#endif 328