aboutsummaryrefslogtreecommitdiff
path: root/drivers/block/drbd/drbd_vli.h
blob: fc824006e7217179e2ef8bb7552363efe8208827 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
/*
-*- linux-c -*-
   drbd_receiver.c
   This file is part of DRBD by Philipp Reisner and Lars Ellenberg.

   Copyright (C) 2001-2008, LINBIT Information Technologies GmbH.
   Copyright (C) 1999-2008, Philipp Reisner <philipp.reisner@linbit.com>.
   Copyright (C) 2002-2008, Lars Ellenberg <lars.ellenberg@linbit.com>.

   drbd is free software; you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation; either version 2, or (at your option)
   any later version.

   drbd is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License
   along with drbd; see the file COPYING.  If not, write to
   the Free Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA.
 */

#ifndef _DRBD_VLI_H
#define _DRBD_VLI_H

/*
 * At a granularity of 4KiB storage represented per bit,
 * and stroage sizes of several TiB,
 * and possibly small-bandwidth replication,
 * the bitmap transfer time can take much too long,
 * if transmitted in plain text.
 *
 * We try to reduce the transfered bitmap information
 * by encoding runlengths of bit polarity.
 *
 * We never actually need to encode a "zero" (runlengths are positive).
 * But then we have to store the value of the first bit.
 * The first bit of information thus shall encode if the first runlength
 * gives the number of set or unset bits.
 *
 * We assume that large areas are either completely set or unset,
 * which gives good compression with any runlength method,
 * even when encoding the runlength as fixed size 32bit/64bit integers.
 *
 * Still, there may be areas where the polarity flips every few bits,
 * and encoding the runlength sequence of those areas with fix size
 * integers would be much worse than plaintext.
 *
 * We want to encode small runlength values with minimum code length,
 * while still being able to encode a Huge run of all zeros.
 *
 * Thus we need a Variable Length Integer encoding, VLI.
 *
 * For some cases, we produce more code bits than plaintext input.
 * We need to send incompressible chunks as plaintext, skip over them
 * and then see if the next chunk compresses better.
 *
 * We don't care too much about "excellent" compression ratio for large
 * runlengths (all set/all clear): whether we achieve a factor of 100
 * or 1000 is not that much of an issue.
 * We do not want to waste too much on short runlengths in the "noisy"
 * parts of the bitmap, though.
 *
 * There are endless variants of VLI, we experimented with:
 *  * simple byte-based
 *  * various bit based with different code word length.
 *
 * To avoid yet an other configuration parameter (choice of bitmap compression
 * algorithm) which was difficult to explain and tune, we just chose the one
 * variant that turned out best in all test cases.
 * Based on real world usage patterns, with device sizes ranging from a few GiB
 * to several TiB, file server/mailserver/webserver/mysql/postgress,
 * mostly idle to really busy, the all time winner (though sometimes only
 * marginally better) is:
 */

/*
 * encoding is "visualised" as
 * __little endian__ bitstream, least significant bit first (left most)
 *
 * this particular encoding is chosen so that the prefix code
 * starts as unary encoding the level, then modified so that
 * 10 levels can be described in 8bit, with minimal overhead
 * for the smaller levels.
 *
 * Number of data bits follow fibonacci sequence, with the exception of the
 * last level (+1 data bit, so it makes 64bit total).  The only worse code when
 * encoding bit polarity runlength is 1 plain bits => 2 code bits.
prefix    data bits                                    max val  Nº data bits
0 x                                                         0x2            1
10 x                                                        0x4            1
110 xx                                                      0x8            2
1110 xxx                                                   0x10            3
11110 xxx xx                                               0x30            5
111110 xx xxxxxx                                          0x130            8
11111100  xxxxxxxx xxxxx                                 0x2130           13
11111110  xxxxxxxx xxxxxxxx xxxxx                      0x202130           21
11111101  xxxxxxxx xxxxxxxx xxxxxxxx  xxxxxxxx xx   0x400202130           34
11111111  xxxxxxxx xxxxxxxx xxxxxxxx  xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx 56
 * maximum encodable value: 0x100000400202130 == 2**56 + some */

/* compression "table":
 transmitted   x                                0.29
 as plaintext x                                  ........................
             x                                   ........................
            x                                    ........................
           x    0.59                         0.21........................
          x      ........................................................
         x       .. c ...................................................
        x    0.44.. o ...................................................
       x .......... d ...................................................
      x  .......... e ...................................................
     X.............   ...................................................
    x.............. b ...................................................
2.0x............... i ...................................................
 #X................ t ...................................................
 #................. s ...........................  plain bits  ..........
-+-----------------------------------------------------------------------
 1             16              32                              64
*/

/* LEVEL: (total bits, prefix bits, prefix value),
 * sorted ascending by number of total bits.
 * The rest of the code table is calculated at compiletime from this. */

/* fibonacci data 1, 1, ... */
#define VLI_L_1_1() do { \
	LEVEL( 2, 1, 0x00); \
	LEVEL( 3, 2, 0x01); \
	LEVEL( 5, 3, 0x03); \
	LEVEL( 7, 4, 0x07); \
	LEVEL(10, 5, 0x0f); \
	LEVEL(14, 6, 0x1f); \
	LEVEL(21, 8, 0x3f); \
	LEVEL(29, 8, 0x7f); \
	LEVEL(42, 8, 0xbf); \
	LEVEL(64, 8, 0xff); \
	} while (0)

/* finds a suitable level to decode the least significant part of in.
 * returns number of bits consumed.
 *
 * BUG() for bad input, as that would mean a buggy code table. */
static inline int vli_decode_bits(u64 *out, const u64 in)
{
	u64 adj = 1;

#define LEVEL(t,b,v)					\
	do {						\
		if ((in & ((1 << b) -1)) == v) {	\
			*out = ((in & ((~0ULL) >> (64-t))) >> b) + adj;	\
			return t;			\
		}					\
		adj += 1ULL << (t - b);			\
	} while (0)

	VLI_L_1_1();

	/* NOT REACHED, if VLI_LEVELS code table is defined properly */
	BUG();
#undef LEVEL
}

/* return number of code bits needed,
 * or negative error number */
static inline int __vli_encode_bits(u64 *out, const u64 in)
{
	u64 max = 0;
	u64 adj = 1;

	if (in == 0)
		return -EINVAL;

#define LEVEL(t,b,v) do {		\
		max += 1ULL << (t - b);	\
		if (in <= max) {	\
			if (out)	\
				*out = ((in - adj) << b) | v;	\
			return t;	\
		}			\
		adj = max + 1;		\
	} while (0)

	VLI_L_1_1();

	return -EOVERFLOW;
#undef LEVEL
}

#undef VLI_L_1_1

/* code from here down is independend of actually used bit code */

/*
 * Code length is determined by some unique (e.g. unary) prefix.
 * This encodes arbitrary bit length, not whole bytes: we have a bit-stream,
 * not a byte stream.
 */

/* for the bitstream, we need a cursor */
struct bitstream_cursor {
	/* the current byte */
	u8 *b;
	/* the current bit within *b, nomalized: 0..7 */
	unsigned int bit;
};

/* initialize cursor to point to first bit of stream */
static inline void bitstream_cursor_reset(struct bitstream_cursor *cur, void *s)
{
	cur->b = s;
	cur->bit = 0;
}

/* advance cursor by that many bits; maximum expected input value: 64,
 * but depending on VLI implementation, it may be more. */
static inline void bitstream_cursor_advance(struct bitstream_cursor *cur, unsigned int bits)
{
	bits += cur->bit;
	cur->b = cur->b + (bits >> 3);
	cur->bit = bits & 7;
}

/* the bitstream itself knows its length */
struct bitstream {
	struct bitstream_cursor cur;
	unsigned char *buf;
	size_t buf_len;		/* in bytes */

	/* for input stream:
	 * number of trailing 0 bits for padding
	 * total number of valid bits in stream: buf_len * 8 - pad_bits */
	unsigned int pad_bits;
};

static inline void bitstream_init(struct bitstream *bs, void *s, size_t len, unsigned int pad_bits)
{
	bs->buf = s;
	bs->buf_len = len;
	bs->pad_bits = pad_bits;
	bitstream_cursor_reset(&bs->cur, bs->buf);
}

static inline void bitstream_rewind(struct bitstream *bs)
{
	bitstream_cursor_reset(&bs->cur, bs->buf);
	memset(bs->buf, 0, bs->buf_len);
}

/* Put (at most 64) least significant bits of val into bitstream, and advance cursor.
 * Ignores "pad_bits".
 * Returns zero if bits == 0 (nothing to do).
 * Returns number of bits used if successful.
 *
 * If there is not enough room left in bitstream,
 * leaves bitstream unchanged and returns -ENOBUFS.
 */
static inline int bitstream_put_bits(struct bitstream *bs, u64 val, const unsigned int bits)
{
	unsigned char *b = bs->cur.b;
	unsigned int tmp;

	if (bits == 0)
		return 0;

	if ((bs->cur.b + ((bs->cur.bit + bits -1) >> 3)) - bs->buf >= bs->buf_len)
		return -ENOBUFS;

	/* paranoia: strip off hi bits; they should not be set anyways. */
	if (bits < 64)
		val &= ~0ULL >> (64 - bits);

	*b++ |= (val & 0xff) << bs->cur.bit;

	for (tmp = 8 - bs->cur.bit; tmp < bits; tmp += 8)
		*b++ |= (val >> tmp) & 0xff;

	bitstream_cursor_advance(&bs->cur, bits);
	return bits;
}

/* Fetch (at most 64) bits from bitstream into *out, and advance cursor.
 *
 * If more than 64 bits are requested, returns -EINVAL and leave *out unchanged.
 *
 * If there are less than the requested number of valid bits left in the
 * bitstream, still fetches all available bits.
 *
 * Returns number of actually fetched bits.
 */
static inline int bitstream_get_bits(struct bitstream *bs, u64 *out, int bits)
{
	u64 val;
	unsigned int n;

	if (bits > 64)
		return -EINVAL;

	if (bs->cur.b + ((bs->cur.bit + bs->pad_bits + bits -1) >> 3) - bs->buf >= bs->buf_len)
		bits = ((bs->buf_len - (bs->cur.b - bs->buf)) << 3)
			- bs->cur.bit - bs->pad_bits;

	if (bits == 0) {
		*out = 0;
		return 0;
	}

	/* get the high bits */
	val = 0;
	n = (bs->cur.bit + bits + 7) >> 3;
	/* n may be at most 9, if cur.bit + bits > 64 */
	/* which means this copies at most 8 byte */
	if (n) {
		memcpy(&val, bs->cur.b+1, n - 1);
		val = le64_to_cpu(val) << (8 - bs->cur.bit);
	}

	/* we still need the low bits */
	val |= bs->cur.b[0] >> bs->cur.bit;

	/* and mask out bits we don't want */
	val &= ~0ULL >> (64 - bits);

	bitstream_cursor_advance(&bs->cur, bits);
	*out = val;

	return bits;
}

/* encodes @in as vli into @bs;

 * return values
 *  > 0: number of bits successfully stored in bitstream
 * -ENOBUFS @bs is full
 * -EINVAL input zero (invalid)
 * -EOVERFLOW input too large for this vli code (invalid)
 */
static inline int vli_encode_bits(struct bitstream *bs, u64 in)
{
	u64 code = code;
	int bits = __vli_encode_bits(&code, in);

	if (bits <= 0)
		return bits;

	return bitstream_put_bits(bs, code, bits);
}

#endif