- 根目录:
- drivers
- block
- drbd
- drbd_vli.h
/*
-*- 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 transferred 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