340 lines
11 KiB
C
340 lines
11 KiB
C
/* SPDX-License-Identifier: GPL-2.0-or-later */
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/*
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-*- linux-c -*-
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drbd_receiver.c
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This file is part of DRBD by Philipp Reisner and Lars Ellenberg.
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Copyright (C) 2001-2008, LINBIT Information Technologies GmbH.
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Copyright (C) 1999-2008, Philipp Reisner <philipp.reisner@linbit.com>.
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Copyright (C) 2002-2008, Lars Ellenberg <lars.ellenberg@linbit.com>.
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*/
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#ifndef _DRBD_VLI_H
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#define _DRBD_VLI_H
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/*
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* At a granularity of 4KiB storage represented per bit,
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* and stroage sizes of several TiB,
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* and possibly small-bandwidth replication,
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* the bitmap transfer time can take much too long,
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* if transmitted in plain text.
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*
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* We try to reduce the transferred bitmap information
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* by encoding runlengths of bit polarity.
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*
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* We never actually need to encode a "zero" (runlengths are positive).
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* But then we have to store the value of the first bit.
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* The first bit of information thus shall encode if the first runlength
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* gives the number of set or unset bits.
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*
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* We assume that large areas are either completely set or unset,
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* which gives good compression with any runlength method,
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* even when encoding the runlength as fixed size 32bit/64bit integers.
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*
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* Still, there may be areas where the polarity flips every few bits,
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* and encoding the runlength sequence of those areas with fix size
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* integers would be much worse than plaintext.
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*
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* We want to encode small runlength values with minimum code length,
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* while still being able to encode a Huge run of all zeros.
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*
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* Thus we need a Variable Length Integer encoding, VLI.
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*
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* For some cases, we produce more code bits than plaintext input.
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* We need to send incompressible chunks as plaintext, skip over them
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* and then see if the next chunk compresses better.
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*
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* We don't care too much about "excellent" compression ratio for large
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* runlengths (all set/all clear): whether we achieve a factor of 100
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* or 1000 is not that much of an issue.
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* We do not want to waste too much on short runlengths in the "noisy"
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* parts of the bitmap, though.
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*
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* There are endless variants of VLI, we experimented with:
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* * simple byte-based
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* * various bit based with different code word length.
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*
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* To avoid yet an other configuration parameter (choice of bitmap compression
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* algorithm) which was difficult to explain and tune, we just chose the one
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* variant that turned out best in all test cases.
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* Based on real world usage patterns, with device sizes ranging from a few GiB
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* to several TiB, file server/mailserver/webserver/mysql/postgress,
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* mostly idle to really busy, the all time winner (though sometimes only
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* marginally better) is:
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*/
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/*
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* encoding is "visualised" as
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* __little endian__ bitstream, least significant bit first (left most)
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*
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* this particular encoding is chosen so that the prefix code
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* starts as unary encoding the level, then modified so that
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* 10 levels can be described in 8bit, with minimal overhead
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* for the smaller levels.
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*
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* Number of data bits follow fibonacci sequence, with the exception of the
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* last level (+1 data bit, so it makes 64bit total). The only worse code when
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* encoding bit polarity runlength is 1 plain bits => 2 code bits.
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prefix data bits max val Nº data bits
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0 x 0x2 1
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10 x 0x4 1
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110 xx 0x8 2
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1110 xxx 0x10 3
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11110 xxx xx 0x30 5
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111110 xx xxxxxx 0x130 8
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11111100 xxxxxxxx xxxxx 0x2130 13
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11111110 xxxxxxxx xxxxxxxx xxxxx 0x202130 21
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11111101 xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx xx 0x400202130 34
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11111111 xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx 56
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* maximum encodable value: 0x100000400202130 == 2**56 + some */
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/* compression "table":
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transmitted x 0.29
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as plaintext x ........................
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x ........................
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x ........................
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x 0.59 0.21........................
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x ........................................................
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x .. c ...................................................
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x 0.44.. o ...................................................
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x .......... d ...................................................
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x .......... e ...................................................
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X............. ...................................................
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x.............. b ...................................................
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2.0x............... i ...................................................
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#X................ t ...................................................
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#................. s ........................... plain bits ..........
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-+-----------------------------------------------------------------------
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1 16 32 64
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*/
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/* LEVEL: (total bits, prefix bits, prefix value),
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* sorted ascending by number of total bits.
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* The rest of the code table is calculated at compiletime from this. */
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/* fibonacci data 1, 1, ... */
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#define VLI_L_1_1() do { \
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LEVEL( 2, 1, 0x00); \
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LEVEL( 3, 2, 0x01); \
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LEVEL( 5, 3, 0x03); \
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LEVEL( 7, 4, 0x07); \
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LEVEL(10, 5, 0x0f); \
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LEVEL(14, 6, 0x1f); \
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LEVEL(21, 8, 0x3f); \
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LEVEL(29, 8, 0x7f); \
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LEVEL(42, 8, 0xbf); \
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LEVEL(64, 8, 0xff); \
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} while (0)
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/* finds a suitable level to decode the least significant part of in.
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* returns number of bits consumed.
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*
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* BUG() for bad input, as that would mean a buggy code table. */
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static inline int vli_decode_bits(u64 *out, const u64 in)
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{
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u64 adj = 1;
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#define LEVEL(t,b,v) \
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do { \
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if ((in & ((1 << b) -1)) == v) { \
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*out = ((in & ((~0ULL) >> (64-t))) >> b) + adj; \
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return t; \
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} \
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adj += 1ULL << (t - b); \
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} while (0)
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VLI_L_1_1();
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/* NOT REACHED, if VLI_LEVELS code table is defined properly */
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BUG();
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#undef LEVEL
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}
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/* return number of code bits needed,
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* or negative error number */
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static inline int __vli_encode_bits(u64 *out, const u64 in)
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{
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u64 max = 0;
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u64 adj = 1;
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if (in == 0)
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return -EINVAL;
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#define LEVEL(t,b,v) do { \
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max += 1ULL << (t - b); \
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if (in <= max) { \
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if (out) \
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*out = ((in - adj) << b) | v; \
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return t; \
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} \
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adj = max + 1; \
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} while (0)
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VLI_L_1_1();
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return -EOVERFLOW;
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#undef LEVEL
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}
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#undef VLI_L_1_1
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/* code from here down is independend of actually used bit code */
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/*
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* Code length is determined by some unique (e.g. unary) prefix.
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* This encodes arbitrary bit length, not whole bytes: we have a bit-stream,
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* not a byte stream.
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*/
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/* for the bitstream, we need a cursor */
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struct bitstream_cursor {
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/* the current byte */
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u8 *b;
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/* the current bit within *b, nomalized: 0..7 */
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unsigned int bit;
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};
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/* initialize cursor to point to first bit of stream */
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static inline void bitstream_cursor_reset(struct bitstream_cursor *cur, void *s)
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{
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cur->b = s;
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cur->bit = 0;
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}
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/* advance cursor by that many bits; maximum expected input value: 64,
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* but depending on VLI implementation, it may be more. */
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static inline void bitstream_cursor_advance(struct bitstream_cursor *cur, unsigned int bits)
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{
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bits += cur->bit;
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cur->b = cur->b + (bits >> 3);
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cur->bit = bits & 7;
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}
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/* the bitstream itself knows its length */
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struct bitstream {
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struct bitstream_cursor cur;
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unsigned char *buf;
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size_t buf_len; /* in bytes */
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/* for input stream:
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* number of trailing 0 bits for padding
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* total number of valid bits in stream: buf_len * 8 - pad_bits */
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unsigned int pad_bits;
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};
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static inline void bitstream_init(struct bitstream *bs, void *s, size_t len, unsigned int pad_bits)
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{
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bs->buf = s;
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bs->buf_len = len;
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bs->pad_bits = pad_bits;
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bitstream_cursor_reset(&bs->cur, bs->buf);
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}
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static inline void bitstream_rewind(struct bitstream *bs)
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{
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bitstream_cursor_reset(&bs->cur, bs->buf);
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memset(bs->buf, 0, bs->buf_len);
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}
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/* Put (at most 64) least significant bits of val into bitstream, and advance cursor.
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* Ignores "pad_bits".
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* Returns zero if bits == 0 (nothing to do).
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* Returns number of bits used if successful.
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*
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* If there is not enough room left in bitstream,
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* leaves bitstream unchanged and returns -ENOBUFS.
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*/
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static inline int bitstream_put_bits(struct bitstream *bs, u64 val, const unsigned int bits)
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{
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unsigned char *b = bs->cur.b;
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unsigned int tmp;
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if (bits == 0)
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return 0;
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if ((bs->cur.b + ((bs->cur.bit + bits -1) >> 3)) - bs->buf >= bs->buf_len)
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return -ENOBUFS;
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/* paranoia: strip off hi bits; they should not be set anyways. */
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if (bits < 64)
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val &= ~0ULL >> (64 - bits);
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*b++ |= (val & 0xff) << bs->cur.bit;
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for (tmp = 8 - bs->cur.bit; tmp < bits; tmp += 8)
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*b++ |= (val >> tmp) & 0xff;
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bitstream_cursor_advance(&bs->cur, bits);
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return bits;
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}
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/* Fetch (at most 64) bits from bitstream into *out, and advance cursor.
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*
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* If more than 64 bits are requested, returns -EINVAL and leave *out unchanged.
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*
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* If there are less than the requested number of valid bits left in the
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* bitstream, still fetches all available bits.
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*
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* Returns number of actually fetched bits.
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*/
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static inline int bitstream_get_bits(struct bitstream *bs, u64 *out, int bits)
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{
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u64 val;
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unsigned int n;
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if (bits > 64)
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return -EINVAL;
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if (bs->cur.b + ((bs->cur.bit + bs->pad_bits + bits -1) >> 3) - bs->buf >= bs->buf_len)
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bits = ((bs->buf_len - (bs->cur.b - bs->buf)) << 3)
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- bs->cur.bit - bs->pad_bits;
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if (bits == 0) {
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*out = 0;
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return 0;
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}
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/* get the high bits */
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val = 0;
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n = (bs->cur.bit + bits + 7) >> 3;
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/* n may be at most 9, if cur.bit + bits > 64 */
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/* which means this copies at most 8 byte */
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if (n) {
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memcpy(&val, bs->cur.b+1, n - 1);
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val = le64_to_cpu(val) << (8 - bs->cur.bit);
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}
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/* we still need the low bits */
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val |= bs->cur.b[0] >> bs->cur.bit;
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/* and mask out bits we don't want */
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val &= ~0ULL >> (64 - bits);
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bitstream_cursor_advance(&bs->cur, bits);
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*out = val;
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return bits;
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}
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/* encodes @in as vli into @bs;
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* return values
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* > 0: number of bits successfully stored in bitstream
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* -ENOBUFS @bs is full
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* -EINVAL input zero (invalid)
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* -EOVERFLOW input too large for this vli code (invalid)
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*/
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static inline int vli_encode_bits(struct bitstream *bs, u64 in)
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{
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u64 code = code;
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int bits = __vli_encode_bits(&code, in);
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if (bits <= 0)
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return bits;
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return bitstream_put_bits(bs, code, bits);
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}
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#endif
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