Line data Source code
1 : /*
2 : *
3 : * checksum_impl.h
4 : * Checksum implementation for data pages.
5 : *
6 : * This file exists for the benefit of external programs that may wish to
7 : * check Postgres page checksums. They can #include this to get the code
8 : * referenced by storage/checksum.h. (Note: you may need to redefine
9 : * Assert() as empty to compile this successfully externally.)
10 : *
11 : * Portions Copyright (c) 19962019, PostgreSQL Global Development Group
12 : * Portions Copyright (c) 1994, Regents of the University of California
13 : *
14 : * src/include/storage/checksum_impl.h
15 : *
16 : *
17 : */
18 :
19 : /*
20 : * The algorithm used to checksum pages is chosen for very fast calculation.
21 : * Workloads where the database working set fits into OS file cache but not
22 : * into shared buffers can read in pages at a very fast pace and the checksum
23 : * algorithm itself can become the largest bottleneck.
24 : *
25 : * The checksum algorithm itself is based on the FNV1a hash (FNV is shorthand
26 : * for Fowler/Noll/Vo). The primitive of a plain FNV1a hash folds in data 1
27 : * byte at a time according to the formula:
28 : *
29 : * hash = (hash ^ value) * FNV_PRIME
30 : *
31 : * FNV1a algorithm is described at http://www.isthe.com/chongo/tech/comp/fnv/
32 : *
33 : * PostgreSQL doesn't use FNV1a hash directly because it has bad mixing of
34 : * high bits  high order bits in input data only affect high order bits in
35 : * output data. To resolve this we xor in the value prior to multiplication
36 : * shifted right by 17 bits. The number 17 was chosen because it doesn't
37 : * have common denominator with set bit positions in FNV_PRIME and empirically
38 : * provides the fastest mixing for high order bits of final iterations quickly
39 : * avalanche into lower positions. For performance reasons we choose to combine
40 : * 4 bytes at a time. The actual hash formula used as the basis is:
41 : *
42 : * hash = (hash ^ value) * FNV_PRIME ^ ((hash ^ value) >> 17)
43 : *
44 : * The main bottleneck in this calculation is the multiplication latency. To
45 : * hide the latency and to make use of SIMD parallelism multiple hash values
46 : * are calculated in parallel. The page is treated as a 32 column two
47 : * dimensional array of 32 bit values. Each column is aggregated separately
48 : * into a partial checksum. Each partial checksum uses a different initial
49 : * value (offset basis in FNV terminology). The initial values actually used
50 : * were chosen randomly, as the values themselves don't matter as much as that
51 : * they are different and don't match anything in real data. After initializing
52 : * partial checksums each value in the column is aggregated according to the
53 : * above formula. Finally two more iterations of the formula are performed with
54 : * value 0 to mix the bits of the last value added.
55 : *
56 : * The partial checksums are then folded together using xor to form a single
57 : * 32bit checksum. The caller can safely reduce the value to 16 bits
58 : * using modulo 2^161. That will cause a very slight bias towards lower
59 : * values but this is not significant for the performance of the
60 : * checksum.
61 : *
62 : * The algorithm choice was based on what instructions are available in SIMD
63 : * instruction sets. This meant that a fast and good algorithm needed to use
64 : * multiplication as the main mixing operator. The simplest multiplication
65 : * based checksum primitive is the one used by FNV. The prime used is chosen
66 : * for good dispersion of values. It has no known simple patterns that result
67 : * in collisions. Test of 5bit differentials of the primitive over 64bit keys
68 : * reveals no differentials with 3 or more values out of 100000 random keys
69 : * colliding. Avalanche test shows that only high order bits of the last word
70 : * have a bias. Tests of 14 uncorrelated bit errors, stray 0 and 0xFF bytes,
71 : * overwriting page from random position to end with 0 bytes, and overwriting
72 : * random segments of page with 0x00, 0xFF and random data all show optimal
73 : * 2e16 false positive rate within margin of error.
74 : *
75 : * Vectorization of the algorithm requires 32bit x 32bit > 32bit integer
76 : * multiplication instruction. As of 2013 the corresponding instruction is
77 : * available on x86 SSE4.1 extensions (pmulld) and ARM NEON (vmul.i32).
78 : * Vectorization requires a compiler to do the vectorization for us. For recent
79 : * GCC versions the flags msse4.1 funrollloops ftreevectorize are enough
80 : * to achieve vectorization.
81 : *
82 : * The optimal amount of parallelism to use depends on CPU specific instruction
83 : * latency, SIMD instruction width, throughput and the amount of registers
84 : * available to hold intermediate state. Generally, more parallelism is better
85 : * up to the point that state doesn't fit in registers and extra loadstore
86 : * instructions are needed to swap values in/out. The number chosen is a fixed
87 : * part of the algorithm because changing the parallelism changes the checksum
88 : * result.
89 : *
90 : * The parallelism number 32 was chosen based on the fact that it is the
91 : * largest state that fits into architecturally visible x86 SSE registers while
92 : * leaving some free registers for intermediate values. For future processors
93 : * with 256bit vector registers this will leave some performance on the table.
94 : * When vectorization is not available it might be beneficial to restructure
95 : * the computation to calculate a subset of the columns at a time and perform
96 : * multiple passes to avoid register spilling. This optimization opportunity
97 : * is not used. Current coding also assumes that the compiler has the ability
98 : * to unroll the inner loop to avoid loop overhead and minimize register
99 : * spilling. For less sophisticated compilers it might be beneficial to
100 : * manually unroll the inner loop.
101 : */
102 :
103 : #include "storage/bufpage.h"
104 :
105 : /* number of checksums to calculate in parallel */
106 : #define N_SUMS 32
107 : /* prime multiplier of FNV1a hash */
108 : #define FNV_PRIME 16777619
109 :
110 : /* Use a union so that this code is valid under strict aliasing */
111 : typedef union
112 : {
113 : PageHeaderData phdr;
114 : uint32 data[BLCKSZ / (sizeof(uint32) * N_SUMS)][N_SUMS];
115 : } PGChecksummablePage;
116 :
117 : /*
118 : * Base offsets to initialize each of the parallel FNV hashes into a
119 : * different initial state.
120 : */
121 : static const uint32 checksumBaseOffsets[N_SUMS] = {
122 : 0x5B1F36E9, 0xB8525960, 0x02AB50AA, 0x1DE66D2A,
123 : 0x79FF467A, 0x9BB9F8A3, 0x217E7CD2, 0x83E13D2C,
124 : 0xF8D4474F, 0xE39EB970, 0x42C6AE16, 0x993216FA,
125 : 0x7B093B5D, 0x98DAFF3C, 0xF718902A, 0x0B1C9CDB,
126 : 0xE58F764B, 0x187636BC, 0x5D7B3BB1, 0xE73DE7DE,
127 : 0x92BEC979, 0xCCA6C0B2, 0x304A0979, 0x85AA43D4,
128 : 0x783125BB, 0x6CA8EAA2, 0xE407EAC6, 0x4B5CFC3E,
129 : 0x9FBF8C76, 0x15CA20BE, 0xF2CA9FD3, 0x959BD756
130 : };
131 :
132 : /*
133 : * Calculate one round of the checksum.
134 : */
135 : #define CHECKSUM_COMP(checksum, value) \
136 : do { \
137 : uint32 __tmp = (checksum) ^ (value); \
138 : (checksum) = __tmp * FNV_PRIME ^ (__tmp >> 17); \
139 : } while (0)
140 :
141 : /*
142 : * Block checksum algorithm. The page must be adequately aligned
143 : * (at least on 4byte boundary).
144 : */
145 : static uint32
146 167266 : pg_checksum_block(const PGChecksummablePage *page)
147 : {
148 : uint32 sums[N_SUMS];
149 167266 : uint32 result = 0;
150 : uint32 i,
151 : j;
152 :
153 : /* ensure that the size is compatible with the algorithm */
154 : Assert(sizeof(PGChecksummablePage) == BLCKSZ);
155 :
156 : /* initialize partial checksums to their corresponding offsets */
157 167266 : memcpy(sums, checksumBaseOffsets, sizeof(checksumBaseOffsets));
158 :
159 : /* main checksum calculation */
160 10872290 : for (i = 0; i < (uint32) (BLCKSZ / (sizeof(uint32) * N_SUMS)); i++)
161 353265792 : for (j = 0; j < N_SUMS; j++)
162 342560768 : CHECKSUM_COMP(sums[j], page>data[i][j]);
163 :
164 : /* finally add in two rounds of zeroes for additional mixing */
165 501798 : for (i = 0; i < 2; i++)
166 11039556 : for (j = 0; j < N_SUMS; j++)
167 10705024 : CHECKSUM_COMP(sums[j], 0);
168 :
169 : /* xor fold partial checksums together */
170 5519778 : for (i = 0; i < N_SUMS; i++)
171 5352512 : result ^= sums[i];
172 :
173 167266 : return result;
174 : }
175 :
176 : /*
177 : * Compute the checksum for a Postgres page.
178 : *
179 : * The page must be adequately aligned (at least on a 4byte boundary).
180 : * Beware also that the checksum field of the page is transiently zeroed.
181 : *
182 : * The checksum includes the block number (to detect the case where a page is
183 : * somehow moved to a different location), the page header (excluding the
184 : * checksum itself), and the page data.
185 : */
186 : uint16
187 167266 : pg_checksum_page(char *page, BlockNumber blkno)
188 : {
189 167266 : PGChecksummablePage *cpage = (PGChecksummablePage *) page;
190 : uint16 save_checksum;
191 : uint32 checksum;
192 :
193 : /* We only calculate the checksum for properlyinitialized pages */
194 : Assert(!PageIsNew(&cpage>phdr));
195 :
196 : /*
197 : * Save pd_checksum and temporarily set it to zero, so that the checksum
198 : * calculation isn't affected by the old checksum stored on the page.
199 : * Restore it after, because actually updating the checksum is NOT part of
200 : * the API of this function.
201 : */
202 167266 : save_checksum = cpage>phdr.pd_checksum;
203 167266 : cpage>phdr.pd_checksum = 0;
204 167266 : checksum = pg_checksum_block(cpage);
205 167266 : cpage>phdr.pd_checksum = save_checksum;
206 :
207 : /* Mix in the block number to detect transposed pages */
208 167266 : checksum ^= blkno;
209 :
210 : /*
211 : * Reduce to a uint16 (to fit in the pd_checksum field) with an offset of
212 : * one. That avoids checksums of zero, which seems like a good idea.
213 : */
214 167266 : return (checksum % 65535) + 1;
215 : }
