LCOV - code coverage report
Current view: top level - src/include/storage - checksum_impl.h (source / functions) Hit Total Coverage
Test: PostgreSQL 13devel Lines: 20 20 100.0 %
Date: 2019-11-13 22:07:24 Functions: 2 2 100.0 %
Legend: Lines: hit not hit

          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) 1996-2019, 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 FNV-1a hash (FNV is shorthand
      26             :  * for Fowler/Noll/Vo).  The primitive of a plain FNV-1a hash folds in data 1
      27             :  * byte at a time according to the formula:
      28             :  *
      29             :  *     hash = (hash ^ value) * FNV_PRIME
      30             :  *
      31             :  * FNV-1a algorithm is described at http://www.isthe.com/chongo/tech/comp/fnv/
      32             :  *
      33             :  * PostgreSQL doesn't use FNV-1a 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             :  * 32-bit checksum. The caller can safely reduce the value to 16 bits
      58             :  * using modulo 2^16-1. 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 5-bit 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 1-4 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             :  * 2e-16 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 -funroll-loops -ftree-vectorize 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 load-store
      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 FNV-1a 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 4-byte 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 4-byte 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 properly-initialized 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             : }

Generated by: LCOV version 1.13