Line data    Source code

       1             : /*-------------------------------------------------------------------------
       2             :  *
       3             :  * buf_table.c
       4             :  *    routines for mapping BufferTags to buffer indexes.
       5             :  *
       6             :  * Note: the routines in this file do no locking of their own.  The caller
       7             :  * must hold a suitable lock on the appropriate BufMappingLock, as specified
       8             :  * in the comments.  We can't do the locking inside these functions because
       9             :  * in most cases the caller needs to adjust the buffer header contents
      10             :  * before the lock is released (see notes in README).
      11             :  *
      12             :  *
      13             :  * Portions Copyright (c) 1996-2025, PostgreSQL Global Development Group
      14             :  * Portions Copyright (c) 1994, Regents of the University of California
      15             :  *
      16             :  *
      17             :  * IDENTIFICATION
      18             :  *    src/backend/storage/buffer/buf_table.c
      19             :  *
      20             :  *-------------------------------------------------------------------------
      21             :  */
      22             : #include "postgres.h"
      23             : 
      24             : #include "storage/buf_internals.h"
      25             : 
      26             : /* entry for buffer lookup hashtable */
      27             : typedef struct
      28             : {
      29             :     BufferTag   key;            /* Tag of a disk page */
      30             :     int         id;             /* Associated buffer ID */
      31             : } BufferLookupEnt;
      32             : 
      33             : static HTAB *SharedBufHash;
      34             : 
      35             : 
      36             : /*
      37             :  * Estimate space needed for mapping hashtable
      38             :  *      size is the desired hash table size (possibly more than NBuffers)
      39             :  */
      40             : Size
      41        3906 : BufTableShmemSize(int size)
      42             : {
      43        3906 :     return hash_estimate_size(size, sizeof(BufferLookupEnt));
      44             : }
      45             : 
      46             : /*
      47             :  * Initialize shmem hash table for mapping buffers
      48             :  *      size is the desired hash table size (possibly more than NBuffers)
      49             :  */
      50             : void
      51        2100 : InitBufTable(int size)
      52             : {
      53             :     HASHCTL     info;
      54             : 
      55             :     /* assume no locking is needed yet */
      56             : 
      57             :     /* BufferTag maps to Buffer */
      58        2100 :     info.keysize = sizeof(BufferTag);
      59        2100 :     info.entrysize = sizeof(BufferLookupEnt);
      60        2100 :     info.num_partitions = NUM_BUFFER_PARTITIONS;
      61             : 
      62        2100 :     SharedBufHash = ShmemInitHash("Shared Buffer Lookup Table",
      63             :                                   size, size,
      64             :                                   &info,
      65             :                                   HASH_ELEM | HASH_BLOBS | HASH_PARTITION);
      66        2100 : }
      67             : 
      68             : /*
      69             :  * BufTableHashCode
      70             :  *      Compute the hash code associated with a BufferTag
      71             :  *
      72             :  * This must be passed to the lookup/insert/delete routines along with the
      73             :  * tag.  We do it like this because the callers need to know the hash code
      74             :  * in order to determine which buffer partition to lock, and we don't want
      75             :  * to do the hash computation twice (hash_any is a bit slow).
      76             :  */
      77             : uint32
      78   120278222 : BufTableHashCode(BufferTag *tagPtr)
      79             : {
      80   120278222 :     return get_hash_value(SharedBufHash, tagPtr);
      81             : }
      82             : 
      83             : /*
      84             :  * BufTableLookup
      85             :  *      Lookup the given BufferTag; return buffer ID, or -1 if not found
      86             :  *
      87             :  * Caller must hold at least share lock on BufMappingLock for tag's partition
      88             :  */
      89             : int
      90   117243738 : BufTableLookup(BufferTag *tagPtr, uint32 hashcode)
      91             : {
      92             :     BufferLookupEnt *result;
      93             : 
      94             :     result = (BufferLookupEnt *)
      95   117243738 :         hash_search_with_hash_value(SharedBufHash,
      96             :                                     tagPtr,
      97             :                                     hashcode,
      98             :                                     HASH_FIND,
      99             :                                     NULL);
     100             : 
     101   117243738 :     if (!result)
     102     3365414 :         return -1;
     103             : 
     104   113878324 :     return result->id;
     105             : }
     106             : 
     107             : /*
     108             :  * BufTableInsert
     109             :  *      Insert a hashtable entry for given tag and buffer ID,
     110             :  *      unless an entry already exists for that tag
     111             :  *
     112             :  * Returns -1 on successful insertion.  If a conflicting entry exists
     113             :  * already, returns the buffer ID in that entry.
     114             :  *
     115             :  * Caller must hold exclusive lock on BufMappingLock for tag's partition
     116             :  */
     117             : int
     118     3802048 : BufTableInsert(BufferTag *tagPtr, uint32 hashcode, int buf_id)
     119             : {
     120             :     BufferLookupEnt *result;
     121             :     bool        found;
     122             : 
     123             :     Assert(buf_id >= 0);     /* -1 is reserved for not-in-table */
     124             :     Assert(tagPtr->blockNum != P_NEW);   /* invalid tag */
     125             : 
     126             :     result = (BufferLookupEnt *)
     127     3802048 :         hash_search_with_hash_value(SharedBufHash,
     128             :                                     tagPtr,
     129             :                                     hashcode,
     130             :                                     HASH_ENTER,
     131             :                                     &found);
     132             : 
     133     3802048 :     if (found)                  /* found something already in the table */
     134        1146 :         return result->id;
     135             : 
     136     3800902 :     result->id = buf_id;
     137             : 
     138     3800902 :     return -1;
     139             : }
     140             : 
     141             : /*
     142             :  * BufTableDelete
     143             :  *      Delete the hashtable entry for given tag (which must exist)
     144             :  *
     145             :  * Caller must hold exclusive lock on BufMappingLock for tag's partition
     146             :  */
     147             : void
     148     2579814 : BufTableDelete(BufferTag *tagPtr, uint32 hashcode)
     149             : {
     150             :     BufferLookupEnt *result;
     151             : 
     152             :     result = (BufferLookupEnt *)
     153     2579814 :         hash_search_with_hash_value(SharedBufHash,
     154             :                                     tagPtr,
     155             :                                     hashcode,
     156             :                                     HASH_REMOVE,
     157             :                                     NULL);
     158             : 
     159     2579814 :     if (!result)                /* shouldn't happen */
     160           0 :         elog(ERROR, "shared buffer hash table corrupted");
     161     2579814 : }