ext/misc/mmapwarm.c

   1 /*
   2 ** 2017-09-18
   3 **
   4 ** The author disclaims copyright to this source code.  In place of
   5 ** a legal notice, here is a blessing:
   6 **
   7 **    May you do good and not evil.
   8 **    May you find forgiveness for yourself and forgive others.
   9 **    May you share freely, never taking more than you give.
  10 **
  11 *************************************************************************
  12 **
  13 */
  14
  15 #include "sqlite3.h"
  16
  17
  18 /*
  19 ** This function is used to touch each page of a mapping of a memory
  20 ** mapped SQLite database. Assuming that the system has sufficient free
  21 ** memory and supports sufficiently large mappings, this causes the OS
  22 ** to cache the entire database in main memory, making subsequent
  23 ** database accesses faster.
  24 **
  25 ** If the second parameter to this function is not NULL, it is the name of
  26 ** the specific database to operate on (i.e. "main" or the name of an
  27 ** attached database).
  28 **
  29 ** SQLITE_OK is returned if successful, or an SQLite error code otherwise.
  30 ** It is not considered an error if the file is not memory-mapped, or if
  31 ** the mapping does not span the entire file. If an error does occur, a
  32 ** transaction may be left open on the database file.
  33 **
  34 ** It is illegal to call this function when the database handle has an
  35 ** open transaction. SQLITE_MISUSE is returned in this case.
  36 */
  37 int sqlite3_mmap_warm(sqlite3 *db, const char *zDb){
  38   int rc = SQLITE_OK;
  39   char *zSql = 0;
  40   int pgsz = 0;
  41   int nTotal = 0;
  42
  43   if( 0==sqlite3_get_autocommit(db) ) return SQLITE_MISUSE;
  44
  45   /* Open a read-only transaction on the file in question */
  46   zSql = sqlite3_mprintf("BEGIN; SELECT * FROM %s%q%ssqlite_master",
  47       (zDb ? "'" : ""), (zDb ? zDb : ""), (zDb ? "'." : "")
  48   );
  49   if( zSql==0 ) return SQLITE_NOMEM;
  50   rc = sqlite3_exec(db, zSql, 0, 0, 0);
  51   sqlite3_free(zSql);
  52
  53   /* Find the SQLite page size of the file */
  54   if( rc==SQLITE_OK ){
  55     zSql = sqlite3_mprintf("PRAGMA %s%q%spage_size",
  56         (zDb ? "'" : ""), (zDb ? zDb : ""), (zDb ? "'." : "")
  57     );
  58     if( zSql==0 ){
  59       rc = SQLITE_NOMEM;
  60     }else{
  61       sqlite3_stmt *pPgsz = 0;
  62       rc = sqlite3_prepare_v2(db, zSql, -1, &pPgsz, 0);
  63       sqlite3_free(zSql);
  64       if( rc==SQLITE_OK ){
  65         if( sqlite3_step(pPgsz)==SQLITE_ROW ){
  66           pgsz = sqlite3_column_int(pPgsz, 0);
  67         }
  68         rc = sqlite3_finalize(pPgsz);
  69       }
  70       if( rc==SQLITE_OK && pgsz==0 ){
  71         rc = SQLITE_ERROR;
  72       }
  73     }
  74   }
  75
  76   /* Touch each mmap'd page of the file */
  77   if( rc==SQLITE_OK ){
  78     int rc2;
  79     sqlite3_file *pFd = 0;
  80     rc = sqlite3_file_control(db, zDb, SQLITE_FCNTL_FILE_POINTER, &pFd);
  81     if( rc==SQLITE_OK && pFd->pMethods->iVersion>=3 ){
  82       sqlite3_int64 iPg = 1;
  83       sqlite3_io_methods const *p = pFd->pMethods;
  84       while( 1 ){
  85         unsigned char *pMap;
  86         rc = p->xFetch(pFd, pgsz*iPg, pgsz, (void**)&pMap);
  87         if( rc!=SQLITE_OK || pMap==0 ) break;
  88
  89         nTotal += pMap[0];
  90         nTotal += pMap[pgsz-1];
  91
  92         rc = p->xUnfetch(pFd, pgsz*iPg, (void*)pMap);
  93         if( rc!=SQLITE_OK ) break;
  94         iPg++;
  95       }
  96       sqlite3_log(SQLITE_OK,
  97           "sqlite3_mmap_warm_cache: Warmed up %d pages of %s", iPg==1?0:iPg,
  98           sqlite3_db_filename(db, zDb)
  99       );
 100     }
 101
 102     rc2 = sqlite3_exec(db, "END", 0, 0, 0);
 103     if( rc==SQLITE_OK ) rc = rc2;
 104   }
 105
 106   return rc;
 107 }
 108