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Overview
Comment:Comment and function naming tweaks to pcache. No functionality changes. (CVS 5572)
Downloads: Tarball | ZIP archive | SQL archive
Timelines: family | ancestors | descendants | both | trunk
Files: files | file ages | folders
SHA1: 4b0e67d397236c740bea88ad3a5912abe896322a
User & Date: drh 2008-08-20 21:47:46
Context
2008-08-20
22:06
Modify the sqlite3VdbeUnpackRecord() interface to force the temporary space to be 8-byte aligned. This might be important for Sparc. Ticket #3318. (CVS 5573) check-in: 00b17798 user: drh tags: trunk
21:47
Comment and function naming tweaks to pcache. No functionality changes. (CVS 5572) check-in: 4b0e67d3 user: drh tags: trunk
17:48
Modify the lemon parser template to avoid using zero-initialized constants when compiled with C++. Ticket #3288. (CVS 5571) check-in: 71992f4a user: drh tags: trunk
Changes
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Changes to src/pager.c.

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** The pager is used to access a database disk file.  It implements
** atomic commit and rollback through the use of a journal file that
** is separate from the database file.  The pager also implements file
** locking to prevent two processes from writing the same database
** file simultaneously, or one process from reading the database while
** another is writing.
**
** @(#) $Id: pager.c,v 1.470 2008/08/20 14:49:24 danielk1977 Exp $
*/
#ifndef SQLITE_OMIT_DISKIO
#include "sqliteInt.h"

/*
** Macros for troubleshooting.  Normally turned off
*/
................................................................................
      || (pPg->flags&PGHDR_DIRTY) || pPg->pageHash==pager_pagehash(pPg) );
}

#else
#define pager_datahash(X,Y)  0
#define pager_pagehash(X)  0
#define CHECK_PAGE(x)
#endif

/*
** When this is called the journal file for pager pPager must be open.
** The master journal file name is read from the end of the file and 
** written into memory supplied by the caller. 
**
** zMaster must point to a buffer of at least nMaster bytes allocated by
................................................................................
  */
  if( !pPager || !pPager->pTmpSpace ){
    sqlite3OsClose(pPager->fd);
    sqlite3_free(pPager);
    return ((rc==SQLITE_OK)?SQLITE_NOMEM:rc);
  }
  nExtra = FORCE_ALIGNMENT(nExtra);

  sqlite3PcacheOpen(szPageDflt, nExtra, !memDb, xDesc, !memDb?pagerStress:0, (void *)pPager, pPager->pPCache);

  PAGERTRACE3("OPEN %d %s\n", FILEHANDLEID(pPager->fd), pPager->zFilename);
  IOTRACE(("OPEN %p %s\n", pPager, pPager->zFilename))

  /* Fill in Pager.zDirectory[] */
  memcpy(pPager->zDirectory, pPager->zFilename, nPathname+1);
  for(i=strlen(pPager->zDirectory); i>0 && pPager->zDirectory[i-1]!='/'; i--){}







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** The pager is used to access a database disk file.  It implements
** atomic commit and rollback through the use of a journal file that
** is separate from the database file.  The pager also implements file
** locking to prevent two processes from writing the same database
** file simultaneously, or one process from reading the database while
** another is writing.
**
** @(#) $Id: pager.c,v 1.471 2008/08/20 21:47:46 drh Exp $
*/
#ifndef SQLITE_OMIT_DISKIO
#include "sqliteInt.h"

/*
** Macros for troubleshooting.  Normally turned off
*/
................................................................................
      || (pPg->flags&PGHDR_DIRTY) || pPg->pageHash==pager_pagehash(pPg) );
}

#else
#define pager_datahash(X,Y)  0
#define pager_pagehash(X)  0
#define CHECK_PAGE(x)
#endif  /* SQLITE_CHECK_PAGES */

/*
** When this is called the journal file for pager pPager must be open.
** The master journal file name is read from the end of the file and 
** written into memory supplied by the caller. 
**
** zMaster must point to a buffer of at least nMaster bytes allocated by
................................................................................
  */
  if( !pPager || !pPager->pTmpSpace ){
    sqlite3OsClose(pPager->fd);
    sqlite3_free(pPager);
    return ((rc==SQLITE_OK)?SQLITE_NOMEM:rc);
  }
  nExtra = FORCE_ALIGNMENT(nExtra);
  sqlite3PcacheOpen(szPageDflt, nExtra, !memDb, xDesc, 
                    !memDb?pagerStress:0, (void *)pPager, pPager->pPCache);

  PAGERTRACE3("OPEN %d %s\n", FILEHANDLEID(pPager->fd), pPager->zFilename);
  IOTRACE(("OPEN %p %s\n", pPager, pPager->zFilename))

  /* Fill in Pager.zDirectory[] */
  memcpy(pPager->zDirectory, pPager->zFilename, nPathname+1);
  for(i=strlen(pPager->zDirectory); i>0 && pPager->zDirectory[i-1]!='/'; i--){}

Changes to src/pcache.c.

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**    May you do good and not evil.
**    May you find forgiveness for yourself and forgive others.
**    May you share freely, never taking more than you give.
**
*************************************************************************
** This file implements that page cache.
**
** @(#) $Id: pcache.c,v 1.1 2008/08/20 14:49:25 danielk1977 Exp $
*/
#include "sqliteInt.h"

/*
** A complete page cache is an instance of this structure.
*/
struct PCache {
................................................................................
static struct PCacheGlobal {
  int isInit;                         /* True when initialized */
  sqlite3_mutex *mutex_mem2;          /* static mutex MUTEX_STATIC_MEM2 */
  sqlite3_mutex *mutex_lru;           /* static mutex MUTEX_STATIC_LRU */
  PCache *pAll;                       /* list of all page caches */
  int nPage;                          /* Number of pages */
  int nPurgeable;                     /* Number of pages in purgable caches */
  int mxPage;                   /* Globally configured page maximum */
  int mxPagePurgeable;                /* Purgeable page maximum */
  PgHdr *pLruHead, *pLruTail;         /* Global LRU list of unused pages */
  int szSlot;                         /* Size of each free slot */
  void *pStart, *pEnd;                /* Bounds of pagecache malloc range */
  PgFreeslot *pFree;                  /* Free page blocks */
} pcache = {0};

................................................................................
** underway.
**
** Before the xStress callback of a pager-cache (PCache) is invoked, the
** SQLITE_MUTEX_STATIC_MEM2 mutex is obtained and the SQLITE_MUTEX_STATIC_LRU 
** mutex released (in that order) before making the call.
*/

#define enterPcacheGlobal() sqlite3_mutex_enter(pcache.mutex_lru)
#define exitPcacheGlobal()  sqlite3_mutex_leave(pcache.mutex_lru)

/*
** Increment the reference count on both page p and its cache by n.
*/
static void page_ref(PgHdr *p, int n){
  /* This next block assert()s that the number of references to the 
  ** PCache is the sum of the number of references to all pages in
  ** the PCache. This is a bit expensive to leave turned on all the 
  ** time, even in debugging builds.
  */
#if 0
  PgHdr *pHdr;
................................................................................
#endif
  p->nRef += n;
  p->pCache->nRef += n;
}

/********************************** Linked List Management ********************/







static int check_hash_count(PCache *pCache){
  int i;
  int nPage = 0;
  for(i=0; i<pCache->nHash; i++){
    PgHdr *p;
    for(p=pCache->apHash[i]; p; p=p->pNextHash){
      nPage++;
    }
  }
  assert( nPage==pCache->nPage );
  return 1;
}


/*
** Remove a page from its hash table (PCache.apHash[]).
*/
static void pcacheRemoveFromHash(PgHdr *pPage){
  if( pPage->pPrevHash ){
    pPage->pPrevHash->pNextHash = pPage->pNextHash;
................................................................................
    assert( pCache->apHash[h]==pPage );
    pCache->apHash[h] = pPage->pNextHash;
  }
  if( pPage->pNextHash ){
    pPage->pNextHash->pPrevHash = pPage->pPrevHash;
  }
  pPage->pCache->nPage--;
  assert( check_hash_count(pPage->pCache) );
}

/*
** Insert a page into the hash table
*/
static void pcacheAddToHash(PgHdr *pPage){
  PCache *pCache = pPage->pCache;
................................................................................
  pPage->pNextHash = pCache->apHash[h];
  pPage->pPrevHash = 0;
  if( pCache->apHash[h] ){
    pCache->apHash[h]->pPrevHash = pPage;
  }
  pCache->apHash[h] = pPage;
  pCache->nPage++;
  assert( check_hash_count(pCache) );
}

/*
** Resize the hash table to nHash buckets.

*/
static int pcacheResizeHash(PCache *pCache, int nHash){
#ifdef SQLITE_MALLOC_SOFT_LIMIT
  if( nHash*sizeof(PgHdr *)>SQLITE_MALLOC_SOFT_LIMIT ){
    nHash = SQLITE_MALLOC_SOFT_LIMIT/sizeof(PgHdr *);
  }
#endif
  if( nHash>pCache->nHash ){
    PgHdr *p;
    PgHdr **pNew = (PgHdr **)sqlite3_malloc(sizeof(PgHdr *)*nHash);
    if( !pNew ){
      return SQLITE_NOMEM;
    }
    memset(pNew, 0, sizeof(PgHdr *)*nHash);
    sqlite3_free(pCache->apHash);
    pCache->apHash = pNew;
    pCache->nHash = nHash;
................................................................................
      sqlite3StatusAdd(SQLITE_STATUS_PAGECACHE_OVERFLOW, sz);
    }
    return p;
  }
}
void *sqlite3PageMalloc(sz){
  void *p;
  enterPcacheGlobal();
  p = pcacheMalloc(sz);
  exitPcacheGlobal();
  return p;
}

/*
** Release a pager memory allocation
*/
void pcacheFree(void *p){
................................................................................
  }else{
    int iSize = sqlite3MallocSize(p);
    sqlite3StatusAdd(SQLITE_STATUS_PAGECACHE_OVERFLOW, -iSize);
    sqlite3_free(p);
  }
}
void sqlite3PageFree(void *p){
  enterPcacheGlobal();
  pcacheFree(p);
  exitPcacheGlobal();
}

/*
** Allocate a new page.
*/
static PgHdr *pcachePageAlloc(int szPage, int szExtra, int bPurgeable){
  PgHdr *p;
................................................................................
  int szPage = pCache->szPage;
  int szExtra = pCache->szExtra;
  int bPurg = pCache->bPurgeable;

  assert( pcache.isInit );
  assert( sqlite3_mutex_notheld(pcache.mutex_lru) );

  enterPcacheGlobal();

  if( (pcache.mxPage && pcache.nPage>=pcache.mxPage) 
   || (!pcache.mxPage && bPurg && pcache.nPurgeable>=pcache.mxPagePurgeable)
  ){
    PCache *pCsr;

    /* If the above test succeeds, then a page will be obtained by recycling
................................................................................
      ** page that can be reused or until the xStress() callback of all
      ** caches has been invoked.
      */
      for(pCsr=pcache.pAll; pCsr&&!pcache.pLruTail; pCsr=pCsr->pNextAll){
        assert( pCsr->iInUseMM==0 );
        pCsr->iInUseMM = 1;
        if( pCsr->xStress && (pCsr->iInUseDB==0 || pCache==pCsr) ){
          exitPcacheGlobal();
          pCsr->xStress(pCsr->pStress);
          enterPcacheGlobal();
        }
        pCsr->iInUseMM = 0;
      }

      sqlite3_mutex_leave(pcache.mutex_mem2);
    }

................................................................................
  }

  if( !p ){
    /* Allocate a new page object. */
    p = pcachePageAlloc(szPage, szExtra, bPurg);
  }

  exitPcacheGlobal();
  return p;
}

/*************************************************** General Interfaces ******
**
** Initialize and shutdown the page cache subsystem. Neither of these 
** functions are threadsafe.
................................................................................
  p->bPurgeable = bPurgeable;
  p->xDestroy = xDestroy;
  p->xStress = xStress;
  p->pStress = pStress;
  p->nMax = 100;

  if( bPurgeable ){
    enterPcacheGlobal();
    pcache.mxPagePurgeable += p->nMax;
    exitPcacheGlobal();
  }

  /* Add the new pager-cache to the list of caches starting at pcache.pAll */
  sqlite3_mutex_enter(pcache.mutex_mem2);
  p->pNextAll = pcache.pAll;
  if( pcache.pAll ){
    pcache.pAll->pPrevAll = p;
  }
  p->pPrevAll = 0;
  pcache.pAll = p;
  sqlite3_mutex_leave(pcache.mutex_mem2);
}





void sqlite3PcacheSetPageSize(PCache *pCache, int szPage){
  assert(pCache->nPage==0);
  pCache->szPage = szPage;
}

/*
** Try to obtain a page from the cache.
................................................................................

  /* Search the hash table for the requested page. Exit early if it is found. */
  if( pCache->apHash ){
    u32 h = pgno % pCache->nHash;
    for(pPage=pCache->apHash[h]; pPage; pPage=pPage->pNextHash){
      if( pPage->pgno==pgno ){
        if( pPage->nRef==0 && (pPage->flags & PGHDR_DIRTY)==0 ){
          enterPcacheGlobal();
          pcacheRemoveFromLruList(pPage);
          exitPcacheGlobal();
        }
        page_ref(pPage, 1);
        *ppPage = pPage;
        return SQLITE_OK;
      }
    }
  }

  if( createFlag ){
................................................................................

    pPage->pPager = 0;
    pPage->flags = 0;
    pPage->pDirty = 0;
    pPage->nRef = 0;
    pPage->pgno = pgno;
    pPage->pCache = pCache;
    page_ref(pPage, 1);
    pcacheAddToList(&pCache->pClean, pPage);
    pcacheAddToHash(pPage);
  }else{
    *ppPage = 0;
  }

  return SQLITE_OK;
................................................................................
/*
** Dereference a page.  When the reference count reaches zero,
** move the page to the LRU list if it is clean.
*/
void sqlite3PcacheRelease(PgHdr *p){
  assert( p->nRef>0 );
  assert( p->pCache->iInUseDB || p->pCache->iInUseMM );
  page_ref(p, -1);
  if( p->nRef!=0 ) return;
  if( p->pCache->xDestroy ){
    p->pCache->xDestroy(p);
  }
  if( (p->flags & PGHDR_DIRTY)!=0 ) return;
  enterPcacheGlobal();
  pcacheAddToLruList(p);
  exitPcacheGlobal();
}

void sqlite3PcacheRef(PgHdr *p){
  assert(p->nRef>=0);
  page_ref(p, 1);
}

/*
** Drop a page from the cache.  This should be the only reference to
** the page.
*/
void sqlite3PcacheDrop(PgHdr *p){
................................................................................
  pCache->nRef--;
  if( p->flags & PGHDR_DIRTY ){
    pcacheRemoveFromList(&pCache->pDirty, p);
  }else{
    pcacheRemoveFromList(&pCache->pClean, p);
  }
  pcacheRemoveFromHash(p);
  enterPcacheGlobal();
  pcachePageFree(p);
  exitPcacheGlobal();
}

/*
** Make sure the page is marked as dirty.  If it isn't dirty already,
** make it so.
*/
void sqlite3PcacheMakeDirty(PgHdr *p){
................................................................................
  assert( p->flags & PGHDR_DIRTY );
  /* assert( p->nRef>0 ); */
  pCache = p->pCache;
  pcacheRemoveFromList(&pCache->pDirty, p);
  pcacheAddToList(&pCache->pClean, p);
  p->flags &= ~PGHDR_DIRTY;
  if( p->nRef==0 ){
    enterPcacheGlobal();
    pcacheAddToLruList(p);
    exitPcacheGlobal();
  }
}

/*
** Make every page in the cache clean.
*/
void sqlite3PcacheCleanAll(PCache *pCache){
................................................................................
  assert( pCache->iInUseDB );
  while( (p = pCache->pDirty)!=0 ){
    assert( p->apSave[0]==0 && p->apSave[1]==0 );
    pcacheRemoveFromList(&pCache->pDirty, p);
    pcacheAddToList(&pCache->pClean, p);
    p->flags &= ~PGHDR_DIRTY;
    if( p->nRef==0 ){
      enterPcacheGlobal();
      pcacheAddToLruList(p);
      exitPcacheGlobal();
    }
  }
}

/*
** Change the page number of page p to newPgno. If newPgno is 0, then the
** page object is added to the clean-list and the PGHDR_REUSE_UNLIKELY 
................................................................................
*/
void sqlite3PcacheMove(PgHdr *p, Pgno newPgno){
  assert( p->pCache->iInUseDB );
  pcacheRemoveFromHash(p);
  p->pgno = newPgno;
  if( newPgno==0 ){
    p->flags |= PGHDR_REUSE_UNLIKELY;
    enterPcacheGlobal();
    pcacheFree(p->apSave[0]);
    pcacheFree(p->apSave[1]);
    exitPcacheGlobal();
    p->apSave[0] = 0;
    p->apSave[1] = 0;
    sqlite3PcacheMakeClean(p);
  }
  pcacheAddToHash(p);
}

/*
** Set the global maximum number of pages. Return the previous value.
*/
void sqlite3PcacheGlobalMax(int mx){
  enterPcacheGlobal();
  pcache.mxPage = mx;
  exitPcacheGlobal();
}

/*
** Remove all content from a page cache
*/
void pcacheClear(PCache *pCache){
  PgHdr *p, *pNext;
................................................................................
/*
** Drop every cache entry whose page number is greater than "pgno".
*/
void sqlite3PcacheTruncate(PCache *pCache, Pgno pgno){
  PgHdr *p, *pNext;
  PgHdr *pDirty = pCache->pDirty;
  assert( pCache->iInUseDB );
  enterPcacheGlobal();
  for(p=pCache->pClean; p||pDirty; p=pNext){
    if( !p ){
      p = pDirty;
      pDirty = 0;
    }
    pNext = p->pNext;
    if( p->pgno>pgno ){
................................................................................
        /* If there are references to the page, it cannot be freed. In this
        ** case, zero the page content instead.
        */
        memset(p->pData, 0, pCache->szPage);
      }
    }
  }
  exitPcacheGlobal();
}


/*
** Close a cache.
*/
void sqlite3PcacheClose(PCache *pCache){
  assert( pCache->iInUseDB==1 );

  /* Free all the pages used by this pager and remove them from the LRU
  ** list. This requires the protection of the MUTEX_STATIC_LRU mutex.
  */
  enterPcacheGlobal();
  pcacheClear(pCache);
  if( pCache->bPurgeable ){
    pcache.mxPagePurgeable -= pCache->nMax;
  }
  sqlite3_free(pCache->apHash);
  exitPcacheGlobal();

  /* Now remove the pager-cache structure itself from the list of
  ** all such structures headed by pcache.pAll. This required the
  ** MUTEX_STATIC_MEM2 mutex.
  */
  sqlite3_mutex_enter(pcache.mutex_mem2);
  assert(pCache==pcache.pAll || pCache->pPrevAll);
................................................................................

/*
** Commit a change previously preserved.
*/
void sqlite3PcacheCommit(PCache *pCache, int idJournal){
  PgHdr *p;
  assert( pCache->iInUseDB );
  enterPcacheGlobal();     /* Mutex is required to call pcacheFree() */
  for(p=pCache->pDirty; p; p=p->pNext){
    if( p->apSave[idJournal] ){
      pcacheFree(p->apSave[idJournal]);
      p->apSave[idJournal] = 0;
    }
  }
  exitPcacheGlobal();
}

/*
** Rollback a change previously preserved.
*/
void sqlite3PcacheRollback(PCache *pCache, int idJournal){
  PgHdr *p;
  int sz;
  assert( pCache->iInUseDB );
  enterPcacheGlobal();     /* Mutex is required to call pcacheFree() */
  sz = pCache->szPage;
  for(p=pCache->pDirty; p; p=p->pNext){
    if( p->apSave[idJournal] ){
      memcpy(p->pData, p->apSave[idJournal], sz);
      pcacheFree(p->apSave[idJournal]);
      p->apSave[idJournal] = 0;
    }
  }
  exitPcacheGlobal();
}

/* 
** Assert flags settings on all pages.  Debugging only.
*/
void sqlite3PcacheAssertFlags(PCache *pCache, int trueMask, int falseMask){
  PgHdr *p;
................................................................................

/* 
** Discard the contents of the cache.
*/
int sqlite3PcacheClear(PCache *pCache){
  assert( pCache->iInUseDB );
  assert(pCache->nRef==0);
  enterPcacheGlobal();
  pcacheClear(pCache);
  exitPcacheGlobal();
  return SQLITE_OK;
}

/*
** Merge two lists of pages connected by pDirty and in pgno order.
** Do not both fixing the pPrevDirty pointers.
*/
................................................................................
** Set the suggested cache-size value.
*/
void sqlite3PcacheSetCachesize(PCache *pCache, int mxPage){
  if( mxPage<10 ){
    mxPage = 10;
  }
  if( pCache->bPurgeable ){
    enterPcacheGlobal();
    pcache.mxPagePurgeable -= pCache->nMax;
    pcache.mxPagePurgeable += mxPage;
    exitPcacheGlobal();
  }
  pCache->nMax = mxPage;
}

/*
** Lock a pager-cache.
*/
................................................................................
/*
** Unlock a pager-cache.
*/
void sqlite3PcacheUnlock(PCache *pCache){
  pCache->iInUseDB--;
  assert( pCache->iInUseDB>=0 );
}








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**    May you do good and not evil.
**    May you find forgiveness for yourself and forgive others.
**    May you share freely, never taking more than you give.
**
*************************************************************************
** This file implements that page cache.
**
** @(#) $Id: pcache.c,v 1.2 2008/08/20 21:47:46 drh Exp $
*/
#include "sqliteInt.h"

/*
** A complete page cache is an instance of this structure.
*/
struct PCache {
................................................................................
static struct PCacheGlobal {
  int isInit;                         /* True when initialized */
  sqlite3_mutex *mutex_mem2;          /* static mutex MUTEX_STATIC_MEM2 */
  sqlite3_mutex *mutex_lru;           /* static mutex MUTEX_STATIC_LRU */
  PCache *pAll;                       /* list of all page caches */
  int nPage;                          /* Number of pages */
  int nPurgeable;                     /* Number of pages in purgable caches */
  int mxPage;                         /* Globally configured page maximum */
  int mxPagePurgeable;                /* Purgeable page maximum */
  PgHdr *pLruHead, *pLruTail;         /* Global LRU list of unused pages */
  int szSlot;                         /* Size of each free slot */
  void *pStart, *pEnd;                /* Bounds of pagecache malloc range */
  PgFreeslot *pFree;                  /* Free page blocks */
} pcache = {0};

................................................................................
** underway.
**
** Before the xStress callback of a pager-cache (PCache) is invoked, the
** SQLITE_MUTEX_STATIC_MEM2 mutex is obtained and the SQLITE_MUTEX_STATIC_LRU 
** mutex released (in that order) before making the call.
*/

#define pcacheEnterGlobal() sqlite3_mutex_enter(pcache.mutex_lru)
#define pcacheExitGlobal()  sqlite3_mutex_leave(pcache.mutex_lru)

/*
** Increment the reference count on both page p and its cache by n.
*/
static void pcacheRef(PgHdr *p, int n){
  /* This next block assert()s that the number of references to the 
  ** PCache is the sum of the number of references to all pages in
  ** the PCache. This is a bit expensive to leave turned on all the 
  ** time, even in debugging builds.
  */
#if 0
  PgHdr *pHdr;
................................................................................
#endif
  p->nRef += n;
  p->pCache->nRef += n;
}

/********************************** Linked List Management ********************/

#ifndef NDEBUG
/*
** This routine verifies that the number of entries in the hash table
** is pCache->nPage.  This routine is used within assert() statements
** only and is therefore disabled during production builds.
*/
static int pcacheCheckHashCount(PCache *pCache){
  int i;
  int nPage = 0;
  for(i=0; i<pCache->nHash; i++){
    PgHdr *p;
    for(p=pCache->apHash[i]; p; p=p->pNextHash){
      nPage++;
    }
  }
  assert( nPage==pCache->nPage );
  return 1;
}
#endif

/*
** Remove a page from its hash table (PCache.apHash[]).
*/
static void pcacheRemoveFromHash(PgHdr *pPage){
  if( pPage->pPrevHash ){
    pPage->pPrevHash->pNextHash = pPage->pNextHash;
................................................................................
    assert( pCache->apHash[h]==pPage );
    pCache->apHash[h] = pPage->pNextHash;
  }
  if( pPage->pNextHash ){
    pPage->pNextHash->pPrevHash = pPage->pPrevHash;
  }
  pPage->pCache->nPage--;
  assert( pcacheCheckHashCount(pPage->pCache) );
}

/*
** Insert a page into the hash table
*/
static void pcacheAddToHash(PgHdr *pPage){
  PCache *pCache = pPage->pCache;
................................................................................
  pPage->pNextHash = pCache->apHash[h];
  pPage->pPrevHash = 0;
  if( pCache->apHash[h] ){
    pCache->apHash[h]->pPrevHash = pPage;
  }
  pCache->apHash[h] = pPage;
  pCache->nPage++;
  assert( pcacheCheckHashCount(pCache) );
}

/*
** Attempt to increase the size the hash table to contain
** at least nHash buckets.
*/
static int pcacheResizeHash(PCache *pCache, int nHash){
#ifdef SQLITE_MALLOC_SOFT_LIMIT
  if( nHash*sizeof(PgHdr*)>SQLITE_MALLOC_SOFT_LIMIT ){
    nHash = SQLITE_MALLOC_SOFT_LIMIT/sizeof(PgHdr *);
  }
#endif
  if( nHash>pCache->nHash ){
    PgHdr *p;
    PgHdr **pNew = (PgHdr **)sqlite3_malloc(sizeof(PgHdr*)*nHash);
    if( !pNew ){
      return SQLITE_NOMEM;
    }
    memset(pNew, 0, sizeof(PgHdr *)*nHash);
    sqlite3_free(pCache->apHash);
    pCache->apHash = pNew;
    pCache->nHash = nHash;
................................................................................
      sqlite3StatusAdd(SQLITE_STATUS_PAGECACHE_OVERFLOW, sz);
    }
    return p;
  }
}
void *sqlite3PageMalloc(sz){
  void *p;
  pcacheEnterGlobal();
  p = pcacheMalloc(sz);
  pcacheExitGlobal();
  return p;
}

/*
** Release a pager memory allocation
*/
void pcacheFree(void *p){
................................................................................
  }else{
    int iSize = sqlite3MallocSize(p);
    sqlite3StatusAdd(SQLITE_STATUS_PAGECACHE_OVERFLOW, -iSize);
    sqlite3_free(p);
  }
}
void sqlite3PageFree(void *p){
  pcacheEnterGlobal();
  pcacheFree(p);
  pcacheExitGlobal();
}

/*
** Allocate a new page.
*/
static PgHdr *pcachePageAlloc(int szPage, int szExtra, int bPurgeable){
  PgHdr *p;
................................................................................
  int szPage = pCache->szPage;
  int szExtra = pCache->szExtra;
  int bPurg = pCache->bPurgeable;

  assert( pcache.isInit );
  assert( sqlite3_mutex_notheld(pcache.mutex_lru) );

  pcacheEnterGlobal();

  if( (pcache.mxPage && pcache.nPage>=pcache.mxPage) 
   || (!pcache.mxPage && bPurg && pcache.nPurgeable>=pcache.mxPagePurgeable)
  ){
    PCache *pCsr;

    /* If the above test succeeds, then a page will be obtained by recycling
................................................................................
      ** page that can be reused or until the xStress() callback of all
      ** caches has been invoked.
      */
      for(pCsr=pcache.pAll; pCsr&&!pcache.pLruTail; pCsr=pCsr->pNextAll){
        assert( pCsr->iInUseMM==0 );
        pCsr->iInUseMM = 1;
        if( pCsr->xStress && (pCsr->iInUseDB==0 || pCache==pCsr) ){
          pcacheExitGlobal();
          pCsr->xStress(pCsr->pStress);
          pcacheEnterGlobal();
        }
        pCsr->iInUseMM = 0;
      }

      sqlite3_mutex_leave(pcache.mutex_mem2);
    }

................................................................................
  }

  if( !p ){
    /* Allocate a new page object. */
    p = pcachePageAlloc(szPage, szExtra, bPurg);
  }

  pcacheExitGlobal();
  return p;
}

/*************************************************** General Interfaces ******
**
** Initialize and shutdown the page cache subsystem. Neither of these 
** functions are threadsafe.
................................................................................
  p->bPurgeable = bPurgeable;
  p->xDestroy = xDestroy;
  p->xStress = xStress;
  p->pStress = pStress;
  p->nMax = 100;

  if( bPurgeable ){
    pcacheEnterGlobal();
    pcache.mxPagePurgeable += p->nMax;
    pcacheExitGlobal();
  }

  /* Add the new pager-cache to the list of caches starting at pcache.pAll */
  sqlite3_mutex_enter(pcache.mutex_mem2);
  p->pNextAll = pcache.pAll;
  if( pcache.pAll ){
    pcache.pAll->pPrevAll = p;
  }
  p->pPrevAll = 0;
  pcache.pAll = p;
  sqlite3_mutex_leave(pcache.mutex_mem2);
}

/*
** Change the page size for PCache object.  This can only happen
** when the cache is empty.
*/
void sqlite3PcacheSetPageSize(PCache *pCache, int szPage){
  assert(pCache->nPage==0);
  pCache->szPage = szPage;
}

/*
** Try to obtain a page from the cache.
................................................................................

  /* Search the hash table for the requested page. Exit early if it is found. */
  if( pCache->apHash ){
    u32 h = pgno % pCache->nHash;
    for(pPage=pCache->apHash[h]; pPage; pPage=pPage->pNextHash){
      if( pPage->pgno==pgno ){
        if( pPage->nRef==0 && (pPage->flags & PGHDR_DIRTY)==0 ){
          pcacheEnterGlobal();
          pcacheRemoveFromLruList(pPage);
          pcacheExitGlobal();
        }
        pcacheRef(pPage, 1);
        *ppPage = pPage;
        return SQLITE_OK;
      }
    }
  }

  if( createFlag ){
................................................................................

    pPage->pPager = 0;
    pPage->flags = 0;
    pPage->pDirty = 0;
    pPage->nRef = 0;
    pPage->pgno = pgno;
    pPage->pCache = pCache;
    pcacheRef(pPage, 1);
    pcacheAddToList(&pCache->pClean, pPage);
    pcacheAddToHash(pPage);
  }else{
    *ppPage = 0;
  }

  return SQLITE_OK;
................................................................................
/*
** Dereference a page.  When the reference count reaches zero,
** move the page to the LRU list if it is clean.
*/
void sqlite3PcacheRelease(PgHdr *p){
  assert( p->nRef>0 );
  assert( p->pCache->iInUseDB || p->pCache->iInUseMM );
  pcacheRef(p, -1);
  if( p->nRef!=0 ) return;
  if( p->pCache->xDestroy ){
    p->pCache->xDestroy(p);
  }
  if( (p->flags & PGHDR_DIRTY)!=0 ) return;
  pcacheEnterGlobal();
  pcacheAddToLruList(p);
  pcacheExitGlobal();
}

void sqlite3PcacheRef(PgHdr *p){
  assert(p->nRef>=0);
  pcacheRef(p, 1);
}

/*
** Drop a page from the cache.  This should be the only reference to
** the page.
*/
void sqlite3PcacheDrop(PgHdr *p){
................................................................................
  pCache->nRef--;
  if( p->flags & PGHDR_DIRTY ){
    pcacheRemoveFromList(&pCache->pDirty, p);
  }else{
    pcacheRemoveFromList(&pCache->pClean, p);
  }
  pcacheRemoveFromHash(p);
  pcacheEnterGlobal();
  pcachePageFree(p);
  pcacheExitGlobal();
}

/*
** Make sure the page is marked as dirty.  If it isn't dirty already,
** make it so.
*/
void sqlite3PcacheMakeDirty(PgHdr *p){
................................................................................
  assert( p->flags & PGHDR_DIRTY );
  /* assert( p->nRef>0 ); */
  pCache = p->pCache;
  pcacheRemoveFromList(&pCache->pDirty, p);
  pcacheAddToList(&pCache->pClean, p);
  p->flags &= ~PGHDR_DIRTY;
  if( p->nRef==0 ){
    pcacheEnterGlobal();
    pcacheAddToLruList(p);
    pcacheExitGlobal();
  }
}

/*
** Make every page in the cache clean.
*/
void sqlite3PcacheCleanAll(PCache *pCache){
................................................................................
  assert( pCache->iInUseDB );
  while( (p = pCache->pDirty)!=0 ){
    assert( p->apSave[0]==0 && p->apSave[1]==0 );
    pcacheRemoveFromList(&pCache->pDirty, p);
    pcacheAddToList(&pCache->pClean, p);
    p->flags &= ~PGHDR_DIRTY;
    if( p->nRef==0 ){
      pcacheEnterGlobal();
      pcacheAddToLruList(p);
      pcacheExitGlobal();
    }
  }
}

/*
** Change the page number of page p to newPgno. If newPgno is 0, then the
** page object is added to the clean-list and the PGHDR_REUSE_UNLIKELY 
................................................................................
*/
void sqlite3PcacheMove(PgHdr *p, Pgno newPgno){
  assert( p->pCache->iInUseDB );
  pcacheRemoveFromHash(p);
  p->pgno = newPgno;
  if( newPgno==0 ){
    p->flags |= PGHDR_REUSE_UNLIKELY;
    pcacheEnterGlobal();
    pcacheFree(p->apSave[0]);
    pcacheFree(p->apSave[1]);
    pcacheExitGlobal();
    p->apSave[0] = 0;
    p->apSave[1] = 0;
    sqlite3PcacheMakeClean(p);
  }
  pcacheAddToHash(p);
}

/*
** Set the global maximum number of pages. Return the previous value.
*/
void sqlite3PcacheGlobalMax(int mx){
  pcacheEnterGlobal();
  pcache.mxPage = mx;
  pcacheExitGlobal();
}

/*
** Remove all content from a page cache
*/
void pcacheClear(PCache *pCache){
  PgHdr *p, *pNext;
................................................................................
/*
** Drop every cache entry whose page number is greater than "pgno".
*/
void sqlite3PcacheTruncate(PCache *pCache, Pgno pgno){
  PgHdr *p, *pNext;
  PgHdr *pDirty = pCache->pDirty;
  assert( pCache->iInUseDB );
  pcacheEnterGlobal();
  for(p=pCache->pClean; p||pDirty; p=pNext){
    if( !p ){
      p = pDirty;
      pDirty = 0;
    }
    pNext = p->pNext;
    if( p->pgno>pgno ){
................................................................................
        /* If there are references to the page, it cannot be freed. In this
        ** case, zero the page content instead.
        */
        memset(p->pData, 0, pCache->szPage);
      }
    }
  }
  pcacheExitGlobal();
}


/*
** Close a cache.
*/
void sqlite3PcacheClose(PCache *pCache){
  assert( pCache->iInUseDB==1 );

  /* Free all the pages used by this pager and remove them from the LRU
  ** list. This requires the protection of the MUTEX_STATIC_LRU mutex.
  */
  pcacheEnterGlobal();
  pcacheClear(pCache);
  if( pCache->bPurgeable ){
    pcache.mxPagePurgeable -= pCache->nMax;
  }
  sqlite3_free(pCache->apHash);
  pcacheExitGlobal();

  /* Now remove the pager-cache structure itself from the list of
  ** all such structures headed by pcache.pAll. This required the
  ** MUTEX_STATIC_MEM2 mutex.
  */
  sqlite3_mutex_enter(pcache.mutex_mem2);
  assert(pCache==pcache.pAll || pCache->pPrevAll);
................................................................................

/*
** Commit a change previously preserved.
*/
void sqlite3PcacheCommit(PCache *pCache, int idJournal){
  PgHdr *p;
  assert( pCache->iInUseDB );
  pcacheEnterGlobal();     /* Mutex is required to call pcacheFree() */
  for(p=pCache->pDirty; p; p=p->pNext){
    if( p->apSave[idJournal] ){
      pcacheFree(p->apSave[idJournal]);
      p->apSave[idJournal] = 0;
    }
  }
  pcacheExitGlobal();
}

/*
** Rollback a change previously preserved.
*/
void sqlite3PcacheRollback(PCache *pCache, int idJournal){
  PgHdr *p;
  int sz;
  assert( pCache->iInUseDB );
  pcacheEnterGlobal();     /* Mutex is required to call pcacheFree() */
  sz = pCache->szPage;
  for(p=pCache->pDirty; p; p=p->pNext){
    if( p->apSave[idJournal] ){
      memcpy(p->pData, p->apSave[idJournal], sz);
      pcacheFree(p->apSave[idJournal]);
      p->apSave[idJournal] = 0;
    }
  }
  pcacheExitGlobal();
}

/* 
** Assert flags settings on all pages.  Debugging only.
*/
void sqlite3PcacheAssertFlags(PCache *pCache, int trueMask, int falseMask){
  PgHdr *p;
................................................................................

/* 
** Discard the contents of the cache.
*/
int sqlite3PcacheClear(PCache *pCache){
  assert( pCache->iInUseDB );
  assert(pCache->nRef==0);
  pcacheEnterGlobal();
  pcacheClear(pCache);
  pcacheExitGlobal();
  return SQLITE_OK;
}

/*
** Merge two lists of pages connected by pDirty and in pgno order.
** Do not both fixing the pPrevDirty pointers.
*/
................................................................................
** Set the suggested cache-size value.
*/
void sqlite3PcacheSetCachesize(PCache *pCache, int mxPage){
  if( mxPage<10 ){
    mxPage = 10;
  }
  if( pCache->bPurgeable ){
    pcacheEnterGlobal();
    pcache.mxPagePurgeable -= pCache->nMax;
    pcache.mxPagePurgeable += mxPage;
    pcacheExitGlobal();
  }
  pCache->nMax = mxPage;
}

/*
** Lock a pager-cache.
*/
................................................................................
/*
** Unlock a pager-cache.
*/
void sqlite3PcacheUnlock(PCache *pCache){
  pCache->iInUseDB--;
  assert( pCache->iInUseDB>=0 );
}