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Comment:Enhance sqlite3PcacheTruncate() to run faster in the common case where the cutoff is just a few pages less than the page number highwater mark.
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Timelines: family | ancestors | descendants | both | branch-3.14
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SHA1:03dceaea15aa357097a4615ef6044be36d505044
User & Date: drh 2016-08-11 13:03:51
Context
2016-08-11
13:08
Increase the version number to 3.14.1. check-in: 34aed3a3 user: drh tags: branch-3.14
13:03
Enhance sqlite3PcacheTruncate() to run faster in the common case where the cutoff is just a few pages less than the page number highwater mark. check-in: 03dceaea user: drh tags: branch-3.14
2016-08-08
13:40
Version 3.14 check-in: d5e98057 user: drh tags: trunk, release, version-3.14.0
Changes
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Changes to src/pcache1.c.

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**
** The PCache mutex must be held when this function is called.
*/
static void pcache1TruncateUnsafe(
  PCache1 *pCache,             /* The cache to truncate */
  unsigned int iLimit          /* Drop pages with this pgno or larger */
){
  TESTONLY( unsigned int nPage = 0; )  /* To assert pCache->nPage is correct */
  unsigned int h;
  assert( sqlite3_mutex_held(pCache->pGroup->mutex) );













  for(h=0; h<pCache->nHash; h++){



    PgHdr1 **pp = &pCache->apHash[h]; 
    PgHdr1 *pPage;


    while( (pPage = *pp)!=0 ){
      if( pPage->iKey>=iLimit ){
        pCache->nPage--;
        *pp = pPage->pNext;
        if( !pPage->isPinned ) pcache1PinPage(pPage);
        pcache1FreePage(pPage);
      }else{
        pp = &pPage->pNext;
        TESTONLY( nPage++; )
      }
    }


  }
  assert( pCache->nPage==nPage );
}

/******************************************************************************/
/******** sqlite3_pcache Methods **********************************************/

/*
** Implementation of the sqlite3_pcache.xInit method.
................................................................................
** Destroy a cache allocated using pcache1Create().
*/
static void pcache1Destroy(sqlite3_pcache *p){
  PCache1 *pCache = (PCache1 *)p;
  PGroup *pGroup = pCache->pGroup;
  assert( pCache->bPurgeable || (pCache->nMax==0 && pCache->nMin==0) );
  pcache1EnterMutex(pGroup);
  pcache1TruncateUnsafe(pCache, 0);
  assert( pGroup->nMaxPage >= pCache->nMax );
  pGroup->nMaxPage -= pCache->nMax;
  assert( pGroup->nMinPage >= pCache->nMin );
  pGroup->nMinPage -= pCache->nMin;
  pGroup->mxPinned = pGroup->nMaxPage + 10 - pGroup->nMinPage;
  pcache1EnforceMaxPage(pCache);
  pcache1LeaveMutex(pGroup);







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**
** The PCache mutex must be held when this function is called.
*/
static void pcache1TruncateUnsafe(
  PCache1 *pCache,             /* The cache to truncate */
  unsigned int iLimit          /* Drop pages with this pgno or larger */
){
  TESTONLY( int nPage = 0; )  /* To assert pCache->nPage is correct */
  unsigned int h, iStop;
  assert( sqlite3_mutex_held(pCache->pGroup->mutex) );
  assert( pCache->iMaxKey >= iLimit );
  assert( pCache->nHash > 0 );
  if( pCache->iMaxKey - iLimit < pCache->nHash ){
    /* If we are just shaving the last few pages off the end of the
    ** cache, then there is no point in scanning the entire hash table.
    ** Only scan those hash slots that might contain pages that need to
    ** be removed. */
    h = iLimit % pCache->nHash;
    iStop = pCache->iMaxKey % pCache->nHash;
    TESTONLY( nPage = -10; )  /* Disable the pCache->nPage validity check */
  }else{
    /* This is the general case where many pages are being removed.
    ** It is necessary to scan the entire hash table */
    h = pCache->nHash/2;
    iStop = h - 1;
  }
  for(;;){
    PgHdr1 **pp;
    PgHdr1 *pPage;
    assert( h<pCache->nHash );
    pp = &pCache->apHash[h]; 
    while( (pPage = *pp)!=0 ){
      if( pPage->iKey>=iLimit ){
        pCache->nPage--;
        *pp = pPage->pNext;
        if( !pPage->isPinned ) pcache1PinPage(pPage);
        pcache1FreePage(pPage);
      }else{
        pp = &pPage->pNext;
        TESTONLY( if( nPage>=0 ) nPage++; )
      }
    }
    if( h==iStop ) break;
    h = (h+1) % pCache->nHash;
  }
  assert( nPage<0 || pCache->nPage==(unsigned)nPage );
}

/******************************************************************************/
/******** sqlite3_pcache Methods **********************************************/

/*
** Implementation of the sqlite3_pcache.xInit method.
................................................................................
** Destroy a cache allocated using pcache1Create().
*/
static void pcache1Destroy(sqlite3_pcache *p){
  PCache1 *pCache = (PCache1 *)p;
  PGroup *pGroup = pCache->pGroup;
  assert( pCache->bPurgeable || (pCache->nMax==0 && pCache->nMin==0) );
  pcache1EnterMutex(pGroup);
  if( pCache->nPage ) pcache1TruncateUnsafe(pCache, 0);
  assert( pGroup->nMaxPage >= pCache->nMax );
  pGroup->nMaxPage -= pCache->nMax;
  assert( pGroup->nMinPage >= pCache->nMin );
  pGroup->nMinPage -= pCache->nMin;
  pGroup->mxPinned = pGroup->nMaxPage + 10 - pGroup->nMinPage;
  pcache1EnforceMaxPage(pCache);
  pcache1LeaveMutex(pGroup);