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SQLite training in Houston TX on 2019-11-05 (details)
Part of the 2019 Tcl Conference

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Overview
Comment:Faster memory allocation from lookaside by not trying to keep track of the number of outstanding allocations, and rather computing that value only when requested.
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SHA3-256: a06263f1efd2d45eac88b8d59e8fe8e458670fa3808c795feaa7f247fc36cbe9
User & Date: drh 2017-08-29 20:21:12
Context
2017-08-30
04:44
Small performance optimization in pcache1. check-in: ffd437da user: drh tags: trunk
2017-08-29
20:21
Faster memory allocation from lookaside by not trying to keep track of the number of outstanding allocations, and rather computing that value only when requested. check-in: a06263f1 user: drh tags: trunk
2017-08-28
17:00
Add the --enable-update-limit option to the ./configure script. check-in: 64a8ae68 user: drh tags: trunk
Changes
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Changes to src/build.c.

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** contains lookaside memory.  (Table objects in the schema do not use
** lookaside memory, but some ephemeral Table objects do.)  Or the
** db parameter can be used with db->pnBytesFreed to measure the memory
** used by the Table object.
*/
static void SQLITE_NOINLINE deleteTable(sqlite3 *db, Table *pTable){
  Index *pIndex, *pNext;
  TESTONLY( int nLookaside; ) /* Used to verify lookaside not used for schema */


  /* Record the number of outstanding lookaside allocations in schema Tables
  ** prior to doing any free() operations.  Since schema Tables do not use
  ** lookaside, this number should not change. */

  TESTONLY( nLookaside = (db && (pTable->tabFlags & TF_Ephemeral)==0) ?
                         db->lookaside.nOut : 0 );




  /* Delete all indices associated with this table. */
  for(pIndex = pTable->pIndex; pIndex; pIndex=pNext){
    pNext = pIndex->pNext;
    assert( pIndex->pSchema==pTable->pSchema
         || (IsVirtual(pTable) && pIndex->idxType!=SQLITE_IDXTYPE_APPDEF) );
    if( (db==0 || db->pnBytesFreed==0) && !IsVirtual(pTable) ){
................................................................................
  sqlite3ExprListDelete(db, pTable->pCheck);
#ifndef SQLITE_OMIT_VIRTUALTABLE
  sqlite3VtabClear(db, pTable);
#endif
  sqlite3DbFree(db, pTable);

  /* Verify that no lookaside memory was used by schema tables */
  assert( nLookaside==0 || nLookaside==db->lookaside.nOut );
}
void sqlite3DeleteTable(sqlite3 *db, Table *pTable){
  /* Do not delete the table until the reference count reaches zero. */
  if( !pTable ) return;
  if( ((!db || db->pnBytesFreed==0) && (--pTable->nTabRef)>0) ) return;
  deleteTable(db, pTable);
}







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** contains lookaside memory.  (Table objects in the schema do not use
** lookaside memory, but some ephemeral Table objects do.)  Or the
** db parameter can be used with db->pnBytesFreed to measure the memory
** used by the Table object.
*/
static void SQLITE_NOINLINE deleteTable(sqlite3 *db, Table *pTable){
  Index *pIndex, *pNext;


#ifdef SQLITE_DEBUG
  /* Record the number of outstanding lookaside allocations in schema Tables
  ** prior to doing any free() operations.  Since schema Tables do not use
  ** lookaside, this number should not change. */
  int nLookaside = 0;
  if( db && (pTable->tabFlags & TF_Ephemeral)==0 ){

    nLookaside = sqlite3LookasideUsed(db, 0);
  }
#endif

  /* Delete all indices associated with this table. */
  for(pIndex = pTable->pIndex; pIndex; pIndex=pNext){
    pNext = pIndex->pNext;
    assert( pIndex->pSchema==pTable->pSchema
         || (IsVirtual(pTable) && pIndex->idxType!=SQLITE_IDXTYPE_APPDEF) );
    if( (db==0 || db->pnBytesFreed==0) && !IsVirtual(pTable) ){
................................................................................
  sqlite3ExprListDelete(db, pTable->pCheck);
#ifndef SQLITE_OMIT_VIRTUALTABLE
  sqlite3VtabClear(db, pTable);
#endif
  sqlite3DbFree(db, pTable);

  /* Verify that no lookaside memory was used by schema tables */
  assert( nLookaside==0 || nLookaside==sqlite3LookasideUsed(db,0) );
}
void sqlite3DeleteTable(sqlite3 *db, Table *pTable){
  /* Do not delete the table until the reference count reaches zero. */
  if( !pTable ) return;
  if( ((!db || db->pnBytesFreed==0) && (--pTable->nTabRef)>0) ) return;
  deleteTable(db, pTable);
}

Changes to src/main.c.

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** space for the lookaside memory is obtained from sqlite3_malloc().
** If pStart is not NULL then it is sz*cnt bytes of memory to use for
** the lookaside memory.
*/
static int setupLookaside(sqlite3 *db, void *pBuf, int sz, int cnt){
#ifndef SQLITE_OMIT_LOOKASIDE
  void *pStart;
  if( db->lookaside.nOut ){

    return SQLITE_BUSY;
  }
  /* Free any existing lookaside buffer for this handle before
  ** allocating a new one so we don't have to have space for 
  ** both at the same time.
  */
  if( db->lookaside.bMalloced ){
................................................................................
    pStart = sqlite3Malloc( sz*cnt );  /* IMP: R-61949-35727 */
    sqlite3EndBenignMalloc();
    if( pStart ) cnt = sqlite3MallocSize(pStart)/sz;
  }else{
    pStart = pBuf;
  }
  db->lookaside.pStart = pStart;

  db->lookaside.pFree = 0;
  db->lookaside.sz = (u16)sz;
  if( pStart ){
    int i;
    LookasideSlot *p;
    assert( sz > (int)sizeof(LookasideSlot*) );

    p = (LookasideSlot*)pStart;
    for(i=cnt-1; i>=0; i--){
      p->pNext = db->lookaside.pFree;
      db->lookaside.pFree = p;
      p = (LookasideSlot*)&((u8*)p)[sz];
    }
    db->lookaside.pEnd = p;
    db->lookaside.bDisable = 0;
    db->lookaside.bMalloced = pBuf==0 ?1:0;
  }else{
    db->lookaside.pStart = db;
    db->lookaside.pEnd = db;
    db->lookaside.bDisable = 1;
    db->lookaside.bMalloced = 0;

  }
#endif /* SQLITE_OMIT_LOOKASIDE */
  return SQLITE_OK;
}

/*
** Return the mutex associated with a database connection.
................................................................................
  ** the same sqliteMalloc() as the one that allocates the database 
  ** structure?
  */
  sqlite3DbFree(db, db->aDb[1].pSchema);
  sqlite3_mutex_leave(db->mutex);
  db->magic = SQLITE_MAGIC_CLOSED;
  sqlite3_mutex_free(db->mutex);
  assert( db->lookaside.nOut==0 );  /* Fails on a lookaside memory leak */
  if( db->lookaside.bMalloced ){
    sqlite3_free(db->lookaside.pStart);
  }
  sqlite3_free(db);
}

/*







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** space for the lookaside memory is obtained from sqlite3_malloc().
** If pStart is not NULL then it is sz*cnt bytes of memory to use for
** the lookaside memory.
*/
static int setupLookaside(sqlite3 *db, void *pBuf, int sz, int cnt){
#ifndef SQLITE_OMIT_LOOKASIDE
  void *pStart;
  
  if( sqlite3LookasideUsed(db,0)>0 ){
    return SQLITE_BUSY;
  }
  /* Free any existing lookaside buffer for this handle before
  ** allocating a new one so we don't have to have space for 
  ** both at the same time.
  */
  if( db->lookaside.bMalloced ){
................................................................................
    pStart = sqlite3Malloc( sz*cnt );  /* IMP: R-61949-35727 */
    sqlite3EndBenignMalloc();
    if( pStart ) cnt = sqlite3MallocSize(pStart)/sz;
  }else{
    pStart = pBuf;
  }
  db->lookaside.pStart = pStart;
  db->lookaside.pInit = 0;
  db->lookaside.pFree = 0;
  db->lookaside.sz = (u16)sz;
  if( pStart ){
    int i;
    LookasideSlot *p;
    assert( sz > (int)sizeof(LookasideSlot*) );
    db->lookaside.nSlot = cnt;
    p = (LookasideSlot*)pStart;
    for(i=cnt-1; i>=0; i--){
      p->pNext = db->lookaside.pInit;
      db->lookaside.pInit = p;
      p = (LookasideSlot*)&((u8*)p)[sz];
    }
    db->lookaside.pEnd = p;
    db->lookaside.bDisable = 0;
    db->lookaside.bMalloced = pBuf==0 ?1:0;
  }else{
    db->lookaside.pStart = db;
    db->lookaside.pEnd = db;
    db->lookaside.bDisable = 1;
    db->lookaside.bMalloced = 0;
    db->lookaside.nSlot = 0;
  }
#endif /* SQLITE_OMIT_LOOKASIDE */
  return SQLITE_OK;
}

/*
** Return the mutex associated with a database connection.
................................................................................
  ** the same sqliteMalloc() as the one that allocates the database 
  ** structure?
  */
  sqlite3DbFree(db, db->aDb[1].pSchema);
  sqlite3_mutex_leave(db->mutex);
  db->magic = SQLITE_MAGIC_CLOSED;
  sqlite3_mutex_free(db->mutex);
  assert( sqlite3LookasideUsed(db,0)==0 );
  if( db->lookaside.bMalloced ){
    sqlite3_free(db->lookaside.pStart);
  }
  sqlite3_free(db);
}

/*

Changes to src/malloc.c.

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      LookasideSlot *pBuf = (LookasideSlot*)p;
#ifdef SQLITE_DEBUG
      /* Trash all content in the buffer being freed */
      memset(p, 0xaa, db->lookaside.sz);
#endif
      pBuf->pNext = db->lookaside.pFree;
      db->lookaside.pFree = pBuf;
      db->lookaside.nOut--;
      return;
    }
  }
  assert( sqlite3MemdebugHasType(p, (MEMTYPE_LOOKASIDE|MEMTYPE_HEAP)) );
  assert( sqlite3MemdebugNoType(p, (u8)~(MEMTYPE_LOOKASIDE|MEMTYPE_HEAP)) );
  assert( db!=0 || sqlite3MemdebugNoType(p, MEMTYPE_LOOKASIDE) );
  sqlite3MemdebugSetType(p, MEMTYPE_HEAP);
................................................................................
  assert( db!=0 );
  assert( sqlite3_mutex_held(db->mutex) );
  assert( db->pnBytesFreed==0 );
  if( db->lookaside.bDisable==0 ){
    assert( db->mallocFailed==0 );
    if( n>db->lookaside.sz ){
      db->lookaside.anStat[1]++;
    }else if( (pBuf = db->lookaside.pFree)==0 ){
      db->lookaside.anStat[2]++;
    }else{
      db->lookaside.pFree = pBuf->pNext;
      db->lookaside.nOut++;
      db->lookaside.anStat[0]++;
      if( db->lookaside.nOut>db->lookaside.mxOut ){
        db->lookaside.mxOut = db->lookaside.nOut;
      }
      return (void*)pBuf;
    }
  }else if( db->mallocFailed ){
    return 0;
  }
#else
  assert( db!=0 );
  assert( sqlite3_mutex_held(db->mutex) );







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      LookasideSlot *pBuf = (LookasideSlot*)p;
#ifdef SQLITE_DEBUG
      /* Trash all content in the buffer being freed */
      memset(p, 0xaa, db->lookaside.sz);
#endif
      pBuf->pNext = db->lookaside.pFree;
      db->lookaside.pFree = pBuf;

      return;
    }
  }
  assert( sqlite3MemdebugHasType(p, (MEMTYPE_LOOKASIDE|MEMTYPE_HEAP)) );
  assert( sqlite3MemdebugNoType(p, (u8)~(MEMTYPE_LOOKASIDE|MEMTYPE_HEAP)) );
  assert( db!=0 || sqlite3MemdebugNoType(p, MEMTYPE_LOOKASIDE) );
  sqlite3MemdebugSetType(p, MEMTYPE_HEAP);
................................................................................
  assert( db!=0 );
  assert( sqlite3_mutex_held(db->mutex) );
  assert( db->pnBytesFreed==0 );
  if( db->lookaside.bDisable==0 ){
    assert( db->mallocFailed==0 );
    if( n>db->lookaside.sz ){
      db->lookaside.anStat[1]++;
    }else if( (pBuf = db->lookaside.pFree)!=0 ){
      db->lookaside.pFree = pBuf->pNext;
      db->lookaside.anStat[0]++;
      return (void*)pBuf;
    }else if( (pBuf = db->lookaside.pInit)!=0 ){
      db->lookaside.pInit = pBuf->pNext;
      db->lookaside.anStat[0]++;
      return (void*)pBuf;
    }else{
      db->lookaside.anStat[2]++;
    }
  }else if( db->mallocFailed ){
    return 0;
  }
#else
  assert( db!=0 );
  assert( sqlite3_mutex_held(db->mutex) );

Changes to src/sqliteInt.h.

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** schema information, the Lookaside.bEnabled flag is cleared so that
** lookaside allocations are not used to construct the schema objects.
*/
struct Lookaside {
  u32 bDisable;           /* Only operate the lookaside when zero */
  u16 sz;                 /* Size of each buffer in bytes */
  u8 bMalloced;           /* True if pStart obtained from sqlite3_malloc() */
  int nOut;               /* Number of buffers currently checked out */
  int mxOut;              /* Highwater mark for nOut */
  int anStat[3];          /* 0: hits.  1: size misses.  2: full misses */

  LookasideSlot *pFree;   /* List of available buffers */
  void *pStart;           /* First byte of available memory space */
  void *pEnd;             /* First byte past end of available space */
};
struct LookasideSlot {
  LookasideSlot *pNext;    /* Next buffer in the list of free buffers */
};
................................................................................
# define sqlite3MemoryBarrier()
#endif

sqlite3_int64 sqlite3StatusValue(int);
void sqlite3StatusUp(int, int);
void sqlite3StatusDown(int, int);
void sqlite3StatusHighwater(int, int);


/* Access to mutexes used by sqlite3_status() */
sqlite3_mutex *sqlite3Pcache1Mutex(void);
sqlite3_mutex *sqlite3MallocMutex(void);

#ifndef SQLITE_OMIT_FLOATING_POINT
  int sqlite3IsNaN(double);







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** schema information, the Lookaside.bEnabled flag is cleared so that
** lookaside allocations are not used to construct the schema objects.
*/
struct Lookaside {
  u32 bDisable;           /* Only operate the lookaside when zero */
  u16 sz;                 /* Size of each buffer in bytes */
  u8 bMalloced;           /* True if pStart obtained from sqlite3_malloc() */
  u32 nSlot;              /* Number of lookaside slots allocated */

  u32 anStat[3];          /* 0: hits.  1: size misses.  2: full misses */
  LookasideSlot *pInit;   /* List of buffers not previously used */
  LookasideSlot *pFree;   /* List of available buffers */
  void *pStart;           /* First byte of available memory space */
  void *pEnd;             /* First byte past end of available space */
};
struct LookasideSlot {
  LookasideSlot *pNext;    /* Next buffer in the list of free buffers */
};
................................................................................
# define sqlite3MemoryBarrier()
#endif

sqlite3_int64 sqlite3StatusValue(int);
void sqlite3StatusUp(int, int);
void sqlite3StatusDown(int, int);
void sqlite3StatusHighwater(int, int);
int sqlite3LookasideUsed(sqlite3*,int*);

/* Access to mutexes used by sqlite3_status() */
sqlite3_mutex *sqlite3Pcache1Mutex(void);
sqlite3_mutex *sqlite3MallocMutex(void);

#ifndef SQLITE_OMIT_FLOATING_POINT
  int sqlite3IsNaN(double);

Changes to src/status.c.

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  rc = sqlite3_status64(op, &iCur, &iHwtr, resetFlag);
  if( rc==0 ){
    *pCurrent = (int)iCur;
    *pHighwater = (int)iHwtr;
  }
  return rc;
}























/*
** Query status information for a single database connection
*/
int sqlite3_db_status(
  sqlite3 *db,          /* The database connection whose status is desired */
  int op,               /* Status verb */
................................................................................
  if( !sqlite3SafetyCheckOk(db) || pCurrent==0|| pHighwater==0 ){
    return SQLITE_MISUSE_BKPT;
  }
#endif
  sqlite3_mutex_enter(db->mutex);
  switch( op ){
    case SQLITE_DBSTATUS_LOOKASIDE_USED: {
      *pCurrent = db->lookaside.nOut;
      *pHighwater = db->lookaside.mxOut;
      if( resetFlag ){



        db->lookaside.mxOut = db->lookaside.nOut;



      }
      break;
    }

    case SQLITE_DBSTATUS_LOOKASIDE_HIT:
    case SQLITE_DBSTATUS_LOOKASIDE_MISS_SIZE:
    case SQLITE_DBSTATUS_LOOKASIDE_MISS_FULL: {







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  rc = sqlite3_status64(op, &iCur, &iHwtr, resetFlag);
  if( rc==0 ){
    *pCurrent = (int)iCur;
    *pHighwater = (int)iHwtr;
  }
  return rc;
}

/*
** Return the number of LookasideSlot elements on the linked list
*/
static u32 countLookasideSlots(LookasideSlot *p){
  u32 cnt = 0;
  while( p ){
    p = p->pNext;
    cnt++;
  }
  return cnt;
}

/*
** Count the number of slots of lookaside memory that are outstanding
*/
int sqlite3LookasideUsed(sqlite3 *db, int *pHighwater){
  u32 nInit = countLookasideSlots(db->lookaside.pInit);
  u32 nFree = countLookasideSlots(db->lookaside.pFree);
  if( pHighwater ) *pHighwater = db->lookaside.nSlot - nInit;
  return db->lookaside.nSlot - (nInit+nFree);
}

/*
** Query status information for a single database connection
*/
int sqlite3_db_status(
  sqlite3 *db,          /* The database connection whose status is desired */
  int op,               /* Status verb */
................................................................................
  if( !sqlite3SafetyCheckOk(db) || pCurrent==0|| pHighwater==0 ){
    return SQLITE_MISUSE_BKPT;
  }
#endif
  sqlite3_mutex_enter(db->mutex);
  switch( op ){
    case SQLITE_DBSTATUS_LOOKASIDE_USED: {
      *pCurrent = sqlite3LookasideUsed(db, pHighwater);

      if( resetFlag ){
        LookasideSlot *p = db->lookaside.pFree;
        if( p ){
          while( p->pNext ) p = p->pNext;
          p->pNext = db->lookaside.pInit;
          db->lookaside.pInit = db->lookaside.pFree;
          db->lookaside.pFree = 0;
        }
      }
      break;
    }

    case SQLITE_DBSTATUS_LOOKASIDE_HIT:
    case SQLITE_DBSTATUS_LOOKASIDE_MISS_SIZE:
    case SQLITE_DBSTATUS_LOOKASIDE_MISS_FULL: {