SQLite 学习之路 第五节 Page Cache脏页

所有的脏页通过一个双向链表来连接在一起，其结构关系如下图所示：

脏页的添加和删除：
按照LRU的顺序来维护，新的链表元素是从头部插入，即页面p比p->DirtyNext更新。pCache->pDirty指向最新的页面，pCache->pDirtyTail指向最老的页面。通过pcacheManageDirtyList()函数来添加和删除：

/* Allowed values for second argument to pcacheManageDirtyList() */
#define PCACHE_DIRTYLIST_REMOVE   1    /* Remove pPage from dirty list */
#define PCACHE_DIRTYLIST_ADD      2    /* Add pPage to the dirty list */
#define PCACHE_DIRTYLIST_FRONT    3    /* Move pPage to the front of the list *//*
** Manage pPage's participation on the dirty list.  Bits of the addRemove
** argument determines what operation to do.  The 0x01 bit means first
** remove pPage from the dirty list.  The 0x02 means add pPage back to
** the dirty list.  Doing both moves pPage to the front of the dirty list.
*/
static void pcacheManageDirtyList(PgHdr *pPage, u8 addRemove){PCache *p = pPage->pCache;pcacheTrace(("%p.DIRTYLIST.%s %d\n", p,addRemove==1 ? "REMOVE" : addRemove==2 ? "ADD" : "FRONT",pPage->pgno));//打印调试信息//把页面从链表移除if( addRemove & PCACHE_DIRTYLIST_REMOVE ){ assert( pPage->pDirtyNext || pPage==p->pDirtyTail );assert( pPage->pDirtyPrev || pPage==p->pDirty );/* Update the PCache1.pSynced variable if necessary. */if( p->pSynced==pPage ){p->pSynced = pPage->pDirtyPrev;}if( pPage->pDirtyNext ){pPage->pDirtyNext->pDirtyPrev = pPage->pDirtyPrev;//让下一个节点指向前一个节点}else{assert( pPage==p->pDirtyTail );//如果被删除的页面是最后一个，那么更新链表尾部p->pDirtyTail = pPage->pDirtyPrev;}
if( pPage->pDirtyPrev ){//让前一个节点指向后一个节点pPage->pDirtyPrev->pDirtyNext = pPage->pDirtyNext;}else{/* If there are now no dirty pages in the cache, set eCreate to 2. ** This is an optimization that allows sqlite3PcacheFetch() to skip** searching for a dirty page to eject from the cache when it might** otherwise have to.  */assert( pPage==p->pDirty );//如果被删的是头部，那么更新链表头部p->pDirty = pPage->pDirtyNext;assert( p->bPurgeable || p->eCreate==2 );if( p->pDirty==0 ){         /*OPTIMIZATION-IF-TRUE*/assert( p->bPurgeable==0 || p->eCreate==1 );//没有脏页的情况下，p->eCreate被设为2p->eCreate = 2;}}pPage->pDirtyNext = 0;pPage->pDirtyPrev = 0;}//在链表头部插入新的页面if( addRemove & PCACHE_DIRTYLIST_ADD ){assert( pPage->pDirtyNext==0 && pPage->pDirtyPrev==0 && p->pDirty!=pPage );pPage->pDirtyNext = p->pDirty;if( pPage->pDirtyNext ){assert( pPage->pDirtyNext->pDirtyPrev==0 );//让上一个节点指向下一个节点pPage->pDirtyNext->pDirtyPrev = pPage;
}else{//如果是第一个节点，那么添加尾部p->pDirtyTail = pPage;if( p->bPurgeable ){assert( p->eCreate==2 );//有脏页存在时，p->eCreate置1p->eCreate = 1;}
}//更新链表头部p->pDirty = pPage;/* If pSynced is NULL and this page has a clear NEED_SYNC flag, set** pSynced to point to it. Checking the NEED_SYNC flag is an ** optimization, as if pSynced points to a page with the NEED_SYNC** flag set sqlite3PcacheFetchStress() searches through all newer ** entries of the dirty-list for a page with NEED_SYNC clear anyway.  */if( !p->pSynced && 0==(pPage->flags&PGHDR_NEED_SYNC)   /*OPTIMIZATION-IF-FALSE*/
){// p->pSynced是一个标记页，用来快速查找最新的已被同步的页p->pSynced = pPage;}}pcacheDump(p);
}

通过sqlite3PcacheFetch()来实现页面的读取：

sqlite3_pcache_page *sqlite3PcacheFetch(PCache *pCache,       /* Obtain the page from this cache */Pgno pgno,            /* Page number to obtain */// createFlag传入的值是0或3(即二进制11)int createFlag        /* If true, create page if it does not exist already */
){int eCreate;sqlite3_pcache_page *pRes;assert( pCache!=0 );assert( pCache->pCache!=0 );assert( createFlag==3 || createFlag==0 );//见第一个真值表第3行assert( pCache->eCreate==((pCache->bPurgeable && pCache->pDirty) ? 1 : 2) );//对于eCreate的具体处理见上一篇文章/* eCreate defines what to do if the page does not exist.**    0     Do not allocate a new page.  (createFlag==0)**    1     Allocate a new page if doing so is inexpensive.**          (createFlag==1 AND bPurgeable AND pDirty)**    2     Allocate a new page even it doing so is difficult.**          (createFlag==1 AND !(bPurgeable AND pDirty)*//*上面的注释的意思是说如果cache slot可回收，并且存在脏页的情况下，**如果缓存页的数量达到最大时需要预留一些slot，不再回收或创建新的**缓存页*///见第2个真值表eCreate = createFlag & pCache->eCreate;assert( eCreate==0 || eCreate==1 || eCreate==2 );assert( createFlag==0 || pCache->eCreate==eCreate );//即eCreate==1+!(pCache->bPurgeable&&pCache->pDirty)//即bPurgeable和pDirty都满足的情况下，eCreate是1assert( createFlag==0 || eCreate==1+(!pCache->bPurgeable||!pCache->pDirty) );pRes = sqlite3GlobalConfig.pcache2.xFetch(pCache->pCache, pgno, eCreate);pcacheTrace(("%p.FETCH %d%s (result: %p)\n",pCache,pgno,createFlag?" create":"",pRes));return pRes;
}