pcache lru 注释
parent
6044cf0556
commit
c08c2c1620
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@ -167,13 +167,10 @@ char *sqlite3_temp_directory = 0;
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char *sqlite3_data_directory = 0;
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/*
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** Initialize SQLite.
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** 初始化 SQLITE3 入口
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**
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** This routine must be called to initialize the memory allocation,
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** VFS, and mutex subsystems prior to doing any serious work with
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** SQLite. But as long as you do not compile with SQLITE_OMIT_AUTOINIT
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** this routine will be called automatically by key routines such as
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** sqlite3_open().
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** 由 SQLITE_OMIT_AUTOINIT 宏控制
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** 初始化 内存，VFS，mutex 子系统
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**
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** This routine is a no-op except on its very first call for the process,
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** or for the first call after a call to sqlite3_shutdown.
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15
src/pcache.c
15
src/pcache.c
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@ -39,7 +39,7 @@
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** pointers).
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*/
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struct PCache {
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PgHdr *pDirty, *pDirtyTail; /* List of dirty pages in LRU order */
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PgHdr *pDirty, *pDirtyTail; /* LRU 顺序排列的脏页 */
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PgHdr *pSynced; /* Last synced page in dirty page list */
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i64 nRefSum; /* Sum of ref counts over all pages */
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int szCache; /* Configured cache size */
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@ -47,10 +47,10 @@ struct PCache {
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int szPage; /* Size of every page in this cache */
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int szExtra; /* Size of extra space for each page */
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u8 bPurgeable; /* True if pages are on backing store */
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u8 eCreate; /* eCreate value for for xFetch() */
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u8 eCreate; /* xFetch 函数使用，是否创建页面 */
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int (*xStress)(void*,PgHdr*); /* Call to try make a page clean */
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void *pStress; /* Argument to xStress */
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sqlite3_pcache *pCache; /* Pluggable cache module */
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sqlite3_pcache *pCache; /* 可插拔的 cache 模块实现 */
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};
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/********************************** Test and Debug Logic **********************/
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@ -403,17 +403,18 @@ sqlite3_pcache_page *sqlite3PcacheFetch(
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assert( createFlag==3 || createFlag==0 );
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assert( pCache->eCreate==((pCache->bPurgeable && pCache->pDirty) ? 1 : 2) );
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/* eCreate defines what to do if the page does not exist.
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** 0 Do not allocate a new page. (createFlag==0)
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** 1 Allocate a new page if doing so is inexpensive.
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/* eCreate 定义页面不存在时的操作
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** 0 不分配新页面. (createFlag==0)
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** 1 开销较小时，分配新页面.
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** (createFlag==1 AND bPurgeable AND pDirty)
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** 2 Allocate a new page even it doing so is difficult.
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** 2 分配新页面.
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** (createFlag==1 AND !(bPurgeable AND pDirty)
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*/
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eCreate = createFlag & pCache->eCreate;
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assert( eCreate==0 || eCreate==1 || eCreate==2 );
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assert( createFlag==0 || pCache->eCreate==eCreate );
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assert( createFlag==0 || eCreate==1+(!pCache->bPurgeable||!pCache->pDirty) );
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/* 调用 pcache1.c 中的 pcache1Fetch 函数 */
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pRes = sqlite3GlobalConfig.pcache2.xFetch(pCache->pCache, pgno, eCreate);
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pcacheTrace(("%p.FETCH %d%s (result: %p) ",pCache,pgno,
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createFlag?" create":"",pRes));
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@ -119,16 +119,16 @@ struct PgHdr1 {
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unsigned int iKey; /* Key value (page number) */
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u16 isBulkLocal; /* This page from bulk local storage */
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u16 isAnchor; /* This is the PGroup.lru element */
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PgHdr1 *pNext; /* Next in hash table chain */
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PCache1 *pCache; /* Cache that currently owns this page */
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PgHdr1 *pLruNext; /* Next in circular LRU list of unpinned pages */
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PgHdr1 *pLruPrev; /* Previous in LRU list of unpinned pages */
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PgHdr1 *pNext; /* Hash 表中的 next */
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PCache1 *pCache; /* 当前页面所属的 cache */
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PgHdr1 *pLruNext; /* LRU 链表中的 next */
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PgHdr1 *pLruPrev; /* LRU 链表中的 prev */
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/* NB: pLruPrev is only valid if pLruNext!=0 */
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};
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/*
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** A page is pinned if it is not on the LRU list. To be "pinned" means
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** that the page is in active use and must not be deallocated.
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** PINNED 页面在使用中，不会被释放，即不在 LRU 链表中
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** UNPINNED 页面不在使用中，即在 LRU 链表中，如果可能可以被替换掉
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*/
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#define PAGE_IS_PINNED(p) ((p)->pLruNext==0)
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#define PAGE_IS_UNPINNED(p) ((p)->pLruNext!=0)
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@ -160,7 +160,7 @@ struct PGroup {
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unsigned int nMaxPage; /* Sum of nMax for purgeable caches */
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unsigned int nMinPage; /* Sum of nMin for purgeable caches */
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unsigned int mxPinned; /* nMaxpage + 10 - nMinPage */
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unsigned int nPurgeable; /* Number of purgeable pages allocated */
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unsigned int nPurgeable; /* 可替换页的数量 */
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PgHdr1 lru; /* The beginning and end of the LRU list */
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};
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@ -180,11 +180,11 @@ struct PCache1 {
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** The PGroup mutex must be held when accessing nMax.
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*/
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PGroup *pGroup; /* PGroup this cache belongs to */
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unsigned int *pnPurgeable; /* Pointer to pGroup->nPurgeable */
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int szPage; /* Size of database content section */
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unsigned int *pnPurgeable; /* 指向 pGroup->nPurgeable 总可替换页数量 */
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int szPage; /* 页面大小 */
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int szExtra; /* sizeof(MemPage)+sizeof(PgHdr) */
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int szAlloc; /* Total size of one pcache line */
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int bPurgeable; /* True if cache is purgeable */
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int szAlloc; /* 总大小，包含页面和元数据 */
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int bPurgeable; /* 页面是否能被重用，即淘汰替换页内容 */
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unsigned int nMin; /* Minimum number of pages reserved */
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unsigned int nMax; /* Configured "cache_size" value */
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unsigned int n90pct; /* nMax*9/10 */
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@ -194,10 +194,10 @@ struct PCache1 {
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/* Hash table of all pages. The following variables may only be accessed
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** when the accessor is holding the PGroup mutex.
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*/
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unsigned int nRecyclable; /* Number of pages in the LRU list */
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unsigned int nPage; /* Total number of pages in apHash */
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unsigned int nHash; /* Number of slots in apHash[] */
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PgHdr1 **apHash; /* Hash table for fast lookup by key */
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unsigned int nRecyclable; /* LRU 链表中剩余的节点 */
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unsigned int nPage; /* Hash 表中的节点数量，即所有页数量 */
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unsigned int nHash; /* Hash 表 slot 数量 */
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PgHdr1 **apHash; /* Hash 表用于快速查找 key */
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PgHdr1 *pFree; /* List of unused pcache-local pages */
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void *pBulk; /* Bulk memory used by pcache-local */
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};
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@ -211,7 +211,7 @@ struct PgFreeslot {
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};
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/*
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** Global data used by this cache.
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** cache 使用的全局变量 - pcache1
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*/
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static SQLITE_WSD struct PCacheGlobal {
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PGroup grp; /* The global PGroup for mode (2) */
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@ -427,7 +427,7 @@ static int pcache1MemSize(void *p){
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#endif /* SQLITE_ENABLE_MEMORY_MANAGEMENT */
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/*
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** Allocate a new page object initially associated with cache pCache.
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** 分配新页面到 cache 中
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*/
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static PgHdr1 *pcache1AllocPage(PCache1 *pCache, int benignMalloc){
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PgHdr1 *p = 0;
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@ -541,6 +541,7 @@ static void pcache1ResizeHash(PCache1 *p){
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assert( sqlite3_mutex_held(p->pGroup->mutex) );
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/* 将 slot 扩大 2 倍 */
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nNew = p->nHash*2;
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if( nNew<256 ){
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nNew = 256;
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@ -551,6 +552,7 @@ static void pcache1ResizeHash(PCache1 *p){
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apNew = (PgHdr1 **)sqlite3MallocZero(sizeof(PgHdr1 *)*nNew);
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if( p->nHash ){ sqlite3EndBenignMalloc(); }
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pcache1EnterMutex(p->pGroup);
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/* 拷贝到新 hash table */
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if( apNew ){
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for(i=0; i<p->nHash; i++){
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PgHdr1 *pPage;
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@ -880,8 +882,10 @@ static SQLITE_NOINLINE PgHdr1 *pcache1FetchStage2(
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PGroup *pGroup = pCache->pGroup;
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PgHdr1 *pPage = 0;
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/* Step 3: Abort if createFlag is 1 but the cache is nearly full */
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/* Step 3: cache 已经快满了，但是任然请求分配新页，直接 abort */
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/* 控制内存申请 */
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assert( pCache->nPage >= pCache->nRecyclable );
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/* nPinned: 使用中的页面 */
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nPinned = pCache->nPage - pCache->nRecyclable;
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assert( pGroup->mxPinned == pGroup->nMaxPage + 10 - pGroup->nMinPage );
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assert( pCache->n90pct == pCache->nMax*9/10 );
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@ -892,11 +896,11 @@ static SQLITE_NOINLINE PgHdr1 *pcache1FetchStage2(
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)){
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return 0;
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}
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/* hash 优化，如果 hash 中节点数量比 slot 多，做一次 resize */
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if( pCache->nPage>=pCache->nHash ) pcache1ResizeHash(pCache);
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assert( pCache->nHash>0 && pCache->apHash );
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/* Step 4. Try to recycle a page. */
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/* Step 4. page 是否能够重用 */
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if( pCache->bPurgeable
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&& !pGroup->lru.pLruPrev->isAnchor
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&& ((pCache->nPage+1>=pCache->nMax) || pcache1UnderMemoryPressure(pCache))
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@ -915,8 +919,7 @@ static SQLITE_NOINLINE PgHdr1 *pcache1FetchStage2(
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}
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}
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/* Step 5. If a usable page buffer has still not been found,
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** attempt to allocate a new one.
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/* Step 5. 任然没有 page 可用，申请新页
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*/
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if( !pPage ){
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pPage = pcache1AllocPage(pCache, createFlag==1);
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@ -941,14 +944,14 @@ static SQLITE_NOINLINE PgHdr1 *pcache1FetchStage2(
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}
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/*
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** Implementation of the sqlite3_pcache.xFetch method.
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** sqlite3_pcache.xFetch 方法的实现.
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**
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** Fetch a page by key value.
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** 通过 key 值获取页面.
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**
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** Whether or not a new page may be allocated by this function depends on
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** the value of the createFlag argument. 0 means do not allocate a new
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** page. 1 means allocate a new page if space is easily available. 2
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** means to try really hard to allocate a new page.
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** 由 createFlag 控制是否在 cache 中分配新页
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** 0 不申请
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** 1 如果分配空间算法比较容易，申请
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** 2 申请
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**
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** For a non-purgeable cache (a cache used as the storage for an in-memory
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** database) there is really no difference between createFlag 1 and 2. So
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@ -994,10 +997,9 @@ static SQLITE_NOINLINE PgHdr1 *pcache1FetchStage2(
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**
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** 5. Otherwise, allocate and return a new page buffer.
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**
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** There are two versions of this routine. pcache1FetchWithMutex() is
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** the general case. pcache1FetchNoMutex() is a faster implementation for
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** the common case where pGroup->mutex is NULL. The pcache1Fetch() wrapper
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** invokes the appropriate routine.
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** 由函数 pcache1Fetch 选择调用路径：
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** pcache1FetchWithMutex()
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** pcache1FetchNoMutex() pGroup->mutex == NULL
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*/
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static PgHdr1 *pcache1FetchNoMutex(
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sqlite3_pcache *p,
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@ -1007,14 +1009,14 @@ static PgHdr1 *pcache1FetchNoMutex(
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PCache1 *pCache = (PCache1 *)p;
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PgHdr1 *pPage = 0;
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/* Step 1: Search the hash table for an existing entry. */
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/* Step 1: 查找 hash 表，key 是否存在 */
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pPage = pCache->apHash[iKey % pCache->nHash];
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while( pPage && pPage->iKey!=iKey ){ pPage = pPage->pNext; }
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/* Step 2: If the page was found in the hash table, then return it.
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** If the page was not in the hash table and createFlag is 0, abort.
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** Otherwise (page not in hash and createFlag!=0) continue with
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** subsequent steps to try to create the page. */
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/* Step 2: 查找到该页，直接返回。]
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** 页面不存在且不能分配，直接 abort
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** 否则分配新页面
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*/
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if( pPage ){
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if( PAGE_IS_UNPINNED(pPage) ){
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return pcache1PinPage(pPage);
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