Change the pcache module to keep track of the total number of references to

all pages rather than the number of pages references, for a performance
improvement and size reduction.

FossilOrigin-Name: f00a9e1e998c4bd249a45444dc2d71a7e4903b8b

manifest

@@ -1,5 +1,5 @@
-C A\ssimple\soptimization\sand\ssize\sreduction\sin\ssqlite3PagerAcquire().
-D 2015-09-03T18:20:10.962
+C Change\sthe\spcache\smodule\sto\skeep\strack\sof\sthe\stotal\snumber\sof\sreferences\sto\nall\spages\srather\sthan\sthe\snumber\sof\spages\sreferences,\sfor\sa\sperformance\nimprovement\sand\ssize\sreduction.
+D 2015-09-03T20:43:55.204
 F Makefile.arm-wince-mingw32ce-gcc d6df77f1f48d690bd73162294bbba7f59507c72f
 F Makefile.in f85066ce844a28b671aaeeff320921cd0ce36239
 F Makefile.linux-gcc 91d710bdc4998cb015f39edf3cb314ec4f4d7e23
@@ -324,12 +324,12 @@ F src/os_setup.h c9d4553b5aaa6f73391448b265b89bed0b890faa
 F src/os_unix.c 76f493ed71c4154338049dee1bf6e47f69c74a55
 F src/os_win.c 40b3af7a47eb1107d0d69e592bec345a3b7b798a
 F src/os_win.h eb7a47aa17b26b77eb97e4823f20a00b8bda12ca
-F src/pager.c 748ff3e183e0a311328af902c0819c6a8e3f22ca
+F src/pager.c 4784012f80b2197c61ff6eaf4f5c7026d93253fd
 F src/pager.h 6d435f563b3f7fcae4b84433b76a6ac2730036e2
 F src/parse.y f599aa5e871a493330d567ced93de696f61f48f7
-F src/pcache.c cde06aa50962595e412d497e22fd2e07878ba1f0
+F src/pcache.c 24be750c79272e0ca7b6e007bc94999700f3e5ef
 F src/pcache.h 9968603796240cdf83da7e7bef76edf90619cea9
-F src/pcache1.c b31af9dbc83b9c68e87d681b8453a9605f28e734
+F src/pcache1.c e1529369c047ac645e6a28196f25b7e936c46b82
 F src/pragma.c d71b813e67bf03f3116b9dd5164fbfd81ec673a2
 F src/pragma.h 631a91c8b0e6ca8f051a1d8a4a0da4150e04620a
 F src/prepare.c 82e5db1013846a819f198336fed72c44c974e7b1
@@ -1380,7 +1380,7 @@ F tool/vdbe_profile.tcl 67746953071a9f8f2f668b73fe899074e2c6d8c1
 F tool/warnings-clang.sh f6aa929dc20ef1f856af04a730772f59283631d4
 F tool/warnings.sh 48bd54594752d5be3337f12c72f28d2080cb630b
 F tool/win/sqlite.vsix deb315d026cc8400325c5863eef847784a219a2f
-P b79096be7cb02aae2f303db33a8bf19e69204374
-R d1173d7149bc899e819d5c3a3bb552ec
+P 618d8dd4ff44cce10cc4688a2134715ff66cc562
+R 7304361de7c7fa82f683343fecc699dd
 U drh
-Z 442809bedf9761a63ee7744b080363fa
+Z 8da3824804c6c5593515c7564fca1c5b

manifest.uuid

@@ -1 +1 @@
-618d8dd4ff44cce10cc4688a2134715ff66cc562
+f00a9e1e998c4bd249a45444dc2d71a7e4903b8b

src/pager.c

@@ -5261,6 +5261,10 @@ int sqlite3PagerAcquire(
 #endif
   );
 
+  /* Optimization note:  Adding the "pgno<=1" term before "pgno==0" here
+  ** allows the compiler optimizer to reuse the results of the "pgno>1"
+  ** test in the previous statement, and avoid testing pgno==0 in the
+  ** common case where pgno is large. */
   if( pgno<=1 && pgno==0 ){
     return SQLITE_CORRUPT_BKPT;
   }
@@ -6390,7 +6394,7 @@ u8 sqlite3PagerIsreadonly(Pager *pPager){
 
 #ifdef SQLITE_DEBUG
 /*
-** Return the number of references to the pager.
+** Return the sum of the reference counts for all pages held by pPager.
 */
 int sqlite3PagerRefcount(Pager *pPager){
   return sqlite3PcacheRefCount(pPager->pPCache);

src/pcache.c

@@ -19,7 +19,7 @@
 struct PCache {
   PgHdr *pDirty, *pDirtyTail;         /* List of dirty pages in LRU order */
   PgHdr *pSynced;                     /* Last synced page in dirty page list */
-  int nRef;                           /* Number of referenced pages */
+  int nRefSum;                        /* Sum of ref counts over all pages */
   int szCache;                        /* Configured cache size */
   int szPage;                         /* Size of every page in this cache */
   int szExtra;                        /* Size of extra space for each page */
@@ -184,7 +184,7 @@ int sqlite3PcacheOpen(
 ** are no outstanding page references when this function is called.
 */
 int sqlite3PcacheSetPageSize(PCache *pCache, int szPage){
-  assert( pCache->nRef==0 && pCache->pDirty==0 );
+  assert( pCache->nRefSum==0 && pCache->pDirty==0 );
   if( pCache->szPage ){
     sqlite3_pcache *pNew;
     pNew = sqlite3GlobalConfig.pcache2.xCreate(
@@ -351,9 +351,7 @@ PgHdr *sqlite3PcacheFetchFinish(
   if( !pPgHdr->pPage ){
     return pcacheFetchFinishWithInit(pCache, pgno, pPage);
   }
-  if( 0==pPgHdr->nRef ){
-    pCache->nRef++;
-  }
+  pCache->nRefSum++;
   pPgHdr->nRef++;
   return pPgHdr;
 }
@@ -364,9 +362,8 @@ PgHdr *sqlite3PcacheFetchFinish(
 */
 void SQLITE_NOINLINE sqlite3PcacheRelease(PgHdr *p){
   assert( p->nRef>0 );
-  p->nRef--;
-  if( p->nRef==0 ){
-    p->pCache->nRef--;
+  p->pCache->nRefSum--;
+  if( (--p->nRef)==0 ){
     if( p->flags&PGHDR_CLEAN ){
       pcacheUnpin(p);
     }else if( p->pDirtyPrev!=0 ){
@@ -382,6 +379,7 @@ void SQLITE_NOINLINE sqlite3PcacheRelease(PgHdr *p){
 void sqlite3PcacheRef(PgHdr *p){
   assert(p->nRef>0);
   p->nRef++;
+  p->pCache->nRefSum++;
 }
 
 /*
@@ -394,7 +392,7 @@ void sqlite3PcacheDrop(PgHdr *p){
   if( p->flags&PGHDR_DIRTY ){
     pcacheManageDirtyList(p, PCACHE_DIRTYLIST_REMOVE);
   }
-  p->pCache->nRef--;
+  p->pCache->nRefSum--;
   sqlite3GlobalConfig.pcache2.xUnpin(p->pCache->pCache, p->pPage, 1);
 }
 
@@ -490,11 +488,11 @@ void sqlite3PcacheTruncate(PCache *pCache, Pgno pgno){
         sqlite3PcacheMakeClean(p);
       }
     }
-    if( pgno==0 && pCache->nRef ){
+    if( pgno==0 && pCache->nRefSum ){
       sqlite3_pcache_page *pPage1;
       pPage1 = sqlite3GlobalConfig.pcache2.xFetch(pCache->pCache,1,0);
       if( ALWAYS(pPage1) ){  /* Page 1 is always available in cache, because
-                             ** pCache->nRef>0 */
+                             ** pCache->nRefSum>0 */
         memset(pPage1->pBuf, 0, pCache->szPage);
         pgno = 1;
       }
@@ -600,10 +598,13 @@ PgHdr *sqlite3PcacheDirtyList(PCache *pCache){
 }
 
 /* 
-** Return the total number of referenced pages held by the cache.
+** Return the total number of references to all pages held by the cache.
+**
+** This is not the total number of pages referenced, but the sum of the
+** reference count for all pages.
 */
 int sqlite3PcacheRefCount(PCache *pCache){
-  return pCache->nRef;
+  return pCache->nRefSum;
 }
 
 /*

src/pcache1.c

@@ -961,7 +961,10 @@ static PgHdr1 *pcache1FetchNoMutex(
   pPage = pCache->apHash[iKey % pCache->nHash];
   while( pPage && pPage->iKey!=iKey ){ pPage = pPage->pNext; }
 
-  /* Step 2: Abort if no existing page is found and createFlag is 0 */
+  /* Step 2: If the page was found in the hash table, then return it.
+  ** If the page was not in the hash table and createFlag is 0, abort.
+  ** Otherwise (page not in hash and createFlag!=0) continue with
+  ** subsequent steps to try to create the page. */
   if( pPage ){
     if( !pPage->isPinned ){
       return pcache1PinPage(pPage);