Remove unnecessary code from malloc.c. Enhance pcache1.c so that is tries

to reuse existing pages, rather than create new pages, when SQLite is under
memory pressure.  "Memory pressure" means that SQLITE_CONFIG_PAGECACHE memory
is nearly exhausted or sqlite3_soft_heap_limit() has been reached.

FossilOrigin-Name: 51049479a8577e03cc353f71f6e13a10c8323d91

1 files changed, 26 insertions, 14 deletions

diff --git a/src/malloc.c b/src/malloc.c
index b36b44f7f..751d9b174 100644
--- a/src/malloc.c
+++ b/src/malloc.c
@@ -97,7 +97,12 @@ static SQLITE_WSD struct Mem0Global {
   ** which pages are available.
   */
   u32 *aScratchFree;
-  u32 *aPageFree;
+
+  /*
+  ** True if heap is nearly "full" where "full" is defined by the
+  ** sqlite3_soft_heap_limit() setting.
+  */
+  int nearlyFull;
 } mem0 = { 0, 0, 0, 0, 0, 0, 0, 0 };
 
 #define mem0 GLOBAL(struct Mem0Global, mem0)
@@ -125,26 +130,25 @@ int sqlite3MallocInit(void){
     sqlite3GlobalConfig.pScratch = 0;
     sqlite3GlobalConfig.szScratch = 0;
   }
-  if( sqlite3GlobalConfig.pPage && sqlite3GlobalConfig.szPage>=512
-      && sqlite3GlobalConfig.nPage>=1 ){
-    int i;
-    int overhead;
-    int sz = ROUNDDOWN8(sqlite3GlobalConfig.szPage);
-    int n = sqlite3GlobalConfig.nPage;
-    overhead = (4*n + sz - 1)/sz;
-    sqlite3GlobalConfig.nPage -= overhead;
-    mem0.aPageFree = (u32*)&((char*)sqlite3GlobalConfig.pPage)
-                  [sqlite3GlobalConfig.szPage*sqlite3GlobalConfig.nPage];
-    for(i=0; i<sqlite3GlobalConfig.nPage; i++){ mem0.aPageFree[i] = i; }
-    mem0.nPageFree = sqlite3GlobalConfig.nPage;
-  }else{
+  if( sqlite3GlobalConfig.pPage==0 || sqlite3GlobalConfig.szPage<512
+      || sqlite3GlobalConfig.nPage<1 ){
     sqlite3GlobalConfig.pPage = 0;
     sqlite3GlobalConfig.szPage = 0;
+    sqlite3GlobalConfig.nPage = 0;
   }
   return sqlite3GlobalConfig.m.xInit(sqlite3GlobalConfig.m.pAppData);
 }
 
 /*
+** Return true if the heap is currently under memory pressure - in other
+** words if the amount of heap used is close to the limit set by
+** sqlite3_soft_heap_limit().
+*/
+int sqlite3HeapNearlyFull(void){
+  return mem0.nearlyFull;
+}
+
+/*
 ** Deinitialize the memory allocation subsystem.
 */
 void sqlite3MallocEnd(void){
@@ -186,10 +190,13 @@ int sqlite3MemoryAlarm(
   void *pArg,
   sqlite3_int64 iThreshold
 ){
+  int nUsed;
   sqlite3_mutex_enter(mem0.mutex);
   mem0.alarmCallback = xCallback;
   mem0.alarmArg = pArg;
   mem0.alarmThreshold = iThreshold;
+  nUsed = sqlite3StatusValue(SQLITE_STATUS_MEMORY_USED);
+  mem0.nearlyFull = (iThreshold>0 && iThreshold<=nUsed);
   sqlite3_mutex_leave(mem0.mutex);
   return SQLITE_OK;
 }
@@ -240,14 +247,19 @@ static int mallocWithAlarm(int n, void **pp){
   if( mem0.alarmCallback!=0 ){
     int nUsed = sqlite3StatusValue(SQLITE_STATUS_MEMORY_USED);
     if( nUsed+nFull >= mem0.alarmThreshold ){
+      mem0.nearlyFull = 1;
       sqlite3MallocAlarm(nFull);
+    }else{
+      mem0.nearlyFull = 0;
     }
   }
   p = sqlite3GlobalConfig.m.xMalloc(nFull);
+#ifdef SQLITE_ENABLE_MEMORY_MANAGEMENT
   if( p==0 && mem0.alarmCallback ){
     sqlite3MallocAlarm(nFull);
     p = sqlite3GlobalConfig.m.xMalloc(nFull);
   }
+#endif
   if( p ){
     nFull = sqlite3MallocSize(p);
     sqlite3StatusAdd(SQLITE_STATUS_MEMORY_USED, nFull);