1 files changed, 107 insertions, 0 deletions
diff --git a/ext/misc/mmapwarm.c b/ext/misc/mmapwarm.c
new file mode 100644
index 0000000..5afa47b
--- /dev/null
+++ b/ext/misc/mmapwarm.c
@@ -0,0 +1,107 @@
+/*
+** 2017-09-18
+**
+** The author disclaims copyright to this source code.  In place of
+** a legal notice, here is a blessing:
+**
+**    May you do good and not evil.
+**    May you find forgiveness for yourself and forgive others.
+**    May you share freely, never taking more than you give.
+**
+*************************************************************************
+**
+*/
+
+#include "sqlite3.h"
+
+
+/*
+** This function is used to touch each page of a mapping of a memory
+** mapped SQLite database. Assuming that the system has sufficient free
+** memory and supports sufficiently large mappings, this causes the OS 
+** to cache the entire database in main memory, making subsequent 
+** database accesses faster.
+**
+** If the second parameter to this function is not NULL, it is the name of
+** the specific database to operate on (i.e. "main" or the name of an
+** attached database).
+**
+** SQLITE_OK is returned if successful, or an SQLite error code otherwise.
+** It is not considered an error if the file is not memory-mapped, or if
+** the mapping does not span the entire file. If an error does occur, a
+** transaction may be left open on the database file.
+**
+** It is illegal to call this function when the database handle has an 
+** open transaction. SQLITE_MISUSE is returned in this case.
+*/
+int sqlite3_mmap_warm(sqlite3 *db, const char *zDb){
+  int rc = SQLITE_OK;
+  char *zSql = 0;
+  int pgsz = 0;
+  int nTotal = 0;
+
+  if( 0==sqlite3_get_autocommit(db) ) return SQLITE_MISUSE;
+
+  /* Open a read-only transaction on the file in question */
+  zSql = sqlite3_mprintf("BEGIN; SELECT * FROM %s%q%ssqlite_schema", 
+      (zDb ? "'" : ""), (zDb ? zDb : ""), (zDb ? "'." : "")
+  );
+  if( zSql==0 ) return SQLITE_NOMEM;
+  rc = sqlite3_exec(db, zSql, 0, 0, 0);
+  sqlite3_free(zSql);
+
+  /* Find the SQLite page size of the file */
+  if( rc==SQLITE_OK ){
+    zSql = sqlite3_mprintf("PRAGMA %s%q%spage_size", 
+        (zDb ? "'" : ""), (zDb ? zDb : ""), (zDb ? "'." : "")
+    );
+    if( zSql==0 ){
+      rc = SQLITE_NOMEM;
+    }else{
+      sqlite3_stmt *pPgsz = 0;
+      rc = sqlite3_prepare_v2(db, zSql, -1, &pPgsz, 0);
+      sqlite3_free(zSql);
+      if( rc==SQLITE_OK ){
+        if( sqlite3_step(pPgsz)==SQLITE_ROW ){
+          pgsz = sqlite3_column_int(pPgsz, 0);
+        }
+        rc = sqlite3_finalize(pPgsz);
+      }
+      if( rc==SQLITE_OK && pgsz==0 ){
+        rc = SQLITE_ERROR;
+      }
+    }
+  }
+
+  /* Touch each mmap'd page of the file */
+  if( rc==SQLITE_OK ){
+    int rc2;
+    sqlite3_file *pFd = 0;
+    rc = sqlite3_file_control(db, zDb, SQLITE_FCNTL_FILE_POINTER, &pFd);
+    if( rc==SQLITE_OK && pFd->pMethods->iVersion>=3 ){
+      sqlite3_int64 iPg = 1;
+      sqlite3_io_methods const *p = pFd->pMethods;
+      while( 1 ){
+        unsigned char *pMap;
+        rc = p->xFetch(pFd, pgsz*iPg, pgsz, (void**)&pMap);
+        if( rc!=SQLITE_OK || pMap==0 ) break;
+
+        nTotal += pMap[0];
+        nTotal += pMap[pgsz-1];
+
+        rc = p->xUnfetch(pFd, pgsz*iPg, (void*)pMap);
+        if( rc!=SQLITE_OK ) break;
+        iPg++;
+      }
+      sqlite3_log(SQLITE_OK, 
+          "sqlite3_mmap_warm_cache: Warmed up %d pages of %s", iPg==1?0:iPg,
+          sqlite3_db_filename(db, zDb)
+      );
+    }
+
+    rc2 = sqlite3_exec(db, "END", 0, 0, 0);
+    if( rc==SQLITE_OK ) rc = rc2;
+  }
+
+  return rc;
+}