Static snapshot data - Mailing list pgsql-patches

Up to now, SerializableSnapshot and QuerySnapshot are malloc'ed and
free'd for every transaction or statement, respectively.  This patch
puts these data structures into static memory, thus saving a few CPU
cycles and two malloc calls per transaction or (in isolation level
READ COMMITTED) per query.

Servus
 Manfred
diff -ruN ../base/src/backend/storage/ipc/sinval.c src/backend/storage/ipc/sinval.c
--- ../base/src/backend/storage/ipc/sinval.c    2003-04-04 15:45:37.000000000 +0200
+++ src/backend/storage/ipc/sinval.c    2003-05-08 21:39:15.000000000 +0200
@@ -305,9 +305,8 @@
  *----------
  */
 Snapshot
-GetSnapshotData(bool serializable)
+GetSnapshotData(Snapshot snapshot, bool serializable)
 {
-    Snapshot    snapshot = (Snapshot) malloc(sizeof(SnapshotData));
     SISeg       *segP = shmInvalBuffer;
     ProcState  *stateP = segP->procState;
     TransactionId xmin;
@@ -316,18 +315,29 @@
     int            index;
     int            count = 0;

-    if (snapshot == NULL)
-        elog(ERROR, "Memory exhausted in GetSnapshotData");
+    Assert(snapshot != NULL);

     /*
      * Allocating space for MaxBackends xids is usually overkill;
      * lastBackend would be sufficient.  But it seems better to do the
      * malloc while not holding the lock, so we can't look at lastBackend.
+     *
+     * if (snapshot->xip != NULL)
+     *     no need to free and reallocate xip;
+     *
+     * We can reuse the old xip array, because MaxBackends does not change
+     * at runtime.
      */
-    snapshot->xip = (TransactionId *)
-        malloc(MaxBackends * sizeof(TransactionId));
     if (snapshot->xip == NULL)
-        elog(ERROR, "Memory exhausted in GetSnapshotData");
+    {
+        /*
+         * First call for this snapshot
+         */
+        snapshot->xip = (TransactionId *)
+            malloc(MaxBackends * sizeof(TransactionId));
+        if (snapshot->xip == NULL)
+            elog(ERROR, "Memory exhausted in GetSnapshotData");
+    }

     globalxmin = xmin = GetCurrentTransactionId();

diff -ruN ../base/src/backend/utils/time/tqual.c src/backend/utils/time/tqual.c
--- ../base/src/backend/utils/time/tqual.c    2003-04-04 15:45:45.000000000 +0200
+++ src/backend/utils/time/tqual.c    2003-05-08 21:45:55.000000000 +0200
@@ -30,6 +30,8 @@
 static SnapshotData SnapshotDirtyData;
 Snapshot    SnapshotDirty = &SnapshotDirtyData;

+static SnapshotData QuerySnapshotData;
+static SnapshotData SerializableSnapshotData;
 Snapshot    QuerySnapshot = NULL;
 Snapshot    SerializableSnapshot = NULL;

@@ -941,23 +943,16 @@
     /* 1st call in xaction? */
     if (SerializableSnapshot == NULL)
     {
-        SerializableSnapshot = GetSnapshotData(true);
+        SerializableSnapshot = GetSnapshotData(&SerializableSnapshotData, true);
         QuerySnapshot = SerializableSnapshot;
         Assert(QuerySnapshot != NULL);
         return;
     }

-    if (QuerySnapshot != SerializableSnapshot)
-    {
-        free(QuerySnapshot->xip);
-        free(QuerySnapshot);
-        QuerySnapshot = NULL;
-    }
-
     if (XactIsoLevel == XACT_SERIALIZABLE)
         QuerySnapshot = SerializableSnapshot;
     else
-        QuerySnapshot = GetSnapshotData(false);
+        QuerySnapshot = GetSnapshotData(&QuerySnapshotData, false);

     Assert(QuerySnapshot != NULL);
 }
@@ -1003,19 +998,11 @@
 void
 FreeXactSnapshot(void)
 {
-    if (QuerySnapshot != NULL && QuerySnapshot != SerializableSnapshot)
-    {
-        free(QuerySnapshot->xip);
-        free(QuerySnapshot);
-    }
-
+    /*
+     * We do not free(QuerySnapshot->xip);
+     *        or free(SerializableSnapshot->xip);
+     * they will be reused soon
+     */
     QuerySnapshot = NULL;
-
-    if (SerializableSnapshot != NULL)
-    {
-        free(SerializableSnapshot->xip);
-        free(SerializableSnapshot);
-    }
-
     SerializableSnapshot = NULL;
 }
diff -ruN ../base/src/include/utils/tqual.h src/include/utils/tqual.h
--- ../base/src/include/utils/tqual.h    2003-04-04 15:45:50.000000000 +0200
+++ src/include/utils/tqual.h    2003-05-08 21:40:04.000000000 +0200
@@ -113,7 +113,7 @@
 extern HTSV_Result HeapTupleSatisfiesVacuum(HeapTupleHeader tuple,
                          TransactionId OldestXmin);

-extern Snapshot GetSnapshotData(bool serializable);
+extern Snapshot GetSnapshotData(Snapshot snapshot, bool serializable);
 extern void SetQuerySnapshot(void);
 extern Snapshot CopyQuerySnapshot(void);
 extern void FreeXactSnapshot(void);