Re: Static snapshot data - Mailing list pgsql-patches

After lots of discussion, it seems this is to be applied.

Your patch has been added to the PostgreSQL unapplied patches list at:

    http://momjian.postgresql.org/cgi-bin/pgpatches

I will try to apply it within the next 48 hours.

---------------------------------------------------------------------------


Manfred Koizar wrote:
> 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);

>
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