TransactionIdIsInProgress can skip scanning the ProcArray if the target XID is

later than latestCompletedXid, per Florian Pflug.  Also some minor
improvements in the XIDCACHE_DEBUG code --- make sure each call of
TransactionIdIsInProgress is counted one way or another.

diff --git a/src/backend/storage/ipc/procarray.c b/src/backend/storage/ipc/procarray.c
index dbac4a8fd05de662eda005deb1a8bffc98253c3c..90ba09d5a881d87b9b4fcb238c4f72014166c805 100644 (file)
--- a/src/backend/storage/ipc/procarray.c
+++ b/src/backend/storage/ipc/procarray.c
@@ -23,7 +23,7 @@
  *
  *
  * IDENTIFICATION
- *       $PostgreSQL: pgsql/src/backend/storage/ipc/procarray.c,v 1.34 2007/09/21 17:36:53 tgl Exp $
+ *       $PostgreSQL: pgsql/src/backend/storage/ipc/procarray.c,v 1.35 2007/09/23 18:50:38 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -61,14 +61,18 @@ static ProcArrayStruct *procArray;
 /* counters for XidCache measurement */
 static long xc_by_recent_xmin = 0;
 static long xc_by_my_xact = 0;
+static long xc_by_latest_xid = 0;
 static long xc_by_main_xid = 0;
 static long xc_by_child_xid = 0;
+static long xc_no_overflow = 0;
 static long xc_slow_answer = 0;
 
 #define xc_by_recent_xmin_inc()                (xc_by_recent_xmin++)
 #define xc_by_my_xact_inc()                    (xc_by_my_xact++)
+#define xc_by_latest_xid_inc()         (xc_by_latest_xid++)
 #define xc_by_main_xid_inc()           (xc_by_main_xid++)
 #define xc_by_child_xid_inc()          (xc_by_child_xid++)
+#define xc_no_overflow_inc()           (xc_no_overflow++)
 #define xc_slow_answer_inc()           (xc_slow_answer++)
 
 static void DisplayXidCache(void);
@@ -76,8 +80,10 @@ static void DisplayXidCache(void);
 
 #define xc_by_recent_xmin_inc()                ((void) 0)
 #define xc_by_my_xact_inc()                    ((void) 0)
+#define xc_by_latest_xid_inc()         ((void) 0)
 #define xc_by_main_xid_inc()           ((void) 0)
 #define xc_by_child_xid_inc()          ((void) 0)
+#define xc_no_overflow_inc()           ((void) 0)
 #define xc_slow_answer_inc()           ((void) 0)
 #endif   /* XIDCACHE_DEBUG */
 
@@ -302,7 +308,8 @@ ProcArrayClearTransaction(PGPROC *proc)
 /*
  * TransactionIdIsInProgress -- is given transaction running in some backend
  *
- * There are three possibilities for finding a running transaction:
+ * Aside from some shortcuts such as checking RecentXmin and our own Xid,
+ * there are three possibilities for finding a running transaction:
  *
  * 1. the given Xid is a main transaction Id.  We will find this out cheaply
  * by looking at the PGPROC struct for each backend.
@@ -368,6 +375,18 @@ TransactionIdIsInProgress(TransactionId xid)
 
        LWLockAcquire(ProcArrayLock, LW_SHARED);
 
+       /*
+        * Now that we have the lock, we can check latestCompletedXid; if the
+        * target Xid is after that, it's surely still running.
+        */
+       if (TransactionIdPrecedes(ShmemVariableCache->latestCompletedXid, xid))
+       {
+               LWLockRelease(ProcArrayLock);
+               xc_by_latest_xid_inc();
+               return true;
+       }
+
+       /* No shortcuts, gotta grovel through the array */
        for (i = 0; i < arrayP->numProcs; i++)
        {
                volatile PGPROC    *proc = arrayP->procs[i];
@@ -434,7 +453,10 @@ TransactionIdIsInProgress(TransactionId xid)
         * running without looking at pg_subtrans.
         */
        if (nxids == 0)
+       {
+               xc_no_overflow_inc();
                return false;
+       }
 
        /*
         * Step 3: have to check pg_subtrans.
@@ -451,8 +473,8 @@ TransactionIdIsInProgress(TransactionId xid)
 
        /*
         * It isn't aborted, so check whether the transaction tree it belongs to
-        * is still running (or, more precisely, whether it was running when this
-        * routine started -- note that we already released ProcArrayLock).
+        * is still running (or, more precisely, whether it was running when
+        * we held ProcArrayLock).
         */
        topxid = SubTransGetTopmostTransaction(xid);
        Assert(TransactionIdIsValid(topxid));
@@ -1300,11 +1322,13 @@ static void
 DisplayXidCache(void)
 {
        fprintf(stderr,
-                       "XidCache: xmin: %ld, myxact: %ld, mainxid: %ld, childxid: %ld, slow: %ld\n",
+                       "XidCache: xmin: %ld, myxact: %ld, latest: %ld, mainxid: %ld, childxid: %ld, nooflo: %ld, slow: %ld\n",
                        xc_by_recent_xmin,
                        xc_by_my_xact,
+                       xc_by_latest_xid,
                        xc_by_main_xid,
                        xc_by_child_xid,
+                       xc_no_overflow,
                        xc_slow_answer);
 }