Even when autovacuum=off, force it for members as we do in other cases.

Thomas Munro, with some adjustments by me.

diff --git a/src/backend/access/transam/multixact.c b/src/backend/access/transam/multixact.c
index be94a87c5c81e4266a3d07d3cb095af005a83417..fc0e4e740044aa1af1c8b2a6d606269225262211 100644 (file)
--- a/src/backend/access/transam/multixact.c
+++ b/src/backend/access/transam/multixact.c
@@ -204,6 +204,7 @@ typedef struct MultiXactStateData
     */
    MultiXactId oldestMultiXactId;
    Oid         oldestMultiXactDB;
+   MultiXactOffset oldestOffset;
 
    /*
     * This is what the previous checkpoint stored as the truncate position.
@@ -954,14 +955,17 @@ GetNewMultiXactId(int nmembers, MultiXactOffset *offset)
     * against catastrophic data loss due to multixact wraparound.  The basic
     * rules are:
     *
-    * If we're past multiVacLimit, start trying to force autovacuum cycles.
+    * If we're past multiVacLimit or the safe threshold for member storage space,
+    * start trying to force autovacuum cycles.
     * If we're past multiWarnLimit, start issuing warnings.
     * If we're past multiStopLimit, refuse to create new MultiXactIds.
     *
     * Note these are pretty much the same protections in GetNewTransactionId.
     *----------
     */
-   if (!MultiXactIdPrecedes(result, MultiXactState->multiVacLimit))
+   if (!MultiXactIdPrecedes(result, MultiXactState->multiVacLimit) ||
+       (MultiXactState->nextOffset - MultiXactState->oldestOffset
+           > MULTIXACT_MEMBER_SAFE_THRESHOLD))
    {
        /*
         * For safety's sake, we release MultiXactGenLock while sending
@@ -2142,6 +2146,8 @@ SetMultiXactIdLimit(MultiXactId oldest_datminmxid, Oid oldest_datoid)
    MultiXactId multiStopLimit;
    MultiXactId multiWrapLimit;
    MultiXactId curMulti;
+   MultiXactOffset oldestOffset;
+   MultiXactOffset nextOffset;
 
    Assert(MultiXactIdIsValid(oldest_datminmxid));
 
@@ -2194,6 +2200,35 @@ SetMultiXactIdLimit(MultiXactId oldest_datminmxid, Oid oldest_datoid)
    if (multiVacLimit < FirstMultiXactId)
        multiVacLimit += FirstMultiXactId;
 
+   /*
+    * Determine the offset of the oldest multixact that might still be
+    * referenced.  Normally, we can read the offset from the multixact itself,
+    * but there's an important special case: if there are no multixacts in
+    * existence at all, oldest_datminmxid obviously can't point to one.  It
+    * will instead point to the multixact ID that will be assigned the next
+    * time one is needed.
+    *
+    * NB: oldest_dataminmxid is the oldest multixact that might still be
+    * referenced from a table, unlike in DetermineSafeOldestOffset, where we
+    * do this same computation based on the oldest value that might still
+    * exist in the SLRU.  This is because here we're trying to compute a
+    * threshold for activating autovacuum, which can only remove references
+    * to multixacts, whereas there we are computing a threshold for creating
+    * new multixacts, which requires the old ones to have first been
+    * truncated away by a checkpoint.
+    */
+   LWLockAcquire(MultiXactGenLock, LW_SHARED);
+   if (MultiXactState->nextMXact == oldest_datminmxid)
+   {
+       oldestOffset = MultiXactState->nextOffset;
+       LWLockRelease(MultiXactGenLock);
+   }
+   else
+   {
+       LWLockRelease(MultiXactGenLock);
+       oldestOffset = find_multixact_start(oldest_datminmxid);
+   }
+
    /* Grab lock for just long enough to set the new limit values */
    LWLockAcquire(MultiXactGenLock, LW_EXCLUSIVE);
    MultiXactState->oldestMultiXactId = oldest_datminmxid;
@@ -2202,7 +2237,9 @@ SetMultiXactIdLimit(MultiXactId oldest_datminmxid, Oid oldest_datoid)
    MultiXactState->multiWarnLimit = multiWarnLimit;
    MultiXactState->multiStopLimit = multiStopLimit;
    MultiXactState->multiWrapLimit = multiWrapLimit;
+   MultiXactState->oldestOffset = oldestOffset;
    curMulti = MultiXactState->nextMXact;
+   nextOffset = MultiXactState->nextOffset;
    LWLockRelease(MultiXactGenLock);
 
    /* Log the info */
@@ -2217,7 +2254,8 @@ SetMultiXactIdLimit(MultiXactId oldest_datminmxid, Oid oldest_datoid)
     * database, it'll call here, and we'll signal the postmaster to start
     * another iteration immediately if there are still any old databases.
     */
-   if (MultiXactIdPrecedes(multiVacLimit, curMulti) &&
+   if ((MultiXactIdPrecedes(multiVacLimit, curMulti) ||
+        (nextOffset - oldestOffset > MULTIXACT_MEMBER_SAFE_THRESHOLD)) &&
        IsUnderPostmaster && !InRecovery)
        SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);
 
@@ -2476,11 +2514,16 @@ DetermineSafeOldestOffset(MultiXactId oldestMXact)
    MultiXactOffset oldestOffset;
 
    /*
-    * We determine the safe upper bound for offsets of new xacts by reading
-    * the offset of the oldest multixact, and going back one segment.  This
-    * way, the sequence of multixact member segments will always have a
-    * one-segment hole at a minimum.  We start spewing warnings a few
-    * complete segments before that.
+    * Determine the offset of the oldest multixact.  Normally, we can read
+    * the offset from the multixact itself, but there's an important special
+    * case: if there are no multixacts in existence at all, oldestMXact
+    * obviously can't point to one.  It will instead point to the multixact
+    * ID that will be assigned the next time one is needed.
+    *
+    * NB: oldestMXact should be the oldest multixact that still exists in
+    * the SLRU, unlike in SetMultiXactIdLimit, where we do this same
+    * computation based on the oldest value that might be referenced in a
+    * table.
     */
    LWLockAcquire(MultiXactGenLock, LW_SHARED);
    if (MultiXactState->nextMXact == oldestMXact)
@@ -2594,9 +2637,9 @@ ReadMultiXactCounts(uint32 *multixacts, MultiXactOffset *members)
    nextOffset = MultiXactState->nextOffset;
    oldestMultiXactId = MultiXactState->oldestMultiXactId;
    nextMultiXactId = MultiXactState->nextMXact;
+   oldestOffset = MultiXactState->oldestOffset;
    LWLockRelease(MultiXactGenLock);
 
-   oldestOffset = find_multixact_start(oldestMultiXactId);
    *members = nextOffset - oldestOffset;
    *multixacts = nextMultiXactId - oldestMultiXactId;
 }