8000 Avoid SnapshotResetXmin() during AtEOXact_Snapshot() · postgrespro/postgres@42b4b0b · GitHub
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    simonat2ndQuadrantsimonat2ndQuadrant
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    Avoid SnapshotResetXmin() during AtEOXact_Snapshot()
    For normal commits and aborts we already reset PgXact->xmin Avoiding touching highly contented shmem improves concurrent performance. Simon Riggs Discussion: CANP8+jJdXE9b+b9F8CQT-LuxxO0PBCB-SZFfMVAdp+akqo4zfg@mail.gmail.com
    1 parent 8398c83 commit 42b4b0b

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    +22
    -7
    lines changed

    3 files changed

    +22
    -7
    lines changed

    ‎src/backend/access/transam/xact.c

    Lines changed: 3 additions & 3 deletions
    Original file line numberDiff line numberDiff line change
    @@ -2137,7 +2137,7 @@ CommitTransaction(void)
    21372137
    AtEOXact_ComboCid();
    21382138
    AtEOXact_HashTables(true);
    21392139
    AtEOXact_PgStat(true);
    2140-
    AtEOXact_Snapshot(true);
    2140+
    AtEOXact_Snapshot(true, false);
    21412141
    AtCommit_ApplyLauncher();
    21422142
    pgstat_report_xact_timestamp(0);
    21432143

    @@ -2409,7 +2409,7 @@ PrepareTransaction(void)
    24092409
    AtEOXact_ComboCid();
    24102410
    AtEOXact_HashTables(true);
    24112411
    /* don't call AtEOXact_PgStat here; we fixed pgstat state above */
    2412-
    AtEOXact_Snapshot(true);
    2412+
    AtEOXact_Snapshot(true, true);
    24132413
    pgstat_report_xact_timestamp(0);
    24142414

    24152415
    CurrentResourceOwner = NULL;
    @@ -2640,7 +2640,7 @@ CleanupTransaction(void)
    26402640
    * do abort cleanup processing
    26412641
    */
    26422642
    AtCleanup_Portals(); /* now safe to release portal memory */
    2643-
    AtEOXact_Snapshot(false); /* and release the transaction's snapshots */
    2643+
    AtEOXact_Snapshot(false, false); /* and release the transaction's snapshots */
    26442644

    26452645
    CurrentResourceOwner = NULL; /* and resource owner */
    26462646
    if (TopTransactionResourceOwner)

    ‎src/backend/utils/time/snapmgr.c

    Lines changed: 18 additions & 3 deletions
    Original file line numberDiff line numberDiff line change
    @@ -954,7 +954,12 @@ xmin_cmp(const pairingheap_node *a, const pairingheap_node *b, void *arg)
    954954
    *
    955955
    * If there are no more snapshots, we can reset our PGXACT->xmin to InvalidXid.
    956956
    * Note we can do this without locking because we assume that storing an Xid
    957-
    * is atomic.
    957+
    * is atomic. We do this because it will allow multi-statement transactions to
    958+
    * reset their xmin and prevent us from holding back removal of dead rows;
    959+
    * this has little purpose when we are dealing with very fast statements in
    960+
    * read committed mode since the xmin will advance quickly anyway. It has no
    961+
    * use at all when we are running single statement transactions since the xmin
    962+
    * is reset as part of end of transaction anyway.
    958963
    *
    959964
    * Even if there are some remaining snapshots, we may be able to advance our
    960965
    * PGXACT->xmin to some degree. This typically happens when a portal is
    @@ -1051,7 +1056,7 @@ AtSubAbort_Snapshot(int level)
    10511056
    * Snapshot manager's cleanup function for end of transaction
    10521057
    */
    10531058
    void
    1054-
    AtEOXact_Snapshot(bool isCommit)
    1059+
    AtEOXact_Snapshot(bool isCommit, bool isPrepare)
    10551060
    {
    10561061
    /*
    10571062
    * In transaction-snapshot mode we must release our privately-managed
    @@ -1136,7 +1141,17 @@ AtEOXact_Snapshot(bool isCommit)
    11361141

    11371142
    FirstSnapshotSet = false;
    11381143

    1139-
    SnapshotResetXmin();
    1144+
    /*
    1145+
    * During normal commit and abort processing, we call
    1146+
    * ProcArrayEndTransaction() or ProcArrayClearTransaction() to
    1147+
    * reset the PgXact->xmin. That call happens prior to the call to
    1148+
    * AtEOXact_Snapshot(), so we need not touch xmin here at all,
    1149+
    * accept when we are preparing a transaction.
    1150+
    */
    1151+
    if (isPrepare)
    1152+
    SnapshotResetXmin();
    1153+
    1154+
    Assert(MyPgXact->xmin == 0);
    11401155
    }
    11411156

    11421157

    ‎src/include/utils/snapmgr.h

    Lines changed: 1 addition & 1 deletion
    Original file line numberDiff line numberDiff line change
    @@ -85,7 +85,7 @@ extern void UnregisterSnapshotFromOwner(Snapshot snapshot, ResourceOwner owner);
    8585

    8686
    extern void AtSubCommit_Snapshot(int level);
    8787
    extern void AtSubAbort_Snapshot(int level);
    88-
    extern void AtEOXact_Snapshot(bool isCommit);
    88+
    extern void AtEOXact_Snapshot(bool isCommit, bool isPrepare);
    8989

    9090
    extern void ImportSnapshot(const char *idstr);
    9191
    extern bool XactHasExportedSnapshots(void);

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