From f88add9c98f1563a602fbd4dd34d18149d6c7058 Mon Sep 17 00:00:00 2001 From: Petr Jelinek Date: Tue, 21 Feb 2017 19:58:18 +0100 Subject: [PATCH 4/4] Skip unnecessary snapshot builds When doing initial snapshot build during logical decoding initialization, don't build snapshots for transactions where we know the transaction didn't do any catalog changes. Otherwise we might end up with thousands of useless snapshots on busy server which can be quite expensive. --- src/backend/replication/logical/snapbuild.c | 82 +++++++++++++++++++---------- 1 file changed, 53 insertions(+), 29 deletions(-) diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 40937fb..9f536b0 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -955,6 +955,7 @@ SnapBuildCommitTxn(SnapBuild *builder, XLogRecPtr lsn, TransactionId xid, bool forced_timetravel = false; bool sub_needs_timetravel = false; bool top_needs_timetravel = false; + bool skip_forced_snapshot = false; TransactionId xmax = xid; @@ -976,10 +977,19 @@ SnapBuildCommitTxn(SnapBuild *builder, XLogRecPtr lsn, TransactionId xid, /* * We could avoid treating !SnapBuildTxnIsRunning transactions as * timetravel ones, but we want to be able to export a snapshot when - * we reached consistency. + * we reached consistency so we need to keep track of them. */ forced_timetravel = true; elog(DEBUG1, "forced to assume catalog changes for xid %u because it was running too early", xid); + + /* + * It is however desirable to skip building new snapshot for + * !SnapBuildTxnIsRunning transactions as otherwise we might end up + * building thousands of unused snapshots on busy servers which can + * be very expensive. + */ + if (!SnapBuildTxnIsRunning(builder, xid)) + skip_forced_snapshot = true; } for (nxact = 0; nxact < nsubxacts; nxact++) @@ -992,21 +1002,10 @@ SnapBuildCommitTxn(SnapBuild *builder, XLogRecPtr lsn, TransactionId xid, SnapBuildEndTxn(builder, lsn, subxid); /* - * If we're forcing timetravel we also need visibility information - * about subtransaction, so keep track of subtransaction's state. - */ - if (forced_timetravel) - { - SnapBuildAddCommittedTxn(builder, subxid); - if (NormalTransactionIdFollows(subxid, xmax)) - xmax = subxid; - } - - /* * Add subtransaction to base snapshot if it DDL, we don't distinguish * to toplevel transactions there. */ - else if (ReorderBufferXidHasCatalogChanges(builder->reorder, subxid)) + if (ReorderBufferXidHasCatalogChanges(builder->reorder, subxid)) { sub_needs_timetravel = true; @@ -1018,6 +1017,16 @@ SnapBuildCommitTxn(SnapBuild *builder, XLogRecPtr lsn, TransactionId xid, if (NormalTransactionIdFollows(subxid, xmax)) xmax = subxid; } + /* + * If we're forcing timetravel we also need visibility information + * about subtransaction, so keep track of subtransaction's state. + */ + else if (forced_timetravel) + { + SnapBuildAddCommittedTxn(builder, subxid); + if (NormalTransactionIdFollows(subxid, xmax)) + xmax = subxid; + } } /* @@ -1026,14 +1035,8 @@ SnapBuildCommitTxn(SnapBuild *builder, XLogRecPtr lsn, TransactionId xid, */ SnapBuildEndTxn(builder, lsn, xid); - if (forced_timetravel) - { - elog(DEBUG2, "forced transaction %u to do timetravel.", xid); - - SnapBuildAddCommittedTxn(builder, xid); - } /* add toplevel transaction to base snapshot */ - else if (ReorderBufferXidHasCatalogChanges(builder->reorder, xid)) + if (ReorderBufferXidHasCatalogChanges(builder->reorder, xid)) { elog(DEBUG2, "found top level transaction %u, with catalog changes!", xid); @@ -1046,10 +1049,18 @@ SnapBuildCommitTxn(SnapBuild *builder, XLogRecPtr lsn, TransactionId xid, /* mark toplevel txn as timetravel as well */ SnapBuildAddCommittedTxn(builder, xid); } + else if (forced_timetravel) + { + elog(DEBUG2, "forced transaction %u to do timetravel.", xid); + + SnapBuildAddCommittedTxn(builder, xid); + } /* if there's any reason to build a historic snapshot, do so now */ if (forced_timetravel || top_needs_timetravel || sub_needs_timetravel) { + bool build_snapshot; + /* * Adjust xmax of the snapshot builder, we only do that for committed, * catalog modifying, transactions, everything else isn't interesting @@ -1070,14 +1081,29 @@ SnapBuildCommitTxn(SnapBuild *builder, XLogRecPtr lsn, TransactionId xid, return; /* - * Decrease the snapshot builder's refcount of the old snapshot, note - * that it still will be used if it has been handed out to the - * reorderbuffer earlier. + * Build snapshot if needed. We need to build it if there isn't one + * already built, or if the transaction has made catalog changes or + * when we can't know if transaction made catalog changes. */ - if (builder->snapshot) + build_snapshot = !builder->snapshot || top_needs_timetravel || + sub_needs_timetravel || !skip_forced_snapshot; + + /* + * Decrease the snapshot builder's refcount of the old snapshot if we + * plan to build new one, note that it still will be used if it has + * been handed out to the reorderbuffer earlier. + */ + if (builder->snapshot && build_snapshot) SnapBuildSnapDecRefcount(builder->snapshot); - builder->snapshot = SnapBuildBuildSnapshot(builder, xid); + /* Build new snapshot unless asked not to. */ + if (build_snapshot) + { + builder->snapshot = SnapBuildBuildSnapshot(builder, xid); + + /* refcount of the snapshot builder for the new snapshot */ + SnapBuildSnapIncRefcount(builder->snapshot); + } /* we might need to execute invalidations, add snapshot */ if (!ReorderBufferXidHasBaseSnapshot(builder->reorder, xid)) @@ -1087,11 +1113,9 @@ SnapBuildCommitTxn(SnapBuild *builder, XLogRecPtr lsn, TransactionId xid, builder->snapshot); } - /* refcount of the snapshot builder for the new snapshot */ - SnapBuildSnapIncRefcount(builder->snapshot); - /* add a new Snapshot to all currently running transactions */ - SnapBuildDistributeNewCatalogSnapshot(builder, lsn); + if (build_snapshot) + SnapBuildDistributeNewCatalogSnapshot(builder, lsn); } else { -- 2.7.4