From 7c3e8039f450eb99b3a73272d0a1661195747d1b Mon Sep 17 00:00:00 2001 From: Kevin Grittner Date: Fri, 29 Apr 2016 16:46:08 -0500 Subject: [PATCH] Add a few entries to the tail of time mapping, to see old values. Without a few entries beyond old_snapshot_threshold, the lookup would often fail, resulting in the more aggressive pruning or vacuum being skipped often enough to matter. This was very clearly shown by a python test script posted by Ants Aasma, and was likely a factor in an earlier but somewhat less clear-cut test case posted by Jeff Janes. This patch makes no change to the logic, per se -- it just makes the array of mapping entries big enough to make lookup misses based on timing much less likely. An occasional miss is still possible if a thread stalls for more than 10 minutes, but that does not create any problem with correctness of behavior. Besides, if things are so busy that a thread is stalling for more than 10 minutes, it is probably OK to skip the more aggressive cleanup at that particular point in time. --- src/backend/utils/time/snapmgr.c | 26 +++++++++++++------------- src/include/utils/snapmgr.h | 13 +++++++++++++ 2 files changed, 26 insertions(+), 13 deletions(-) diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 35f338133aa..0a9a231f597 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -92,10 +92,10 @@ typedef struct OldSnapshotControlData * Use a circular buffer with a head offset, a count of entries currently * used, and a timestamp corresponding to the xid at the head offset. A * count_used value of zero means that there are no times stored; a - * count_used value of old_snapshot_threshold means that the buffer is - * full and the head must be advanced to add new entries. Use timestamps - * aligned to minute boundaries, since that seems less surprising than - * aligning based on the first usage timestamp. + * count_used value of OLD_SNAPSHOT_TIME_MAP_ENTRIES means that the buffer + * is full and the head must be advanced to add new entries. Use + * timestamps aligned to minute boundaries, since that seems less + * surprising than aligning based on the first usage timestamp. * * It is OK if the xid for a given time slot is from earlier than * calculated by adding the number of minutes corresponding to the @@ -243,7 +243,7 @@ SnapMgrShmemSize(void) size = offsetof(OldSnapshotControlData, xid_by_minute); if (old_snapshot_threshold > 0) size = add_size(size, mul_size(sizeof(TransactionId), - old_snapshot_threshold)); + OLD_SNAPSHOT_TIME_MAP_ENTRIES)); return size; } @@ -1643,7 +1643,7 @@ TransactionIdLimitedForOldSnapshots(TransactionId recentXmin, if (offset > oldSnapshotControl->count_used - 1) offset = oldSnapshotControl->count_used - 1; offset = (oldSnapshotControl->head_offset + offset) - % old_snapshot_threshold; + % OLD_SNAPSHOT_TIME_MAP_ENTRIES; xlimit = oldSnapshotControl->xid_by_minute[offset]; if (NormalTransactionIdFollows(xlimit, recentXmin)) @@ -1720,10 +1720,10 @@ MaintainOldSnapshotTimeMapping(int64 whenTaken, TransactionId xmin) LWLockAcquire(OldSnapshotTimeMapLock, LW_EXCLUSIVE); Assert(oldSnapshotControl->head_offset >= 0); - Assert(oldSnapshotControl->head_offset < old_snapshot_threshold); + Assert(oldSnapshotControl->head_offset < OLD_SNAPSHOT_TIME_MAP_ENTRIES); Assert((oldSnapshotControl->head_timestamp % USECS_PER_MINUTE) == 0); Assert(oldSnapshotControl->count_used >= 0); - Assert(oldSnapshotControl->count_used <= old_snapshot_threshold); + Assert(oldSnapshotControl->count_used <= OLD_SNAPSHOT_TIME_MAP_ENTRIES); if (oldSnapshotControl->count_used == 0) { @@ -1750,7 +1750,7 @@ MaintainOldSnapshotTimeMapping(int64 whenTaken, TransactionId xmin) int bucket = (oldSnapshotControl->head_offset + ((ts - oldSnapshotControl->head_timestamp) / USECS_PER_MINUTE)) - % old_snapshot_threshold; + % OLD_SNAPSHOT_TIME_MAP_ENTRIES; if (TransactionIdPrecedes(oldSnapshotControl->xid_by_minute[bucket], xmin)) oldSnapshotControl->xid_by_minute[bucket] = xmin; @@ -1763,7 +1763,7 @@ MaintainOldSnapshotTimeMapping(int64 whenTaken, TransactionId xmin) oldSnapshotControl->head_timestamp = ts; - if (advance >= old_snapshot_threshold) + if (advance >= OLD_SNAPSHOT_TIME_MAP_ENTRIES) { /* Advance is so far that all old data is junk; start over. */ oldSnapshotControl->head_offset = 0; @@ -1777,12 +1777,12 @@ MaintainOldSnapshotTimeMapping(int64 whenTaken, TransactionId xmin) for (i = 0; i < advance; i++) { - if (oldSnapshotControl->count_used == old_snapshot_threshold) + if (oldSnapshotControl->count_used == OLD_SNAPSHOT_TIME_MAP_ENTRIES) { /* Map full and new value replaces old head. */ int old_head = oldSnapshotControl->head_offset; - if (old_head == (old_snapshot_threshold - 1)) + if (old_head == (OLD_SNAPSHOT_TIME_MAP_ENTRIES - 1)) oldSnapshotControl->head_offset = 0; else oldSnapshotControl->head_offset = old_head + 1; @@ -1793,7 +1793,7 @@ MaintainOldSnapshotTimeMapping(int64 whenTaken, TransactionId xmin) /* Extend map to unused entry. */ int new_tail = (oldSnapshotControl->head_offset + oldSnapshotControl->count_used) - % old_snapshot_threshold; + % OLD_SNAPSHOT_TIME_MAP_ENTRIES; oldSnapshotControl->count_used++; oldSnapshotControl->xid_by_minute[new_tail] = xmin; diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index 6a80f0ba550..42706966f10 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -19,6 +19,19 @@ #include "utils/snapshot.h" +/* + * The structure used to map times to TransactionId values for the "snapshot + * too old" feature must have a few entries at the tail to hold old values; + * otherwise the lookup will often fail and the expected early pruning or + * vacuum will not usually occur. It is best if this padding is for a number + * of minutes greater than a thread would normally be stalled, but it's OK if + * early vacuum opportunities are occasionally missed, so there's no need to + * use an extreme value or get too fancy. 10 minutes seems plenty. + */ +#define OLD_SNAPSHOT_PADDING_ENTRIES 10 +#define OLD_SNAPSHOT_TIME_MAP_ENTRIES (old_snapshot_threshold + OLD_SNAPSHOT_PADDING_ENTRIES) + + /* GUC variables */ extern PGDLLIMPORT int old_snapshot_threshold; -- 2.30.2