diff options
Diffstat (limited to 'src/backend/access/transam/commit_ts.c')
| -rw-r--r-- | src/backend/access/transam/commit_ts.c | 35 |
1 files changed, 25 insertions, 10 deletions
diff --git a/src/backend/access/transam/commit_ts.c b/src/backend/access/transam/commit_ts.c index b786eeff7a1..9f42461e12c 100644 --- a/src/backend/access/transam/commit_ts.c +++ b/src/backend/access/transam/commit_ts.c @@ -557,6 +557,7 @@ CommitTsShmemInit(void) CommitTsSLRULock, "pg_commit_ts", LWTRANCHE_COMMITTS_BUFFER, SYNC_HANDLER_COMMIT_TS); + SlruPagePrecedesUnitTests(CommitTsCtl, COMMIT_TS_XACTS_PER_PAGE); commitTsShared = ShmemInitStruct("CommitTs shared", sizeof(CommitTimestampShared), @@ -927,14 +928,27 @@ AdvanceOldestCommitTsXid(TransactionId oldestXact) /* - * Decide which of two commitTS page numbers is "older" for truncation - * purposes. + * Decide whether a commitTS page number is "older" for truncation purposes. + * Analogous to CLOGPagePrecedes(). * - * We need to use comparison of TransactionIds here in order to do the right - * thing with wraparound XID arithmetic. However, if we are asked about - * page number zero, we don't want to hand InvalidTransactionId to - * TransactionIdPrecedes: it'll get weird about permanent xact IDs. So, - * offset both xids by FirstNormalTransactionId to avoid that. + * At default BLCKSZ, (1 << 31) % COMMIT_TS_XACTS_PER_PAGE == 128. This + * introduces differences compared to CLOG and the other SLRUs having (1 << + * 31) % per_page == 0. This function never tests exactly + * TransactionIdPrecedes(x-2^31, x). When the system reaches xidStopLimit, + * there are two possible counts of page boundaries between oldestXact and the + * latest XID assigned, depending on whether oldestXact is within the first + * 128 entries of its page. Since this function doesn't know the location of + * oldestXact within page2, it returns false for one page that actually is + * expendable. This is a wider (yet still negligible) version of the + * truncation opportunity that CLOGPagePrecedes() cannot recognize. + * + * For the sake of a worked example, number entries with decimal values such + * that page1==1 entries range from 1.0 to 1.999. Let N+0.15 be the number of + * pages that 2^31 entries will span (N is an integer). If oldestXact=N+2.1, + * then the final safe XID assignment leaves newestXact=1.95. We keep page 2, + * because entry=2.85 is the border that toggles whether entries precede the + * last entry of the oldestXact page. While page 2 is expendable at + * oldestXact=N+2.1, it would be precious at oldestXact=N+2.9. */ static bool CommitTsPagePrecedes(int page1, int page2) @@ -943,11 +957,12 @@ CommitTsPagePrecedes(int page1, int page2) TransactionId xid2; xid1 = ((TransactionId) page1) * COMMIT_TS_XACTS_PER_PAGE; - xid1 += FirstNormalTransactionId; + xid1 += FirstNormalTransactionId + 1; xid2 = ((TransactionId) page2) * COMMIT_TS_XACTS_PER_PAGE; - xid2 += FirstNormalTransactionId; + xid2 += FirstNormalTransactionId + 1; - return TransactionIdPrecedes(xid1, xid2); + return (TransactionIdPrecedes(xid1, xid2) && + TransactionIdPrecedes(xid1, xid2 + COMMIT_TS_XACTS_PER_PAGE - 1)); } |