From d703fe4538754534817596a0d4f51e06a8c3293f Mon Sep 17 00:00:00 2001 From: Maxim Orlov Date: Wed, 23 Oct 2024 18:23:39 +0300 Subject: [PATCH v9 4/7] Get rid of MultiXactMemberFreezeThreshold call. Since MaxMultiXactOffset are UINT64_MAX now, MULTIXACT_MEMBER_SAFE_THRESHOLD and MULTIXACT_MEMBER_DANGER_THRESHOLD values are not meaningful any more. Thus, MultiXactMemberFreezeThreshold is not needed too. Instead, switch to MULTIXACT_MEMBER_AUTOVAC_THRESHOLD (eq 2^32) members threshold. It is used to determine if we need to force autovacuum or not. Author: Maxim Orlov --- src/backend/access/transam/multixact.c | 117 +++---------------------- src/backend/commands/vacuum.c | 2 +- src/backend/postmaster/autovacuum.c | 4 +- src/include/access/multixact.h | 1 - 4 files changed, 15 insertions(+), 109 deletions(-) diff --git a/src/backend/access/transam/multixact.c b/src/backend/access/transam/multixact.c index 48e1c0160a..a817f539ee 100644 --- a/src/backend/access/transam/multixact.c +++ b/src/backend/access/transam/multixact.c @@ -204,10 +204,14 @@ MXOffsetToMemberOffset(MultiXactOffset offset) member_in_group * sizeof(TransactionId); } -/* Multixact members wraparound thresholds. */ -#define MULTIXACT_MEMBER_SAFE_THRESHOLD (MaxMultiXactOffset / 2) -#define MULTIXACT_MEMBER_DANGER_THRESHOLD \ - (MaxMultiXactOffset - MaxMultiXactOffset / 4) +/* + * Multixact members warning threshold. + * + * If difference bettween nextOffset and oldestOffset exceed this value, we + * trigger autovacuumin order to release the disk space, reduce table bloat if + * possible. + */ +#define MULTIXACT_MEMBER_AUTOVAC_THRESHOLD UINT64CONST(0xFFFFFFFF) static inline MultiXactId PreviousMultiXactId(MultiXactId multi) @@ -2616,15 +2620,13 @@ GetOldestMultiXactId(void) } /* - * Determine how aggressively we need to vacuum in order to prevent member - * wraparound. + * Determine if we need to vacuum for member or not. * * To do so determine what's the oldest member offset and install the limit * info in MultiXactState, where it can be used to prevent overrun of old data * in the members SLRU area. * - * The return value is true if emergency autovacuum is required and false - * otherwise. + * The return value is true if autovacuum is required and false otherwise. */ static bool SetOffsetVacuumLimit(bool is_startup) @@ -2712,10 +2714,10 @@ SetOffsetVacuumLimit(bool is_startup) LWLockRelease(MultiXactGenLock); /* - * Do we need an emergency autovacuum? If we're not sure, assume yes. + * Do we need autovacuum? If we're not sure, assume yes. */ return !oldestOffsetKnown || - (nextOffset - oldestOffset > MULTIXACT_MEMBER_SAFE_THRESHOLD); + (nextOffset - oldestOffset > MULTIXACT_MEMBER_AUTOVAC_THRESHOLD); } /* @@ -2761,101 +2763,6 @@ find_multixact_start(MultiXactId multi, MultiXactOffset *result) return true; } -/* - * Determine how many multixacts, and how many multixact members, currently - * exist. Return false if unable to determine. - */ -static bool -ReadMultiXactCounts(uint32 *multixacts, MultiXactOffset *members) -{ - MultiXactOffset nextOffset; - MultiXactOffset oldestOffset; - MultiXactId oldestMultiXactId; - MultiXactId nextMultiXactId; - bool oldestOffsetKnown; - - LWLockAcquire(MultiXactGenLock, LW_SHARED); - nextOffset = MultiXactState->nextOffset; - oldestMultiXactId = MultiXactState->oldestMultiXactId; - nextMultiXactId = MultiXactState->nextMXact; - oldestOffset = MultiXactState->oldestOffset; - oldestOffsetKnown = MultiXactState->oldestOffsetKnown; - LWLockRelease(MultiXactGenLock); - - if (!oldestOffsetKnown) - return false; - - *members = nextOffset - oldestOffset; - *multixacts = nextMultiXactId - oldestMultiXactId; - return true; -} - -/* - * Multixact members can be removed once the multixacts that refer to them - * are older than every datminmxid. autovacuum_multixact_freeze_max_age and - * vacuum_multixact_freeze_table_age work together to make sure we never have - * too many multixacts; we hope that, at least under normal circumstances, - * this will also be sufficient to keep us from using too many offsets. - * However, if the average multixact has many members, we might exhaust the - * members space while still using few enough members that these limits fail - * to trigger relminmxid advancement by VACUUM. At that point, we'd have no - * choice but to start failing multixact-creating operations with an error. - * - * To prevent that, if more than a threshold portion of the members space is - * used, we effectively reduce autovacuum_multixact_freeze_max_age and - * to a value just less than the number of multixacts in use. We hope that - * this will quickly trigger autovacuuming on the table or tables with the - * oldest relminmxid, thus allowing datminmxid values to advance and removing - * some members. - * - * As the fraction of the member space currently in use grows, we become - * more aggressive in clamping this value. That not only causes autovacuum - * to ramp up, but also makes any manual vacuums the user issues more - * aggressive. This happens because vacuum_get_cutoffs() will clamp the - * freeze table and the minimum freeze age cutoffs based on the effective - * autovacuum_multixact_freeze_max_age this function returns. In the worst - * case, we'll claim the freeze_max_age to zero, and every vacuum of any - * table will freeze every multixact. - */ -int -MultiXactMemberFreezeThreshold(void) -{ - MultiXactOffset members; - uint32 multixacts; - uint32 victim_multixacts; - double fraction; - int result; - - /* If we can't determine member space utilization, assume the worst. */ - if (!ReadMultiXactCounts(&multixacts, &members)) - return 0; - - /* If member space utilization is low, no special action is required. */ - if (members <= MULTIXACT_MEMBER_SAFE_THRESHOLD) - return autovacuum_multixact_freeze_max_age; - - /* - * Compute a target for relminmxid advancement. The number of multixacts - * we try to eliminate from the system is based on how far we are past - * MULTIXACT_MEMBER_SAFE_THRESHOLD. - */ - fraction = (double) (members - MULTIXACT_MEMBER_SAFE_THRESHOLD); - fraction /= (double) (MULTIXACT_MEMBER_DANGER_THRESHOLD - MULTIXACT_MEMBER_SAFE_THRESHOLD); - - victim_multixacts = multixacts * fraction; - - /* fraction could be > 1.0, but lowest possible freeze age is zero */ - if (victim_multixacts > multixacts) - return 0; - result = multixacts - victim_multixacts; - - /* - * Clamp to autovacuum_multixact_freeze_max_age, so that we never make - * autovacuum less aggressive than it would otherwise be. - */ - return Min(result, autovacuum_multixact_freeze_max_age); -} - typedef struct mxtruncinfo { int64 earliestExistingPage; diff --git a/src/backend/commands/vacuum.c b/src/backend/commands/vacuum.c index 86f36b3695..e7506e268a 100644 --- a/src/backend/commands/vacuum.c +++ b/src/backend/commands/vacuum.c @@ -1133,7 +1133,7 @@ vacuum_get_cutoffs(Relation rel, const VacuumParams *params, * normally autovacuum_multixact_freeze_max_age, but may be less if we are * short of multixact member space. */ - effective_multixact_freeze_max_age = MultiXactMemberFreezeThreshold(); + effective_multixact_freeze_max_age = autovacuum_multixact_freeze_max_age; /* * Almost ready to set freeze output parameters; check if OldestXmin or diff --git a/src/backend/postmaster/autovacuum.c b/src/backend/postmaster/autovacuum.c index dc3cf87aba..180bb7e96e 100644 --- a/src/backend/postmaster/autovacuum.c +++ b/src/backend/postmaster/autovacuum.c @@ -1122,7 +1122,7 @@ do_start_worker(void) /* Also determine the oldest datminmxid we will consider. */ recentMulti = ReadNextMultiXactId(); - multiForceLimit = recentMulti - MultiXactMemberFreezeThreshold(); + multiForceLimit = recentMulti - autovacuum_multixact_freeze_max_age; if (multiForceLimit < FirstMultiXactId) multiForceLimit -= FirstMultiXactId; @@ -1915,7 +1915,7 @@ do_autovacuum(void) * normally autovacuum_multixact_freeze_max_age, but may be less if we are * short of multixact member space. */ - effective_multixact_freeze_max_age = MultiXactMemberFreezeThreshold(); + effective_multixact_freeze_max_age = autovacuum_multixact_freeze_max_age; /* * Find the pg_database entry and select the default freeze ages. We use diff --git a/src/include/access/multixact.h b/src/include/access/multixact.h index 90583634ec..5aefbddce3 100644 --- a/src/include/access/multixact.h +++ b/src/include/access/multixact.h @@ -143,7 +143,6 @@ extern void MultiXactSetNextMXact(MultiXactId nextMulti, extern void MultiXactAdvanceNextMXact(MultiXactId minMulti, MultiXactOffset minMultiOffset); extern void MultiXactAdvanceOldest(MultiXactId oldestMulti, Oid oldestMultiDB); -extern int MultiXactMemberFreezeThreshold(void); extern void multixact_twophase_recover(TransactionId xid, uint16 info, void *recdata, uint32 len); -- 2.43.0