From 8484ae3f2c2f8af20ae8ce2f6d7960b6519e65c0 Mon Sep 17 00:00:00 2001 From: Sokolov Yura aka funny_falcon Date: Fri, 9 Mar 2018 22:49:01 +0300 Subject: [PATCH] Make hash table for xip in XidInMVCCSnapshot When lot of concurrent transactions attempts to update single row, then linear scan through running list in XidInMVCCSnapshot became noticebale bottleneck. With this change, hash table is built on first search of xid in snapshot->xip and snapshot->subxip arrays. If size of array is smaller than 60, then linear scan is still used, cause there is no much benefits from building hash then. (at least on Intel Xeon). --- src/backend/storage/ipc/procarray.c | 67 ++++++++++++++++++++++------ src/backend/utils/time/snapmgr.c | 25 +++++++---- src/backend/utils/time/tqual.c | 88 ++++++++++++++++++++++++++++--------- src/include/utils/snapshot.h | 11 +++++ 4 files changed, 148 insertions(+), 43 deletions(-) diff --git a/src/backend/storage/ipc/procarray.c b/src/backend/storage/ipc/procarray.c index afe1c03aa3..2020b7482a 100644 --- a/src/backend/storage/ipc/procarray.c +++ b/src/backend/storage/ipc/procarray.c @@ -1469,6 +1469,54 @@ GetMaxSnapshotSubxidCount(void) return TOTAL_MAX_CACHED_SUBXIDS; } +/* + * ExtendXipSizeForHash - calculate xip array size with space for hash table. + * + * Hash table should be at least twice larger than array to not depend on + * cleverness of algorithm. + * + * But if xipcnt < SnapshotMinHash, then no need in hash-table at all. + */ +int +ExtendXipSizeForHash(int xipcnt, uint8* plog) +{ + int size; + uint8 log = 0; + + size = xipcnt; + if (xipcnt >= SnapshotMinHash) + { + log = 1; + while (xipcnt) { + log++; + xipcnt >>= 1; + } + size += 1<xip = (TransactionId *) - malloc(GetMaxSnapshotXidCount() * sizeof(TransactionId)); - if (snapshot->xip == NULL) - ereport(ERROR, - (errcode(ERRCODE_OUT_OF_MEMORY), - errmsg("out of memory"))); - Assert(snapshot->subxip == NULL); - snapshot->subxip = (TransactionId *) - malloc(GetMaxSnapshotSubxidCount() * sizeof(TransactionId)); - if (snapshot->subxip == NULL) - ereport(ERROR, - (errcode(ERRCODE_OUT_OF_MEMORY), - errmsg("out of memory"))); + snapshot->xip = AllocateXip(GetMaxSnapshotXidCount(), + &snapshot->xhlog); + snapshot->subxip = AllocateXip(GetMaxSnapshotSubxidCount(), + &snapshot->subxhlog); } /* @@ -1758,6 +1797,8 @@ GetSnapshotData(Snapshot snapshot) snapshot->active_count = 0; snapshot->regd_count = 0; snapshot->copied = false; + snapshot->xhlog &= ~SnapshotHashBuilt; + snapshot->subxhlog &= ~SnapshotHashBuilt; if (old_snapshot_threshold < 0) { diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index e58c69dbd7..f8212bdd60 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -662,14 +662,16 @@ CopySnapshot(Snapshot snapshot) Snapshot newsnap; Size subxipoff; Size size; + int xcnt, subxcnt; + uint8 xhlog, subxhlog; Assert(snapshot != InvalidSnapshot); + xcnt = ExtendXipSizeForHash(snapshot->xcnt, &xhlog); + subxcnt = ExtendXipSizeForHash(snapshot->subxcnt, &subxhlog); /* We allocate any XID arrays needed in the same palloc block. */ - size = subxipoff = sizeof(SnapshotData) + - snapshot->xcnt * sizeof(TransactionId); - if (snapshot->subxcnt > 0) - size += snapshot->subxcnt * sizeof(TransactionId); + size = subxipoff = sizeof(SnapshotData) + xcnt * sizeof(TransactionId); + size += subxcnt * sizeof(TransactionId); newsnap = (Snapshot) MemoryContextAlloc(TopTransactionContext, size); memcpy(newsnap, snapshot, sizeof(SnapshotData)); @@ -677,6 +679,8 @@ CopySnapshot(Snapshot snapshot) newsnap->regd_count = 0; newsnap->active_count = 0; newsnap->copied = true; + newsnap->xhlog = xhlog; + newsnap->subxhlog = subxhlog; /* setup XID array */ if (snapshot->xcnt > 0) @@ -2130,16 +2134,18 @@ RestoreSnapshot(char *start_address) Size size; Snapshot snapshot; TransactionId *serialized_xids; + int xcnt, subxcnt; + uint8 xhlog, subxhlog; memcpy(&serialized_snapshot, start_address, sizeof(SerializedSnapshotData)); serialized_xids = (TransactionId *) (start_address + sizeof(SerializedSnapshotData)); + xcnt = ExtendXipSizeForHash(serialized_snapshot.xcnt, &xhlog); + subxcnt = ExtendXipSizeForHash(serialized_snapshot.subxcnt, &subxhlog); /* We allocate any XID arrays needed in the same palloc block. */ - size = sizeof(SnapshotData) - + serialized_snapshot.xcnt * sizeof(TransactionId) - + serialized_snapshot.subxcnt * sizeof(TransactionId); + size = sizeof(SnapshotData) + (xcnt + subxcnt) * sizeof(TransactionId); /* Copy all required fields */ snapshot = (Snapshot) MemoryContextAlloc(TopTransactionContext, size); @@ -2148,8 +2154,10 @@ RestoreSnapshot(char *start_address) snapshot->xmax = serialized_snapshot.xmax; snapshot->xip = NULL; snapshot->xcnt = serialized_snapshot.xcnt; + snapshot->xhlog = xhlog; snapshot->subxip = NULL; snapshot->subxcnt = serialized_snapshot.subxcnt; + snapshot->subxhlog = subxhlog; snapshot->suboverflowed = serialized_snapshot.suboverflowed; snapshot->takenDuringRecovery = serialized_snapshot.takenDuringRecovery; snapshot->curcid = serialized_snapshot.curcid; @@ -2167,8 +2175,7 @@ RestoreSnapshot(char *start_address) /* Copy SubXIDs, if present. */ if (serialized_snapshot.subxcnt > 0) { - snapshot->subxip = ((TransactionId *) (snapshot + 1)) + - serialized_snapshot.xcnt; + snapshot->subxip = ((TransactionId *) (snapshot + 1)) + xcnt; memcpy(snapshot->subxip, serialized_xids + serialized_snapshot.xcnt, serialized_snapshot.subxcnt * sizeof(TransactionId)); } diff --git a/src/backend/utils/time/tqual.c b/src/backend/utils/time/tqual.c index f7c4c9188c..3fbd9327b3 100644 --- a/src/backend/utils/time/tqual.c +++ b/src/backend/utils/time/tqual.c @@ -1461,6 +1461,66 @@ HeapTupleIsSurelyDead(HeapTuple htup, TransactionId OldestXmin) return TransactionIdPrecedes(HeapTupleHeaderGetRawXmax(tuple), OldestXmin); } +/* + * XidInXip searches xid in xip array. + * + * If xcnt is smaller than SnapshotMinHash (60), or *xhlog is zero, then simple + * linear scan of xip array used. Looks like there is no benifit in hash table + * for small xip array. + * + * If xhlog points to non-zero byte, then space for hash table is allocated + * after xip array, and logarithm of its size is in xhlog. + * Most significant xhlog bit should be set if hash table is already built. + * + * Hash table is built using simple linear probing. It allows keep both + * insertion and lookup loop small. + */ +static inline bool +XidInXip(TransactionId xid, TransactionId *xip, uint32 xcnt, uint8 *xhlog) +{ + TransactionId *xiphash; + uint32 i, pos, mask; + int found = 0; + + if (*xhlog == 0) + { + /* full scan for small snapshots and if xiphash is not allocated */ + for (i = 0; i < xcnt; i++) + if (TransactionIdEquals(xid, xip[i])) + return true; + return false; + } + + mask = ((uint32)1 << (*xhlog & ~SnapshotHashBuilt)) - 1; + xiphash = xip + xcnt; + + if (*xhlog & SnapshotHashBuilt) + { + /* Hash table already built. Do lookup */ + pos = xid & mask; + while (xiphash[pos] != InvalidTransactionId) + { + if (TransactionIdEquals(xiphash[pos], xid)) + return true; + pos = (pos + 1) & mask; + } + return false; + } + + /* Build hash table. */ + *xhlog |= SnapshotHashBuilt; + memset(xiphash, 0, (mask+1) * sizeof(TransactionId)); + for (i = 0; i < xcnt; i++) + { + found |= TransactionIdEquals(xip[i], xid); + pos = xip[i] & mask; + while (xiphash[pos] != InvalidTransactionId) + pos = (pos + 1) & mask; + xiphash[pos] = xip[i]; + } + return found != 0; +} + /* * XidInMVCCSnapshot * Is the given XID still-in-progress according to the snapshot? @@ -1474,8 +1534,6 @@ HeapTupleIsSurelyDead(HeapTuple htup, TransactionId OldestXmin) bool XidInMVCCSnapshot(TransactionId xid, Snapshot snapshot) { - uint32 i; - /* * Make a quick range check to eliminate most XIDs without looking at the * xip arrays. Note that this is OK even if we convert a subxact XID to @@ -1507,13 +1565,8 @@ XidInMVCCSnapshot(TransactionId xid, Snapshot snapshot) if (!snapshot->suboverflowed) { /* we have full data, so search subxip */ - int32 j; - - for (j = 0; j < snapshot->subxcnt; j++) - { - if (TransactionIdEquals(xid, snapshot->subxip[j])) - return true; - } + if (XidInXip(xid, snapshot->subxip, snapshot->subxcnt, &snapshot->subxhlog)) + return true; /* not there, fall through to search xip[] */ } @@ -1534,16 +1587,12 @@ XidInMVCCSnapshot(TransactionId xid, Snapshot snapshot) return false; } - for (i = 0; i < snapshot->xcnt; i++) - { - if (TransactionIdEquals(xid, snapshot->xip[i])) - return true; - } + + if (XidInXip(xid, snapshot->xip, snapshot->xcnt, &snapshot->xhlog)) + return true; } else { - int32 j; - /* * In recovery we store all xids in the subxact array because it is by * far the bigger array, and we mostly don't know which xids are @@ -1573,11 +1622,8 @@ XidInMVCCSnapshot(TransactionId xid, Snapshot snapshot) * indeterminate xid. We don't know whether it's top level or subxact * but it doesn't matter. If it's present, the xid is visible. */ - for (j = 0; j < snapshot->subxcnt; j++) - { - if (TransactionIdEquals(xid, snapshot->subxip[j])) - return true; - } + if (XidInXip(xid, snapshot->subxip, snapshot->subxcnt, &snapshot->subxhlog)) + return true; } return false; diff --git a/src/include/utils/snapshot.h b/src/include/utils/snapshot.h index a8a5a8f4c0..332561e82b 100644 --- a/src/include/utils/snapshot.h +++ b/src/include/utils/snapshot.h @@ -23,6 +23,15 @@ typedef struct SnapshotData *Snapshot; #define InvalidSnapshot ((Snapshot) NULL) +/* + * Big running list will be converted into hash table on demand. + * SnapshotMinHash is threshold between "linear scan" and "hashtable on demand". + * SnapshotHashBuilt indicates that hash table was already built. + */ +#define SnapshotMinHash (60) +#define SnapshotHashBuilt ((uint8)0x80) +/* Calculate size to hold both array and hash table (if needed) */ +int ExtendXipSizeForHash(int xipcnt, uint8* plog); /* * We use SnapshotData structures to represent both "regular" (MVCC) @@ -78,6 +87,7 @@ typedef struct SnapshotData */ TransactionId *xip; uint32 xcnt; /* # of xact ids in xip[] */ + uint8 xhlog; /* log2 of allocated xip hash part */ /* * For non-historic MVCC snapshots, this contains subxact IDs that are in @@ -90,6 +100,7 @@ typedef struct SnapshotData */ TransactionId *subxip; int32 subxcnt; /* # of xact ids in subxip[] */ + uint8 subxhlog; /* log2 of allocated subxip hash part */ bool suboverflowed; /* has the subxip array overflowed? */ bool takenDuringRecovery; /* recovery-shaped snapshot? */ -- 2.14.1