diff --git a/src/backend/access/transam/clog.c b/src/backend/access/transam/clog.c index c34e7e1..01c6a29 100644 --- a/src/backend/access/transam/clog.c +++ b/src/backend/access/transam/clog.c @@ -40,6 +40,7 @@ #include "access/xlogutils.h" #include "miscadmin.h" #include "pg_trace.h" +#include "storage/proc.h" /* * Defines for CLOG page sizes. A page is the same BLCKSZ as is used @@ -71,6 +72,13 @@ #define GetLSNIndex(slotno, xid) ((slotno) * CLOG_LSNS_PER_PAGE + \ ((xid) % (TransactionId) CLOG_XACTS_PER_PAGE) / CLOG_XACTS_PER_LSN_GROUP) +/* + * The number of subtransactions below which we consider to apply clog group + * update optimization. Testing reveals that the number higher than this can + * hurt performance. Note that this threshold value must always be lesser + * than PGPROC_MAX_CACHED_SUBXIDS. + */ +#define THRESHOLD_SUBTRANS_CLOG_OPT 5 /* * Link to shared-memory data structures for CLOG control @@ -87,11 +95,17 @@ static void WriteTruncateXlogRec(int pageno, TransactionId oldestXact, Oid oldestXidDb); static void TransactionIdSetPageStatus(TransactionId xid, int nsubxids, TransactionId *subxids, XidStatus status, - XLogRecPtr lsn, int pageno); + XLogRecPtr lsn, int pageno, + bool all_xact_same_page); static void TransactionIdSetStatusBit(TransactionId xid, XidStatus status, XLogRecPtr lsn, int slotno); static void set_status_by_pages(int nsubxids, TransactionId *subxids, XidStatus status, XLogRecPtr lsn); +static bool TransactionGroupUpdateXidStatus(TransactionId xid, XidStatus status, + XLogRecPtr lsn, int pageno); +static void TransactionIdSetPageStatusInternal(TransactionId xid, int nsubxids, + TransactionId *subxids, XidStatus status, + XLogRecPtr lsn, int pageno); /* @@ -174,7 +188,7 @@ TransactionIdSetTreeStatus(TransactionId xid, int nsubxids, * Set the parent and all subtransactions in a single call */ TransactionIdSetPageStatus(xid, nsubxids, subxids, status, lsn, - pageno); + pageno, true); } else { @@ -201,7 +215,7 @@ TransactionIdSetTreeStatus(TransactionId xid, int nsubxids, */ pageno = TransactionIdToPage(xid); TransactionIdSetPageStatus(xid, nsubxids_on_first_page, subxids, status, - lsn, pageno); + lsn, pageno, false); /* * Now work through the rest of the subxids one clog page at a time, @@ -239,7 +253,7 @@ set_status_by_pages(int nsubxids, TransactionId *subxids, TransactionIdSetPageStatus(InvalidTransactionId, num_on_page, subxids + offset, - status, lsn, pageno); + status, lsn, pageno, false); offset = i; pageno = TransactionIdToPage(subxids[offset]); } @@ -249,12 +263,77 @@ set_status_by_pages(int nsubxids, TransactionId *subxids, * Record the final state of transaction entries in the commit log for * all entries on a single page. Atomic only on this page. * - * Otherwise API is same as TransactionIdSetTreeStatus() + * When there is contention on CLogControlLock, we try to group multiple + * updates; a single leader process will perform transaction status updates + * for multiple backends so that the number of times CLogControlLock needs + * to be acquired is reduced. We try to do this iff the transaction id and + * subtransaction ids whose status needs to be updated matches with the one + * we have in PGPROC/PGXACT. The reason of this restriction is that this + * optimization relies on subxids stored in PGPROC. We skip it if a process + * has XIDs on more than one CLOG page, or on a different CLOG page than + * processes already waiting for a group update. This latter condition has a + * race condition (see TransactionGroupUpdateXidStatus) but the worst thing + * that happens if we mess up is a small loss of efficiency; the intent is to + * avoid having the leader access pages it wouldn't otherwise need to touch. */ static void TransactionIdSetPageStatus(TransactionId xid, int nsubxids, TransactionId *subxids, XidStatus status, - XLogRecPtr lsn, int pageno) + XLogRecPtr lsn, int pageno, + bool all_xact_same_page) +{ + if (all_xact_same_page && + xid == MyPgXact->xid && + nsubxids <= THRESHOLD_SUBTRANS_CLOG_OPT && + nsubxids == MyPgXact->nxids && + memcmp(subxids, MyProc->subxids.xids, nsubxids * sizeof(TransactionId)) == 0) + { + /* + * We don't try to do group update optimization if a process has + * overflowed the subxids array in its PGPROC, since in that case we + * don't have a complete list of XIDs for it. + */ + Assert(THRESHOLD_SUBTRANS_CLOG_OPT <= PGPROC_MAX_CACHED_SUBXIDS); + + /* + * If we can immediately acquire CLogControlLock, we update the status + * of our own XID and release the lock. If not, try use group XID + * update. If that doesn't work out, fall back to waiting for the + * lock to perform an update for this transaction only. + */ + if (LWLockConditionalAcquire(CLogControlLock, LW_EXCLUSIVE)) + { + TransactionIdSetPageStatusInternal(xid, nsubxids, subxids, status, lsn, pageno); + LWLockRelease(CLogControlLock); + } + else if (!TransactionGroupUpdateXidStatus(xid, status, lsn, pageno)) + { + LWLockAcquire(CLogControlLock, LW_EXCLUSIVE); + + TransactionIdSetPageStatusInternal(xid, nsubxids, subxids, status, lsn, pageno); + + LWLockRelease(CLogControlLock); + } + } + else + { + LWLockAcquire(CLogControlLock, LW_EXCLUSIVE); + + TransactionIdSetPageStatusInternal(xid, nsubxids, subxids, status, lsn, pageno); + + LWLockRelease(CLogControlLock); + } +} + +/* + * Record the final state of transaction entry in the commit log + * + * We don't do any locking here; caller must handle that. + */ +static void +TransactionIdSetPageStatusInternal(TransactionId xid, int nsubxids, + TransactionId *subxids, XidStatus status, + XLogRecPtr lsn, int pageno) { int slotno; int i; @@ -262,8 +341,7 @@ TransactionIdSetPageStatus(TransactionId xid, int nsubxids, Assert(status == TRANSACTION_STATUS_COMMITTED || status == TRANSACTION_STATUS_ABORTED || (status == TRANSACTION_STATUS_SUB_COMMITTED && !TransactionIdIsValid(xid))); - - LWLockAcquire(CLogControlLock, LW_EXCLUSIVE); + Assert(LWLockHeldByMeInMode(CLogControlLock, LW_EXCLUSIVE)); /* * If we're doing an async commit (ie, lsn is valid), then we must wait @@ -311,8 +389,166 @@ TransactionIdSetPageStatus(TransactionId xid, int nsubxids, } ClogCtl->shared->page_dirty[slotno] = true; +} + +/* + * When we cannot immediately acquire CLogControlLock in exclusive mode at + * commit time, add ourselves to a list of processes that need their XIDs + * status update. The first process to add itself to the list will acquire + * CLogControlLock in exclusive mode and set transaction status as required + * on behalf of all group members. This avoids a great deal of contention + * around CLogControlLock when many processes are trying to commit at once, + * since the lock need not be repeatedly handed off from one committing + * process to the next. + * + * Returns true when transaction status has been updated in clog; returns + * false if we decided against applying the optimization because the page + * number we need to update differs from those processes already waiting. + */ +static bool +TransactionGroupUpdateXidStatus(TransactionId xid, XidStatus status, + XLogRecPtr lsn, int pageno) +{ + volatile PROC_HDR *procglobal = ProcGlobal; + PGPROC *proc = MyProc; + uint32 nextidx; + uint32 wakeidx; + + /* We should definitely have an XID whose status needs to be updated. */ + Assert(TransactionIdIsValid(xid)); + + /* + * Add ourselves to the list of processes needing a group XID status + * update. + */ + proc->clogGroupMember = true; + proc->clogGroupMemberXid = xid; + proc->clogGroupMemberXidStatus = status; + proc->clogGroupMemberPage = pageno; + proc->clogGroupMemberLsn = lsn; + + nextidx = pg_atomic_read_u32(&procglobal->clogGroupFirst); + + while (true) + { + /* + * Add the proc to list, if the clog page where we need to update the + * current transaction status is same as group leader's clog page. + * + * There is a race condition here, which is that after doing the below + * check and before adding this proc's clog update to a group, the + * group leader might have already finished the group update for this + * page and becomes group leader of another group. This will lead to a + * situation where a single group can have different clog page + * updates. This isn't likely and will still work, just maybe a bit + * less efficiently. + */ + if (nextidx != INVALID_PGPROCNO && + ProcGlobal->allProcs[nextidx].clogGroupMemberPage != proc->clogGroupMemberPage) + { + proc->clogGroupMember = false; + return false; + } + + pg_atomic_write_u32(&proc->clogGroupNext, nextidx); + + if (pg_atomic_compare_exchange_u32(&procglobal->clogGroupFirst, + &nextidx, + (uint32) proc->pgprocno)) + break; + } + + /* + * If the list was not empty, the leader will update the status of our + * XID. It is impossible to have followers without a leader because the + * first process that has added itself to the list will always have + * nextidx as INVALID_PGPROCNO. + */ + if (nextidx != INVALID_PGPROCNO) + { + int extraWaits = 0; + + /* Sleep until the leader updates our XID status. */ + for (;;) + { + /* acts as a read barrier */ + PGSemaphoreLock(proc->sem); + if (!proc->clogGroupMember) + break; + extraWaits++; + } + + Assert(pg_atomic_read_u32(&proc->clogGroupNext) == INVALID_PGPROCNO); + + /* Fix semaphore count for any absorbed wakeups */ + while (extraWaits-- > 0) + PGSemaphoreUnlock(proc->sem); + return true; + } + + /* We are the leader. Acquire the lock on behalf of everyone. */ + LWLockAcquire(CLogControlLock, LW_EXCLUSIVE); + /* + * Now that we've got the lock, clear the list of processes waiting for + * group XID status update, saving a pointer to the head of the list. + * Trying to pop elements one at a time could lead to an ABA problem. + */ + nextidx = pg_atomic_exchange_u32(&procglobal->clogGroupFirst, INVALID_PGPROCNO); + + /* Remember head of list so we can perform wakeups after dropping lock. */ + wakeidx = nextidx; + + /* Walk the list and update the status of all XIDs. */ + while (nextidx != INVALID_PGPROCNO) + { + PGPROC *proc = &ProcGlobal->allProcs[nextidx]; + PGXACT *pgxact = &ProcGlobal->allPgXact[nextidx]; + + /* + * Overflowed transactions should not use group XID status update + * mechanism. + */ + Assert(!pgxact->overflowed); + + TransactionIdSetPageStatusInternal(proc->clogGroupMemberXid, + pgxact->nxids, + proc->subxids.xids, + proc->clogGroupMemberXidStatus, + proc->clogGroupMemberLsn, + proc->clogGroupMemberPage); + + /* Move to next proc in list. */ + nextidx = pg_atomic_read_u32(&proc->clogGroupNext); + } + + /* We're done with the lock now. */ LWLockRelease(CLogControlLock); + + /* + * Now that we've released the lock, go back and wake everybody up. We + * don't do this under the lock so as to keep lock hold times to a + * minimum. The system calls we need to perform to wake other processes + * up are probably slower and can cause performance slowdown if done under + * lock. + */ + while (wakeidx != INVALID_PGPROCNO) + { + PGPROC *proc = &ProcGlobal->allProcs[wakeidx]; + + wakeidx = pg_atomic_read_u32(&proc->clogGroupNext); + pg_atomic_write_u32(&proc->clogGroupNext, INVALID_PGPROCNO); + + /* ensure all previous writes are visible before follower continues. */ + pg_write_barrier(); + + proc->clogGroupMember = false; + + if (proc != MyProc) + PGSemaphoreUnlock(proc->sem); + } + + return true; } /* diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c index bfa8499..5f6727d 100644 --- a/src/backend/storage/lmgr/proc.c +++ b/src/backend/storage/lmgr/proc.c @@ -186,6 +186,7 @@ InitProcGlobal(void) ProcGlobal->walwriterLatch = NULL; ProcGlobal->checkpointerLatch = NULL; pg_atomic_init_u32(&ProcGlobal->procArrayGroupFirst, INVALID_PGPROCNO); + pg_atomic_init_u32(&ProcGlobal->clogGroupFirst, INVALID_PGPROCNO); /* * Create and initialize all the PGPROC structures we'll need. There are @@ -408,6 +409,14 @@ InitProcess(void) /* Initialize wait event information. */ MyProc->wait_event_info = 0; + /* Initialize fields for group transaction status update. */ + MyProc->clogGroupMember = false; + MyProc->clogGroupMemberXid = InvalidTransactionId; + MyProc->clogGroupMemberXidStatus = TRANSACTION_STATUS_IN_PROGRESS; + MyProc->clogGroupMemberPage = -1; + MyProc->clogGroupMemberLsn = InvalidXLogRecPtr; + pg_atomic_init_u32(&MyProc->clogGroupNext, INVALID_PGPROCNO); + /* * Acquire ownership of the PGPROC's latch, so that we can use WaitLatch * on it. That allows us to repoint the process latch, which so far diff --git a/src/include/storage/proc.h b/src/include/storage/proc.h index 7dbaa81..205f484 100644 --- a/src/include/storage/proc.h +++ b/src/include/storage/proc.h @@ -14,6 +14,7 @@ #ifndef _PROC_H_ #define _PROC_H_ +#include "access/clog.h" #include "access/xlogdefs.h" #include "lib/ilist.h" #include "storage/latch.h" @@ -171,6 +172,17 @@ struct PGPROC uint32 wait_event_info; /* proc's wait information */ + /* Support for group transaction status update. */ + bool clogGroupMember; /* true, if member of clog group */ + pg_atomic_uint32 clogGroupNext; /* next clog group member */ + TransactionId clogGroupMemberXid; /* transaction id of clog group member */ + XidStatus clogGroupMemberXidStatus; /* transaction status of clog + * group member */ + int clogGroupMemberPage; /* clog page corresponding to + * transaction id of clog group member */ + XLogRecPtr clogGroupMemberLsn; /* WAL location of commit record for clog + * group member */ + /* Per-backend LWLock. Protects fields below (but not group fields). */ LWLock backendLock; @@ -242,6 +254,8 @@ typedef struct PROC_HDR PGPROC *bgworkerFreeProcs; /* First pgproc waiting for group XID clear */ pg_atomic_uint32 procArrayGroupFirst; + /* First pgproc waiting for group transaction status update */ + pg_atomic_uint32 clogGroupFirst; /* WALWriter process's latch */ Latch *walwriterLatch; /* Checkpointer process's latch */