snapshot scalability: Move subxact info to ProcGlobal, remove PGXACT.

Similar to the previous changes this increases the chance that data
frequently needed by GetSnapshotData() stays in l2 cache. In many
workloads subtransactions are very rare, and this makes the check for
that considerably cheaper.

As this removes the last member of PGXACT, there is no need to keep it
around anymore.

On a larger 2 socket machine this and the two preceding commits result
in a ~1.07x performance increase in read-only pgbench. For read-heavy
mixed r/w workloads without row level contention, I see about 1.1x.

Author: Andres Freund <andres@anarazel.de>
Reviewed-By: Robert Haas <robertmhaas@gmail.com>
Reviewed-By: Thomas Munro <thomas.munro@gmail.com>
Reviewed-By: David Rowley <dgrowleyml@gmail.com>
Discussion: https://postgr.es/m/20200301083601.ews6hz5dduc3w2se@alap3.anarazel.de

1 files changed, 4 insertions, 20 deletions

diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index f6113b2d243..aa9fbd80545 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -63,9 +63,8 @@ int			LockTimeout = 0;
 int			IdleInTransactionSessionTimeout = 0;
 bool		log_lock_waits = false;
 
-/* Pointer to this process's PGPROC and PGXACT structs, if any */
+/* Pointer to this process's PGPROC struct, if any */
 PGPROC	   *MyProc = NULL;
-PGXACT	   *MyPgXact = NULL;
 
 /*
  * This spinlock protects the freelist of recycled PGPROC structures.
@@ -110,10 +109,8 @@ ProcGlobalShmemSize(void)
 	size = add_size(size, mul_size(TotalProcs, sizeof(PGPROC)));
 	size = add_size(size, sizeof(slock_t));
 
-	size = add_size(size, mul_size(MaxBackends, sizeof(PGXACT)));
-	size = add_size(size, mul_size(NUM_AUXILIARY_PROCS, sizeof(PGXACT)));
-	size = add_size(size, mul_size(max_prepared_xacts, sizeof(PGXACT)));
 	size = add_size(size, mul_size(TotalProcs, sizeof(*ProcGlobal->xids)));
+	size = add_size(size, mul_size(TotalProcs, sizeof(*ProcGlobal->subxidStates)));
 	size = add_size(size, mul_size(TotalProcs, sizeof(*ProcGlobal->vacuumFlags)));
 
 	return size;
@@ -161,7 +158,6 @@ void
 InitProcGlobal(void)
 {
 	PGPROC	   *procs;
-	PGXACT	   *pgxacts;
 	int			i,
 				j;
 	bool		found;
@@ -203,18 +199,6 @@ InitProcGlobal(void)
 	ProcGlobal->allProcCount = MaxBackends + NUM_AUXILIARY_PROCS;
 
 	/*
-	 * Also allocate a separate array of PGXACT structures.  This is separate
-	 * from the main PGPROC array so that the most heavily accessed data is
-	 * stored contiguously in memory in as few cache lines as possible. This
-	 * provides significant performance benefits, especially on a
-	 * multiprocessor system.  There is one PGXACT structure for every PGPROC
-	 * structure.
-	 */
-	pgxacts = (PGXACT *) ShmemAlloc(TotalProcs * sizeof(PGXACT));
-	MemSet(pgxacts, 0, TotalProcs * sizeof(PGXACT));
-	ProcGlobal->allPgXact = pgxacts;
-
-	/*
 	 * Allocate arrays mirroring PGPROC fields in a dense manner. See
 	 * PROC_HDR.
 	 *
@@ -224,6 +208,8 @@ InitProcGlobal(void)
 	ProcGlobal->xids =
 		(TransactionId *) ShmemAlloc(TotalProcs * sizeof(*ProcGlobal->xids));
 	MemSet(ProcGlobal->xids, 0, TotalProcs * sizeof(*ProcGlobal->xids));
+	ProcGlobal->subxidStates = (XidCacheStatus *) ShmemAlloc(TotalProcs * sizeof(*ProcGlobal->subxidStates));
+	MemSet(ProcGlobal->subxidStates, 0, TotalProcs * sizeof(*ProcGlobal->subxidStates));
 	ProcGlobal->vacuumFlags = (uint8 *) ShmemAlloc(TotalProcs * sizeof(*ProcGlobal->vacuumFlags));
 	MemSet(ProcGlobal->vacuumFlags, 0, TotalProcs * sizeof(*ProcGlobal->vacuumFlags));
 
@@ -372,7 +358,6 @@ InitProcess(void)
 				(errcode(ERRCODE_TOO_MANY_CONNECTIONS),
 				 errmsg("sorry, too many clients already")));
 	}
-	MyPgXact = &ProcGlobal->allPgXact[MyProc->pgprocno];
 
 	/*
 	 * Cross-check that the PGPROC is of the type we expect; if this were not
@@ -569,7 +554,6 @@ InitAuxiliaryProcess(void)
 	((volatile PGPROC *) auxproc)->pid = MyProcPid;
 
 	MyProc = auxproc;
-	MyPgXact = &ProcGlobal->allPgXact[auxproc->pgprocno];
 
 	SpinLockRelease(ProcStructLock);