On 7/17/26 11:54, Matthias van de Meent wrote:
On Thu, 11 Jun 2026 at 16:31, Maksim.Melnikov <[email protected]> wrote:
Patch with fixes was attached. Thanks for review.
You removed the pgxactoff field with its comment from PGPROC, but
haven't yet added a comment on PROC_HDR's pgxactoffs. Please add a
comment to the pgxactoffs array that describes its contents. A simple
adaptation of the comment removed with PGPROC's pgxactoff field should
be sufficient.
The comments on PROC_HDR and PGPROC also reference the deleted
PGPROC->pgxactoff field, so that also must be adjusted; there may be
several other places, as I didn't do a full check on the codebase.
Once the relevant comments are added and adjusted I think this is
ready for a committer.
Kind regards,
Matthias van de Meent
Databricks (https://www.databricks.com)
Sorry for the delay, was unavailable for several weeks.
Thanks for review, patch was updated.
Best regards,
Maksim Melnikov
From 142b29615475aeec750b9cc92c6f41ddb6d68041 Mon Sep 17 00:00:00 2001
From: Maksim Melnikov <[email protected]>
Date: Tue, 18 Nov 2025 17:20:09 +0300
Subject: [PATCH v3] This patch reduce connection init/close time.
ProcArrayRemove/ProcArrayAdd are expensive in terms of accessing pgxactoff
field in PGPROC, because its are placed on different pages and it the
reason of page_faults occurence. Now one of the use case with pgxactoff
is iteration with gaps over PGPROCs and read/write pgxactoff, PGPROC
allocate ~1KB and it quite enough to have page_faults in case of such
accessing.
So we placed all pgxactoff in the separate array pgxactoffs to have adjacent
pages for them all. Indexes in allProcs aligned with pgxactoffs array, so
the Nth element in pgxactoffs refer to Nth PGPROC.
Eventually it helps to avoid extra page_faults and reduce connection
init/close time. The main benefit is seen on configurations without huge
pages.
---
src/backend/access/transam/varsup.c | 10 ++---
src/backend/commands/indexcmds.c | 2 +-
src/backend/commands/vacuum.c | 2 +-
src/backend/replication/logical/logical.c | 2 +-
src/backend/replication/slot.c | 2 +-
src/backend/replication/walsender.c | 2 +-
src/backend/storage/ipc/procarray.c | 53 +++++++++++++----------
src/backend/storage/lmgr/proc.c | 6 ++-
src/include/storage/proc.h | 22 +++++++---
9 files changed, 61 insertions(+), 40 deletions(-)
diff --git a/src/backend/access/transam/varsup.c b/src/backend/access/transam/varsup.c
index dc5e32d86f3..1bbc1bcd665 100644
--- a/src/backend/access/transam/varsup.c
+++ b/src/backend/access/transam/varsup.c
@@ -85,7 +85,7 @@ GetNewTransactionId(bool isSubXact)
{
Assert(!isSubXact);
MyProc->xid = BootstrapTransactionId;
- ProcGlobal->xids[MyProc->pgxactoff] = BootstrapTransactionId;
+ ProcGlobal->xids[ProcGetMyXactOff()] = BootstrapTransactionId;
return FullTransactionIdFromEpochAndXid(0, BootstrapTransactionId);
}
@@ -244,18 +244,18 @@ GetNewTransactionId(bool isSubXact)
*/
if (!isSubXact)
{
- Assert(ProcGlobal->subxidStates[MyProc->pgxactoff].count == 0);
- Assert(!ProcGlobal->subxidStates[MyProc->pgxactoff].overflowed);
+ Assert(ProcGlobal->subxidStates[ProcGetMyXactOff()].count == 0);
+ Assert(!ProcGlobal->subxidStates[ProcGetMyXactOff()].overflowed);
Assert(MyProc->subxidStatus.count == 0);
Assert(!MyProc->subxidStatus.overflowed);
/* LWLockRelease acts as barrier */
MyProc->xid = xid;
- ProcGlobal->xids[MyProc->pgxactoff] = xid;
+ ProcGlobal->xids[ProcGetMyXactOff()] = xid;
}
else
{
- XidCacheStatus *substat = &ProcGlobal->subxidStates[MyProc->pgxactoff];
+ XidCacheStatus *substat = &ProcGlobal->subxidStates[ProcGetMyXactOff()];
int nxids = MyProc->subxidStatus.count;
Assert(substat->count == MyProc->subxidStatus.count);
diff --git a/src/backend/commands/indexcmds.c b/src/backend/commands/indexcmds.c
index 713bb5d10f1..046c28a10e7 100644
--- a/src/backend/commands/indexcmds.c
+++ b/src/backend/commands/indexcmds.c
@@ -4779,6 +4779,6 @@ set_indexsafe_procflags(void)
LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
MyProc->statusFlags |= PROC_IN_SAFE_IC;
- ProcGlobal->statusFlags[MyProc->pgxactoff] = MyProc->statusFlags;
+ ProcGlobal->statusFlags[ProcGetMyXactOff()] = MyProc->statusFlags;
LWLockRelease(ProcArrayLock);
}
diff --git a/src/backend/commands/vacuum.c b/src/backend/commands/vacuum.c
index 38539a6fd3d..fc49b8e14d4 100644
--- a/src/backend/commands/vacuum.c
+++ b/src/backend/commands/vacuum.c
@@ -2059,7 +2059,7 @@ vacuum_rel(Oid relid, RangeVar *relation, VacuumParams params,
MyProc->statusFlags |= PROC_IN_VACUUM;
if (params.is_wraparound)
MyProc->statusFlags |= PROC_VACUUM_FOR_WRAPAROUND;
- ProcGlobal->statusFlags[MyProc->pgxactoff] = MyProc->statusFlags;
+ ProcGlobal->statusFlags[ProcGetMyXactOff()] = MyProc->statusFlags;
LWLockRelease(ProcArrayLock);
}
diff --git a/src/backend/replication/logical/logical.c b/src/backend/replication/logical/logical.c
index 3541fc793e4..c2c96a70c92 100644
--- a/src/backend/replication/logical/logical.c
+++ b/src/backend/replication/logical/logical.c
@@ -188,7 +188,7 @@ StartupDecodingContext(List *output_plugin_options,
{
LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
MyProc->statusFlags |= PROC_IN_LOGICAL_DECODING;
- ProcGlobal->statusFlags[MyProc->pgxactoff] = MyProc->statusFlags;
+ ProcGlobal->statusFlags[ProcGetMyXactOff()] = MyProc->statusFlags;
LWLockRelease(ProcArrayLock);
}
diff --git a/src/backend/replication/slot.c b/src/backend/replication/slot.c
index c7ab82a28e1..da4e01f0caf 100644
--- a/src/backend/replication/slot.c
+++ b/src/backend/replication/slot.c
@@ -832,7 +832,7 @@ ReplicationSlotRelease(void)
/* might not have been set when we've been a plain slot */
LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
MyProc->statusFlags &= ~PROC_IN_LOGICAL_DECODING;
- ProcGlobal->statusFlags[MyProc->pgxactoff] = MyProc->statusFlags;
+ ProcGlobal->statusFlags[ProcGetMyXactOff()] = MyProc->statusFlags;
LWLockRelease(ProcArrayLock);
if (am_walsender)
diff --git a/src/backend/replication/walsender.c b/src/backend/replication/walsender.c
index 35ebc7e61c8..6b84de1bcfb 100644
--- a/src/backend/replication/walsender.c
+++ b/src/backend/replication/walsender.c
@@ -358,7 +358,7 @@ InitWalSender(void)
Assert(MyProc->xmin == InvalidTransactionId);
LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
MyProc->statusFlags |= PROC_AFFECTS_ALL_HORIZONS;
- ProcGlobal->statusFlags[MyProc->pgxactoff] = MyProc->statusFlags;
+ ProcGlobal->statusFlags[ProcGetMyXactOff()] = MyProc->statusFlags;
LWLockRelease(ProcArrayLock);
}
diff --git a/src/backend/storage/ipc/procarray.c b/src/backend/storage/ipc/procarray.c
index 60336b31803..e6075dafe9f 100644
--- a/src/backend/storage/ipc/procarray.c
+++ b/src/backend/storage/ipc/procarray.c
@@ -284,6 +284,12 @@ typedef enum KAXCompressReason
static PGPROC *allProcs;
+/*
+ * offsets into various ProcGlobal->arrays with data mirrored from appropriate PGPROCs,
+ * procno define offset index in pgxactoffs array(See PROC_HDR for details).
+ */
+static int *pgxactoffs;
+
/*
* Cache to reduce overhead of repeated calls to TransactionIdIsInProgress()
*/
@@ -449,6 +455,7 @@ ProcArrayShmemInit(void *arg)
TransamVariables->xactCompletionCount = 1;
allProcs = ProcGlobal->allProcs;
+ pgxactoffs = ProcGlobal->pgxactoffs;
}
static void
@@ -498,7 +505,7 @@ ProcArrayAdd(PGPROC *proc)
int this_procno = arrayP->pgprocnos[index];
Assert(this_procno >= 0 && this_procno < (arrayP->maxProcs + NUM_AUXILIARY_PROCS));
- Assert(allProcs[this_procno].pgxactoff == index);
+ Assert(pgxactoffs[this_procno] == index);
/* If we have found our right position in the array, break */
if (this_procno > pgprocno)
@@ -520,23 +527,23 @@ ProcArrayAdd(PGPROC *proc)
movecount * sizeof(*ProcGlobal->statusFlags));
arrayP->pgprocnos[index] = GetNumberFromPGProc(proc);
- proc->pgxactoff = index;
+ pgxactoffs[arrayP->pgprocnos[index]] = index;
ProcGlobal->xids[index] = proc->xid;
ProcGlobal->subxidStates[index] = proc->subxidStatus;
ProcGlobal->statusFlags[index] = proc->statusFlags;
arrayP->numProcs++;
- /* adjust pgxactoff for all following PGPROCs */
+ /* adjust appropriate pgxactoff for all following PGPROCs */
index++;
for (; index < arrayP->numProcs; index++)
{
int procno = arrayP->pgprocnos[index];
Assert(procno >= 0 && procno < (arrayP->maxProcs + NUM_AUXILIARY_PROCS));
- Assert(allProcs[procno].pgxactoff == index - 1);
+ Assert(pgxactoffs[procno] == index - 1);
- allProcs[procno].pgxactoff = index;
+ pgxactoffs[procno] = index;
}
/*
@@ -574,10 +581,10 @@ ProcArrayRemove(PGPROC *proc, TransactionId latestXid)
LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
LWLockAcquire(XidGenLock, LW_EXCLUSIVE);
- myoff = proc->pgxactoff;
+ myoff = ProcGetXactOff(GetNumberFromPGProc(proc));
Assert(myoff >= 0 && myoff < arrayP->numProcs);
- Assert(ProcGlobal->allProcs[arrayP->pgprocnos[myoff]].pgxactoff == myoff);
+ Assert(ProcGlobal->pgxactoffs[arrayP->pgprocnos[myoff]] == myoff);
if (TransactionIdIsValid(latestXid))
{
@@ -624,17 +631,17 @@ ProcArrayRemove(PGPROC *proc, TransactionId latestXid)
arrayP->numProcs--;
/*
- * Adjust pgxactoff of following procs for removed PGPROC (note that
- * numProcs already has been decremented).
+ * Adjust appropriate pgxactoff of following procs for removed PGPROC
+ * (note that numProcs already has been decremented).
*/
for (int index = myoff; index < arrayP->numProcs; index++)
{
int procno = arrayP->pgprocnos[index];
Assert(procno >= 0 && procno < (arrayP->maxProcs + NUM_AUXILIARY_PROCS));
- Assert(allProcs[procno].pgxactoff - 1 == index);
+ Assert(pgxactoffs[procno] - 1 == index);
- allProcs[procno].pgxactoff = index;
+ pgxactoffs[procno] = index;
}
/*
@@ -706,11 +713,13 @@ ProcArrayEndTransaction(PGPROC *proc, TransactionId latestXid)
/* avoid unnecessarily dirtying shared cachelines */
if (proc->statusFlags & PROC_VACUUM_STATE_MASK)
{
+ int pgxactoff = ProcGetXactOff(GetNumberFromPGProc(proc));
+
Assert(!LWLockHeldByMe(ProcArrayLock));
LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
- Assert(proc->statusFlags == ProcGlobal->statusFlags[proc->pgxactoff]);
+ Assert(proc->statusFlags == ProcGlobal->statusFlags[pgxactoff]);
proc->statusFlags &= ~PROC_VACUUM_STATE_MASK;
- ProcGlobal->statusFlags[proc->pgxactoff] = proc->statusFlags;
+ ProcGlobal->statusFlags[pgxactoff] = proc->statusFlags;
LWLockRelease(ProcArrayLock);
}
}
@@ -724,7 +733,7 @@ ProcArrayEndTransaction(PGPROC *proc, TransactionId latestXid)
static inline void
ProcArrayEndTransactionInternal(PGPROC *proc, TransactionId latestXid)
{
- int pgxactoff = proc->pgxactoff;
+ int pgxactoff = ProcGetXactOff(GetNumberFromPGProc(proc));
/*
* Note: we need exclusive lock here because we're going to change other
@@ -747,7 +756,7 @@ ProcArrayEndTransactionInternal(PGPROC *proc, TransactionId latestXid)
if (proc->statusFlags & PROC_VACUUM_STATE_MASK)
{
proc->statusFlags &= ~PROC_VACUUM_STATE_MASK;
- ProcGlobal->statusFlags[proc->pgxactoff] = proc->statusFlags;
+ ProcGlobal->statusFlags[pgxactoff] = proc->statusFlags;
}
/* Clear the subtransaction-XID cache too while holding the lock */
@@ -903,7 +912,7 @@ ProcArrayClearTransaction(PGPROC *proc)
/*
* Currently we need to lock ProcArrayLock exclusively here, as we
* increment xactCompletionCount below. We also need it at least in shared
- * mode for pgproc->pgxactoff to stay the same below.
+ * mode for proc's pgxactoff to stay the same below.
*
* We could however, as this action does not actually change anyone's view
* of the set of running XIDs (our entry is duplicate with the gxact that
@@ -916,7 +925,7 @@ ProcArrayClearTransaction(PGPROC *proc)
*/
LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
- pgxactoff = proc->pgxactoff;
+ pgxactoff = ProcGetXactOff(GetNumberFromPGProc(proc));
ProcGlobal->xids[pgxactoff] = InvalidTransactionId;
proc->xid = InvalidTransactionId;
@@ -1475,7 +1484,7 @@ TransactionIdIsInProgress(TransactionId xid)
}
/* No shortcuts, gotta grovel through the array */
- mypgxactoff = MyProc->pgxactoff;
+ mypgxactoff = ProcGetMyXactOff();
numProcs = arrayP->numProcs;
for (int pgxactoff = 0; pgxactoff < numProcs; pgxactoff++)
{
@@ -2184,7 +2193,7 @@ GetSnapshotData(Snapshot snapshot)
}
latest_completed = TransamVariables->latestCompletedXid;
- mypgxactoff = MyProc->pgxactoff;
+ mypgxactoff = ProcGetMyXactOff();
myxid = other_xids[mypgxactoff];
Assert(myxid == MyProc->xid);
@@ -2223,7 +2232,7 @@ GetSnapshotData(Snapshot snapshot)
TransactionId xid = UINT32_ACCESS_ONCE(other_xids[pgxactoff]);
uint8 statusFlags;
- Assert(allProcs[arrayP->pgprocnos[pgxactoff]].pgxactoff == pgxactoff);
+ Assert(pgxactoffs[arrayP->pgprocnos[pgxactoff]] == pgxactoff);
/*
* If the transaction has no XID assigned, we can skip it; it
@@ -2591,7 +2600,7 @@ ProcArrayInstallRestoredXmin(TransactionId xmin, PGPROC *proc)
MyProc->xmin = TransactionXmin = xmin;
MyProc->statusFlags = (MyProc->statusFlags & ~PROC_XMIN_FLAGS) |
(proc->statusFlags & PROC_XMIN_FLAGS);
- ProcGlobal->statusFlags[MyProc->pgxactoff] = MyProc->statusFlags;
+ ProcGlobal->statusFlags[ProcGetMyXactOff()] = MyProc->statusFlags;
result = true;
}
@@ -4019,7 +4028,7 @@ XidCacheRemoveRunningXids(TransactionId xid,
*/
LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
- mysubxidstat = &ProcGlobal->subxidStates[MyProc->pgxactoff];
+ mysubxidstat = &ProcGlobal->subxidStates[ProcGetMyXactOff()];
/*
* Under normal circumstances xid and xids[] will be in increasing order,
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index 9d6e69175a5..e64ab6e597c 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -164,6 +164,7 @@ ProcGlobalShmemRequest(void *arg)
size = add_size(size, mul_size(TotalProcs, sizeof(PGPROC)));
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->pgxactoffs)));
size = add_size(size, mul_size(TotalProcs, sizeof(*ProcGlobal->statusFlags)));
ProcGlobalAllProcsShmemSize = size;
ShmemRequestStruct(.name = "PGPROC structures",
@@ -271,6 +272,9 @@ ProcGlobalShmemInit(void *arg)
ProcGlobal->subxidStates = (XidCacheStatus *) ptr;
ptr = ptr + (TotalProcs * sizeof(*ProcGlobal->subxidStates));
+ ProcGlobal->pgxactoffs = (int *) ptr;
+ ptr = (char *) ptr + TotalProcs * sizeof(*ProcGlobal->pgxactoffs);
+
ProcGlobal->statusFlags = (uint8 *) ptr;
ptr = ptr + (TotalProcs * sizeof(*ProcGlobal->statusFlags));
@@ -1561,7 +1565,7 @@ ProcSleep(LOCALLOCK *locallock)
* the lock held, which is much more undesirable.
*/
LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
- statusFlags = ProcGlobal->statusFlags[autovac->pgxactoff];
+ statusFlags = ProcGlobal->statusFlags[ProcGetXactOff(GetNumberFromPGProc(autovac))];
lockmethod_copy = lock->tag.locktag_lockmethodid;
locktag_copy = lock->tag;
LWLockRelease(ProcArrayLock);
diff --git a/src/include/storage/proc.h b/src/include/storage/proc.h
index 03a1a466fa8..e08d8dd8a4a 100644
--- a/src/include/storage/proc.h
+++ b/src/include/storage/proc.h
@@ -174,7 +174,8 @@ typedef enum
*
* Some fields in PGPROC (see "mirrored in ..." comment) are mirrored into an
* element of more densely packed ProcGlobal arrays. These arrays are indexed
- * by PGPROC->pgxactoff. Both copies need to be maintained coherently.
+ * by PROC_HDR->pgxactoffs array elements. Both copies need to be maintained
+ * coherently.
*
* NB: The pgxactoff indexed value can *never* be accessed without holding
* locks.
@@ -210,9 +211,6 @@ typedef struct PGPROC
Oid tempNamespaceId; /* OID of temp schema this backend is
* using */
- int pgxactoff; /* offset into various ProcGlobal->arrays with
- * data mirrored from this PGPROC */
-
uint8 statusFlags; /* this backend's status flags, see PROC_*
* above. mirrored in
* ProcGlobal->statusFlags[pgxactoff] */
@@ -402,10 +400,11 @@ extern PGDLLIMPORT PGPROC *MyProc;
* for PGPROCs that have been added to the shared array with ProcArrayAdd()
* (in contrast to PGPROC array which has unused PGPROCs interspersed).
*
- * The dense arrays are indexed by PGPROC->pgxactoff. Any concurrent
+ * The dense arrays are indexed by PROC_HDR->pgxactoffs array elements, procno
+ * define position of appropriate offset in pgxactoffs array. Any concurrent
* ProcArrayAdd() / ProcArrayRemove() can lead to pgxactoff of a procarray
- * member to change. Therefore it is only safe to use PGPROC->pgxactoff to
- * access the dense array while holding either ProcArrayLock or XidGenLock.
+ * member to change. Therefore it is only safe to use appropriate proc's pgxactoff
+ * to access the dense array while holding either ProcArrayLock or XidGenLock.
*
* As long as a PGPROC is in the procarray, the mirrored values need to be
* maintained in both places in a coherent manner.
@@ -446,6 +445,13 @@ typedef struct PROC_HDR
/* Array of PGPROC structures (not including dummies for prepared txns) */
PGPROC *allProcs;
+ /*
+ * offsets into various ProcGlobal->arrays with data mirrored from
+ * appropriate PGPROCs. The PGPROC's appropriate pgxactoff element index
+ * is the same as PGPROC index in allProcs(check ProcGetMyXactOff).
+ */
+ int *pgxactoffs;
+
/* Array mirroring PGPROC.xid for each PGPROC currently in the procarray */
TransactionId *xids;
@@ -511,6 +517,8 @@ extern PGDLLIMPORT PGPROC *PreparedXactProcs;
*/
#define GetPGProcByNumber(n) (&ProcGlobal->allProcs[(n)])
#define GetNumberFromPGProc(proc) ((proc) - &ProcGlobal->allProcs[0])
+#define ProcGetXactOff(procno) (ProcGlobal->pgxactoffs[(procno)])
+#define ProcGetMyXactOff() (ProcGetXactOff(GetNumberFromPGProc(MyProc)))
/*
* We set aside some extra PGPROC structures for "special worker" processes,
--
2.43.0
