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Replace known_assigned_xids_lck with memory barriers.

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This lock was introduced before memory barrier support was added,
and it is only used to guarantee proper memory ordering when
KnownAssignedXidsAdd() appends to the array without a lock.  Now
that such memory barrier support exists, we can remove the lock and
use barriers instead.

Suggested-by: Tom Lane
Author: Michail Nikolaev
Reviewed-by: Robert Haas
Discussion: https://postgr.es/m/CANtu0oh0si%3DjG5z_fLeFtmYcETssQ08kLEa8b6TQqDm_cinroA%40mail.gmail.com
This commit is contained in:
Nathan Bossart 2023-09-05 13:59:06 -07:00
parent a899d07b24
commit 119c23eb98
1 changed files with 28 additions and 48 deletions
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76
src/backend/storage/ipc/procarray.c
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@ -61,7 +61,6 @@
#include "port/pg_lfind.h"
#include "storage/proc.h"
#include "storage/procarray.h"
#include "storage/spin.h"
#include "utils/acl.h"
#include "utils/builtins.h"
#include "utils/rel.h"
@ -82,7 +81,6 @@ typedef struct ProcArrayStruct
int numKnownAssignedXids; /* current # of valid entries */
int tailKnownAssignedXids; /* index of oldest valid element */
int headKnownAssignedXids; /* index of newest element, + 1 */
slock_t known_assigned_xids_lck; /* protects head/tail pointers */
/*
* Highest subxid that has been removed from KnownAssignedXids array to
@ -441,7 +439,6 @@ CreateSharedProcArray(void)
procArray->numKnownAssignedXids = 0;
procArray->tailKnownAssignedXids = 0;
procArray->headKnownAssignedXids = 0;
SpinLockInit(&procArray->known_assigned_xids_lck);
procArray->lastOverflowedXid = InvalidTransactionId;
procArray->replication_slot_xmin = InvalidTransactionId;
procArray->replication_slot_catalog_xmin = InvalidTransactionId;
@ -4533,22 +4530,19 @@ KnownAssignedTransactionIdsIdleMaintenance(void)
* during normal running). Compressing unused entries out of the array
* likewise requires exclusive lock. To add XIDs to the array, we just insert
* them into slots to the right of the head pointer and then advance the head
* pointer. This wouldn't require any lock at all, except that on machines
* with weak memory ordering we need to be careful that other processors
* see the array element changes before they see the head pointer change.
* We handle this by using a spinlock to protect reads and writes of the
* head/tail pointers. (We could dispense with the spinlock if we were to
* create suitable memory access barrier primitives and use those instead.)
* The spinlock must be taken to read or write the head/tail pointers unless
* the caller holds ProcArrayLock exclusively.
* pointer. This doesn't require any lock at all, but on machines with weak
* memory ordering, we need to be careful that other processors see the array
* element changes before they see the head pointer change. We handle this by
* using memory barriers when reading or writing the head/tail pointers (unless
* the caller holds ProcArrayLock exclusively).
*
* Algorithmic analysis:
*
* If we have a maximum of M slots, with N XIDs currently spread across
* S elements then we have N <= S <= M always.
*
* * Adding a new XID is O(1) and needs little locking (unless compression
* must happen)
* * Adding a new XID is O(1) and needs no lock (unless compression must
* happen)
* * Compressing the array is O(S) and requires exclusive lock
* * Removing an XID is O(logS) and requires exclusive lock
* * Taking a snapshot is O(S) and requires shared lock
@ -4778,22 +4772,15 @@ KnownAssignedXidsAdd(TransactionId from_xid, TransactionId to_xid,
pArray->numKnownAssignedXids += nxids;
/*
* Now update the head pointer. We use a spinlock to protect this
* pointer, not because the update is likely to be non-atomic, but to
* ensure that other processors see the above array updates before they
* see the head pointer change.
*
* If we're holding ProcArrayLock exclusively, there's no need to take the
* spinlock.
* Now update the head pointer. We use a write barrier to ensure that
* other processors see the above array updates before they see the head
* pointer change. The barrier isn't required if we're holding
* ProcArrayLock exclusively.
*/
if (exclusive_lock)
pArray->headKnownAssignedXids = head;
else
{
SpinLockAcquire(&pArray->known_assigned_xids_lck);
pArray->headKnownAssignedXids = head;
SpinLockRelease(&pArray->known_assigned_xids_lck);
}
if (!exclusive_lock)
pg_write_barrier();
pArray->headKnownAssignedXids = head;
}
/*
@ -4815,20 +4802,15 @@ KnownAssignedXidsSearch(TransactionId xid, bool remove)
int tail;
int result_index = -1;
if (remove)
{
/* we hold ProcArrayLock exclusively, so no need for spinlock */
tail = pArray->tailKnownAssignedXids;
head = pArray->headKnownAssignedXids;
}
else
{
/* take spinlock to ensure we see up-to-date array contents */
SpinLockAcquire(&pArray->known_assigned_xids_lck);
tail = pArray->tailKnownAssignedXids;
head = pArray->headKnownAssignedXids;
SpinLockRelease(&pArray->known_assigned_xids_lck);
}
tail = pArray->tailKnownAssignedXids;
head = pArray->headKnownAssignedXids;
/*
* Only the startup process removes entries, so we don't need the read
* barrier in that case.
*/
if (!remove)
pg_read_barrier(); /* pairs with KnownAssignedXidsAdd */
/*
* Standard binary search. Note we can ignore the KnownAssignedXidsValid
@ -5066,13 +5048,11 @@ KnownAssignedXidsGetAndSetXmin(TransactionId *xarray, TransactionId *xmin,
* cannot enter and then leave the array while we hold ProcArrayLock. We
* might miss newly-added xids, but they should be >= xmax so irrelevant
* anyway.
*
* Must take spinlock to ensure we see up-to-date array contents.
*/
SpinLockAcquire(&procArray->known_assigned_xids_lck);
tail = procArray->tailKnownAssignedXids;
head = procArray->headKnownAssignedXids;
SpinLockRelease(&procArray->known_assigned_xids_lck);
pg_read_barrier(); /* pairs with KnownAssignedXidsAdd */
for (i = tail; i < head; i++)
{
@ -5119,10 +5099,10 @@ KnownAssignedXidsGetOldestXmin(void)
/*
* Fetch head just once, since it may change while we loop.
*/
SpinLockAcquire(&procArray->known_assigned_xids_lck);
tail = procArray->tailKnownAssignedXids;
head = procArray->headKnownAssignedXids;
SpinLockRelease(&procArray->known_assigned_xids_lck);
pg_read_barrier(); /* pairs with KnownAssignedXidsAdd */
for (i = tail; i < head; i++)
{