Sorry, brown paper bag bug there. Here's the correct one.
--
Álvaro Herrera 48°01'N 7°57'E — https://www.EnterpriseDB.com/
"I can't go to a restaurant and order food because I keep looking at the
fonts on the menu. Five minutes later I realize that it's also talking
about food" (Donald Knuth)
>From 99cadfdf7475146953e9846c20c4a708a3527937 Mon Sep 17 00:00:00 2001
From: Alvaro Herrera <alvhe...@alvh.no-ip.org>
Date: Wed, 31 Jan 2024 12:27:51 +0100
Subject: [PATCH v3] Use atomics for SlruSharedData->latest_page_number
---
src/backend/access/transam/clog.c | 6 +---
src/backend/access/transam/commit_ts.c | 7 ++---
src/backend/access/transam/multixact.c | 28 +++++++++++-------
src/backend/access/transam/slru.c | 40 +++++++++++++++++++-------
src/include/access/slru.h | 5 +++-
5 files changed, 54 insertions(+), 32 deletions(-)
diff --git a/src/backend/access/transam/clog.c b/src/backend/access/transam/clog.c
index f6e7da7ffc..06fc2989ba 100644
--- a/src/backend/access/transam/clog.c
+++ b/src/backend/access/transam/clog.c
@@ -766,14 +766,10 @@ StartupCLOG(void)
TransactionId xid = XidFromFullTransactionId(TransamVariables->nextXid);
int64 pageno = TransactionIdToPage(xid);
- LWLockAcquire(XactSLRULock, LW_EXCLUSIVE);
-
/*
* Initialize our idea of the latest page number.
*/
- XactCtl->shared->latest_page_number = pageno;
-
- LWLockRelease(XactSLRULock);
+ pg_atomic_write_u64(&XactCtl->shared->latest_page_number, pageno);
}
/*
diff --git a/src/backend/access/transam/commit_ts.c b/src/backend/access/transam/commit_ts.c
index 61b82385f3..6bfe60343e 100644
--- a/src/backend/access/transam/commit_ts.c
+++ b/src/backend/access/transam/commit_ts.c
@@ -689,9 +689,7 @@ ActivateCommitTs(void)
/*
* Re-Initialize our idea of the latest page number.
*/
- LWLockAcquire(CommitTsSLRULock, LW_EXCLUSIVE);
- CommitTsCtl->shared->latest_page_number = pageno;
- LWLockRelease(CommitTsSLRULock);
+ pg_atomic_write_u64(&CommitTsCtl->shared->latest_page_number, pageno);
/*
* If CommitTs is enabled, but it wasn't in the previous server run, we
@@ -1006,7 +1004,8 @@ commit_ts_redo(XLogReaderState *record)
* During XLOG replay, latest_page_number isn't set up yet; insert a
* suitable value to bypass the sanity test in SimpleLruTruncate.
*/
- CommitTsCtl->shared->latest_page_number = trunc->pageno;
+ pg_atomic_write_u64(&CommitTsCtl->shared->latest_page_number,
+ trunc->pageno);
SimpleLruTruncate(CommitTsCtl, trunc->pageno);
}
diff --git a/src/backend/access/transam/multixact.c b/src/backend/access/transam/multixact.c
index 59523be901..febc429f72 100644
--- a/src/backend/access/transam/multixact.c
+++ b/src/backend/access/transam/multixact.c
@@ -2017,13 +2017,15 @@ StartupMultiXact(void)
* Initialize offset's idea of the latest page number.
*/
pageno = MultiXactIdToOffsetPage(multi);
- MultiXactOffsetCtl->shared->latest_page_number = pageno;
+ pg_atomic_write_u64(&MultiXactOffsetCtl->shared->latest_page_number,
+ pageno);
/*
* Initialize member's idea of the latest page number.
*/
pageno = MXOffsetToMemberPage(offset);
- MultiXactMemberCtl->shared->latest_page_number = pageno;
+ pg_atomic_write_u64(&MultiXactMemberCtl->shared->latest_page_number,
+ pageno);
}
/*
@@ -2047,14 +2049,15 @@ TrimMultiXact(void)
oldestMXactDB = MultiXactState->oldestMultiXactDB;
LWLockRelease(MultiXactGenLock);
- /* Clean up offsets state */
- LWLockAcquire(MultiXactOffsetSLRULock, LW_EXCLUSIVE);
-
/*
* (Re-)Initialize our idea of the latest page number for offsets.
*/
pageno = MultiXactIdToOffsetPage(nextMXact);
- MultiXactOffsetCtl->shared->latest_page_number = pageno;
+ pg_atomic_write_u64(&MultiXactOffsetCtl->shared->latest_page_number,
+ pageno);
+
+ /* Clean up offsets state */
+ LWLockAcquire(MultiXactOffsetSLRULock, LW_EXCLUSIVE);
/*
* Zero out the remainder of the current offsets page. See notes in
@@ -2081,14 +2084,16 @@ TrimMultiXact(void)
LWLockRelease(MultiXactOffsetSLRULock);
- /* And the same for members */
- LWLockAcquire(MultiXactMemberSLRULock, LW_EXCLUSIVE);
-
/*
+ * And the same for members.
+ *
* (Re-)Initialize our idea of the latest page number for members.
*/
pageno = MXOffsetToMemberPage(offset);
- MultiXactMemberCtl->shared->latest_page_number = pageno;
+ pg_atomic_write_u64(&MultiXactMemberCtl->shared->latest_page_number,
+ pageno);
+
+ LWLockAcquire(MultiXactMemberSLRULock, LW_EXCLUSIVE);
/*
* Zero out the remainder of the current members page. See notes in
@@ -3333,7 +3338,8 @@ multixact_redo(XLogReaderState *record)
* SimpleLruTruncate.
*/
pageno = MultiXactIdToOffsetPage(xlrec.endTruncOff);
- MultiXactOffsetCtl->shared->latest_page_number = pageno;
+ pg_atomic_write_u64(&MultiXactOffsetCtl->shared->latest_page_number,
+ pageno);
PerformOffsetsTruncation(xlrec.startTruncOff, xlrec.endTruncOff);
LWLockRelease(MultiXactTruncationLock);
diff --git a/src/backend/access/transam/slru.c b/src/backend/access/transam/slru.c
index 9ac4790f16..c1d0dfc73b 100644
--- a/src/backend/access/transam/slru.c
+++ b/src/backend/access/transam/slru.c
@@ -17,7 +17,8 @@
* per-buffer LWLocks that synchronize I/O for each buffer. The control lock
* must be held to examine or modify any shared state. A process that is
* reading in or writing out a page buffer does not hold the control lock,
- * only the per-buffer lock for the buffer it is working on.
+ * only the per-buffer lock for the buffer it is working on. One exception
+ * is latest_page_number, which is read and written using atomic ops.
*
* "Holding the control lock" means exclusive lock in all cases except for
* SimpleLruReadPage_ReadOnly(); see comments for SlruRecentlyUsed() for
@@ -239,8 +240,7 @@ SimpleLruInit(SlruCtl ctl, const char *name, int nslots, int nlsns,
shared->lsn_groups_per_page = nlsns;
shared->cur_lru_count = 0;
-
- /* shared->latest_page_number will be set later */
+ pg_atomic_write_u64(&shared->latest_page_number, 0);
shared->slru_stats_idx = pgstat_get_slru_index(name);
@@ -329,8 +329,16 @@ SimpleLruZeroPage(SlruCtl ctl, int64 pageno)
/* Set the LSNs for this new page to zero */
SimpleLruZeroLSNs(ctl, slotno);
- /* Assume this page is now the latest active page */
- shared->latest_page_number = pageno;
+ /*
+ * Assume this page is now the latest active page.
+ *
+ * Note that because both this routine and SlruSelectLRUPage run with
+ * a bank lock held, it is not possible for this to be zeroing a page
+ * that SlruSelectLRUPage is going to evict simultaneously -- they would
+ * both have to hold the same bank lock! Therefore, there's no memory
+ * barrier here.
+ */
+ pg_atomic_write_u64(&shared->latest_page_number, pageno);
/* update the stats counter of zeroed pages */
pgstat_count_slru_page_zeroed(shared->slru_stats_idx);
@@ -1113,9 +1121,17 @@ SlruSelectLRUPage(SlruCtl ctl, int64 pageno)
shared->page_lru_count[slotno] = cur_count;
this_delta = 0;
}
+
+ /*
+ * If this page is the one most recently zeroed, don't consider it
+ * an eviction candidate. See comments in SimpleLruZeroPage for an
+ * explanation about the lack of a memory barrier here.
+ */
this_page_number = shared->page_number[slotno];
- if (this_page_number == shared->latest_page_number)
+ if (this_page_number ==
+ pg_atomic_read_u64(&shared->latest_page_number))
continue;
+
if (shared->page_status[slotno] == SLRU_PAGE_VALID)
{
if (this_delta > best_valid_delta ||
@@ -1254,7 +1270,6 @@ void
SimpleLruTruncate(SlruCtl ctl, int64 cutoffPage)
{
SlruShared shared = ctl->shared;
- int slotno;
/* update the stats counter of truncates */
pgstat_count_slru_truncate(shared->slru_stats_idx);
@@ -1270,10 +1285,13 @@ SimpleLruTruncate(SlruCtl ctl, int64 cutoffPage)
restart:
/*
- * While we are holding the lock, make an important safety check: the
- * current endpoint page must not be eligible for removal.
+ * An important safety check: the current endpoint page must not be
+ * eligible for removal. Like SlruSelectLRUPage, we don't need a
+ * memory barrier here because for the affected page to be relevant,
+ * we'd have to have the same bank lock as SimpleLruZeroPage.
*/
- if (ctl->PagePrecedes(shared->latest_page_number, cutoffPage))
+ if (ctl->PagePrecedes(pg_atomic_read_u64(&shared->latest_page_number),
+ cutoffPage))
{
LWLockRelease(shared->ControlLock);
ereport(LOG,
@@ -1282,7 +1300,7 @@ restart:
return;
}
- for (slotno = 0; slotno < shared->num_slots; slotno++)
+ for (int slotno = 0; slotno < shared->num_slots; slotno++)
{
if (shared->page_status[slotno] == SLRU_PAGE_EMPTY)
continue;
diff --git a/src/include/access/slru.h b/src/include/access/slru.h
index b05f6bc71d..2109488654 100644
--- a/src/include/access/slru.h
+++ b/src/include/access/slru.h
@@ -49,6 +49,9 @@ typedef enum
/*
* Shared-memory state
+ *
+ * ControlLock is used to protect access to the other fields, except
+ * latest_page_number, which uses atomics; see comment in slru.c.
*/
typedef struct SlruSharedData
{
@@ -95,7 +98,7 @@ typedef struct SlruSharedData
* this is not critical data, since we use it only to avoid swapping out
* the latest page.
*/
- int64 latest_page_number;
+ pg_atomic_uint64 latest_page_number;
/* SLRU's index for statistics purposes (might not be unique) */
int slru_stats_idx;
--
2.39.2
