postgres.git / summary / log / commit / refs
commit 818013665259d4242ba641aad705ebe5a3e2db8e
Author: Noah Misch <noah@leadboat.com>
Date: Sun Apr 20 19:00:17 2025 +0000
Comment on need to MarkBufferDirty() if omitting DELAY_CHKPT_START.
Blocking checkpoint phase 2 requires MarkBufferDirty() and
BUFFER_LOCK_EXCLUSIVE; neither suffices by itself. transam/README documents
this, citing SyncOneBuffer(). Update the DELAY_CHKPT_START documentation to
say this. Expand the heap_inplace_update_and_unlock() comment that cites
XLogSaveBufferForHint() as precedent, since heap_inplace_update_and_unlock()
could have opted not to use DELAY_CHKPT_START.
Commit 8e7e672cdaa6bfec85d4d5dd9be84159df23bb41 added DELAY_CHKPT_START to
heap_inplace_update_and_unlock(). Since commit
bc6bad88572501aecaa2ac5d4bc900ac0fd457d5 reverted it in non-master branches,
no back-patch.
Discussion: https://postgr.es/m/20250406180054.26.nmisch@google.com
src/backend/access/heap/heapam.c | 14 +++++++++++---
src/include/storage/proc.h | 8 ++++----
2 files changed, 15 insertions(+), 7 deletions(-)
diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c
index ed2e3021799..c1a4de14a59 100644
--- a/src/backend/access/heap/heapam.c
+++ b/src/backend/access/heap/heapam.c
@@ -6507,9 +6507,17 @@ heap_inplace_update_and_unlock(Relation relation,
* [crash]
* [recovery restores datfrozenxid w/o relfrozenxid]
*
- * Like in MarkBufferDirtyHint() subroutine XLogSaveBufferForHint(), copy
- * the buffer to the stack before logging. Here, that facilitates a FPI
- * of the post-mutation block before we accept other sessions seeing it.
+ * Mimic MarkBufferDirtyHint() subroutine XLogSaveBufferForHint().
+ * Specifically, use DELAY_CHKPT_START, and copy the buffer to the stack.
+ * The stack copy facilitates a FPI of the post-mutation block before we
+ * accept other sessions seeing it. DELAY_CHKPT_START allows us to
+ * XLogInsert() before MarkBufferDirty(). Since XLogSaveBufferForHint()
+ * can operate under BUFFER_LOCK_SHARED, it can't avoid DELAY_CHKPT_START.
+ * This function, however, likely could avoid it with the following order
+ * of operations: MarkBufferDirty(), XLogInsert(), memcpy(). Opt to use
+ * DELAY_CHKPT_START here, too, as a way to have fewer distinct code
+ * patterns to analyze. Inplace update isn't so frequent that it should
+ * pursue the small optimization of skipping DELAY_CHKPT_START.
*/
Assert((MyProc->delayChkptFlags & DELAY_CHKPT_START) == 0);
START_CRIT_SECTION();
diff --git a/src/include/storage/proc.h b/src/include/storage/proc.h
index f51b03d3822..86c5f998c77 100644
--- a/src/include/storage/proc.h
+++ b/src/include/storage/proc.h
@@ -110,10 +110,10 @@ extern PGDLLIMPORT int FastPathLockGroupsPerBackend;
* is inserted prior to the new redo point, the corresponding data changes will
* also be flushed to disk before the checkpoint can complete. (In the
* extremely common case where the data being modified is in shared buffers
- * and we acquire an exclusive content lock on the relevant buffers before
- * writing WAL, this mechanism is not needed, because phase 2 will block
- * until we release the content lock and then flush the modified data to
- * disk.)
+ * and we acquire an exclusive content lock and MarkBufferDirty() on the
+ * relevant buffers before writing WAL, this mechanism is not needed, because
+ * phase 2 will block until we release the content lock and then flush the
+ * modified data to disk. See transam/README and SyncOneBuffer().)
*
* Setting DELAY_CHKPT_COMPLETE prevents the system from moving from phase 2
* to phase 3. This is useful if we are performing a WAL-logged operation that
[parent: 714bd9e3a733]