On Tue, Sep 9, 2025 at 11:16 AM Melanie Plageman
<melanieplage...@gmail.com> wrote:
>
> One more fix and a bit more cleanup in attached v4.
Okay one more version: I updated the thread on eager flushing the
bulkwrite ring [1], and some updates were needed here.
- Melanie
[1]
https://www.postgresql.org/message-id/flat/CAAKRu_Yjn4mvN9NBxtmsCQSGwup45CoA4e05nhR7ADP-v0WCig%40mail.gmail.com
From ab164bbeb8ea4d7718f7f3c2b33893be1e0907dd Mon Sep 17 00:00:00 2001
From: Melanie Plageman <melanieplage...@gmail.com>
Date: Tue, 2 Sep 2025 12:56:38 -0400
Subject: [PATCH v5 4/9] Write combining for BAS_BULKWRITE
Implement write combining for users of the bulkwrite buffer access
strategy (e.g. COPY FROM). When the buffer access strategy needs to
clean a buffer for reuse, it already opportunistically flushes some
other buffers. Now, combine any contiguous blocks from the same relation
into larger writes and issue them with smgrwritev().
The performance benefit for COPY FROM is mostly noticeable for multiple
concurrent COPY FROMs because a single COPY FROM is either CPU bound or
bound by WAL writes.
The infrastructure for flushing larger batches of IOs will be reused by
checkpointer and other processes doing writes of dirty data.
XXX: Because this sets in-place checksums for batches, it is not
committable until additional infrastructure goes in place.
---
src/backend/storage/buffer/bufmgr.c | 198 ++++++++++++++++++++++++--
src/backend/storage/buffer/freelist.c | 26 ++++
src/backend/storage/page/bufpage.c | 20 +++
src/backend/utils/probes.d | 2 +
src/include/storage/buf_internals.h | 32 +++++
src/include/storage/bufpage.h | 1 +
src/tools/pgindent/typedefs.list | 1 +
7 files changed, 269 insertions(+), 11 deletions(-)
diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c
index d0f40b6a3ec..98c03ef1b1a 100644
--- a/src/backend/storage/buffer/bufmgr.c
+++ b/src/backend/storage/buffer/bufmgr.c
@@ -539,6 +539,8 @@ static BufferDesc *PrepareOrRejectEagerFlushBuffer(Buffer bufnum, BlockNumber re
RelFileLocator *rlocator,
bool skip_pinned,
XLogRecPtr *max_lsn);
+static void FindFlushAdjacents(BufferAccessStrategy strategy, BufferDesc *start,
+ uint32 max_batch_size, BufWriteBatch *batch);
static void CleanVictimBuffer(BufferAccessStrategy strategy,
BufferDesc *bufdesc, uint32 *buf_state,
bool from_ring, IOContext io_context);
@@ -4259,10 +4261,73 @@ FlushBuffer(BufferDesc *buf, SMgrRelation reln, IOObject io_object,
DoFlushBuffer(buf, reln, io_object, io_context, lsn);
}
+
+/*
+ * Given a buffer descriptor, start, from a strategy ring, strategy, that
+ * supports eager flushing, find additional buffers from the ring that can be
+ * combined into a single write batch with this buffer.
+ *
+ * max_batch_size is the maximum number of blocks that can be combined into a
+ * single write in general. This function, based on the block number of start,
+ * will determine the maximum IO size for this particular write given how much
+ * of the file remains. max_batch_size is provided by the caller so it doesn't
+ * have to be recalculated for each write.
+ *
+ * batch is an output parameter that this function will fill with the needed
+ * information to write this IO.
+ *
+ * This function will pin and content lock all of the buffers that it
+ * assembles for the IO batch. The caller is responsible for issuing the IO.
+ */
+static void
+FindFlushAdjacents(BufferAccessStrategy strategy, BufferDesc *start,
+ uint32 max_batch_size, BufWriteBatch *batch)
+{
+ BlockNumber limit;
+ uint32 buf_state;
+
+ Assert(start);
+ batch->bufdescs[0] = start;
+
+ buf_state = LockBufHdr(start);
+ batch->max_lsn = BufferGetLSN(start);
+ UnlockBufHdr(start, buf_state);
+
+ batch->start = batch->bufdescs[0]->tag.blockNum;
+ batch->forkno = BufTagGetForkNum(&batch->bufdescs[0]->tag);
+ batch->rlocator = BufTagGetRelFileLocator(&batch->bufdescs[0]->tag);
+ batch->reln = smgropen(batch->rlocator, INVALID_PROC_NUMBER);
+
+ Assert(BlockNumberIsValid(batch->start));
+
+ limit = smgrmaxcombine(batch->reln, batch->forkno, batch->start);
+ limit = Max(limit, 1);
+ limit = Min(max_batch_size, limit);
+
+ /* Now assemble a run of blocks to write out. */
+ for (batch->n = 1; batch->n < limit; batch->n++)
+ {
+ Buffer bufnum;
+
+ if ((bufnum = StrategySweepNextBuffer(strategy)) == InvalidBuffer)
+ break;
+
+ /* Stop when we encounter a buffer that will break the run */
+ if ((batch->bufdescs[batch->n] =
+ PrepareOrRejectEagerFlushBuffer(bufnum,
+ batch->start + batch->n,
+ &batch->rlocator,
+ true,
+ &batch->max_lsn)) == NULL)
+ break;
+ }
+}
+
/*
* Returns the buffer descriptor of the buffer containing the next block we
* should eagerly flush or or NULL when there are no further buffers to
- * consider writing out.
+ * consider writing out. This will be the start of a new batch of buffers to
+ * write out.
*/
static BufferDesc *
next_strat_buf_to_flush(BufferAccessStrategy strategy,
@@ -4296,7 +4361,6 @@ CleanVictimBuffer(BufferAccessStrategy strategy,
XLogRecPtr max_lsn = InvalidXLogRecPtr;
LWLock *content_lock;
- bool first_buffer = true;
Assert(*buf_state & BM_DIRTY);
@@ -4306,19 +4370,22 @@ CleanVictimBuffer(BufferAccessStrategy strategy,
if (from_ring && strategy_supports_eager_flush(strategy))
{
+ uint32 max_batch_size = max_write_batch_size_for_strategy(strategy);
+
+ /* Pin our victim again so it stays ours even after batch released */
+ ReservePrivateRefCountEntry();
+ ResourceOwnerEnlarge(CurrentResourceOwner);
+ IncrBufferRefCount(BufferDescriptorGetBuffer(bufdesc));
+
/* Clean victim buffer and find more to flush opportunistically */
StartStrategySweep(strategy);
do
{
- DoFlushBuffer(bufdesc, NULL, IOOBJECT_RELATION, io_context, max_lsn);
- content_lock = BufferDescriptorGetContentLock(bufdesc);
- LWLockRelease(content_lock);
- ScheduleBufferTagForWriteback(&BackendWritebackContext, io_context,
- &bufdesc->tag);
- /* We leave the first buffer pinned for the caller */
- if (!first_buffer)
- UnpinBuffer(bufdesc);
- first_buffer = false;
+ BufWriteBatch batch;
+
+ FindFlushAdjacents(strategy, bufdesc, max_batch_size, &batch);
+ FlushBufferBatch(&batch, io_context);
+ CompleteWriteBatchIO(&batch, io_context, &BackendWritebackContext);
} while ((bufdesc = next_strat_buf_to_flush(strategy, &max_lsn)) != NULL);
}
else
@@ -4440,6 +4507,73 @@ except_unlock_header:
return NULL;
}
+/*
+ * Given a prepared batch of buffers write them out as a vector.
+ */
+void
+FlushBufferBatch(BufWriteBatch *batch,
+ IOContext io_context)
+{
+ BlockNumber blknums[MAX_IO_COMBINE_LIMIT];
+ Block blocks[MAX_IO_COMBINE_LIMIT];
+ instr_time io_start;
+ ErrorContextCallback errcallback =
+ {
+ .callback = shared_buffer_write_error_callback,
+ .previous = error_context_stack,
+ };
+
+ error_context_stack = &errcallback;
+
+ if (!XLogRecPtrIsInvalid(batch->max_lsn))
+ XLogFlush(batch->max_lsn);
+
+ if (batch->reln == NULL)
+ batch->reln = smgropen(batch->rlocator, INVALID_PROC_NUMBER);
+
+#ifdef USE_ASSERT_CHECKING
+ for (uint32 i = 0; i < batch->n; i++)
+ {
+ BufferDesc *bufdesc = batch->bufdescs[i];
+ uint32 buf_state = LockBufHdr(bufdesc);
+ XLogRecPtr lsn = BufferGetLSN(bufdesc);
+
+ UnlockBufHdr(bufdesc, buf_state);
+ Assert(!(buf_state & BM_PERMANENT) || !XLogNeedsFlush(lsn));
+ }
+#endif
+
+ TRACE_POSTGRESQL_BUFFER_BATCH_FLUSH_START(batch->forkno,
+ batch->reln->smgr_rlocator.locator.spcOid,
+ batch->reln->smgr_rlocator.locator.dbOid,
+ batch->reln->smgr_rlocator.locator.relNumber,
+ batch->reln->smgr_rlocator.backend,
+ batch->n);
+
+ /*
+ * XXX: All blocks should be copied and then checksummed but doing so
+ * takes a lot of extra memory and a future patch will eliminate this
+ * requirement.
+ */
+ for (BlockNumber i = 0; i < batch->n; i++)
+ {
+ blknums[i] = batch->start + i;
+ blocks[i] = BufHdrGetBlock(batch->bufdescs[i]);
+ }
+
+ PageSetBatchChecksumInplace((Page *) blocks, blknums, batch->n);
+
+ io_start = pgstat_prepare_io_time(track_io_timing);
+
+ smgrwritev(batch->reln, batch->forkno,
+ batch->start, (const void **) blocks, batch->n, false);
+
+ pgstat_count_io_op_time(IOOBJECT_RELATION, io_context, IOOP_WRITE,
+ io_start, batch->n, BLCKSZ);
+
+ error_context_stack = errcallback.previous;
+}
+
/*
* Prepare the buffer with budesc for writing. buf_state and lsn are output
* parameters. Returns true if the buffer acutally needs writing and false
@@ -4585,6 +4719,48 @@ DoFlushBuffer(BufferDesc *buf, SMgrRelation reln, IOObject io_object,
error_context_stack = errcallback.previous;
}
+/*
+ * Given a previously initialized batch with buffers that have already been
+ * flushed, terminate the IO on each buffer and then unlock and unpin them.
+ * This assumes all the buffers were locked and pinned. wb_context will be
+ * modified.
+ */
+void
+CompleteWriteBatchIO(BufWriteBatch *batch, IOContext io_context,
+ WritebackContext *wb_context)
+{
+ ErrorContextCallback errcallback =
+ {
+ .callback = shared_buffer_write_error_callback,
+ .previous = error_context_stack,
+ };
+
+ error_context_stack = &errcallback;
+ pgBufferUsage.shared_blks_written += batch->n;
+
+ for (uint32 i = 0; i < batch->n; i++)
+ {
+ Buffer buffer = BufferDescriptorGetBuffer(batch->bufdescs[i]);
+
+ errcallback.arg = batch->bufdescs[i];
+
+ /* Mark the buffer as clean and end the BM_IO_IN_PROGRESS state. */
+ TerminateBufferIO(batch->bufdescs[i], true, 0, true, false);
+ LWLockRelease(BufferDescriptorGetContentLock(batch->bufdescs[i]));
+ ReleaseBuffer(buffer);
+ ScheduleBufferTagForWriteback(wb_context, io_context,
+ &batch->bufdescs[i]->tag);
+ }
+
+ TRACE_POSTGRESQL_BUFFER_BATCH_FLUSH_DONE(batch->forkno,
+ batch->reln->smgr_rlocator.locator.spcOid,
+ batch->reln->smgr_rlocator.locator.dbOid,
+ batch->reln->smgr_rlocator.locator.relNumber,
+ batch->reln->smgr_rlocator.backend,
+ batch->n, batch->start);
+ error_context_stack = errcallback.previous;
+}
+
/*
* RelationGetNumberOfBlocksInFork
* Determines the current number of pages in the specified relation fork.
diff --git a/src/backend/storage/buffer/freelist.c b/src/backend/storage/buffer/freelist.c
index 025592778f7..eadf2899a01 100644
--- a/src/backend/storage/buffer/freelist.c
+++ b/src/backend/storage/buffer/freelist.c
@@ -727,6 +727,32 @@ GetBufferFromRing(BufferAccessStrategy strategy, uint32 *buf_state)
return NULL;
}
+
+/*
+ * Determine the largest IO we can assemble from the given strategy ring given
+ * strategy-specific as well as global constraints on the number of pinned
+ * buffers and max IO size.
+ */
+uint32
+max_write_batch_size_for_strategy(BufferAccessStrategy strategy)
+{
+ uint32 max_possible_buffer_limit;
+ uint32 max_write_batch_size;
+ int strategy_pin_limit;
+
+ max_write_batch_size = io_combine_limit;
+
+ strategy_pin_limit = GetAccessStrategyPinLimit(strategy);
+ max_possible_buffer_limit = GetPinLimit();
+
+ max_write_batch_size = Min(strategy_pin_limit, max_write_batch_size);
+ max_write_batch_size = Min(max_possible_buffer_limit, max_write_batch_size);
+ max_write_batch_size = Max(1, max_write_batch_size);
+ max_write_batch_size = Min(max_write_batch_size, io_combine_limit);
+ Assert(max_write_batch_size < MAX_IO_COMBINE_LIMIT);
+ return max_write_batch_size;
+}
+
/*
* AddBufferToRing -- add a buffer to the buffer ring
*
diff --git a/src/backend/storage/page/bufpage.c b/src/backend/storage/page/bufpage.c
index dbb49ed9197..fc749dd5a50 100644
--- a/src/backend/storage/page/bufpage.c
+++ b/src/backend/storage/page/bufpage.c
@@ -1546,3 +1546,23 @@ PageSetChecksumInplace(Page page, BlockNumber blkno)
((PageHeader) page)->pd_checksum = pg_checksum_page(page, blkno);
}
+
+/*
+ * A helper to set multiple block's checksums.
+ */
+void
+PageSetBatchChecksumInplace(Page *pages, BlockNumber *blknos, uint32 length)
+{
+ /* If we don't need a checksum, just return */
+ if (!DataChecksumsEnabled())
+ return;
+
+ for (uint32 i = 0; i < length; i++)
+ {
+ Page page = pages[i];
+
+ if (PageIsNew(page))
+ continue;
+ ((PageHeader) page)->pd_checksum = pg_checksum_page(page, blknos[i]);
+ }
+}
diff --git a/src/backend/utils/probes.d b/src/backend/utils/probes.d
index e9e413477ba..36dd4f8375b 100644
--- a/src/backend/utils/probes.d
+++ b/src/backend/utils/probes.d
@@ -61,6 +61,8 @@ provider postgresql {
probe buffer__flush__done(ForkNumber, BlockNumber, Oid, Oid, Oid);
probe buffer__extend__start(ForkNumber, Oid, Oid, Oid, int, unsigned int);
probe buffer__extend__done(ForkNumber, Oid, Oid, Oid, int, unsigned int, BlockNumber);
+ probe buffer__batch__flush__start(ForkNumber, Oid, Oid, Oid, int, unsigned int);
+ probe buffer__batch__flush__done(ForkNumber, Oid, Oid, Oid, int, unsigned int, BlockNumber);
probe buffer__checkpoint__start(int);
probe buffer__checkpoint__sync__start();
diff --git a/src/include/storage/buf_internals.h b/src/include/storage/buf_internals.h
index 7963d1189a6..c082a50166f 100644
--- a/src/include/storage/buf_internals.h
+++ b/src/include/storage/buf_internals.h
@@ -416,6 +416,34 @@ ResourceOwnerForgetBufferIO(ResourceOwner owner, Buffer buffer)
ResourceOwnerForget(owner, Int32GetDatum(buffer), &buffer_io_resowner_desc);
}
+/*
+ * Used to write out multiple blocks at a time in a combined IO. bufdescs
+ * contains buffer descriptors for buffers containing adjacent blocks of the
+ * same fork of the same relation.
+ */
+typedef struct BufWriteBatch
+{
+ RelFileLocator rlocator;
+ ForkNumber forkno;
+ SMgrRelation reln;
+
+ /*
+ * The BlockNumber of the first block in the run of contiguous blocks to
+ * be written out as a single IO.
+ */
+ BlockNumber start;
+
+ /*
+ * While assembling the buffers, we keep track of the maximum LSN so that
+ * we can flush WAL through this LSN before flushing the buffers.
+ */
+ XLogRecPtr max_lsn;
+
+ /* The number of valid buffers in bufdescs */
+ uint32 n;
+ BufferDesc *bufdescs[MAX_IO_COMBINE_LIMIT];
+} BufWriteBatch;
+
/*
* Internal buffer management routines
*/
@@ -429,6 +457,7 @@ extern void WritebackContextInit(WritebackContext *context, int *max_pending);
extern void IssuePendingWritebacks(WritebackContext *wb_context, IOContext io_context);
extern void ScheduleBufferTagForWriteback(WritebackContext *wb_context,
IOContext io_context, BufferTag *tag);
+extern void FlushBufferBatch(BufWriteBatch *batch, IOContext io_context);
/* solely to make it easier to write tests */
extern bool StartBufferIO(BufferDesc *buf, bool forInput, bool nowait);
@@ -438,8 +467,11 @@ extern void TerminateBufferIO(BufferDesc *buf, bool clear_dirty, uint32 set_flag
/* freelist.c */
extern bool strategy_supports_eager_flush(BufferAccessStrategy strategy);
+extern uint32 max_write_batch_size_for_strategy(BufferAccessStrategy strategy);
extern Buffer StrategySweepNextBuffer(BufferAccessStrategy strategy);
extern void StartStrategySweep(BufferAccessStrategy strategy);
+extern void CompleteWriteBatchIO(BufWriteBatch *batch, IOContext io_context,
+ WritebackContext *wb_context);
extern IOContext IOContextForStrategy(BufferAccessStrategy strategy);
extern BufferDesc *StrategyGetBuffer(BufferAccessStrategy strategy,
uint32 *buf_state, bool *from_ring);
diff --git a/src/include/storage/bufpage.h b/src/include/storage/bufpage.h
index aeb67c498c5..1020cb3ac78 100644
--- a/src/include/storage/bufpage.h
+++ b/src/include/storage/bufpage.h
@@ -507,5 +507,6 @@ extern bool PageIndexTupleOverwrite(Page page, OffsetNumber offnum,
Item newtup, Size newsize);
extern char *PageSetChecksumCopy(Page page, BlockNumber blkno);
extern void PageSetChecksumInplace(Page page, BlockNumber blkno);
+extern void PageSetBatchChecksumInplace(Page *pages, BlockNumber *blknos, uint32 length);
#endif /* BUFPAGE_H */
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index a13e8162890..9492adeee58 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -349,6 +349,7 @@ BufferManagerRelation
BufferStrategyControl
BufferTag
BufferUsage
+BufWriteBatch
BuildAccumulator
BuiltinScript
BulkInsertState
--
2.43.0
From dae7c82146c2d73729fc12a742d84b660e6db2ad Mon Sep 17 00:00:00 2001
From: Melanie Plageman <melanieplage...@gmail.com>
Date: Tue, 2 Sep 2025 11:00:44 -0400
Subject: [PATCH v5 1/9] Refactor goto into for loop in GetVictimBuffer()
GetVictimBuffer() implemented a loop to optimistically lock a clean
victim buffer using a goto. Future commits will add batch flushing
functionality to GetVictimBuffer. The new logic works better with a
regular for loop flow control.
This commit is only a refactor and does not introduce any new
functionality.
Reviewed-by: Nazir Bilal Yavuz <byavu...@gmail.com>
Discussion: https://postgr.es/m/flat/CAAKRu_bcWRvRwZUop_d9vzF9nHAiT%2B-uPzkJ%3DS3ShZ1GqeAYOw%40mail.gmail.com
---
src/backend/storage/buffer/bufmgr.c | 200 ++++++++++++--------------
src/backend/storage/buffer/freelist.c | 32 ++++-
src/include/storage/buf_internals.h | 5 +
3 files changed, 124 insertions(+), 113 deletions(-)
diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c
index fe470de63f2..f3668051574 100644
--- a/src/backend/storage/buffer/bufmgr.c
+++ b/src/backend/storage/buffer/bufmgr.c
@@ -68,10 +68,6 @@
#include "utils/timestamp.h"
-/* Note: these two macros only work on shared buffers, not local ones! */
-#define BufHdrGetBlock(bufHdr) ((Block) (BufferBlocks + ((Size) (bufHdr)->buf_id) * BLCKSZ))
-#define BufferGetLSN(bufHdr) (PageGetLSN(BufHdrGetBlock(bufHdr)))
-
/* Note: this macro only works on local buffers, not shared ones! */
#define LocalBufHdrGetBlock(bufHdr) \
LocalBufferBlockPointers[-((bufHdr)->buf_id + 2)]
@@ -2344,130 +2340,122 @@ GetVictimBuffer(BufferAccessStrategy strategy, IOContext io_context)
ReservePrivateRefCountEntry();
ResourceOwnerEnlarge(CurrentResourceOwner);
- /* we return here if a prospective victim buffer gets used concurrently */
-again:
-
- /*
- * Select a victim buffer. The buffer is returned with its header
- * spinlock still held!
- */
- buf_hdr = StrategyGetBuffer(strategy, &buf_state, &from_ring);
- buf = BufferDescriptorGetBuffer(buf_hdr);
-
- Assert(BUF_STATE_GET_REFCOUNT(buf_state) == 0);
-
- /* Pin the buffer and then release the buffer spinlock */
- PinBuffer_Locked(buf_hdr);
-
- /*
- * We shouldn't have any other pins for this buffer.
- */
- CheckBufferIsPinnedOnce(buf);
-
- /*
- * If the buffer was dirty, try to write it out. There is a race
- * condition here, in that someone might dirty it after we released the
- * buffer header lock above, or even while we are writing it out (since
- * our share-lock won't prevent hint-bit updates). We will recheck the
- * dirty bit after re-locking the buffer header.
- */
- if (buf_state & BM_DIRTY)
+ /* Select a victim buffer using an optimistic locking scheme. */
+ for (;;)
{
- LWLock *content_lock;
+ /*
+ * Attempt to claim a victim buffer. The buffer is returned with its
+ * header spinlock still held!
+ */
+ buf_hdr = StrategyGetBuffer(strategy, &buf_state, &from_ring);
+ buf = BufferDescriptorGetBuffer(buf_hdr);
- Assert(buf_state & BM_TAG_VALID);
- Assert(buf_state & BM_VALID);
+ Assert(BUF_STATE_GET_REFCOUNT(buf_state) == 0);
+
+ /* Pin the buffer and then release the buffer spinlock */
+ PinBuffer_Locked(buf_hdr);
/*
- * We need a share-lock on the buffer contents to write it out (else
- * we might write invalid data, eg because someone else is compacting
- * the page contents while we write). We must use a conditional lock
- * acquisition here to avoid deadlock. Even though the buffer was not
- * pinned (and therefore surely not locked) when StrategyGetBuffer
- * returned it, someone else could have pinned and exclusive-locked it
- * by the time we get here. If we try to get the lock unconditionally,
- * we'd block waiting for them; if they later block waiting for us,
- * deadlock ensues. (This has been observed to happen when two
- * backends are both trying to split btree index pages, and the second
- * one just happens to be trying to split the page the first one got
- * from StrategyGetBuffer.)
+ * We shouldn't have any other pins for this buffer.
*/
- content_lock = BufferDescriptorGetContentLock(buf_hdr);
- if (!LWLockConditionalAcquire(content_lock, LW_SHARED))
- {
- /*
- * Someone else has locked the buffer, so give it up and loop back
- * to get another one.
- */
- UnpinBuffer(buf_hdr);
- goto again;
- }
+ CheckBufferIsPinnedOnce(buf);
/*
- * If using a nondefault strategy, and writing the buffer would
- * require a WAL flush, let the strategy decide whether to go ahead
- * and write/reuse the buffer or to choose another victim. We need a
- * lock to inspect the page LSN, so this can't be done inside
- * StrategyGetBuffer.
+ * If the buffer was dirty, try to write it out. There is a race
+ * condition here, in that someone might dirty it after we released
+ * the buffer header lock above, or even while we are writing it out
+ * (since our share-lock won't prevent hint-bit updates). We will
+ * recheck the dirty bit after re-locking the buffer header.
*/
- if (strategy != NULL)
+ if (buf_state & BM_DIRTY)
{
- XLogRecPtr lsn;
+ LWLock *content_lock;
- /* Read the LSN while holding buffer header lock */
- buf_state = LockBufHdr(buf_hdr);
- lsn = BufferGetLSN(buf_hdr);
- UnlockBufHdr(buf_hdr, buf_state);
+ Assert(buf_state & BM_TAG_VALID);
+ Assert(buf_state & BM_VALID);
- if (XLogNeedsFlush(lsn)
- && StrategyRejectBuffer(strategy, buf_hdr, from_ring))
+ /*
+ * We need a share-lock on the buffer contents to write it out
+ * (else we might write invalid data, eg because someone else is
+ * compacting the page contents while we write). We must use a
+ * conditional lock acquisition here to avoid deadlock. Even
+ * though the buffer was not pinned (and therefore surely not
+ * locked) when StrategyGetBuffer returned it, someone else could
+ * have pinned and exclusive-locked it by the time we get here. If
+ * we try to get the lock unconditionally, we'd block waiting for
+ * them; if they later block waiting for us, deadlock ensues.
+ * (This has been observed to happen when two backends are both
+ * trying to split btree index pages, and the second one just
+ * happens to be trying to split the page the first one got from
+ * StrategyGetBuffer.)
+ */
+ content_lock = BufferDescriptorGetContentLock(buf_hdr);
+ if (!LWLockConditionalAcquire(content_lock, LW_SHARED))
+ {
+ /*
+ * Someone else has locked the buffer, so give it up and loop
+ * back to get another one.
+ */
+ UnpinBuffer(buf_hdr);
+ continue;
+ }
+
+ /*
+ * If using a nondefault strategy, and writing the buffer would
+ * require a WAL flush, let the strategy decide whether to go
+ * ahead and write/reuse the buffer or to choose another victim.
+ * We need the content lock to inspect the page LSN, so this can't
+ * be done inside StrategyGetBuffer.
+ */
+ if (StrategyRejectBuffer(strategy, buf_hdr, from_ring))
{
LWLockRelease(content_lock);
UnpinBuffer(buf_hdr);
- goto again;
+ continue;
}
- }
- /* OK, do the I/O */
- FlushBuffer(buf_hdr, NULL, IOOBJECT_RELATION, io_context);
- LWLockRelease(content_lock);
+ /* OK, do the I/O */
+ FlushBuffer(buf_hdr, NULL, IOOBJECT_RELATION, io_context);
+ LWLockRelease(content_lock);
- ScheduleBufferTagForWriteback(&BackendWritebackContext, io_context,
- &buf_hdr->tag);
- }
+ ScheduleBufferTagForWriteback(&BackendWritebackContext, io_context,
+ &buf_hdr->tag);
+ }
+ if (buf_state & BM_VALID)
+ {
+ /*
+ * When a BufferAccessStrategy is in use, blocks evicted from
+ * shared buffers are counted as IOOP_EVICT in the corresponding
+ * context (e.g. IOCONTEXT_BULKWRITE). Shared buffers are evicted
+ * by a strategy in two cases: 1) while initially claiming buffers
+ * for the strategy ring 2) to replace an existing strategy ring
+ * buffer because it is pinned or in use and cannot be reused.
+ *
+ * Blocks evicted from buffers already in the strategy ring are
+ * counted as IOOP_REUSE in the corresponding strategy context.
+ *
+ * At this point, we can accurately count evictions and reuses,
+ * because we have successfully claimed the valid buffer.
+ * Previously, we may have been forced to release the buffer due
+ * to concurrent pinners or erroring out.
+ */
+ pgstat_count_io_op(IOOBJECT_RELATION, io_context,
+ from_ring ? IOOP_REUSE : IOOP_EVICT, 1, 0);
+ }
- if (buf_state & BM_VALID)
- {
/*
- * When a BufferAccessStrategy is in use, blocks evicted from shared
- * buffers are counted as IOOP_EVICT in the corresponding context
- * (e.g. IOCONTEXT_BULKWRITE). Shared buffers are evicted by a
- * strategy in two cases: 1) while initially claiming buffers for the
- * strategy ring 2) to replace an existing strategy ring buffer
- * because it is pinned or in use and cannot be reused.
- *
- * Blocks evicted from buffers already in the strategy ring are
- * counted as IOOP_REUSE in the corresponding strategy context.
- *
- * At this point, we can accurately count evictions and reuses,
- * because we have successfully claimed the valid buffer. Previously,
- * we may have been forced to release the buffer due to concurrent
- * pinners or erroring out.
+ * If the buffer has an entry in the buffer mapping table, delete it.
+ * This can fail because another backend could have pinned or dirtied
+ * the buffer. Then loop around and try again.
*/
- pgstat_count_io_op(IOOBJECT_RELATION, io_context,
- from_ring ? IOOP_REUSE : IOOP_EVICT, 1, 0);
- }
+ if ((buf_state & BM_TAG_VALID) && !InvalidateVictimBuffer(buf_hdr))
+ {
+ UnpinBuffer(buf_hdr);
+ continue;
+ }
- /*
- * If the buffer has an entry in the buffer mapping table, delete it. This
- * can fail because another backend could have pinned or dirtied the
- * buffer.
- */
- if ((buf_state & BM_TAG_VALID) && !InvalidateVictimBuffer(buf_hdr))
- {
- UnpinBuffer(buf_hdr);
- goto again;
+ break;
}
/* a final set of sanity checks */
diff --git a/src/backend/storage/buffer/freelist.c b/src/backend/storage/buffer/freelist.c
index 7d59a92bd1a..ce95afe2e94 100644
--- a/src/backend/storage/buffer/freelist.c
+++ b/src/backend/storage/buffer/freelist.c
@@ -15,6 +15,7 @@
*/
#include "postgres.h"
+#include "access/xlog.h"
#include "pgstat.h"
#include "port/atomics.h"
#include "storage/buf_internals.h"
@@ -716,12 +717,21 @@ IOContextForStrategy(BufferAccessStrategy strategy)
* be written out and doing so would require flushing WAL too. This gives us
* a chance to choose a different victim.
*
+ * The buffer must pinned and content locked and the buffer header spinlock
+ * must not be held. We must have the content lock to examine the LSN.
+ *
* Returns true if buffer manager should ask for a new victim, and false
- * if this buffer should be written and re-used.
+ * if this buffer should be written and reused.
*/
bool
StrategyRejectBuffer(BufferAccessStrategy strategy, BufferDesc *buf, bool from_ring)
{
+ uint32 buf_state;
+ XLogRecPtr lsn;
+
+ if (!strategy)
+ return false;
+
/* We only do this in bulkread mode */
if (strategy->btype != BAS_BULKREAD)
return false;
@@ -731,11 +741,19 @@ StrategyRejectBuffer(BufferAccessStrategy strategy, BufferDesc *buf, bool from_r
strategy->buffers[strategy->current] != BufferDescriptorGetBuffer(buf))
return false;
- /*
- * Remove the dirty buffer from the ring; necessary to prevent infinite
- * loop if all ring members are dirty.
- */
- strategy->buffers[strategy->current] = InvalidBuffer;
+ buf_state = LockBufHdr(buf);
+ lsn = BufferGetLSN(buf);
+ UnlockBufHdr(buf, buf_state);
+
+ if (XLogNeedsFlush(lsn))
+ {
+ /*
+ * Remove the dirty buffer from the ring; necessary to prevent an
+ * infinite loop if all ring members are dirty.
+ */
+ strategy->buffers[strategy->current] = InvalidBuffer;
+ return true;
+ }
- return true;
+ return false;
}
diff --git a/src/include/storage/buf_internals.h b/src/include/storage/buf_internals.h
index dfd614f7ca4..b1b81f31419 100644
--- a/src/include/storage/buf_internals.h
+++ b/src/include/storage/buf_internals.h
@@ -419,6 +419,11 @@ ResourceOwnerForgetBufferIO(ResourceOwner owner, Buffer buffer)
/*
* Internal buffer management routines
*/
+
+/* Note: these two macros only work on shared buffers, not local ones! */
+#define BufHdrGetBlock(bufHdr) ((Block) (BufferBlocks + ((Size) (bufHdr)->buf_id) * BLCKSZ))
+#define BufferGetLSN(bufHdr) (PageGetLSN(BufHdrGetBlock(bufHdr)))
+
/* bufmgr.c */
extern void WritebackContextInit(WritebackContext *context, int *max_pending);
extern void IssuePendingWritebacks(WritebackContext *wb_context, IOContext io_context);
--
2.43.0
From bf4b108cd97f710e89f27be69709c7960764a932 Mon Sep 17 00:00:00 2001
From: Melanie Plageman <melanieplage...@gmail.com>
Date: Tue, 2 Sep 2025 10:01:17 -0400
Subject: [PATCH v5 5/9] Fix XLogNeedsFlush() for checkpointer
XLogNeedsFlush() takes an LSN and compares it to either the flush pointer or the
min recovery point, depending on whether it is in normal operation or recovery.
Even though it is technically recovery, the checkpointer must flush WAL during
an end-of-recovery checkpoint, so in this case, it should compare the provided
LSN to the flush pointer and not the min recovery point.
If it compares the LSN to the min recovery point when the control file's min
recovery point has been updated to an incorrect value, XLogNeedsFlush() can
return an incorrect result of true -- even after just having flushed WAL.
Change this to only compare the LSN to min recovery point -- and, potentially
update the local copy of min recovery point, when xlog inserts are allowed --
which is true for the checkpointer during an end-of-recovery checkpoint, but
false during crash recovery otherwise.
Author: Melanie Plageman <melanieplage...@gmail.com>
Reported-by: Melanie Plageman <melanieplage...@gmail.com>
Discussion: https://postgr.es/m/CAAKRu_a1vZRZRWO3_jv_X13RYoqLRVipGO0237g5PKzPa2YX6g%40mail.gmail.com
---
src/backend/access/transam/xlog.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 7ffb2179151..16ef6d2cd64 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -3115,7 +3115,7 @@ XLogNeedsFlush(XLogRecPtr record)
* instead. So "needs flush" is taken to mean whether minRecoveryPoint
* would need to be updated.
*/
- if (RecoveryInProgress())
+ if (RecoveryInProgress() && !XLogInsertAllowed())
{
/*
* An invalid minRecoveryPoint means that we need to recover all the
--
2.43.0
From 2c8aafe30fb58516654e7d0cfdbfbb15a6a00498 Mon Sep 17 00:00:00 2001
From: Melanie Plageman <melanieplage...@gmail.com>
Date: Tue, 2 Sep 2025 11:32:24 -0400
Subject: [PATCH v5 2/9] Split FlushBuffer() into two parts
Before adding write combining to write a batch of blocks when flushing
dirty buffers, refactor FlushBuffer() into the preparatory step and
actual buffer flushing step. This provides better symmetry with the
batch flushing code.
---
src/backend/storage/buffer/bufmgr.c | 103 ++++++++++++++++++++--------
1 file changed, 76 insertions(+), 27 deletions(-)
diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c
index f3668051574..27cc418ef61 100644
--- a/src/backend/storage/buffer/bufmgr.c
+++ b/src/backend/storage/buffer/bufmgr.c
@@ -529,8 +529,13 @@ static inline BufferDesc *BufferAlloc(SMgrRelation smgr,
static bool AsyncReadBuffers(ReadBuffersOperation *operation, int *nblocks_progress);
static void CheckReadBuffersOperation(ReadBuffersOperation *operation, bool is_complete);
static Buffer GetVictimBuffer(BufferAccessStrategy strategy, IOContext io_context);
+static bool PrepareFlushBuffer(BufferDesc *bufdesc, uint32 *buf_state, XLogRecPtr *lsn);
+static void DoFlushBuffer(BufferDesc *buf, SMgrRelation reln, IOObject io_object,
+ IOContext io_context, XLogRecPtr buffer_lsn);
static void FlushBuffer(BufferDesc *buf, SMgrRelation reln,
IOObject io_object, IOContext io_context);
+static void CleanVictimBuffer(BufferDesc *bufdesc, uint32 *buf_state,
+ bool from_ring, IOContext io_context);
static void FindAndDropRelationBuffers(RelFileLocator rlocator,
ForkNumber forkNum,
BlockNumber nForkBlock,
@@ -2414,12 +2419,7 @@ GetVictimBuffer(BufferAccessStrategy strategy, IOContext io_context)
continue;
}
- /* OK, do the I/O */
- FlushBuffer(buf_hdr, NULL, IOOBJECT_RELATION, io_context);
- LWLockRelease(content_lock);
-
- ScheduleBufferTagForWriteback(&BackendWritebackContext, io_context,
- &buf_hdr->tag);
+ CleanVictimBuffer(buf_hdr, &buf_state, from_ring, io_context);
}
if (buf_state & BM_VALID)
@@ -4246,20 +4246,81 @@ static void
FlushBuffer(BufferDesc *buf, SMgrRelation reln, IOObject io_object,
IOContext io_context)
{
- XLogRecPtr recptr;
- ErrorContextCallback errcallback;
- instr_time io_start;
- Block bufBlock;
- char *bufToWrite;
uint32 buf_state;
+ XLogRecPtr lsn;
+
+ if (PrepareFlushBuffer(buf, &buf_state, &lsn))
+ DoFlushBuffer(buf, reln, io_object, io_context, lsn);
+}
+
+/*
+ * Prepare to write and write a dirty victim buffer.
+ * bufdesc and buf_state may be modified.
+ */
+static void
+CleanVictimBuffer(BufferDesc *bufdesc, uint32 *buf_state,
+ bool from_ring, IOContext io_context)
+{
+
+ XLogRecPtr max_lsn = InvalidXLogRecPtr;
+ LWLock *content_lock;
+ Assert(*buf_state & BM_DIRTY);
+
+ /* Set up this victim buffer to be flushed */
+ if (!PrepareFlushBuffer(bufdesc, buf_state, &max_lsn))
+ return;
+
+ DoFlushBuffer(bufdesc, NULL, IOOBJECT_RELATION, io_context, max_lsn);
+ content_lock = BufferDescriptorGetContentLock(bufdesc);
+ LWLockRelease(content_lock);
+ ScheduleBufferTagForWriteback(&BackendWritebackContext, io_context,
+ &bufdesc->tag);
+}
+
+/*
+ * Prepare the buffer with budesc for writing. buf_state and lsn are output
+ * parameters. Returns true if the buffer acutally needs writing and false
+ * otherwise. All three parameters may be modified.
+ */
+static bool
+PrepareFlushBuffer(BufferDesc *bufdesc, uint32 *buf_state, XLogRecPtr *lsn)
+{
/*
* Try to start an I/O operation. If StartBufferIO returns false, then
* someone else flushed the buffer before we could, so we need not do
* anything.
*/
- if (!StartBufferIO(buf, false, false))
- return;
+ if (!StartBufferIO(bufdesc, false, false))
+ return false;
+
+ *lsn = InvalidXLogRecPtr;
+ *buf_state = LockBufHdr(bufdesc);
+
+ /*
+ * Run PageGetLSN while holding header lock, since we don't have the
+ * buffer locked exclusively in all cases.
+ */
+ if (*buf_state & BM_PERMANENT)
+ *lsn = BufferGetLSN(bufdesc);
+
+ /* To check if block content changes while flushing. - vadim 01/17/97 */
+ *buf_state &= ~BM_JUST_DIRTIED;
+ UnlockBufHdr(bufdesc, *buf_state);
+ return true;
+}
+
+/*
+ * Actually do the write I/O to clean a buffer. buf and reln may be modified.
+ */
+static void
+DoFlushBuffer(BufferDesc *buf, SMgrRelation reln, IOObject io_object,
+ IOContext io_context, XLogRecPtr buffer_lsn)
+{
+ ErrorContextCallback errcallback;
+ instr_time io_start;
+ Block bufBlock;
+ char *bufToWrite;
/* Setup error traceback support for ereport() */
errcallback.callback = shared_buffer_write_error_callback;
@@ -4277,18 +4338,6 @@ FlushBuffer(BufferDesc *buf, SMgrRelation reln, IOObject io_object,
reln->smgr_rlocator.locator.dbOid,
reln->smgr_rlocator.locator.relNumber);
- buf_state = LockBufHdr(buf);
-
- /*
- * Run PageGetLSN while holding header lock, since we don't have the
- * buffer locked exclusively in all cases.
- */
- recptr = BufferGetLSN(buf);
-
- /* To check if block content changes while flushing. - vadim 01/17/97 */
- buf_state &= ~BM_JUST_DIRTIED;
- UnlockBufHdr(buf, buf_state);
-
/*
* Force XLOG flush up to buffer's LSN. This implements the basic WAL
* rule that log updates must hit disk before any of the data-file changes
@@ -4306,8 +4355,8 @@ FlushBuffer(BufferDesc *buf, SMgrRelation reln, IOObject io_object,
* disastrous system-wide consequences. To make sure that can't happen,
* skip the flush if the buffer isn't permanent.
*/
- if (buf_state & BM_PERMANENT)
- XLogFlush(recptr);
+ if (!XLogRecPtrIsInvalid(buffer_lsn))
+ XLogFlush(buffer_lsn);
/*
* Now it's safe to write the buffer to disk. Note that no one else should
--
2.43.0
From c060a306293fbcecaab4fd8dd9174860c94ce6be Mon Sep 17 00:00:00 2001
From: Melanie Plageman <melanieplage...@gmail.com>
Date: Tue, 2 Sep 2025 12:43:24 -0400
Subject: [PATCH v5 3/9] Eagerly flush bulkwrite strategy ring
Operations using BAS_BULKWRITE (COPY FROM and createdb) will inevitably
need to flush buffers in the strategy ring in order to reuse
them. By eagerly flushing the buffers in a larger batch, we encourage
larger writes at the kernel level and less interleaving of WAL flushes
and data file writes. The effect is mainly noticeable with multiple
parallel COPY FROMs. In this case, client backends achieve higher write
throughput and end up spending less time waiting on acquiring the lock
to flush WAL. Larger flush operations also mean less time waiting for
flush operations at the kernel level as well.
The heuristic for eager eviction is to only flush buffers in the
strategy ring which flushing does not require flushing WAL.
This patch also is a stepping stone toward AIO writes.
Earlier version
Reviewed-by: Kirill Reshke <reshkekir...@gmail.com>
Discussion: https://postgr.es/m/flat/CAAKRu_Yjn4mvN9NBxtmsCQSGwup45CoA4e05nhR7ADP-v0WCig%40mail.gmail.com
---
src/backend/storage/buffer/bufmgr.c | 174 +++++++++++++++++++++++++-
src/backend/storage/buffer/freelist.c | 63 ++++++++++
src/include/storage/buf_internals.h | 3 +
3 files changed, 234 insertions(+), 6 deletions(-)
diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c
index 27cc418ef61..d0f40b6a3ec 100644
--- a/src/backend/storage/buffer/bufmgr.c
+++ b/src/backend/storage/buffer/bufmgr.c
@@ -534,7 +534,13 @@ static void DoFlushBuffer(BufferDesc *buf, SMgrRelation reln, IOObject io_object
IOContext io_context, XLogRecPtr buffer_lsn);
static void FlushBuffer(BufferDesc *buf, SMgrRelation reln,
IOObject io_object, IOContext io_context);
-static void CleanVictimBuffer(BufferDesc *bufdesc, uint32 *buf_state,
+static BufferDesc *next_strat_buf_to_flush(BufferAccessStrategy strategy, XLogRecPtr *lsn);
+static BufferDesc *PrepareOrRejectEagerFlushBuffer(Buffer bufnum, BlockNumber require,
+ RelFileLocator *rlocator,
+ bool skip_pinned,
+ XLogRecPtr *max_lsn);
+static void CleanVictimBuffer(BufferAccessStrategy strategy,
+ BufferDesc *bufdesc, uint32 *buf_state,
bool from_ring, IOContext io_context);
static void FindAndDropRelationBuffers(RelFileLocator rlocator,
ForkNumber forkNum,
@@ -2419,7 +2425,7 @@ GetVictimBuffer(BufferAccessStrategy strategy, IOContext io_context)
continue;
}
- CleanVictimBuffer(buf_hdr, &buf_state, from_ring, io_context);
+ CleanVictimBuffer(strategy, buf_hdr, &buf_state, from_ring, io_context);
}
if (buf_state & BM_VALID)
@@ -4253,17 +4259,44 @@ FlushBuffer(BufferDesc *buf, SMgrRelation reln, IOObject io_object,
DoFlushBuffer(buf, reln, io_object, io_context, lsn);
}
+/*
+ * Returns the buffer descriptor of the buffer containing the next block we
+ * should eagerly flush or or NULL when there are no further buffers to
+ * consider writing out.
+ */
+static BufferDesc *
+next_strat_buf_to_flush(BufferAccessStrategy strategy,
+ XLogRecPtr *lsn)
+{
+ Buffer bufnum;
+ BufferDesc *bufdesc;
+
+ while ((bufnum = StrategySweepNextBuffer(strategy)) != InvalidBuffer)
+ {
+ if ((bufdesc = PrepareOrRejectEagerFlushBuffer(bufnum,
+ InvalidBlockNumber,
+ NULL,
+ true,
+ lsn)) != NULL)
+ return bufdesc;
+ }
+
+ return NULL;
+}
+
/*
* Prepare to write and write a dirty victim buffer.
* bufdesc and buf_state may be modified.
*/
static void
-CleanVictimBuffer(BufferDesc *bufdesc, uint32 *buf_state,
+CleanVictimBuffer(BufferAccessStrategy strategy,
+ BufferDesc *bufdesc, uint32 *buf_state,
bool from_ring, IOContext io_context)
{
XLogRecPtr max_lsn = InvalidXLogRecPtr;
LWLock *content_lock;
+ bool first_buffer = true;
Assert(*buf_state & BM_DIRTY);
@@ -4271,11 +4304,140 @@ CleanVictimBuffer(BufferDesc *bufdesc, uint32 *buf_state,
if (!PrepareFlushBuffer(bufdesc, buf_state, &max_lsn))
return;
- DoFlushBuffer(bufdesc, NULL, IOOBJECT_RELATION, io_context, max_lsn);
+ if (from_ring && strategy_supports_eager_flush(strategy))
+ {
+ /* Clean victim buffer and find more to flush opportunistically */
+ StartStrategySweep(strategy);
+ do
+ {
+ DoFlushBuffer(bufdesc, NULL, IOOBJECT_RELATION, io_context, max_lsn);
+ content_lock = BufferDescriptorGetContentLock(bufdesc);
+ LWLockRelease(content_lock);
+ ScheduleBufferTagForWriteback(&BackendWritebackContext, io_context,
+ &bufdesc->tag);
+ /* We leave the first buffer pinned for the caller */
+ if (!first_buffer)
+ UnpinBuffer(bufdesc);
+ first_buffer = false;
+ } while ((bufdesc = next_strat_buf_to_flush(strategy, &max_lsn)) != NULL);
+ }
+ else
+ {
+ DoFlushBuffer(bufdesc, NULL, IOOBJECT_RELATION, io_context, max_lsn);
+ content_lock = BufferDescriptorGetContentLock(bufdesc);
+ LWLockRelease(content_lock);
+ ScheduleBufferTagForWriteback(&BackendWritebackContext, io_context,
+ &bufdesc->tag);
+ }
+}
+
+/*
+ * Prepare bufdesc for eager flushing.
+ *
+ * Given bufnum, returns the block -- the pointer to the block data in memory
+ * -- which we will opportunistically flush or NULL if this buffer does not
+ * contain a block that should be flushed.
+ *
+ * require is the BlockNumber required by the caller. Some callers may require
+ * a specific BlockNumber to be in bufnum because they are assembling a
+ * contiguous run of blocks.
+ *
+ * If the caller needs the block to be from a specific relation, rlocator will
+ * be provided.
+ */
+BufferDesc *
+PrepareOrRejectEagerFlushBuffer(Buffer bufnum, BlockNumber require,
+ RelFileLocator *rlocator, bool skip_pinned,
+ XLogRecPtr *max_lsn)
+{
+ BufferDesc *bufdesc;
+ uint32 buf_state;
+ XLogRecPtr lsn;
+ BlockNumber blknum;
+ LWLock *content_lock;
+
+ if (!BufferIsValid(bufnum))
+ return NULL;
+
+ Assert(!BufferIsLocal(bufnum));
+
+ bufdesc = GetBufferDescriptor(bufnum - 1);
+
+ /* Block may need to be in a specific relation */
+ if (rlocator &&
+ !RelFileLocatorEquals(BufTagGetRelFileLocator(&bufdesc->tag),
+ *rlocator))
+ return NULL;
+
+ /* Must do this before taking the buffer header spinlock. */
+ ResourceOwnerEnlarge(CurrentResourceOwner);
+ ReservePrivateRefCountEntry();
+
+ buf_state = LockBufHdr(bufdesc);
+
+ if (!(buf_state & BM_DIRTY) || !(buf_state & BM_VALID))
+ goto except_unlock_header;
+
+ /* We don't include used buffers in batches */
+ if (skip_pinned &&
+ (BUF_STATE_GET_REFCOUNT(buf_state) > 0 ||
+ BUF_STATE_GET_USAGECOUNT(buf_state) > 1))
+ goto except_unlock_header;
+
+ /* Get page LSN while holding header lock */
+ lsn = BufferGetLSN(bufdesc);
+
+ PinBuffer_Locked(bufdesc);
+ CheckBufferIsPinnedOnce(bufnum);
+
+ blknum = BufferGetBlockNumber(bufnum);
+ Assert(BlockNumberIsValid(blknum));
+
+ /* If we'll have to flush WAL to flush the block, we're done */
+ if (buf_state & BM_PERMANENT && XLogNeedsFlush(lsn))
+ goto except_unpin_buffer;
+
+ /* We only include contiguous blocks in the run */
+ if (BlockNumberIsValid(require) && blknum != require)
+ goto except_unpin_buffer;
+
content_lock = BufferDescriptorGetContentLock(bufdesc);
+ if (!LWLockConditionalAcquire(content_lock, LW_SHARED))
+ goto except_unpin_buffer;
+
+ /*
+ * Now that we have the content lock, we need to recheck if we need to
+ * flush WAL.
+ */
+ buf_state = LockBufHdr(bufdesc);
+ lsn = BufferGetLSN(bufdesc);
+ UnlockBufHdr(bufdesc, buf_state);
+
+ if (buf_state & BM_PERMANENT && XLogNeedsFlush(lsn))
+ goto except_unlock_content;
+
+ /* Try to start an I/O operation. */
+ if (!StartBufferIO(bufdesc, false, true))
+ goto except_unlock_content;
+
+ if (lsn > *max_lsn)
+ *max_lsn = lsn;
+ buf_state = LockBufHdr(bufdesc);
+ buf_state &= ~BM_JUST_DIRTIED;
+ UnlockBufHdr(bufdesc, buf_state);
+
+ return bufdesc;
+
+except_unlock_content:
LWLockRelease(content_lock);
- ScheduleBufferTagForWriteback(&BackendWritebackContext, io_context,
- &bufdesc->tag);
+
+except_unpin_buffer:
+ UnpinBuffer(bufdesc);
+ return NULL;
+
+except_unlock_header:
+ UnlockBufHdr(bufdesc, buf_state);
+ return NULL;
}
/*
diff --git a/src/backend/storage/buffer/freelist.c b/src/backend/storage/buffer/freelist.c
index ce95afe2e94..025592778f7 100644
--- a/src/backend/storage/buffer/freelist.c
+++ b/src/backend/storage/buffer/freelist.c
@@ -75,6 +75,15 @@ typedef struct BufferAccessStrategyData
*/
int current;
+ /*
+ * If the strategy supports eager flushing, we may initiate a sweep of the
+ * strategy ring, flushing all the dirty buffers we can cheaply flush.
+ * sweep_start and sweep_current keep track of a given sweep so we don't
+ * loop around the ring infinitely.
+ */
+ int sweep_start;
+ int sweep_current;
+
/*
* Array of buffer numbers. InvalidBuffer (that is, zero) indicates we
* have not yet selected a buffer for this ring slot. For allocation
@@ -156,6 +165,31 @@ ClockSweepTick(void)
return victim;
}
+/*
+ * Some BufferAccessStrategies support eager flushing -- which is flushing
+ * buffers in the ring before they are needed. This can lean to better I/O
+ * patterns than lazily flushing buffers directly before reusing them.
+ */
+bool
+strategy_supports_eager_flush(BufferAccessStrategy strategy)
+{
+ Assert(strategy);
+
+ switch (strategy->btype)
+ {
+ case BAS_BULKWRITE:
+ return true;
+ case BAS_VACUUM:
+ case BAS_NORMAL:
+ case BAS_BULKREAD:
+ return false;
+ default:
+ elog(ERROR, "unrecognized buffer access strategy: %d",
+ (int) strategy->btype);
+ return false;
+ }
+}
+
/*
* StrategyGetBuffer
*
@@ -270,6 +304,35 @@ StrategyGetBuffer(BufferAccessStrategy strategy, uint32 *buf_state, bool *from_r
}
}
+/*
+ * Return the next buffer in the ring or InvalidBuffer if the current sweep is
+ * over.
+ */
+Buffer
+StrategySweepNextBuffer(BufferAccessStrategy strategy)
+{
+ strategy->sweep_current++;
+ if (strategy->sweep_current >= strategy->nbuffers)
+ strategy->sweep_current = 0;
+
+ if (strategy->sweep_current == strategy->sweep_start)
+ return InvalidBuffer;
+
+ return strategy->buffers[strategy->sweep_current];
+}
+
+/*
+ * Start a sweep of the strategy ring.
+ */
+void
+StartStrategySweep(BufferAccessStrategy strategy)
+{
+ if (!strategy)
+ return;
+ strategy->sweep_start = strategy->sweep_current = strategy->current;
+}
+
+
/*
* StrategySyncStart -- tell BgBufferSync where to start syncing
*
diff --git a/src/include/storage/buf_internals.h b/src/include/storage/buf_internals.h
index b1b81f31419..7963d1189a6 100644
--- a/src/include/storage/buf_internals.h
+++ b/src/include/storage/buf_internals.h
@@ -437,6 +437,9 @@ extern void TerminateBufferIO(BufferDesc *buf, bool clear_dirty, uint32 set_flag
/* freelist.c */
+extern bool strategy_supports_eager_flush(BufferAccessStrategy strategy);
+extern Buffer StrategySweepNextBuffer(BufferAccessStrategy strategy);
+extern void StartStrategySweep(BufferAccessStrategy strategy);
extern IOContext IOContextForStrategy(BufferAccessStrategy strategy);
extern BufferDesc *StrategyGetBuffer(BufferAccessStrategy strategy,
uint32 *buf_state, bool *from_ring);
--
2.43.0
From 7e54f2577dce04615b3082fe518490dbd2fa8ad7 Mon Sep 17 00:00:00 2001
From: Melanie Plageman <melanieplage...@gmail.com>
Date: Tue, 2 Sep 2025 15:22:11 -0400
Subject: [PATCH v5 6/9] Add database Oid to CkptSortItem
This is useful for checkpointer write combining -- which will be added
in a future commit.
---
src/backend/storage/buffer/bufmgr.c | 8 ++++++++
src/include/storage/buf_internals.h | 1 +
2 files changed, 9 insertions(+)
diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c
index 98c03ef1b1a..246f675333e 100644
--- a/src/backend/storage/buffer/bufmgr.c
+++ b/src/backend/storage/buffer/bufmgr.c
@@ -3371,6 +3371,7 @@ BufferSync(int flags)
item = &CkptBufferIds[num_to_scan++];
item->buf_id = buf_id;
item->tsId = bufHdr->tag.spcOid;
+ item->db_id = bufHdr->tag.dbOid;
item->relNumber = BufTagGetRelNumber(&bufHdr->tag);
item->forkNum = BufTagGetForkNum(&bufHdr->tag);
item->blockNum = bufHdr->tag.blockNum;
@@ -6691,6 +6692,13 @@ ckpt_buforder_comparator(const CkptSortItem *a, const CkptSortItem *b)
return -1;
else if (a->tsId > b->tsId)
return 1;
+
+ /* compare database */
+ if (a->db_id < b->db_id)
+ return -1;
+ else if (a->db_id > b->db_id)
+ return 1;
+
/* compare relation */
if (a->relNumber < b->relNumber)
return -1;
diff --git a/src/include/storage/buf_internals.h b/src/include/storage/buf_internals.h
index c082a50166f..99f17091a40 100644
--- a/src/include/storage/buf_internals.h
+++ b/src/include/storage/buf_internals.h
@@ -382,6 +382,7 @@ UnlockBufHdr(BufferDesc *desc, uint32 buf_state)
typedef struct CkptSortItem
{
Oid tsId;
+ Oid db_id;
RelFileNumber relNumber;
ForkNumber forkNum;
BlockNumber blockNum;
--
2.43.0
From 5e9ebf1c1cafdb5805671d3c57cfeadab5e8c434 Mon Sep 17 00:00:00 2001
From: Melanie Plageman <melanieplage...@gmail.com>
Date: Tue, 2 Sep 2025 15:42:29 -0400
Subject: [PATCH v5 7/9] Implement checkpointer data write combining
When the checkpointer writes out dirty buffers, writing multiple
contiguous blocks as a single IO is a substantial performance
improvement. The checkpointer is usually bottlenecked on IO, so issuing
larger IOs leads to increased write throughput and faster checkpoints.
---
src/backend/storage/buffer/bufmgr.c | 232 ++++++++++++++++++++++++----
src/backend/utils/probes.d | 2 +-
2 files changed, 207 insertions(+), 27 deletions(-)
diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c
index 246f675333e..e7c789dffd7 100644
--- a/src/backend/storage/buffer/bufmgr.c
+++ b/src/backend/storage/buffer/bufmgr.c
@@ -512,6 +512,7 @@ static bool PinBuffer(BufferDesc *buf, BufferAccessStrategy strategy);
static void PinBuffer_Locked(BufferDesc *buf);
static void UnpinBuffer(BufferDesc *buf);
static void UnpinBufferNoOwner(BufferDesc *buf);
+static uint32 checkpointer_max_batch_size(void);
static void BufferSync(int flags);
static uint32 WaitBufHdrUnlocked(BufferDesc *buf);
static int SyncOneBuffer(int buf_id, bool skip_recently_used,
@@ -3313,7 +3314,6 @@ UnpinBufferNoOwner(BufferDesc *buf)
static void
BufferSync(int flags)
{
- uint32 buf_state;
int buf_id;
int num_to_scan;
int num_spaces;
@@ -3325,6 +3325,8 @@ BufferSync(int flags)
int i;
int mask = BM_DIRTY;
WritebackContext wb_context;
+ uint32 max_batch_size;
+ BufWriteBatch batch;
/*
* Unless this is a shutdown checkpoint or we have been explicitly told,
@@ -3355,6 +3357,7 @@ BufferSync(int flags)
for (buf_id = 0; buf_id < NBuffers; buf_id++)
{
BufferDesc *bufHdr = GetBufferDescriptor(buf_id);
+ uint32 buf_state;
/*
* Header spinlock is enough to examine BM_DIRTY, see comment in
@@ -3495,48 +3498,208 @@ BufferSync(int flags)
*/
num_processed = 0;
num_written = 0;
+ max_batch_size = checkpointer_max_batch_size();
while (!binaryheap_empty(ts_heap))
{
+ BlockNumber limit = max_batch_size;
BufferDesc *bufHdr = NULL;
CkptTsStatus *ts_stat = (CkptTsStatus *)
DatumGetPointer(binaryheap_first(ts_heap));
-
- buf_id = CkptBufferIds[ts_stat->index].buf_id;
- Assert(buf_id != -1);
-
- bufHdr = GetBufferDescriptor(buf_id);
-
- num_processed++;
+ int ts_end = ts_stat->index - ts_stat->num_scanned + ts_stat->num_to_scan;
+ int processed = 0;
/*
- * We don't need to acquire the lock here, because we're only looking
- * at a single bit. It's possible that someone else writes the buffer
- * and clears the flag right after we check, but that doesn't matter
- * since SyncOneBuffer will then do nothing. However, there is a
- * further race condition: it's conceivable that between the time we
- * examine the bit here and the time SyncOneBuffer acquires the lock,
- * someone else not only wrote the buffer but replaced it with another
- * page and dirtied it. In that improbable case, SyncOneBuffer will
- * write the buffer though we didn't need to. It doesn't seem worth
- * guarding against this, though.
+ * Each batch will have exactly one start and one max lsn and one
+ * length.
*/
- if (pg_atomic_read_u32(&bufHdr->state) & BM_CHECKPOINT_NEEDED)
+ batch.start = InvalidBlockNumber;
+ batch.max_lsn = InvalidXLogRecPtr;
+ batch.n = 0;
+
+ while (batch.n < limit)
{
- if (SyncOneBuffer(buf_id, false, &wb_context) & BUF_WRITTEN)
+ uint32 buf_state;
+ XLogRecPtr lsn = InvalidXLogRecPtr;
+ LWLock *content_lock;
+ CkptSortItem item;
+
+ if (ProcSignalBarrierPending)
+ ProcessProcSignalBarrier();
+
+ /* Check if we are done with this tablespace */
+ if (ts_stat->index + processed >= ts_end)
+ break;
+
+ item = CkptBufferIds[ts_stat->index + processed];
+
+ buf_id = item.buf_id;
+ Assert(buf_id != -1);
+
+ bufHdr = GetBufferDescriptor(buf_id);
+
+ /*
+ * If this is the first block of the batch, then check if we need
+ * to open a new relation. Open the relation now because we have
+ * to determine the maximum IO size based on how many blocks
+ * remain in the file.
+ */
+ if (!BlockNumberIsValid(batch.start))
+ {
+ Assert(batch.max_lsn == InvalidXLogRecPtr && batch.n == 0);
+ batch.rlocator.spcOid = item.tsId;
+ batch.rlocator.dbOid = item.db_id;
+ batch.rlocator.relNumber = item.relNumber;
+ batch.forkno = item.forkNum;
+ batch.start = item.blockNum;
+ batch.reln = smgropen(batch.rlocator, INVALID_PROC_NUMBER);
+ limit = smgrmaxcombine(batch.reln, batch.forkno, batch.start);
+ limit = Max(1, limit);
+ limit = Min(limit, max_batch_size);
+ }
+
+ /*
+ * Once we hit blocks from the next relation or fork of the
+ * relation, break out of the loop and issue the IO we've built up
+ * so far. It is important that we don't increment processed
+ * becasue we want to start the next IO with this item.
+ */
+ if (item.db_id != batch.rlocator.dbOid)
+ break;
+
+ if (item.relNumber != batch.rlocator.relNumber)
+ break;
+
+ if (item.forkNum != batch.forkno)
+ break;
+
+ /*
+ * It the next block is not contiguous, we can't include it in the
+ * IO we will issue. Break out of the loop and issue what we have
+ * so far. Do not count this item as processed -- otherwise we
+ * will end up skipping it.
+ */
+ if (item.blockNum != batch.start + batch.n)
+ break;
+
+ /*
+ * We don't need to acquire the lock here, because we're only
+ * looking at a single bit. It's possible that someone else writes
+ * the buffer and clears the flag right after we check, but that
+ * doesn't matter since StartBufferIO will then return false. If
+ * the buffer doesn't need checkpointing, don't include it in the
+ * batch we are building. We're done with the item, so count it as
+ * processed and break out of the loop to issue the IO we have
+ * built so far.
+ */
+ if (!(pg_atomic_read_u32(&bufHdr->state) & BM_CHECKPOINT_NEEDED))
+ {
+ processed++;
+ break;
+ }
+
+ ReservePrivateRefCountEntry();
+ ResourceOwnerEnlarge(CurrentResourceOwner);
+
+ buf_state = LockBufHdr(bufHdr);
+
+ /*
+ * If the buffer doesn't need eviction, we're done with the item,
+ * so count it as processed and break out of the loop to issue the
+ * IO so far.
+ */
+ if (!(buf_state & BM_VALID) || !(buf_state & BM_DIRTY))
{
- TRACE_POSTGRESQL_BUFFER_SYNC_WRITTEN(buf_id);
- PendingCheckpointerStats.buffers_written++;
- num_written++;
+ processed++;
+ UnlockBufHdr(bufHdr, buf_state);
+ break;
+ }
+
+ PinBuffer_Locked(bufHdr);
+
+ /*
+ * There is a race condition here: it's conceivable that between
+ * the time we examine the buffer header for BM_CHECKPOINT_NEEDED
+ * above and when we are now acquiring the lock that, someone else
+ * not only wrote the buffer but replaced it with another page and
+ * dirtied it. In that improbable case, we will write the buffer
+ * though we didn't need to. It doesn't seem worth guarding
+ * against this, though.
+ */
+ content_lock = BufferDescriptorGetContentLock(bufHdr);
+
+ /*
+ * We are willing to wait for the content lock on the first IO in
+ * the batch. However, for subsequent IOs, waiting could lead to
+ * deadlock. We have to eventually flush all eligible buffers,
+ * though. So, if we fail to acquire the lock on a subsequent
+ * buffer, we break out and issue the IO we've built up so far.
+ * Then we come back and start a new IO with that buffer as the
+ * starting buffer. As such, we must not count the item as
+ * processed if we end up failing to acquire the content lock.
+ */
+ if (batch.n == 0)
+ LWLockAcquire(content_lock, LW_SHARED);
+ else if (!LWLockConditionalAcquire(content_lock, LW_SHARED))
+ {
+ UnpinBuffer(bufHdr);
+ break;
+ }
+
+ /*
+ * If the buffer doesn't need IO, count the item as processed,
+ * release the buffer, and break out of the loop to issue the IO
+ * we have built up so far.
+ */
+ if (!StartBufferIO(bufHdr, false, true))
+ {
+ processed++;
+ LWLockRelease(content_lock);
+ UnpinBuffer(bufHdr);
+ break;
}
+
+ buf_state = LockBufHdr(bufHdr);
+ lsn = BufferGetLSN(bufHdr);
+ buf_state &= ~BM_JUST_DIRTIED;
+ UnlockBufHdr(bufHdr, buf_state);
+
+ /*
+ * Keep track of the max LSN so that we can be sure to flush
+ * enough WAL before flushing data from the buffers. See comment
+ * in DoFlushBuffer() for more on why we don't consider the LSNs
+ * of unlogged relations.
+ */
+ if (buf_state & BM_PERMANENT && lsn > batch.max_lsn)
+ batch.max_lsn = lsn;
+
+ batch.bufdescs[batch.n++] = bufHdr;
+ processed++;
}
/*
* Measure progress independent of actually having to flush the buffer
* - otherwise writing become unbalanced.
*/
- ts_stat->progress += ts_stat->progress_slice;
- ts_stat->num_scanned++;
- ts_stat->index++;
+ num_processed += processed;
+ ts_stat->progress += ts_stat->progress_slice * processed;
+ ts_stat->num_scanned += processed;
+ ts_stat->index += processed;
+
+ /*
+ * If we built up an IO, issue it. There's a chance we didn't find any
+ * items referencing buffers that needed flushing this time, but we
+ * still want to check if we should update the heap if we examined and
+ * processed the items.
+ */
+ if (batch.n > 0)
+ {
+ FlushBufferBatch(&batch, IOCONTEXT_NORMAL);
+ CompleteWriteBatchIO(&batch, IOCONTEXT_NORMAL, &wb_context);
+
+ TRACE_POSTGRESQL_BUFFER_BATCH_SYNC_WRITTEN(batch.n);
+ PendingCheckpointerStats.buffers_written += batch.n;
+ num_written += batch.n;
+ }
/* Have all the buffers from the tablespace been processed? */
if (ts_stat->num_scanned == ts_stat->num_to_scan)
@@ -4262,6 +4425,23 @@ FlushBuffer(BufferDesc *buf, SMgrRelation reln, IOObject io_object,
DoFlushBuffer(buf, reln, io_object, io_context, lsn);
}
+/*
+ * The maximum number of blocks that can be written out in a single batch by
+ * the checkpointer.
+ */
+static uint32
+checkpointer_max_batch_size(void)
+{
+ uint32 result;
+ uint32 pin_limit = GetPinLimit();
+
+ result = Max(pin_limit, 1);
+ result = Min(pin_limit, io_combine_limit);
+ result = Max(result, 1);
+ Assert(result < MAX_IO_COMBINE_LIMIT);
+ return result;
+}
+
/*
* Given a buffer descriptor, start, from a strategy ring, strategy, that
diff --git a/src/backend/utils/probes.d b/src/backend/utils/probes.d
index 36dd4f8375b..d6970731ba9 100644
--- a/src/backend/utils/probes.d
+++ b/src/backend/utils/probes.d
@@ -68,7 +68,7 @@ provider postgresql {
probe buffer__checkpoint__sync__start();
probe buffer__checkpoint__done();
probe buffer__sync__start(int, int);
- probe buffer__sync__written(int);
+ probe buffer__batch__sync__written(BlockNumber);
probe buffer__sync__done(int, int, int);
probe deadlock__found();
--
2.43.0
