On Fri, Jan 29, 2016 at 9:13 PM, Michael Paquier wrote:
> Well, to put it short, I am just trying to find a way to make the
> backend skip unnecessary checkpoints on an idle system, which results
> in the following WAL pattern if system is completely idle:
> CHECKPOINT_ONLINE
> RUNNING_XACTS
> RUNNING_XACTS
> [etc..]
>
> The thing is that I am lost with the meaning of this condition to
> decide if a checkpoint should be skipped or not:
> if (prevPtr == ControlFile->checkPointCopy.redo &&
> prevPtr / XLOG_SEG_SIZE == curInsert / XLOG_SEG_SIZE)
> {
> WALInsertLockRelease();
> LWLockRelease(CheckpointLock);
> return;
> }
> As at least one standby snapshot is logged before the checkpoint
> record, the redo position is never going to match the previous insert
> LSN, so checkpoints will never be skipped if wal_level >= hot_standby.
> Skipping such unnecessary checkpoints is what you would like to
> address, no? Because that's what I would like to do here first. And
> once we got that right, we could think about addressing the case where
> WAL segments are forcibly archived for idle systems.
I have put a bit more of brain power into that, and finished with the
patch attached. A new field called chkpProgressLSN is attached to
XLogCtl, which is to the current insert position of the checkpoint
when wal_level <= archive, or the LSN position of the standby snapshot
taken after a checkpoint. The bgwriter code is modified as well so as
it uses this progress LSN and compares it with the current insert LSN
to determine if a standby snapshot should be logged or not. On an
instance of Postgres completely idle, no useless checkpoints, and no
useless standby snapshots are generated anymore.
--
Michael
diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index a2846c4..ccef3f0 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -527,6 +527,8 @@ typedef struct XLogCtlData
TransactionId ckptXid;
XLogRecPtr asyncXactLSN; /* LSN of newest async commit/abort */
XLogRecPtr replicationSlotMinLSN; /* oldest LSN needed by any slot */
+ XLogRecPtr ckptProgressLSN; /* LSN tracking progress of checkpoint
+ * and standby snapshots */
XLogSegNo lastRemovedSegNo; /* latest removed/recycled XLOG
* segment */
@@ -6314,6 +6316,7 @@ StartupXLOG(void)
checkPoint.newestCommitTsXid);
XLogCtl->ckptXidEpoch = checkPoint.nextXidEpoch;
XLogCtl->ckptXid = checkPoint.nextXid;
+ XLogCtl->ckptProgressLSN = checkPoint.redo;
/*
* Initialize replication slots, before there's a chance to remove
@@ -7869,6 +7872,22 @@ GetFlushRecPtr(void)
}
/*
+ * GetProgressRecPtr -- Returns the reference position of last checkpoint,
+ * taking into account standby snapshots generated by checkpoints.
+ */
+XLogRecPtr
+GetProgressRecPtr(void)
+{
+ XLogRecPtr progress_lsn;
+
+ SpinLockAcquire(&XLogCtl->info_lck);
+ progress_lsn = XLogCtl->ckptProgressLSN;
+ SpinLockRelease(&XLogCtl->info_lck);
+
+ return progress_lsn;
+}
+
+/*
* Get the time of the last xlog segment switch
*/
pg_time_t
@@ -8133,6 +8152,7 @@ CreateCheckPoint(int flags)
XLogRecPtr PriorRedoPtr;
XLogRecPtr curInsert;
XLogRecPtr prevPtr;
+ XLogRecPtr progress_lsn;
VirtualTransactionId *vxids;
int nvxids;
@@ -8214,10 +8234,13 @@ CreateCheckPoint(int flags)
/*
* We must block concurrent insertions while examining insert state to
- * determine the checkpoint REDO pointer.
+ * determine the checkpoint REDO pointer. The progress LSN of this
+ * to-be-created checkpoint is for now initialized as the current insert
+ * position in WAL. This will get updated later on depending on if
+ * a standby snapshot is logged.
*/
WALInsertLockAcquireExclusive();
- curInsert = XLogBytePosToRecPtr(Insert->CurrBytePos);
+ curInsert = progress_lsn = XLogBytePosToRecPtr(Insert->CurrBytePos);
prevPtr = XLogBytePosToRecPtr(Insert->PrevBytePos);
/*
@@ -8239,7 +8262,7 @@ CreateCheckPoint(int flags)
if ((flags & (CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_END_OF_RECOVERY |
CHECKPOINT_FORCE)) == 0)
{
- if (prevPtr == ControlFile->checkPointCopy.redo &&
+ if (GetProgressRecPtr() == prevPtr &&
prevPtr / XLOG_SEG_SIZE == curInsert / XLOG_SEG_SIZE)
{
WALInsertLockRelease();
@@ -8406,9 +8429,15 @@ CreateCheckPoint(int flags)
*
* If we are shutting down, or Startup process is completing crash
* recovery we don't need to write running xact data.
+ *
+ * progress_lsn is aimed at tracking the WAL progress that happens
+ * since this checkpoint. If a standby snapshot is logged, its record
+ * postion need to be taken into account in the progress LSN position
+ * that is used to evaluate if checkpoints are necessary or standby
+ * snapshots need to be logged, and skip them on idle systems.
*/
if (!shutdown && XLogStandbyInfoActive())
- LogStandbySnapshot();
+ progress_lsn = LogStandbySnapshot();
START_CRIT_SECTION();
@@ -8478,10 +8507,15 @@ CreateCheckPoint(int flags)
UpdateControlFile();
LWLockRelease(ControlFileLock);
- /* Update shared-memory copy of checkpoint XID/epoch */
+ /*
+ * Update shared-memory copy of checkpoint XID/epoch and the LSN
+ * record tracking progress of checkpoint and standby snapshot
+ * activity.
+ */
SpinLockAcquire(&XLogCtl->info_lck);
XLogCtl->ckptXidEpoch = checkPoint.nextXidEpoch;
XLogCtl->ckptXid = checkPoint.nextXid;
+ XLogCtl->ckptProgressLSN = progress_lsn;
SpinLockRelease(&XLogCtl->info_lck);
/*
diff --git a/src/backend/postmaster/bgwriter.c b/src/backend/postmaster/bgwriter.c
index 010b5fc..22374b0 100644
--- a/src/backend/postmaster/bgwriter.c
+++ b/src/backend/postmaster/bgwriter.c
@@ -78,12 +78,10 @@ int BgWriterDelay = 200;
#define LOG_SNAPSHOT_INTERVAL_MS 15000
/*
- * LSN and timestamp at which we last issued a LogStandbySnapshot(), to avoid
- * doing so too often or repeatedly if there has been no other write activity
- * in the system.
+ * Timestamp at which we last issued a LogStandbySnapshot(), to avoid doing
+ * so too often.
*/
static TimestampTz last_snapshot_ts;
-static XLogRecPtr last_snapshot_lsn = InvalidXLogRecPtr;
/*
* Flags set by interrupt handlers for later service in the main loop.
@@ -301,8 +299,8 @@ BackgroundWriterMain(void)
* check whether there has been any WAL inserted since the last time
* we've logged a running xacts.
*
- * We do this logging in the bgwriter as its the only process thats
- * run regularly and returns to its mainloop all the time. E.g.
+ * We do this logging in the bgwriter as it is the only process that
+ * is run regularly and returns to its mainloop all the time. E.g.
* Checkpointer, when active, is barely ever in its mainloop and thus
* makes it hard to log regularly.
*/
@@ -315,13 +313,12 @@ BackgroundWriterMain(void)
LOG_SNAPSHOT_INTERVAL_MS);
/*
- * only log if enough time has passed and some xlog record has
- * been inserted.
+ * only log if enough time has passed.
*/
if (now >= timeout &&
- last_snapshot_lsn != GetXLogInsertRecPtr())
+ GetProgressRecPtr() != GetInsertRecPtr())
{
- last_snapshot_lsn = LogStandbySnapshot();
+ (void) LogStandbySnapshot();
last_snapshot_ts = now;
}
}
diff --git a/src/include/access/xlog.h b/src/include/access/xlog.h
index ecd30ce..41cb512 100644
--- a/src/include/access/xlog.h
+++ b/src/include/access/xlog.h
@@ -261,6 +261,7 @@ extern void GetFullPageWriteInfo(XLogRecPtr *RedoRecPtr_p, bool *doPageWrites_p)
extern XLogRecPtr GetRedoRecPtr(void);
extern XLogRecPtr GetInsertRecPtr(void);
extern XLogRecPtr GetFlushRecPtr(void);
+extern XLogRecPtr GetProgressRecPtr(void);
extern void GetNextXidAndEpoch(TransactionId *xid, uint32 *epoch);
extern void RemovePromoteSignalFiles(void);
--
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