nbtree: fix read page recheck typo.

[pgsql.git] / src / backend / postmaster / checkpointer.c

/*-------------------------------------------------------------------------
 *
 * checkpointer.c
 *
 * The checkpointer is new as of Postgres 9.2.  It handles all checkpoints.
 * Checkpoints are automatically dispatched after a certain amount of time has
 * elapsed since the last one, and it can be signaled to perform requested
 * checkpoints as well.  (The GUC parameter that mandates a checkpoint every
 * so many WAL segments is implemented by having backends signal when they
 * fill WAL segments; the checkpointer itself doesn't watch for the
 * condition.)
 *
 * Normal termination is by SIGUSR2, which instructs the checkpointer to
 * execute a shutdown checkpoint and then exit(0).  (All backends must be
 * stopped before SIGUSR2 is issued!)  Emergency termination is by SIGQUIT;
 * like any backend, the checkpointer will simply abort and exit on SIGQUIT.
 *
 * If the checkpointer exits unexpectedly, the postmaster treats that the same
 * as a backend crash: shared memory may be corrupted, so remaining backends
 * should be killed by SIGQUIT and then a recovery cycle started.  (Even if
 * shared memory isn't corrupted, we have lost information about which
 * files need to be fsync'd for the next checkpoint, and so a system
 * restart needs to be forced.)
 *
 *
 * Portions Copyright (c) 1996-2024, PostgreSQL Global Development Group
 *
 *
 * IDENTIFICATION
 *        src/backend/postmaster/checkpointer.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include <sys/time.h>
#include <time.h>

#include "access/xlog.h"
#include "access/xlog_internal.h"
#include "access/xlogrecovery.h"
#include "libpq/pqsignal.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "postmaster/auxprocess.h"
#include "postmaster/bgwriter.h"
#include "postmaster/interrupt.h"
#include "replication/syncrep.h"
#include "storage/bufmgr.h"
#include "storage/condition_variable.h"
#include "storage/fd.h"
#include "storage/ipc.h"
#include "storage/lwlock.h"
#include "storage/proc.h"
#include "storage/procsignal.h"
#include "storage/shmem.h"
#include "storage/smgr.h"
#include "storage/spin.h"
#include "utils/guc.h"
#include "utils/memutils.h"
#include "utils/resowner.h"


/*----------
 * Shared memory area for communication between checkpointer and backends
 *
 * The ckpt counters allow backends to watch for completion of a checkpoint
 * request they send.  Here's how it works:
 *      * At start of a checkpoint, checkpointer reads (and clears) the request
 *        flags and increments ckpt_started, while holding ckpt_lck.
 *      * On completion of a checkpoint, checkpointer sets ckpt_done to
 *        equal ckpt_started.
 *      * On failure of a checkpoint, checkpointer increments ckpt_failed
 *        and sets ckpt_done to equal ckpt_started.
 *
 * The algorithm for backends is:
 *      1. Record current values of ckpt_failed and ckpt_started, and
 *         set request flags, while holding ckpt_lck.
 *      2. Send signal to request checkpoint.
 *      3. Sleep until ckpt_started changes.  Now you know a checkpoint has
 *         begun since you started this algorithm (although *not* that it was
 *         specifically initiated by your signal), and that it is using your flags.
 *      4. Record new value of ckpt_started.
 *      5. Sleep until ckpt_done >= saved value of ckpt_started.  (Use modulo
 *         arithmetic here in case counters wrap around.)  Now you know a
 *         checkpoint has started and completed, but not whether it was
 *         successful.
 *      6. If ckpt_failed is different from the originally saved value,
 *         assume request failed; otherwise it was definitely successful.
 *
 * ckpt_flags holds the OR of the checkpoint request flags sent by all
 * requesting backends since the last checkpoint start.  The flags are
 * chosen so that OR'ing is the correct way to combine multiple requests.
 *
 * The requests array holds fsync requests sent by backends and not yet
 * absorbed by the checkpointer.
 *
 * Unlike the checkpoint fields, requests related fields are protected by
 * CheckpointerCommLock.
 *----------
 */
typedef struct
{
        SyncRequestType type;           /* request type */
        FileTag         ftag;                   /* file identifier */
} CheckpointerRequest;

typedef struct
{
        pid_t           checkpointer_pid;       /* PID (0 if not started) */

        slock_t         ckpt_lck;               /* protects all the ckpt_* fields */

        int                     ckpt_started;   /* advances when checkpoint starts */
        int                     ckpt_done;              /* advances when checkpoint done */
        int                     ckpt_failed;    /* advances when checkpoint fails */

        int                     ckpt_flags;             /* checkpoint flags, as defined in xlog.h */

        ConditionVariable start_cv; /* signaled when ckpt_started advances */
        ConditionVariable done_cv;      /* signaled when ckpt_done advances */

        int                     num_requests;   /* current # of requests */
        int                     max_requests;   /* allocated array size */
        CheckpointerRequest requests[FLEXIBLE_ARRAY_MEMBER];
} CheckpointerShmemStruct;

static CheckpointerShmemStruct *CheckpointerShmem;

/* interval for calling AbsorbSyncRequests in CheckpointWriteDelay */
#define WRITES_PER_ABSORB               1000

/*
 * GUC parameters
 */
int                     CheckPointTimeout = 300;
int                     CheckPointWarning = 30;
double          CheckPointCompletionTarget = 0.9;

/*
 * Private state
 */
static bool ckpt_active = false;

/* these values are valid when ckpt_active is true: */
static pg_time_t ckpt_start_time;
static XLogRecPtr ckpt_start_recptr;
static double ckpt_cached_elapsed;

static pg_time_t last_checkpoint_time;
static pg_time_t last_xlog_switch_time;

/* Prototypes for private functions */

static void HandleCheckpointerInterrupts(void);
static void CheckArchiveTimeout(void);
static bool IsCheckpointOnSchedule(double progress);
static bool ImmediateCheckpointRequested(void);
static bool CompactCheckpointerRequestQueue(void);
static void UpdateSharedMemoryConfig(void);

/* Signal handlers */
static void ReqCheckpointHandler(SIGNAL_ARGS);


/*
 * Main entry point for checkpointer process
 *
 * This is invoked from AuxiliaryProcessMain, which has already created the
 * basic execution environment, but not enabled signals yet.
 */
void
CheckpointerMain(char *startup_data, size_t startup_data_len)
{
        sigjmp_buf      local_sigjmp_buf;
        MemoryContext checkpointer_context;

        Assert(startup_data_len == 0);

        MyBackendType = B_CHECKPOINTER;
        AuxiliaryProcessMainCommon();

        CheckpointerShmem->checkpointer_pid = MyProcPid;

        /*
         * Properly accept or ignore signals the postmaster might send us
         *
         * Note: we deliberately ignore SIGTERM, because during a standard Unix
         * system shutdown cycle, init will SIGTERM all processes at once.  We
         * want to wait for the backends to exit, whereupon the postmaster will
         * tell us it's okay to shut down (via SIGUSR2).
         */
        pqsignal(SIGHUP, SignalHandlerForConfigReload);
        pqsignal(SIGINT, ReqCheckpointHandler); /* request checkpoint */
        pqsignal(SIGTERM, SIG_IGN); /* ignore SIGTERM */
        /* SIGQUIT handler was already set up by InitPostmasterChild */
        pqsignal(SIGALRM, SIG_IGN);
        pqsignal(SIGPIPE, SIG_IGN);
        pqsignal(SIGUSR1, procsignal_sigusr1_handler);
        pqsignal(SIGUSR2, SignalHandlerForShutdownRequest);

        /*
         * Reset some signals that are accepted by postmaster but not here
         */
        pqsignal(SIGCHLD, SIG_DFL);

        /*
         * Initialize so that first time-driven event happens at the correct time.
         */
        last_checkpoint_time = last_xlog_switch_time = (pg_time_t) time(NULL);

        /*
         * Write out stats after shutdown. This needs to be called by exactly one
         * process during a normal shutdown, and since checkpointer is shut down
         * very late...
         *
         * Walsenders are shut down after the checkpointer, but currently don't
         * report stats. If that changes, we need a more complicated solution.
         */
        before_shmem_exit(pgstat_before_server_shutdown, 0);

        /*
         * Create a memory context that we will do all our work in.  We do this so
         * that we can reset the context during error recovery and thereby avoid
         * possible memory leaks.  Formerly this code just ran in
         * TopMemoryContext, but resetting that would be a really bad idea.
         */
        checkpointer_context = AllocSetContextCreate(TopMemoryContext,
                                                                                                 "Checkpointer",
                                                                                                 ALLOCSET_DEFAULT_SIZES);
        MemoryContextSwitchTo(checkpointer_context);

        /*
         * If an exception is encountered, processing resumes here.
         *
         * You might wonder why this isn't coded as an infinite loop around a
         * PG_TRY construct.  The reason is that this is the bottom of the
         * exception stack, and so with PG_TRY there would be no exception handler
         * in force at all during the CATCH part.  By leaving the outermost setjmp
         * always active, we have at least some chance of recovering from an error
         * during error recovery.  (If we get into an infinite loop thereby, it
         * will soon be stopped by overflow of elog.c's internal state stack.)
         *
         * Note that we use sigsetjmp(..., 1), so that the prevailing signal mask
         * (to wit, BlockSig) will be restored when longjmp'ing to here.  Thus,
         * signals other than SIGQUIT will be blocked until we complete error
         * recovery.  It might seem that this policy makes the HOLD_INTERRUPTS()
         * call redundant, but it is not since InterruptPending might be set
         * already.
         */
        if (sigsetjmp(local_sigjmp_buf, 1) != 0)
        {
                /* Since not using PG_TRY, must reset error stack by hand */
                error_context_stack = NULL;

                /* Prevent interrupts while cleaning up */
                HOLD_INTERRUPTS();

                /* Report the error to the server log */
                EmitErrorReport();

                /*
                 * These operations are really just a minimal subset of
                 * AbortTransaction().  We don't have very many resources to worry
                 * about in checkpointer, but we do have LWLocks, buffers, and temp
                 * files.
                 */
                LWLockReleaseAll();
                ConditionVariableCancelSleep();
                pgstat_report_wait_end();
                UnlockBuffers();
                ReleaseAuxProcessResources(false);
                AtEOXact_Buffers(false);
                AtEOXact_SMgr();
                AtEOXact_Files(false);
                AtEOXact_HashTables(false);

                /* Warn any waiting backends that the checkpoint failed. */
                if (ckpt_active)
                {
                        SpinLockAcquire(&CheckpointerShmem->ckpt_lck);
                        CheckpointerShmem->ckpt_failed++;
                        CheckpointerShmem->ckpt_done = CheckpointerShmem->ckpt_started;
                        SpinLockRelease(&CheckpointerShmem->ckpt_lck);

                        ConditionVariableBroadcast(&CheckpointerShmem->done_cv);

                        ckpt_active = false;
                }

                /*
                 * Now return to normal top-level context and clear ErrorContext for
                 * next time.
                 */
                MemoryContextSwitchTo(checkpointer_context);
                FlushErrorState();

                /* Flush any leaked data in the top-level context */
                MemoryContextReset(checkpointer_context);

                /* Now we can allow interrupts again */
                RESUME_INTERRUPTS();

                /*
                 * Sleep at least 1 second after any error.  A write error is likely
                 * to be repeated, and we don't want to be filling the error logs as
                 * fast as we can.
                 */
                pg_usleep(1000000L);
        }

        /* We can now handle ereport(ERROR) */
        PG_exception_stack = &local_sigjmp_buf;

        /*
         * Unblock signals (they were blocked when the postmaster forked us)
         */
        sigprocmask(SIG_SETMASK, &UnBlockSig, NULL);

        /*
         * Ensure all shared memory values are set correctly for the config. Doing
         * this here ensures no race conditions from other concurrent updaters.
         */
        UpdateSharedMemoryConfig();

        /*
         * Advertise our latch that backends can use to wake us up while we're
         * sleeping.
         */
        ProcGlobal->checkpointerLatch = &MyProc->procLatch;

        /*
         * Loop forever
         */
        for (;;)
        {
                bool            do_checkpoint = false;
                int                     flags = 0;
                pg_time_t       now;
                int                     elapsed_secs;
                int                     cur_timeout;
                bool            chkpt_or_rstpt_requested = false;
                bool            chkpt_or_rstpt_timed = false;

                /* Clear any already-pending wakeups */
                ResetLatch(MyLatch);

                /*
                 * Process any requests or signals received recently.
                 */
                AbsorbSyncRequests();
                HandleCheckpointerInterrupts();

                /*
                 * Detect a pending checkpoint request by checking whether the flags
                 * word in shared memory is nonzero.  We shouldn't need to acquire the
                 * ckpt_lck for this.
                 */
                if (((volatile CheckpointerShmemStruct *) CheckpointerShmem)->ckpt_flags)
                {
                        do_checkpoint = true;
                        chkpt_or_rstpt_requested = true;
                }

                /*
                 * Force a checkpoint if too much time has elapsed since the last one.
                 * Note that we count a timed checkpoint in stats only when this
                 * occurs without an external request, but we set the CAUSE_TIME flag
                 * bit even if there is also an external request.
                 */
                now = (pg_time_t) time(NULL);
                elapsed_secs = now - last_checkpoint_time;
                if (elapsed_secs >= CheckPointTimeout)
                {
                        if (!do_checkpoint)
                                chkpt_or_rstpt_timed = true;
                        do_checkpoint = true;
                        flags |= CHECKPOINT_CAUSE_TIME;
                }

                /*
                 * Do a checkpoint if requested.
                 */
                if (do_checkpoint)
                {
                        bool            ckpt_performed = false;
                        bool            do_restartpoint;

                        /* Check if we should perform a checkpoint or a restartpoint. */
                        do_restartpoint = RecoveryInProgress();

                        /*
                         * Atomically fetch the request flags to figure out what kind of a
                         * checkpoint we should perform, and increase the started-counter
                         * to acknowledge that we've started a new checkpoint.
                         */
                        SpinLockAcquire(&CheckpointerShmem->ckpt_lck);
                        flags |= CheckpointerShmem->ckpt_flags;
                        CheckpointerShmem->ckpt_flags = 0;
                        CheckpointerShmem->ckpt_started++;
                        SpinLockRelease(&CheckpointerShmem->ckpt_lck);

                        ConditionVariableBroadcast(&CheckpointerShmem->start_cv);

                        /*
                         * The end-of-recovery checkpoint is a real checkpoint that's
                         * performed while we're still in recovery.
                         */
                        if (flags & CHECKPOINT_END_OF_RECOVERY)
                                do_restartpoint = false;

                        if (chkpt_or_rstpt_timed)
                        {
                                chkpt_or_rstpt_timed = false;
                                if (do_restartpoint)
                                        PendingCheckpointerStats.restartpoints_timed++;
                                else
                                        PendingCheckpointerStats.num_timed++;
                        }

                        if (chkpt_or_rstpt_requested)
                        {
                                chkpt_or_rstpt_requested = false;
                                if (do_restartpoint)
                                        PendingCheckpointerStats.restartpoints_requested++;
                                else
                                        PendingCheckpointerStats.num_requested++;
                        }

                        /*
                         * We will warn if (a) too soon since last checkpoint (whatever
                         * caused it) and (b) somebody set the CHECKPOINT_CAUSE_XLOG flag
                         * since the last checkpoint start.  Note in particular that this
                         * implementation will not generate warnings caused by
                         * CheckPointTimeout < CheckPointWarning.
                         */
                        if (!do_restartpoint &&
                                (flags & CHECKPOINT_CAUSE_XLOG) &&
                                elapsed_secs < CheckPointWarning)
                                ereport(LOG,
                                                (errmsg_plural("checkpoints are occurring too frequently (%d second apart)",
                                                                           "checkpoints are occurring too frequently (%d seconds apart)",
                                                                           elapsed_secs,
                                                                           elapsed_secs),
                                                 errhint("Consider increasing the configuration parameter \"%s\".", "max_wal_size")));

                        /*
                         * Initialize checkpointer-private variables used during
                         * checkpoint.
                         */
                        ckpt_active = true;
                        if (do_restartpoint)
                                ckpt_start_recptr = GetXLogReplayRecPtr(NULL);
                        else
                                ckpt_start_recptr = GetInsertRecPtr();
                        ckpt_start_time = now;
                        ckpt_cached_elapsed = 0;

                        /*
                         * Do the checkpoint.
                         */
                        if (!do_restartpoint)
                                ckpt_performed = CreateCheckPoint(flags);
                        else
                                ckpt_performed = CreateRestartPoint(flags);

                        /*
                         * After any checkpoint, free all smgr objects.  Otherwise we
                         * would never do so for dropped relations, as the checkpointer
                         * does not process shared invalidation messages or call
                         * AtEOXact_SMgr().
                         */
                        smgrdestroyall();

                        /*
                         * Indicate checkpoint completion to any waiting backends.
                         */
                        SpinLockAcquire(&CheckpointerShmem->ckpt_lck);
                        CheckpointerShmem->ckpt_done = CheckpointerShmem->ckpt_started;
                        SpinLockRelease(&CheckpointerShmem->ckpt_lck);

                        ConditionVariableBroadcast(&CheckpointerShmem->done_cv);

                        if (!do_restartpoint)
                        {
                                /*
                                 * Note we record the checkpoint start time not end time as
                                 * last_checkpoint_time.  This is so that time-driven
                                 * checkpoints happen at a predictable spacing.
                                 */
                                last_checkpoint_time = now;

                                if (ckpt_performed)
                                        PendingCheckpointerStats.num_performed++;
                        }
                        else
                        {
                                if (ckpt_performed)
                                {
                                        /*
                                         * The same as for checkpoint. Please see the
                                         * corresponding comment.
                                         */
                                        last_checkpoint_time = now;

                                        PendingCheckpointerStats.restartpoints_performed++;
                                }
                                else
                                {
                                        /*
                                         * We were not able to perform the restartpoint
                                         * (checkpoints throw an ERROR in case of error).  Most
                                         * likely because we have not received any new checkpoint
                                         * WAL records since the last restartpoint. Try again in
                                         * 15 s.
                                         */
                                        last_checkpoint_time = now - CheckPointTimeout + 15;
                                }
                        }

                        ckpt_active = false;

                        /* We may have received an interrupt during the checkpoint. */
                        HandleCheckpointerInterrupts();
                }

                /* Check for archive_timeout and switch xlog files if necessary. */
                CheckArchiveTimeout();

                /* Report pending statistics to the cumulative stats system */
                pgstat_report_checkpointer();
                pgstat_report_wal(true);

                /*
                 * If any checkpoint flags have been set, redo the loop to handle the
                 * checkpoint without sleeping.
                 */
                if (((volatile CheckpointerShmemStruct *) CheckpointerShmem)->ckpt_flags)
                        continue;

                /*
                 * Sleep until we are signaled or it's time for another checkpoint or
                 * xlog file switch.
                 */
                now = (pg_time_t) time(NULL);
                elapsed_secs = now - last_checkpoint_time;
                if (elapsed_secs >= CheckPointTimeout)
                        continue;                       /* no sleep for us ... */
                cur_timeout = CheckPointTimeout - elapsed_secs;
                if (XLogArchiveTimeout > 0 && !RecoveryInProgress())
                {
                        elapsed_secs = now - last_xlog_switch_time;
                        if (elapsed_secs >= XLogArchiveTimeout)
                                continue;               /* no sleep for us ... */
                        cur_timeout = Min(cur_timeout, XLogArchiveTimeout - elapsed_secs);
                }

                (void) WaitLatch(MyLatch,
                                                 WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH,
                                                 cur_timeout * 1000L /* convert to ms */ ,
                                                 WAIT_EVENT_CHECKPOINTER_MAIN);
        }
}

/*
 * Process any new interrupts.
 */
static void
HandleCheckpointerInterrupts(void)
{
        if (ProcSignalBarrierPending)
                ProcessProcSignalBarrier();

        if (ConfigReloadPending)
        {
                ConfigReloadPending = false;
                ProcessConfigFile(PGC_SIGHUP);

                /*
                 * Checkpointer is the last process to shut down, so we ask it to hold
                 * the keys for a range of other tasks required most of which have
                 * nothing to do with checkpointing at all.
                 *
                 * For various reasons, some config values can change dynamically so
                 * the primary copy of them is held in shared memory to make sure all
                 * backends see the same value.  We make Checkpointer responsible for
                 * updating the shared memory copy if the parameter setting changes
                 * because of SIGHUP.
                 */
                UpdateSharedMemoryConfig();
        }
        if (ShutdownRequestPending)
        {
                /*
                 * From here on, elog(ERROR) should end with exit(1), not send control
                 * back to the sigsetjmp block above
                 */
                ExitOnAnyError = true;

                /*
                 * Close down the database.
                 *
                 * Since ShutdownXLOG() creates restartpoint or checkpoint, and
                 * updates the statistics, increment the checkpoint request and flush
                 * out pending statistic.
                 */
                PendingCheckpointerStats.num_requested++;
                ShutdownXLOG(0, 0);
                pgstat_report_checkpointer();
                pgstat_report_wal(true);

                /* Normal exit from the checkpointer is here */
                proc_exit(0);                   /* done */
        }

        /* Perform logging of memory contexts of this process */
        if (LogMemoryContextPending)
                ProcessLogMemoryContextInterrupt();
}

/*
 * CheckArchiveTimeout -- check for archive_timeout and switch xlog files
 *
 * This will switch to a new WAL file and force an archive file write if
 * meaningful activity is recorded in the current WAL file. This includes most
 * writes, including just a single checkpoint record, but excludes WAL records
 * that were inserted with the XLOG_MARK_UNIMPORTANT flag being set (like
 * snapshots of running transactions).  Such records, depending on
 * configuration, occur on regular intervals and don't contain important
 * information.  This avoids generating archives with a few unimportant
 * records.
 */
static void
CheckArchiveTimeout(void)
{
        pg_time_t       now;
        pg_time_t       last_time;
        XLogRecPtr      last_switch_lsn;

        if (XLogArchiveTimeout <= 0 || RecoveryInProgress())
                return;

        now = (pg_time_t) time(NULL);

        /* First we do a quick check using possibly-stale local state. */
        if ((int) (now - last_xlog_switch_time) < XLogArchiveTimeout)
                return;

        /*
         * Update local state ... note that last_xlog_switch_time is the last time
         * a switch was performed *or requested*.
         */
        last_time = GetLastSegSwitchData(&last_switch_lsn);

        last_xlog_switch_time = Max(last_xlog_switch_time, last_time);

        /* Now we can do the real checks */
        if ((int) (now - last_xlog_switch_time) >= XLogArchiveTimeout)
        {
                /*
                 * Switch segment only when "important" WAL has been logged since the
                 * last segment switch (last_switch_lsn points to end of segment
                 * switch occurred in).
                 */
                if (GetLastImportantRecPtr() > last_switch_lsn)
                {
                        XLogRecPtr      switchpoint;

                        /* mark switch as unimportant, avoids triggering checkpoints */
                        switchpoint = RequestXLogSwitch(true);

                        /*
                         * If the returned pointer points exactly to a segment boundary,
                         * assume nothing happened.
                         */
                        if (XLogSegmentOffset(switchpoint, wal_segment_size) != 0)
                                elog(DEBUG1, "write-ahead log switch forced (\"archive_timeout\"=%d)",
                                         XLogArchiveTimeout);
                }

                /*
                 * Update state in any case, so we don't retry constantly when the
                 * system is idle.
                 */
                last_xlog_switch_time = now;
        }
}

/*
 * Returns true if an immediate checkpoint request is pending.  (Note that
 * this does not check the *current* checkpoint's IMMEDIATE flag, but whether
 * there is one pending behind it.)
 */
static bool
ImmediateCheckpointRequested(void)
{
        volatile CheckpointerShmemStruct *cps = CheckpointerShmem;

        /*
         * We don't need to acquire the ckpt_lck in this case because we're only
         * looking at a single flag bit.
         */
        if (cps->ckpt_flags & CHECKPOINT_IMMEDIATE)
                return true;
        return false;
}

/*
 * CheckpointWriteDelay -- control rate of checkpoint
 *
 * This function is called after each page write performed by BufferSync().
 * It is responsible for throttling BufferSync()'s write rate to hit
 * checkpoint_completion_target.
 *
 * The checkpoint request flags should be passed in; currently the only one
 * examined is CHECKPOINT_IMMEDIATE, which disables delays between writes.
 *
 * 'progress' is an estimate of how much of the work has been done, as a
 * fraction between 0.0 meaning none, and 1.0 meaning all done.
 */
void
CheckpointWriteDelay(int flags, double progress)
{
        static int      absorb_counter = WRITES_PER_ABSORB;

        /* Do nothing if checkpoint is being executed by non-checkpointer process */
        if (!AmCheckpointerProcess())
                return;

        /*
         * Perform the usual duties and take a nap, unless we're behind schedule,
         * in which case we just try to catch up as quickly as possible.
         */
        if (!(flags & CHECKPOINT_IMMEDIATE) &&
                !ShutdownRequestPending &&
                !ImmediateCheckpointRequested() &&
                IsCheckpointOnSchedule(progress))
        {
                if (ConfigReloadPending)
                {
                        ConfigReloadPending = false;
                        ProcessConfigFile(PGC_SIGHUP);
                        /* update shmem copies of config variables */
                        UpdateSharedMemoryConfig();
                }

                AbsorbSyncRequests();
                absorb_counter = WRITES_PER_ABSORB;

                CheckArchiveTimeout();

                /* Report interim statistics to the cumulative stats system */
                pgstat_report_checkpointer();

                /*
                 * This sleep used to be connected to bgwriter_delay, typically 200ms.
                 * That resulted in more frequent wakeups if not much work to do.
                 * Checkpointer and bgwriter are no longer related so take the Big
                 * Sleep.
                 */
                WaitLatch(MyLatch, WL_LATCH_SET | WL_EXIT_ON_PM_DEATH | WL_TIMEOUT,
                                  100,
                                  WAIT_EVENT_CHECKPOINT_WRITE_DELAY);
                ResetLatch(MyLatch);
        }
        else if (--absorb_counter <= 0)
        {
                /*
                 * Absorb pending fsync requests after each WRITES_PER_ABSORB write
                 * operations even when we don't sleep, to prevent overflow of the
                 * fsync request queue.
                 */
                AbsorbSyncRequests();
                absorb_counter = WRITES_PER_ABSORB;
        }

        /* Check for barrier events. */
        if (ProcSignalBarrierPending)
                ProcessProcSignalBarrier();
}

/*
 * IsCheckpointOnSchedule -- are we on schedule to finish this checkpoint
 *               (or restartpoint) in time?
 *
 * Compares the current progress against the time/segments elapsed since last
 * checkpoint, and returns true if the progress we've made this far is greater
 * than the elapsed time/segments.
 */
static bool
IsCheckpointOnSchedule(double progress)
{
        XLogRecPtr      recptr;
        struct timeval now;
        double          elapsed_xlogs,
                                elapsed_time;

        Assert(ckpt_active);

        /* Scale progress according to checkpoint_completion_target. */
        progress *= CheckPointCompletionTarget;

        /*
         * Check against the cached value first. Only do the more expensive
         * calculations once we reach the target previously calculated. Since
         * neither time or WAL insert pointer moves backwards, a freshly
         * calculated value can only be greater than or equal to the cached value.
         */
        if (progress < ckpt_cached_elapsed)
                return false;

        /*
         * Check progress against WAL segments written and CheckPointSegments.
         *
         * We compare the current WAL insert location against the location
         * computed before calling CreateCheckPoint. The code in XLogInsert that
         * actually triggers a checkpoint when CheckPointSegments is exceeded
         * compares against RedoRecPtr, so this is not completely accurate.
         * However, it's good enough for our purposes, we're only calculating an
         * estimate anyway.
         *
         * During recovery, we compare last replayed WAL record's location with
         * the location computed before calling CreateRestartPoint. That maintains
         * the same pacing as we have during checkpoints in normal operation, but
         * we might exceed max_wal_size by a fair amount. That's because there can
         * be a large gap between a checkpoint's redo-pointer and the checkpoint
         * record itself, and we only start the restartpoint after we've seen the
         * checkpoint record. (The gap is typically up to CheckPointSegments *
         * checkpoint_completion_target where checkpoint_completion_target is the
         * value that was in effect when the WAL was generated).
         */
        if (RecoveryInProgress())
                recptr = GetXLogReplayRecPtr(NULL);
        else
                recptr = GetInsertRecPtr();
        elapsed_xlogs = (((double) (recptr - ckpt_start_recptr)) /
                                         wal_segment_size) / CheckPointSegments;

        if (progress < elapsed_xlogs)
        {
                ckpt_cached_elapsed = elapsed_xlogs;
                return false;
        }

        /*
         * Check progress against time elapsed and checkpoint_timeout.
         */
        gettimeofday(&now, NULL);
        elapsed_time = ((double) ((pg_time_t) now.tv_sec - ckpt_start_time) +
                                        now.tv_usec / 1000000.0) / CheckPointTimeout;

        if (progress < elapsed_time)
        {
                ckpt_cached_elapsed = elapsed_time;
                return false;
        }

        /* It looks like we're on schedule. */
        return true;
}


/* --------------------------------
 *              signal handler routines
 * --------------------------------
 */

/* SIGINT: set flag to run a normal checkpoint right away */
static void
ReqCheckpointHandler(SIGNAL_ARGS)
{
        /*
         * The signaling process should have set ckpt_flags nonzero, so all we
         * need do is ensure that our main loop gets kicked out of any wait.
         */
        SetLatch(MyLatch);
}


/* --------------------------------
 *              communication with backends
 * --------------------------------
 */

/*
 * CheckpointerShmemSize
 *              Compute space needed for checkpointer-related shared memory
 */
Size
CheckpointerShmemSize(void)
{
        Size            size;

        /*
         * Currently, the size of the requests[] array is arbitrarily set equal to
         * NBuffers.  This may prove too large or small ...
         */
        size = offsetof(CheckpointerShmemStruct, requests);
        size = add_size(size, mul_size(NBuffers, sizeof(CheckpointerRequest)));

        return size;
}

/*
 * CheckpointerShmemInit
 *              Allocate and initialize checkpointer-related shared memory
 */
void
CheckpointerShmemInit(void)
{
        Size            size = CheckpointerShmemSize();
        bool            found;

        CheckpointerShmem = (CheckpointerShmemStruct *)
                ShmemInitStruct("Checkpointer Data",
                                                size,
                                                &found);

        if (!found)
        {
                /*
                 * First time through, so initialize.  Note that we zero the whole
                 * requests array; this is so that CompactCheckpointerRequestQueue can
                 * assume that any pad bytes in the request structs are zeroes.
                 */
                MemSet(CheckpointerShmem, 0, size);
                SpinLockInit(&CheckpointerShmem->ckpt_lck);
                CheckpointerShmem->max_requests = NBuffers;
                ConditionVariableInit(&CheckpointerShmem->start_cv);
                ConditionVariableInit(&CheckpointerShmem->done_cv);
        }
}

/*
 * RequestCheckpoint
 *              Called in backend processes to request a checkpoint
 *
 * flags is a bitwise OR of the following:
 *      CHECKPOINT_IS_SHUTDOWN: checkpoint is for database shutdown.
 *      CHECKPOINT_END_OF_RECOVERY: checkpoint is for end of WAL recovery.
 *      CHECKPOINT_IMMEDIATE: finish the checkpoint ASAP,
 *              ignoring checkpoint_completion_target parameter.
 *      CHECKPOINT_FORCE: force a checkpoint even if no XLOG activity has occurred
 *              since the last one (implied by CHECKPOINT_IS_SHUTDOWN or
 *              CHECKPOINT_END_OF_RECOVERY).
 *      CHECKPOINT_WAIT: wait for completion before returning (otherwise,
 *              just signal checkpointer to do it, and return).
 *      CHECKPOINT_CAUSE_XLOG: checkpoint is requested due to xlog filling.
 *              (This affects logging, and in particular enables CheckPointWarning.)
 */
void
RequestCheckpoint(int flags)
{
        int                     ntries;
        int                     old_failed,
                                old_started;

        /*
         * If in a standalone backend, just do it ourselves.
         */
        if (!IsPostmasterEnvironment)
        {
                /*
                 * There's no point in doing slow checkpoints in a standalone backend,
                 * because there's no other backends the checkpoint could disrupt.
                 */
                CreateCheckPoint(flags | CHECKPOINT_IMMEDIATE);

                /* Free all smgr objects, as CheckpointerMain() normally would. */
                smgrdestroyall();

                return;
        }

        /*
         * Atomically set the request flags, and take a snapshot of the counters.
         * When we see ckpt_started > old_started, we know the flags we set here
         * have been seen by checkpointer.
         *
         * Note that we OR the flags with any existing flags, to avoid overriding
         * a "stronger" request by another backend.  The flag senses must be
         * chosen to make this work!
         */
        SpinLockAcquire(&CheckpointerShmem->ckpt_lck);

        old_failed = CheckpointerShmem->ckpt_failed;
        old_started = CheckpointerShmem->ckpt_started;
        CheckpointerShmem->ckpt_flags |= (flags | CHECKPOINT_REQUESTED);

        SpinLockRelease(&CheckpointerShmem->ckpt_lck);

        /*
         * Send signal to request checkpoint.  It's possible that the checkpointer
         * hasn't started yet, or is in process of restarting, so we will retry a
         * few times if needed.  (Actually, more than a few times, since on slow
         * or overloaded buildfarm machines, it's been observed that the
         * checkpointer can take several seconds to start.)  However, if not told
         * to wait for the checkpoint to occur, we consider failure to send the
         * signal to be nonfatal and merely LOG it.  The checkpointer should see
         * the request when it does start, with or without getting a signal.
         */
#define MAX_SIGNAL_TRIES 600    /* max wait 60.0 sec */
        for (ntries = 0;; ntries++)
        {
                if (CheckpointerShmem->checkpointer_pid == 0)
                {
                        if (ntries >= MAX_SIGNAL_TRIES || !(flags & CHECKPOINT_WAIT))
                        {
                                elog((flags & CHECKPOINT_WAIT) ? ERROR : LOG,
                                         "could not signal for checkpoint: checkpointer is not running");
                                break;
                        }
                }
                else if (kill(CheckpointerShmem->checkpointer_pid, SIGINT) != 0)
                {
                        if (ntries >= MAX_SIGNAL_TRIES || !(flags & CHECKPOINT_WAIT))
                        {
                                elog((flags & CHECKPOINT_WAIT) ? ERROR : LOG,
                                         "could not signal for checkpoint: %m");
                                break;
                        }
                }
                else
                        break;                          /* signal sent successfully */

                CHECK_FOR_INTERRUPTS();
                pg_usleep(100000L);             /* wait 0.1 sec, then retry */
        }

        /*
         * If requested, wait for completion.  We detect completion according to
         * the algorithm given above.
         */
        if (flags & CHECKPOINT_WAIT)
        {
                int                     new_started,
                                        new_failed;

                /* Wait for a new checkpoint to start. */
                ConditionVariablePrepareToSleep(&CheckpointerShmem->start_cv);
                for (;;)
                {
                        SpinLockAcquire(&CheckpointerShmem->ckpt_lck);
                        new_started = CheckpointerShmem->ckpt_started;
                        SpinLockRelease(&CheckpointerShmem->ckpt_lck);

                        if (new_started != old_started)
                                break;

                        ConditionVariableSleep(&CheckpointerShmem->start_cv,
                                                                   WAIT_EVENT_CHECKPOINT_START);
                }
                ConditionVariableCancelSleep();

                /*
                 * We are waiting for ckpt_done >= new_started, in a modulo sense.
                 */
                ConditionVariablePrepareToSleep(&CheckpointerShmem->done_cv);
                for (;;)
                {
                        int                     new_done;

                        SpinLockAcquire(&CheckpointerShmem->ckpt_lck);
                        new_done = CheckpointerShmem->ckpt_done;
                        new_failed = CheckpointerShmem->ckpt_failed;
                        SpinLockRelease(&CheckpointerShmem->ckpt_lck);

                        if (new_done - new_started >= 0)
                                break;

                        ConditionVariableSleep(&CheckpointerShmem->done_cv,
                                                                   WAIT_EVENT_CHECKPOINT_DONE);
                }
                ConditionVariableCancelSleep();

                if (new_failed != old_failed)
                        ereport(ERROR,
                                        (errmsg("checkpoint request failed"),
                                         errhint("Consult recent messages in the server log for details.")));
        }
}

/*
 * ForwardSyncRequest
 *              Forward a file-fsync request from a backend to the checkpointer
 *
 * Whenever a backend is compelled to write directly to a relation
 * (which should be seldom, if the background writer is getting its job done),
 * the backend calls this routine to pass over knowledge that the relation
 * is dirty and must be fsync'd before next checkpoint.  We also use this
 * opportunity to count such writes for statistical purposes.
 *
 * To avoid holding the lock for longer than necessary, we normally write
 * to the requests[] queue without checking for duplicates.  The checkpointer
 * will have to eliminate dups internally anyway.  However, if we discover
 * that the queue is full, we make a pass over the entire queue to compact
 * it.  This is somewhat expensive, but the alternative is for the backend
 * to perform its own fsync, which is far more expensive in practice.  It
 * is theoretically possible a backend fsync might still be necessary, if
 * the queue is full and contains no duplicate entries.  In that case, we
 * let the backend know by returning false.
 */
bool
ForwardSyncRequest(const FileTag *ftag, SyncRequestType type)
{
        CheckpointerRequest *request;
        bool            too_full;

        if (!IsUnderPostmaster)
                return false;                   /* probably shouldn't even get here */

        if (AmCheckpointerProcess())
                elog(ERROR, "ForwardSyncRequest must not be called in checkpointer");

        LWLockAcquire(CheckpointerCommLock, LW_EXCLUSIVE);

        /*
         * If the checkpointer isn't running or the request queue is full, the
         * backend will have to perform its own fsync request.  But before forcing
         * that to happen, we can try to compact the request queue.
         */
        if (CheckpointerShmem->checkpointer_pid == 0 ||
                (CheckpointerShmem->num_requests >= CheckpointerShmem->max_requests &&
                 !CompactCheckpointerRequestQueue()))
        {
                LWLockRelease(CheckpointerCommLock);
                return false;
        }

        /* OK, insert request */
        request = &CheckpointerShmem->requests[CheckpointerShmem->num_requests++];
        request->ftag = *ftag;
        request->type = type;

        /* If queue is more than half full, nudge the checkpointer to empty it */
        too_full = (CheckpointerShmem->num_requests >=
                                CheckpointerShmem->max_requests / 2);

        LWLockRelease(CheckpointerCommLock);

        /* ... but not till after we release the lock */
        if (too_full && ProcGlobal->checkpointerLatch)
                SetLatch(ProcGlobal->checkpointerLatch);

        return true;
}

/*
 * CompactCheckpointerRequestQueue
 *              Remove duplicates from the request queue to avoid backend fsyncs.
 *              Returns "true" if any entries were removed.
 *
 * Although a full fsync request queue is not common, it can lead to severe
 * performance problems when it does happen.  So far, this situation has
 * only been observed to occur when the system is under heavy write load,
 * and especially during the "sync" phase of a checkpoint.  Without this
 * logic, each backend begins doing an fsync for every block written, which
 * gets very expensive and can slow down the whole system.
 *
 * Trying to do this every time the queue is full could lose if there
 * aren't any removable entries.  But that should be vanishingly rare in
 * practice: there's one queue entry per shared buffer.
 */
static bool
CompactCheckpointerRequestQueue(void)
{
        struct CheckpointerSlotMapping
        {
                CheckpointerRequest request;
                int                     slot;
        };

        int                     n,
                                preserve_count;
        int                     num_skipped = 0;
        HASHCTL         ctl;
        HTAB       *htab;
        bool       *skip_slot;

        /* must hold CheckpointerCommLock in exclusive mode */
        Assert(LWLockHeldByMe(CheckpointerCommLock));

        /* Avoid memory allocations in a critical section. */
        if (CritSectionCount > 0)
                return false;

        /* Initialize skip_slot array */
        skip_slot = palloc0(sizeof(bool) * CheckpointerShmem->num_requests);

        /* Initialize temporary hash table */
        ctl.keysize = sizeof(CheckpointerRequest);
        ctl.entrysize = sizeof(struct CheckpointerSlotMapping);
        ctl.hcxt = CurrentMemoryContext;

        htab = hash_create("CompactCheckpointerRequestQueue",
                                           CheckpointerShmem->num_requests,
                                           &ctl,
                                           HASH_ELEM | HASH_BLOBS | HASH_CONTEXT);

        /*
         * The basic idea here is that a request can be skipped if it's followed
         * by a later, identical request.  It might seem more sensible to work
         * backwards from the end of the queue and check whether a request is
         * *preceded* by an earlier, identical request, in the hopes of doing less
         * copying.  But that might change the semantics, if there's an
         * intervening SYNC_FORGET_REQUEST or SYNC_FILTER_REQUEST, so we do it
         * this way.  It would be possible to be even smarter if we made the code
         * below understand the specific semantics of such requests (it could blow
         * away preceding entries that would end up being canceled anyhow), but
         * it's not clear that the extra complexity would buy us anything.
         */
        for (n = 0; n < CheckpointerShmem->num_requests; n++)
        {
                CheckpointerRequest *request;
                struct CheckpointerSlotMapping *slotmap;
                bool            found;

                /*
                 * We use the request struct directly as a hashtable key.  This
                 * assumes that any padding bytes in the structs are consistently the
                 * same, which should be okay because we zeroed them in
                 * CheckpointerShmemInit.  Note also that RelFileLocator had better
                 * contain no pad bytes.
                 */
                request = &CheckpointerShmem->requests[n];
                slotmap = hash_search(htab, request, HASH_ENTER, &found);
                if (found)
                {
                        /* Duplicate, so mark the previous occurrence as skippable */
                        skip_slot[slotmap->slot] = true;
                        num_skipped++;
                }
                /* Remember slot containing latest occurrence of this request value */
                slotmap->slot = n;
        }

        /* Done with the hash table. */
        hash_destroy(htab);

        /* If no duplicates, we're out of luck. */
        if (!num_skipped)
        {
                pfree(skip_slot);
                return false;
        }

        /* We found some duplicates; remove them. */
        preserve_count = 0;
        for (n = 0; n < CheckpointerShmem->num_requests; n++)
        {
                if (skip_slot[n])
                        continue;
                CheckpointerShmem->requests[preserve_count++] = CheckpointerShmem->requests[n];
        }
        ereport(DEBUG1,
                        (errmsg_internal("compacted fsync request queue from %d entries to %d entries",
                                                         CheckpointerShmem->num_requests, preserve_count)));
        CheckpointerShmem->num_requests = preserve_count;

        /* Cleanup. */
        pfree(skip_slot);
        return true;
}

/*
 * AbsorbSyncRequests
 *              Retrieve queued sync requests and pass them to sync mechanism.
 *
 * This is exported because it must be called during CreateCheckPoint;
 * we have to be sure we have accepted all pending requests just before
 * we start fsync'ing.  Since CreateCheckPoint sometimes runs in
 * non-checkpointer processes, do nothing if not checkpointer.
 */
void
AbsorbSyncRequests(void)
{
        CheckpointerRequest *requests = NULL;
        CheckpointerRequest *request;
        int                     n;

        if (!AmCheckpointerProcess())
                return;

        LWLockAcquire(CheckpointerCommLock, LW_EXCLUSIVE);

        /*
         * We try to avoid holding the lock for a long time by copying the request
         * array, and processing the requests after releasing the lock.
         *
         * Once we have cleared the requests from shared memory, we have to PANIC
         * if we then fail to absorb them (eg, because our hashtable runs out of
         * memory).  This is because the system cannot run safely if we are unable
         * to fsync what we have been told to fsync.  Fortunately, the hashtable
         * is so small that the problem is quite unlikely to arise in practice.
         */
        n = CheckpointerShmem->num_requests;
        if (n > 0)
        {
                requests = (CheckpointerRequest *) palloc(n * sizeof(CheckpointerRequest));
                memcpy(requests, CheckpointerShmem->requests, n * sizeof(CheckpointerRequest));
        }

        START_CRIT_SECTION();

        CheckpointerShmem->num_requests = 0;

        LWLockRelease(CheckpointerCommLock);

        for (request = requests; n > 0; request++, n--)
                RememberSyncRequest(&request->ftag, request->type);

        END_CRIT_SECTION();

        if (requests)
                pfree(requests);
}

/*
 * Update any shared memory configurations based on config parameters
 */
static void
UpdateSharedMemoryConfig(void)
{
        /* update global shmem state for sync rep */
        SyncRepUpdateSyncStandbysDefined();

        /*
         * If full_page_writes has been changed by SIGHUP, we update it in shared
         * memory and write an XLOG_FPW_CHANGE record.
         */
        UpdateFullPageWrites();

        elog(DEBUG2, "checkpointer updated shared memory configuration values");
}

/*
 * FirstCallSinceLastCheckpoint allows a process to take an action once
 * per checkpoint cycle by asynchronously checking for checkpoint completion.
 */
bool
FirstCallSinceLastCheckpoint(void)
{
        static int      ckpt_done = 0;
        int                     new_done;
        bool            FirstCall = false;

        SpinLockAcquire(&CheckpointerShmem->ckpt_lck);
        new_done = CheckpointerShmem->ckpt_done;
        SpinLockRelease(&CheckpointerShmem->ckpt_lck);

        if (new_done != ckpt_done)
                FirstCall = true;

        ckpt_done = new_done;

        return FirstCall;
}