checkpointer.c

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/*-------------------------------------------------------------------------
 *
 * 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.)
 *
 * The checkpointer is started by the postmaster as soon as the startup
 * subprocess finishes, or as soon as recovery begins if we are doing archive
 * recovery.  It remains alive until the postmaster commands it to terminate.
 * 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) 1996-2020, 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 "libpq/pqsignal.h"
#include "miscadmin.h"
#include "pgstat.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.
 *
 * num_backend_writes is used to count the number of buffer writes performed
 * by user backend processes.  This counter should be wide enough that it
 * can't overflow during a single processing cycle.  num_backend_fsync
 * counts the subset of those writes that also had to do their own fsync,
 * because the checkpointer failed to absorb their request.
 *
 * The requests array holds fsync requests sent by backends and not yet
 * absorbed by the checkpointer.
 *
 * Unlike the checkpoint fields, num_backend_writes, num_backend_fsync, and
 * the requests 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 */

    uint32      num_backend_writes; /* counts user backend buffer writes */
    uint32      num_backend_fsync;  /* counts user backend fsync calls */

    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 = 0.5;

/*
 * 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(void)
{
    sigjmp_buf  local_sigjmp_buf;
    MemoryContext checkpointer_context;

    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 */
    pqsignal(SIGQUIT, SignalHandlerForCrashExit);
    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);

    /* We allow SIGQUIT (quickdie) at all times */
    sigdelset(&BlockSig, SIGQUIT);

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

    /*
     * 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.
     *
     * See notes in postgres.c about the design of this coding.
     */
    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();
        AbortBufferIO();
        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 */
        MemoryContextResetAndDeleteChildren(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);

        /*
         * Close all open files after any error.  This is helpful on Windows,
         * where holding deleted files open causes various strange errors.
         * It's not clear we need it elsewhere, but shouldn't hurt.
         */
        smgrcloseall();
    }

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

    /*
     * Unblock signals (they were blocked when the postmaster forked us)
     */
    PG_SETMASK(&UnBlockSig);

    /*
     * 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;

        /* 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;
            BgWriterStats.m_requested_checkpoints++;
        }

        /*
         * 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)
                BgWriterStats.m_timed_checkpoints++;
            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. As a
             * side-effect, RecoveryInProgress() initializes TimeLineID if
             * it's not set yet.
             */
            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;

            /*
             * 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 \"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)
            {
                CreateCheckPoint(flags);
                ckpt_performed = true;
            }
            else
                ckpt_performed = CreateRestartPoint(flags);

            /*
             * After any checkpoint, close all smgr files.  This is so we
             * won't hang onto smgr references to deleted files indefinitely.
             */
            smgrcloseall();

            /*
             * 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 (ckpt_performed)
            {
                /*
                 * 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;
            }
            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;
        }

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

        /*
         * Send off activity statistics to the stats collector.  (The reason
         * why we re-use bgwriter-related code for this is that the bgwriter
         * and checkpointer used to be just one process.  It's probably not
         * worth the trouble to split the stats support into two independent
         * stats message types.)
         */
        pgstat_send_bgwriter();

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

/*
 * 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 activity statistics to the stats collector.
         */
        pgstat_send_bgwriter();

        /*
         * 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.
         */
        pg_usleep(100000L);
    }
    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)
{
    int         save_errno = errno;

    /*
     * The signalling 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);

    errno = save_errno;
}


/* --------------------------------
 *      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);

        /*
         * After any checkpoint, close all smgr files.  This is so we won't
         * hang onto smgr references to deleted files indefinitely.
         */
        smgrcloseall();

        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);

    /* Count all backend writes regardless of if they fit in the queue */
    if (!AmBackgroundWriterProcess())
        CheckpointerShmem->num_backend_writes++;

    /*
     * 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()))
    {
        /*
         * Count the subset of writes where backends have to do their own
         * fsync
         */
        if (!AmBackgroundWriterProcess())
            CheckpointerShmem->num_backend_fsync++;
        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));

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

    /* Initialize temporary hash table */
    MemSet(&ctl, 0, sizeof(ctl));
    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 RelFileNode 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("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);

    /* Transfer stats counts into pending pgstats message */
    BgWriterStats.m_buf_written_backend += CheckpointerShmem->num_backend_writes;
    BgWriterStats.m_buf_fsync_backend += CheckpointerShmem->num_backend_fsync;

    CheckpointerShmem->num_backend_writes = 0;
    CheckpointerShmem->num_backend_fsync = 0;

    /*
     * 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;
}