[postgresql] / src / backend / postmaster / bgwriter.c

/*-------------------------------------------------------------------------
 *
 * bgwriter.c
 *
 * The background writer (bgwriter) is new in Postgres 8.0.  It attempts
 * to keep regular backends from having to write out dirty shared buffers
 * (which they would only do when needing to free a shared buffer to read in
 * another page).  In the best scenario all writes from shared buffers will
 * be issued by the background writer process.  However, regular backends are
 * still empowered to issue writes if the bgwriter fails to maintain enough
 * clean shared buffers.
 *
 * The bgwriter is also charged with handling all checkpoints.  It will
 * automatically dispatch a checkpoint 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 the bgwriter
 * when they fill WAL segments; the bgwriter itself doesn't watch for the
 * condition.)
 *
 * The bgwriter is started by the postmaster as soon as the startup subprocess
 * finishes.  It remains alive until the postmaster commands it to terminate.
 * Normal termination is by SIGUSR2, which instructs the bgwriter 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 bgwriter will simply abort and exit on SIGQUIT.
 *
 * If the bgwriter 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) 1996-2005, PostgreSQL Global Development Group
 *
 *
 * IDENTIFICATION
 *        $PostgreSQL: pgsql/src/backend/postmaster/bgwriter.c,v 1.20 2005/09/12 22:20:16 tgl Exp $
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include <signal.h>
#include <time.h>

#include "access/xlog.h"
#include "libpq/pqsignal.h"
#include "miscadmin.h"
#include "postmaster/bgwriter.h"
#include "storage/bufmgr.h"
#include "storage/freespace.h"
#include "storage/ipc.h"
#include "storage/pmsignal.h"
#include "storage/smgr.h"
#include "tcop/tcopprot.h"
#include "utils/guc.h"
#include "utils/memutils.h"


/*----------
 * Shared memory area for communication between bgwriter 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, bgwriter increments ckpt_started.
 *      * On completion of a checkpoint, bgwriter sets ckpt_done to
 *        equal ckpt_started.
 *      * On failure of a checkpoint, bgwrite first increments ckpt_failed,
 *        then sets ckpt_done to equal ckpt_started.
 * All three fields are declared sig_atomic_t to ensure they can be read
 * and written without explicit locking.  The algorithm for backends is:
 *      1. Record current values of ckpt_failed and ckpt_started (in that
 *         order!).
 *      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).
 *      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.
 *
 * An additional field is ckpt_time_warn; this is also sig_atomic_t for
 * simplicity, but is only used as a boolean.  If a backend is requesting
 * a checkpoint for which a checkpoints-too-close-together warning is
 * reasonable, it should set this field TRUE just before sending the signal.
 *
 * The requests array holds fsync requests sent by backends and not yet
 * absorbed by the bgwriter.  Unlike the checkpoint fields, the requests
 * fields are protected by BgWriterCommLock.
 *----------
 */
typedef struct
{
        RelFileNode rnode;
        BlockNumber segno;
        /* might add a request-type field later */
} BgWriterRequest;

typedef struct
{
        pid_t           bgwriter_pid;   /* PID of bgwriter (0 if not started) */

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

        sig_atomic_t ckpt_time_warn;    /* warn if too soon since last ckpt? */

        int                     num_requests;   /* current # of requests */
        int                     max_requests;   /* allocated array size */
        BgWriterRequest requests[1];    /* VARIABLE LENGTH ARRAY */
} BgWriterShmemStruct;

static BgWriterShmemStruct *BgWriterShmem;

/*
 * GUC parameters
 */
int                     BgWriterDelay = 200;
int                     CheckPointTimeout = 300;
int                     CheckPointWarning = 30;

/*
 * Flags set by interrupt handlers for later service in the main loop.
 */
static volatile sig_atomic_t got_SIGHUP = false;
static volatile sig_atomic_t checkpoint_requested = false;
static volatile sig_atomic_t shutdown_requested = false;

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

static bool ckpt_active = false;

static time_t last_checkpoint_time;


static void bg_quickdie(SIGNAL_ARGS);
static void BgSigHupHandler(SIGNAL_ARGS);
static void ReqCheckpointHandler(SIGNAL_ARGS);
static void ReqShutdownHandler(SIGNAL_ARGS);


/*
 * Main entry point for bgwriter process
 *
 * This is invoked from BootstrapMain, which has already created the basic
 * execution environment, but not enabled signals yet.
 */
void
BackgroundWriterMain(void)
{
        sigjmp_buf      local_sigjmp_buf;
        MemoryContext bgwriter_context;

        Assert(BgWriterShmem != NULL);
        BgWriterShmem->bgwriter_pid = MyProcPid;
        am_bg_writer = true;

        /*
         * 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).
         *
         * SIGUSR1 is presently unused; keep it spare in case someday we want
         * this process to participate in sinval messaging.
         */
        pqsignal(SIGHUP, BgSigHupHandler);      /* set flag to read config file */
        pqsignal(SIGINT, ReqCheckpointHandler);         /* request checkpoint */
        pqsignal(SIGTERM, SIG_IGN); /* ignore SIGTERM */
        pqsignal(SIGQUIT, bg_quickdie);         /* hard crash time */
        pqsignal(SIGALRM, SIG_IGN);
        pqsignal(SIGPIPE, SIG_IGN);
        pqsignal(SIGUSR1, SIG_IGN); /* reserve for sinval */
        pqsignal(SIGUSR2, ReqShutdownHandler);          /* request shutdown */

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

        /* We allow SIGQUIT (quickdie) at all times */
#ifdef HAVE_SIGPROCMASK
        sigdelset(&BlockSig, SIGQUIT);
#else
        BlockSig &= ~(sigmask(SIGQUIT));
#endif

        /*
         * Initialize so that first time-driven checkpoint happens at the
         * correct time.
         */
        last_checkpoint_time = 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.
         */
        bgwriter_context = AllocSetContextCreate(TopMemoryContext,
                                                                                         "Background Writer",
                                                                                         ALLOCSET_DEFAULT_MINSIZE,
                                                                                         ALLOCSET_DEFAULT_INITSIZE,
                                                                                         ALLOCSET_DEFAULT_MAXSIZE);
        MemoryContextSwitchTo(bgwriter_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 bgwriter, but we do have LWLocks and buffers.
                 */
                LWLockReleaseAll();
                AbortBufferIO();
                UnlockBuffers();

                /* Warn any waiting backends that the checkpoint failed. */
                if (ckpt_active)
                {
                        /* use volatile pointer to prevent code rearrangement */
                        volatile BgWriterShmemStruct *bgs = BgWriterShmem;

                        bgs->ckpt_failed++;
                        bgs->ckpt_done = bgs->ckpt_started;
                        ckpt_active = false;
                }

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

                /* Flush any leaked data in the top-level context */
                MemoryContextResetAndDeleteChildren(bgwriter_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)
         */
        PG_SETMASK(&UnBlockSig);

        /*
         * Loop forever
         */
        for (;;)
        {
                bool            do_checkpoint = false;
                bool            force_checkpoint = false;
                time_t          now;
                int                     elapsed_secs;
                long            udelay;

                /*
                 * Emergency bailout if postmaster has died.  This is to avoid the
                 * necessity for manual cleanup of all postmaster children.
                 */
                if (!PostmasterIsAlive(true))
                        exit(1);

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

                if (got_SIGHUP)
                {
                        got_SIGHUP = false;
                        ProcessConfigFile(PGC_SIGHUP);
                }
                if (checkpoint_requested)
                {
                        checkpoint_requested = false;
                        do_checkpoint = true;
                        force_checkpoint = true;
                }
                if (shutdown_requested)
                {
                        ShutdownXLOG(0, 0);
                        DumpFreeSpaceMap(0, 0);
                        /* Normal exit from the bgwriter is here */
                        proc_exit(0);           /* done */
                }

                /*
                 * Do an unforced checkpoint if too much time has elapsed since
                 * the last one.
                 */
                now = time(NULL);
                elapsed_secs = now - last_checkpoint_time;
                if (elapsed_secs >= CheckPointTimeout)
                        do_checkpoint = true;

                /*
                 * Do a checkpoint if requested, otherwise do one cycle of
                 * dirty-buffer writing.
                 */
                if (do_checkpoint)
                {
                        /*
                         * We will warn if (a) too soon since last checkpoint (whatever
                         * caused it) and (b) somebody has set the ckpt_time_warn flag
                         * since the last checkpoint start.  Note in particular that
                         * this implementation will not generate warnings caused by
                         * CheckPointTimeout < CheckPointWarning.
                         */
                        if (BgWriterShmem->ckpt_time_warn &&
                                elapsed_secs < CheckPointWarning)
                                ereport(LOG,
                                                (errmsg("checkpoints are occurring too frequently (%d seconds apart)",
                                                                elapsed_secs),
                                                 errhint("Consider increasing the configuration parameter \"checkpoint_segments\".")));
                        BgWriterShmem->ckpt_time_warn = false;

                        /*
                         * Indicate checkpoint start to any waiting backends.
                         */
                        ckpt_active = true;
                        BgWriterShmem->ckpt_started++;

                        CreateCheckPoint(false, force_checkpoint);

                        /*
                         * 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.
                         */
                        BgWriterShmem->ckpt_done = BgWriterShmem->ckpt_started;
                        ckpt_active = false;

                        /*
                         * 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
                        BgBufferSync();

                /*
                 * Nap for the configured time, or sleep for 10 seconds if there
                 * is no bgwriter activity configured.
                 *
                 * On some platforms, signals won't interrupt the sleep.  To ensure
                 * we respond reasonably promptly when someone signals us, break
                 * down the sleep into 1-second increments, and check for
                 * interrupts after each nap.
                 *
                 * We absorb pending requests after each short sleep.
                 */
                if ((bgwriter_all_percent > 0.0 && bgwriter_all_maxpages > 0) ||
                        (bgwriter_lru_percent > 0.0 && bgwriter_lru_maxpages > 0))
                        udelay = BgWriterDelay * 1000L;
                else
                        udelay = 10000000L;
                while (udelay > 1000000L)
                {
                        if (got_SIGHUP || checkpoint_requested || shutdown_requested)
                                break;
                        pg_usleep(1000000L);
                        AbsorbFsyncRequests();
                        udelay -= 1000000L;
                }
                if (!(got_SIGHUP || checkpoint_requested || shutdown_requested))
                        pg_usleep(udelay);
        }
}


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

/*
 * bg_quickdie() occurs when signalled SIGQUIT by the postmaster.
 *
 * Some backend has bought the farm,
 * so we need to stop what we're doing and exit.
 */
static void
bg_quickdie(SIGNAL_ARGS)
{
        PG_SETMASK(&BlockSig);

        /*
         * DO NOT proc_exit() -- we're here because shared memory may be
         * corrupted, so we don't want to try to clean up our transaction.
         * Just nail the windows shut and get out of town.
         *
         * Note we do exit(1) not exit(0).      This is to force the postmaster into
         * a system reset cycle if some idiot DBA sends a manual SIGQUIT to a
         * random backend.      This is necessary precisely because we don't clean
         * up our shared memory state.
         */
        exit(1);
}

/* SIGHUP: set flag to re-read config file at next convenient time */
static void
BgSigHupHandler(SIGNAL_ARGS)
{
        got_SIGHUP = true;
}

/* SIGINT: set flag to run a normal checkpoint right away */
static void
ReqCheckpointHandler(SIGNAL_ARGS)
{
        checkpoint_requested = true;
}

/* SIGUSR2: set flag to run a shutdown checkpoint and exit */
static void
ReqShutdownHandler(SIGNAL_ARGS)
{
        shutdown_requested = true;
}


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

/*
 * BgWriterShmemSize
 *              Compute space needed for bgwriter-related shared memory
 */
Size
BgWriterShmemSize(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(BgWriterShmemStruct, requests);
        size = add_size(size, mul_size(NBuffers, sizeof(BgWriterRequest)));

        return size;
}

/*
 * BgWriterShmemInit
 *              Allocate and initialize bgwriter-related shared memory
 */
void
BgWriterShmemInit(void)
{
        bool            found;

        BgWriterShmem = (BgWriterShmemStruct *)
                ShmemInitStruct("Background Writer Data",
                                                BgWriterShmemSize(),
                                                &found);
        if (BgWriterShmem == NULL)
                ereport(FATAL,
                                (errcode(ERRCODE_OUT_OF_MEMORY),
                                 errmsg("not enough shared memory for background writer")));
        if (found)
                return;                                 /* already initialized */

        MemSet(BgWriterShmem, 0, sizeof(BgWriterShmemStruct));
        BgWriterShmem->max_requests = NBuffers;
}

/*
 * RequestCheckpoint
 *              Called in backend processes to request an immediate checkpoint
 *
 * If waitforit is true, wait until the checkpoint is completed
 * before returning; otherwise, just signal the request and return
 * immediately.
 *
 * If warnontime is true, and it's "too soon" since the last checkpoint,
 * the bgwriter will log a warning.  This should be true only for checkpoints
 * caused due to xlog filling, else the warning will be misleading.
 */
void
RequestCheckpoint(bool waitforit, bool warnontime)
{
        /* use volatile pointer to prevent code rearrangement */
        volatile BgWriterShmemStruct *bgs = BgWriterShmem;
        sig_atomic_t old_failed = bgs->ckpt_failed;
        sig_atomic_t old_started = bgs->ckpt_started;

        /*
         * If in a standalone backend, just do it ourselves.
         */
        if (!IsPostmasterEnvironment)
        {
                CreateCheckPoint(false, true);

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

                return;
        }

        /* Set warning request flag if appropriate */
        if (warnontime)
                bgs->ckpt_time_warn = true;

        /*
         * Send signal to request checkpoint.  When waitforit is false, we
         * consider failure to send the signal to be nonfatal.
         */
        if (BgWriterShmem->bgwriter_pid == 0)
                elog(waitforit ? ERROR : LOG,
                         "could not request checkpoint because bgwriter not running");
        if (kill(BgWriterShmem->bgwriter_pid, SIGINT) != 0)
                elog(waitforit ? ERROR : LOG,
                         "could not signal for checkpoint: %m");

        /*
         * If requested, wait for completion.  We detect completion according
         * to the algorithm given above.
         */
        if (waitforit)
        {
                while (bgs->ckpt_started == old_started)
                {
                        CHECK_FOR_INTERRUPTS();
                        pg_usleep(100000L);
                }
                old_started = bgs->ckpt_started;

                /*
                 * We are waiting for ckpt_done >= old_started, in a modulo sense.
                 * This is a little tricky since we don't know the width or
                 * signedness of sig_atomic_t.  We make the lowest common
                 * denominator assumption that it is only as wide as "char".  This
                 * means that this algorithm will cope correctly as long as we
                 * don't sleep for more than 127 completed checkpoints.  (If we
                 * do, we will get another chance to exit after 128 more
                 * checkpoints...)
                 */
                while (((signed char) (bgs->ckpt_done - old_started)) < 0)
                {
                        CHECK_FOR_INTERRUPTS();
                        pg_usleep(100000L);
                }
                if (bgs->ckpt_failed != old_failed)
                        ereport(ERROR,
                                        (errmsg("checkpoint request failed"),
                                         errhint("Consult the server log for details.")));
        }
}

/*
 * ForwardFsyncRequest
 *              Forward a file-fsync request from a backend to the bgwriter
 *
 * Whenever a backend is compelled to write directly to a relation
 * (which should be seldom, if the bgwriter 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.
 *
 * If we are unable to pass over the request (at present, this can happen
 * if the shared memory queue is full), we return false.  That forces
 * the backend to do its own fsync.  We hope that will be even more seldom.
 *
 * Note: we presently make no attempt to eliminate duplicate requests
 * in the requests[] queue.  The bgwriter will have to eliminate dups
 * internally anyway, so we may as well avoid holding the lock longer
 * than we have to here.
 */
bool
ForwardFsyncRequest(RelFileNode rnode, BlockNumber segno)
{
        BgWriterRequest *request;

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

        LWLockAcquire(BgWriterCommLock, LW_EXCLUSIVE);
        if (BgWriterShmem->bgwriter_pid == 0 ||
                BgWriterShmem->num_requests >= BgWriterShmem->max_requests)
        {
                LWLockRelease(BgWriterCommLock);
                return false;
        }
        request = &BgWriterShmem->requests[BgWriterShmem->num_requests++];
        request->rnode = rnode;
        request->segno = segno;
        LWLockRelease(BgWriterCommLock);
        return true;
}

/*
 * AbsorbFsyncRequests
 *              Retrieve queued fsync requests and pass them to local smgr.
 *
 * This is exported because it must be called during CreateCheckPoint;
 * we have to be sure we have accepted all pending requests *after* we
 * establish the checkpoint REDO pointer.  Since CreateCheckPoint
 * sometimes runs in non-bgwriter processes, do nothing if not bgwriter.
 */
void
AbsorbFsyncRequests(void)
{
        BgWriterRequest *requests = NULL;
        BgWriterRequest *request;
        int                     n;

        if (!am_bg_writer)
                return;

        /*
         * We have to PANIC if we fail to absorb all the pending requests
         * (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.
         */
        START_CRIT_SECTION();

        /*
         * We try to avoid holding the lock for a long time by copying the
         * request array.
         */
        LWLockAcquire(BgWriterCommLock, LW_EXCLUSIVE);

        n = BgWriterShmem->num_requests;
        if (n > 0)
        {
                requests = (BgWriterRequest *) palloc(n * sizeof(BgWriterRequest));
                memcpy(requests, BgWriterShmem->requests, n * sizeof(BgWriterRequest));
        }
        BgWriterShmem->num_requests = 0;

        LWLockRelease(BgWriterCommLock);

        for (request = requests; n > 0; request++, n--)
                RememberFsyncRequest(request->rnode, request->segno);

        if (requests)
                pfree(requests);

        END_CRIT_SECTION();
}