walwriter.h File Reference

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Macros
#define	DEFAULT_WAL_WRITER_FLUSH_AFTER   ((1024 * 1024) / XLOG_BLCKSZ)

Functions
pg_noreturn void	WalWriterMain (const void *startup_data, size_t startup_data_len)

Variables
PGDLLIMPORT int	WalWriterDelay
PGDLLIMPORT int	WalWriterFlushAfter

Macro Definition Documentation

DEFAULT_WAL_WRITER_FLUSH_AFTER

#define DEFAULT_WAL_WRITER_FLUSH_AFTER   ((1024 * 1024) / XLOG_BLCKSZ)

Definition at line 15 of file walwriter.h.

Function Documentation

WalWriterMain()

pg_noreturn void WalWriterMain	(	const void *	startup_data,
		size_t	startup_data_len
	)

Definition at line 88 of file walwriter.c.

{
    sigjmp_buf  local_sigjmp_buf;
    MemoryContext walwriter_context;
    int         left_till_hibernate;
    bool        hibernating;
 
    Assert(startup_data_len == 0);
 
    MyBackendType = B_WAL_WRITER;
    AuxiliaryProcessMainCommon();
 
    /*
     * Properly accept or ignore signals the postmaster might send us
     *
     * We have no particular use for SIGINT at the moment, but seems
     * reasonable to treat like SIGTERM.
     */
    pqsignal(SIGHUP, SignalHandlerForConfigReload);
    pqsignal(SIGINT, SignalHandlerForShutdownRequest);
    pqsignal(SIGTERM, SignalHandlerForShutdownRequest);
    /* SIGQUIT handler was already set up by InitPostmasterChild */
    pqsignal(SIGALRM, SIG_IGN);
    pqsignal(SIGPIPE, SIG_IGN);
    pqsignal(SIGUSR1, procsignal_sigusr1_handler);
    pqsignal(SIGUSR2, SIG_IGN); /* not used */
 
    /*
     * Reset some signals that are accepted by postmaster but not here
     */
    pqsignal(SIGCHLD, SIG_DFL);
 
    /*
     * 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.
     */
    walwriter_context = AllocSetContextCreate(TopMemoryContext,
                                              "Wal Writer",
                                              ALLOCSET_DEFAULT_SIZES);
    MemoryContextSwitchTo(walwriter_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 walwriter, but we do have LWLocks, and perhaps buffers?
         */
        LWLockReleaseAll();
        ConditionVariableCancelSleep();
        pgstat_report_wait_end();
        pgaio_error_cleanup();
        UnlockBuffers();
        ReleaseAuxProcessResources(false);
        AtEOXact_Buffers(false);
        AtEOXact_SMgr();
        AtEOXact_Files(false);
        AtEOXact_HashTables(false);
 
        /*
         * Now return to normal top-level context and clear ErrorContext for
         * next time.
         */
        MemoryContextSwitchTo(walwriter_context);
        FlushErrorState();
 
        /* Flush any leaked data in the top-level context */
        MemoryContextReset(walwriter_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);
 
    /*
     * Reset hibernation state after any error.
     */
    left_till_hibernate = LOOPS_UNTIL_HIBERNATE;
    hibernating = false;
    SetWalWriterSleeping(false);
 
    /*
     * Advertise our proc number that backends can use to wake us up while
     * we're sleeping.
     */
    ProcGlobal->walwriterProc = MyProcNumber;
 
    /*
     * Loop forever
     */
    for (;;)
    {
        long        cur_timeout;
 
        /*
         * Advertise whether we might hibernate in this cycle.  We do this
         * before resetting the latch to ensure that any async commits will
         * see the flag set if they might possibly need to wake us up, and
         * that we won't miss any signal they send us.  (If we discover work
         * to do in the last cycle before we would hibernate, the global flag
         * will be set unnecessarily, but little harm is done.)  But avoid
         * touching the global flag if it doesn't need to change.
         */
        if (hibernating != (left_till_hibernate <= 1))
        {
            hibernating = (left_till_hibernate <= 1);
            SetWalWriterSleeping(hibernating);
        }
 
        /* Clear any already-pending wakeups */
        ResetLatch(MyLatch);
 
        /* Process any signals received recently */
        ProcessMainLoopInterrupts();
 
        /*
         * Do what we're here for; then, if XLogBackgroundFlush() found useful
         * work to do, reset hibernation counter.
         */
        if (XLogBackgroundFlush())
            left_till_hibernate = LOOPS_UNTIL_HIBERNATE;
        else if (left_till_hibernate > 0)
            left_till_hibernate--;
 
        /* report pending statistics to the cumulative stats system */
        pgstat_report_wal(false);
 
        /*
         * Sleep until we are signaled or WalWriterDelay has elapsed.  If we
         * haven't done anything useful for quite some time, lengthen the
         * sleep time so as to reduce the server's idle power consumption.
         */
        if (left_till_hibernate > 0)
            cur_timeout = WalWriterDelay;   /* in ms */
        else
            cur_timeout = WalWriterDelay * HIBERNATE_FACTOR;
 
        (void) WaitLatch(MyLatch,
                         WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH,
                         cur_timeout,
                         WAIT_EVENT_WAL_WRITER_MAIN);
    }
}

References ALLOCSET_DEFAULT_SIZES, AllocSetContextCreate, Assert(), AtEOXact_Buffers(), AtEOXact_Files(), AtEOXact_HashTables(), AtEOXact_SMgr(), AuxiliaryProcessMainCommon(), B_WAL_WRITER, ConditionVariableCancelSleep(), EmitErrorReport(), error_context_stack, FlushErrorState(), HIBERNATE_FACTOR, HOLD_INTERRUPTS, LOOPS_UNTIL_HIBERNATE, LWLockReleaseAll(), MemoryContextReset(), MemoryContextSwitchTo(), MyBackendType, MyLatch, MyProcNumber, PG_exception_stack, pg_usleep(), pgaio_error_cleanup(), pgstat_report_wait_end(), pgstat_report_wal(), pqsignal, ProcessMainLoopInterrupts(), ProcGlobal, procsignal_sigusr1_handler(), ReleaseAuxProcessResources(), ResetLatch(), RESUME_INTERRUPTS, SetWalWriterSleeping(), SIGALRM, SIGCHLD, SIGHUP, SignalHandlerForConfigReload(), SignalHandlerForShutdownRequest(), SIGPIPE, SIGUSR1, SIGUSR2, TopMemoryContext, UnBlockSig, UnlockBuffers(), WaitLatch(), WalWriterDelay, PROC_HDR::walwriterProc, WL_EXIT_ON_PM_DEATH, WL_LATCH_SET, WL_TIMEOUT, and XLogBackgroundFlush().

Variable Documentation

WalWriterDelay

PGDLLIMPORT int WalWriterDelay

extern

Definition at line 70 of file walwriter.c.

Referenced by LogicalRepApplyLoop(), WalWriterMain(), and XLogBackgroundFlush().

WalWriterFlushAfter

PGDLLIMPORT int WalWriterFlushAfter

extern

Definition at line 71 of file walwriter.c.

Referenced by XLogBackgroundFlush(), and XLogSetAsyncXactLSN().