LCOV - code coverage report
Current view: top level - src/backend/postmaster - bgwriter.c (source / functions) Hit Total Coverage
Test: PostgreSQL 15devel Lines: 39 59 66.1 %
Date: 2021-12-09 03:08:47 Functions: 1 1 100.0 %
Legend: Lines: hit not hit

          Line data    Source code
       1             : /*-------------------------------------------------------------------------
       2             :  *
       3             :  * bgwriter.c
       4             :  *
       5             :  * The background writer (bgwriter) is new as of Postgres 8.0.  It attempts
       6             :  * to keep regular backends from having to write out dirty shared buffers
       7             :  * (which they would only do when needing to free a shared buffer to read in
       8             :  * another page).  In the best scenario all writes from shared buffers will
       9             :  * be issued by the background writer process.  However, regular backends are
      10             :  * still empowered to issue writes if the bgwriter fails to maintain enough
      11             :  * clean shared buffers.
      12             :  *
      13             :  * As of Postgres 9.2 the bgwriter no longer handles checkpoints.
      14             :  *
      15             :  * Normal termination is by SIGTERM, which instructs the bgwriter to exit(0).
      16             :  * Emergency termination is by SIGQUIT; like any backend, the bgwriter will
      17             :  * simply abort and exit on SIGQUIT.
      18             :  *
      19             :  * If the bgwriter exits unexpectedly, the postmaster treats that the same
      20             :  * as a backend crash: shared memory may be corrupted, so remaining backends
      21             :  * should be killed by SIGQUIT and then a recovery cycle started.
      22             :  *
      23             :  *
      24             :  * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
      25             :  *
      26             :  *
      27             :  * IDENTIFICATION
      28             :  *    src/backend/postmaster/bgwriter.c
      29             :  *
      30             :  *-------------------------------------------------------------------------
      31             :  */
      32             : #include "postgres.h"
      33             : 
      34             : #include "access/xlog.h"
      35             : #include "access/xlog_internal.h"
      36             : #include "libpq/pqsignal.h"
      37             : #include "miscadmin.h"
      38             : #include "pgstat.h"
      39             : #include "postmaster/bgwriter.h"
      40             : #include "postmaster/interrupt.h"
      41             : #include "storage/buf_internals.h"
      42             : #include "storage/bufmgr.h"
      43             : #include "storage/condition_variable.h"
      44             : #include "storage/fd.h"
      45             : #include "storage/ipc.h"
      46             : #include "storage/lwlock.h"
      47             : #include "storage/proc.h"
      48             : #include "storage/procsignal.h"
      49             : #include "storage/shmem.h"
      50             : #include "storage/smgr.h"
      51             : #include "storage/spin.h"
      52             : #include "storage/standby.h"
      53             : #include "utils/guc.h"
      54             : #include "utils/memutils.h"
      55             : #include "utils/resowner.h"
      56             : #include "utils/timestamp.h"
      57             : 
      58             : /*
      59             :  * GUC parameters
      60             :  */
      61             : int         BgWriterDelay = 200;
      62             : 
      63             : /*
      64             :  * Multiplier to apply to BgWriterDelay when we decide to hibernate.
      65             :  * (Perhaps this needs to be configurable?)
      66             :  */
      67             : #define HIBERNATE_FACTOR            50
      68             : 
      69             : /*
      70             :  * Interval in which standby snapshots are logged into the WAL stream, in
      71             :  * milliseconds.
      72             :  */
      73             : #define LOG_SNAPSHOT_INTERVAL_MS 15000
      74             : 
      75             : /*
      76             :  * LSN and timestamp at which we last issued a LogStandbySnapshot(), to avoid
      77             :  * doing so too often or repeatedly if there has been no other write activity
      78             :  * in the system.
      79             :  */
      80             : static TimestampTz last_snapshot_ts;
      81             : static XLogRecPtr last_snapshot_lsn = InvalidXLogRecPtr;
      82             : 
      83             : 
      84             : /*
      85             :  * Main entry point for bgwriter process
      86             :  *
      87             :  * This is invoked from AuxiliaryProcessMain, which has already created the
      88             :  * basic execution environment, but not enabled signals yet.
      89             :  */
      90             : void
      91         550 : BackgroundWriterMain(void)
      92             : {
      93             :     sigjmp_buf  local_sigjmp_buf;
      94             :     MemoryContext bgwriter_context;
      95             :     bool        prev_hibernate;
      96             :     WritebackContext wb_context;
      97             : 
      98             :     /*
      99             :      * Properly accept or ignore signals that might be sent to us.
     100             :      */
     101         550 :     pqsignal(SIGHUP, SignalHandlerForConfigReload);
     102         550 :     pqsignal(SIGINT, SIG_IGN);
     103         550 :     pqsignal(SIGTERM, SignalHandlerForShutdownRequest);
     104             :     /* SIGQUIT handler was already set up by InitPostmasterChild */
     105         550 :     pqsignal(SIGALRM, SIG_IGN);
     106         550 :     pqsignal(SIGPIPE, SIG_IGN);
     107         550 :     pqsignal(SIGUSR1, procsignal_sigusr1_handler);
     108         550 :     pqsignal(SIGUSR2, SIG_IGN);
     109             : 
     110             :     /*
     111             :      * Reset some signals that are accepted by postmaster but not here
     112             :      */
     113         550 :     pqsignal(SIGCHLD, SIG_DFL);
     114             : 
     115             :     /*
     116             :      * We just started, assume there has been either a shutdown or
     117             :      * end-of-recovery snapshot.
     118             :      */
     119         550 :     last_snapshot_ts = GetCurrentTimestamp();
     120             : 
     121             :     /*
     122             :      * Create a memory context that we will do all our work in.  We do this so
     123             :      * that we can reset the context during error recovery and thereby avoid
     124             :      * possible memory leaks.  Formerly this code just ran in
     125             :      * TopMemoryContext, but resetting that would be a really bad idea.
     126             :      */
     127         550 :     bgwriter_context = AllocSetContextCreate(TopMemoryContext,
     128             :                                              "Background Writer",
     129             :                                              ALLOCSET_DEFAULT_SIZES);
     130         550 :     MemoryContextSwitchTo(bgwriter_context);
     131             : 
     132         550 :     WritebackContextInit(&wb_context, &bgwriter_flush_after);
     133             : 
     134             :     /*
     135             :      * If an exception is encountered, processing resumes here.
     136             :      *
     137             :      * You might wonder why this isn't coded as an infinite loop around a
     138             :      * PG_TRY construct.  The reason is that this is the bottom of the
     139             :      * exception stack, and so with PG_TRY there would be no exception handler
     140             :      * in force at all during the CATCH part.  By leaving the outermost setjmp
     141             :      * always active, we have at least some chance of recovering from an error
     142             :      * during error recovery.  (If we get into an infinite loop thereby, it
     143             :      * will soon be stopped by overflow of elog.c's internal state stack.)
     144             :      *
     145             :      * Note that we use sigsetjmp(..., 1), so that the prevailing signal mask
     146             :      * (to wit, BlockSig) will be restored when longjmp'ing to here.  Thus,
     147             :      * signals other than SIGQUIT will be blocked until we complete error
     148             :      * recovery.  It might seem that this policy makes the HOLD_INTERRUPTS()
     149             :      * call redundant, but it is not since InterruptPending might be set
     150             :      * already.
     151             :      */
     152         550 :     if (sigsetjmp(local_sigjmp_buf, 1) != 0)
     153             :     {
     154             :         /* Since not using PG_TRY, must reset error stack by hand */
     155           0 :         error_context_stack = NULL;
     156             : 
     157             :         /* Prevent interrupts while cleaning up */
     158           0 :         HOLD_INTERRUPTS();
     159             : 
     160             :         /* Report the error to the server log */
     161           0 :         EmitErrorReport();
     162             : 
     163             :         /*
     164             :          * These operations are really just a minimal subset of
     165             :          * AbortTransaction().  We don't have very many resources to worry
     166             :          * about in bgwriter, but we do have LWLocks, buffers, and temp files.
     167             :          */
     168           0 :         LWLockReleaseAll();
     169           0 :         ConditionVariableCancelSleep();
     170           0 :         AbortBufferIO();
     171           0 :         UnlockBuffers();
     172           0 :         ReleaseAuxProcessResources(false);
     173           0 :         AtEOXact_Buffers(false);
     174           0 :         AtEOXact_SMgr();
     175           0 :         AtEOXact_Files(false);
     176           0 :         AtEOXact_HashTables(false);
     177             : 
     178             :         /*
     179             :          * Now return to normal top-level context and clear ErrorContext for
     180             :          * next time.
     181             :          */
     182           0 :         MemoryContextSwitchTo(bgwriter_context);
     183           0 :         FlushErrorState();
     184             : 
     185             :         /* Flush any leaked data in the top-level context */
     186           0 :         MemoryContextResetAndDeleteChildren(bgwriter_context);
     187             : 
     188             :         /* re-initialize to avoid repeated errors causing problems */
     189           0 :         WritebackContextInit(&wb_context, &bgwriter_flush_after);
     190             : 
     191             :         /* Now we can allow interrupts again */
     192           0 :         RESUME_INTERRUPTS();
     193             : 
     194             :         /*
     195             :          * Sleep at least 1 second after any error.  A write error is likely
     196             :          * to be repeated, and we don't want to be filling the error logs as
     197             :          * fast as we can.
     198             :          */
     199           0 :         pg_usleep(1000000L);
     200             : 
     201             :         /*
     202             :          * Close all open files after any error.  This is helpful on Windows,
     203             :          * where holding deleted files open causes various strange errors.
     204             :          * It's not clear we need it elsewhere, but shouldn't hurt.
     205             :          */
     206           0 :         smgrcloseall();
     207             : 
     208             :         /* Report wait end here, when there is no further possibility of wait */
     209           0 :         pgstat_report_wait_end();
     210             :     }
     211             : 
     212             :     /* We can now handle ereport(ERROR) */
     213         550 :     PG_exception_stack = &local_sigjmp_buf;
     214             : 
     215             :     /*
     216             :      * Unblock signals (they were blocked when the postmaster forked us)
     217             :      */
     218         550 :     PG_SETMASK(&UnBlockSig);
     219             : 
     220             :     /*
     221             :      * Reset hibernation state after any error.
     222             :      */
     223         550 :     prev_hibernate = false;
     224             : 
     225             :     /*
     226             :      * Loop forever
     227             :      */
     228             :     for (;;)
     229        9840 :     {
     230             :         bool        can_hibernate;
     231             :         int         rc;
     232             : 
     233             :         /* Clear any already-pending wakeups */
     234       10390 :         ResetLatch(MyLatch);
     235             : 
     236       10390 :         HandleMainLoopInterrupts();
     237             : 
     238             :         /*
     239             :          * Do one cycle of dirty-buffer writing.
     240             :          */
     241        9846 :         can_hibernate = BgBufferSync(&wb_context);
     242             : 
     243             :         /*
     244             :          * Send off activity statistics to the stats collector
     245             :          */
     246        9846 :         pgstat_send_bgwriter();
     247             : 
     248        9846 :         if (FirstCallSinceLastCheckpoint())
     249             :         {
     250             :             /*
     251             :              * After any checkpoint, close all smgr files.  This is so we
     252             :              * won't hang onto smgr references to deleted files indefinitely.
     253             :              */
     254         396 :             smgrcloseall();
     255             :         }
     256             : 
     257             :         /*
     258             :          * Log a new xl_running_xacts every now and then so replication can
     259             :          * get into a consistent state faster (think of suboverflowed
     260             :          * snapshots) and clean up resources (locks, KnownXids*) more
     261             :          * frequently. The costs of this are relatively low, so doing it 4
     262             :          * times (LOG_SNAPSHOT_INTERVAL_MS) a minute seems fine.
     263             :          *
     264             :          * We assume the interval for writing xl_running_xacts is
     265             :          * significantly bigger than BgWriterDelay, so we don't complicate the
     266             :          * overall timeout handling but just assume we're going to get called
     267             :          * often enough even if hibernation mode is active. It's not that
     268             :          * important that LOG_SNAPSHOT_INTERVAL_MS is met strictly. To make
     269             :          * sure we're not waking the disk up unnecessarily on an idle system
     270             :          * we check whether there has been any WAL inserted since the last
     271             :          * time we've logged a running xacts.
     272             :          *
     273             :          * We do this logging in the bgwriter as it is the only process that
     274             :          * is run regularly and returns to its mainloop all the time. E.g.
     275             :          * Checkpointer, when active, is barely ever in its mainloop and thus
     276             :          * makes it hard to log regularly.
     277             :          */
     278        9846 :         if (XLogStandbyInfoActive() && !RecoveryInProgress())
     279             :         {
     280        7486 :             TimestampTz timeout = 0;
     281        7486 :             TimestampTz now = GetCurrentTimestamp();
     282             : 
     283        7486 :             timeout = TimestampTzPlusMilliseconds(last_snapshot_ts,
     284             :                                                   LOG_SNAPSHOT_INTERVAL_MS);
     285             : 
     286             :             /*
     287             :              * Only log if enough time has passed and interesting records have
     288             :              * been inserted since the last snapshot.  Have to compare with <=
     289             :              * instead of < because GetLastImportantRecPtr() points at the
     290             :              * start of a record, whereas last_snapshot_lsn points just past
     291             :              * the end of the record.
     292             :              */
     293        7486 :             if (now >= timeout &&
     294          46 :                 last_snapshot_lsn <= GetLastImportantRecPtr())
     295             :             {
     296          46 :                 last_snapshot_lsn = LogStandbySnapshot();
     297          46 :                 last_snapshot_ts = now;
     298             :             }
     299             :         }
     300             : 
     301             :         /*
     302             :          * Sleep until we are signaled or BgWriterDelay has elapsed.
     303             :          *
     304             :          * Note: the feedback control loop in BgBufferSync() expects that we
     305             :          * will call it every BgWriterDelay msec.  While it's not critical for
     306             :          * correctness that that be exact, the feedback loop might misbehave
     307             :          * if we stray too far from that.  Hence, avoid loading this process
     308             :          * down with latch events that are likely to happen frequently during
     309             :          * normal operation.
     310             :          */
     311        9846 :         rc = WaitLatch(MyLatch,
     312             :                        WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH,
     313             :                        BgWriterDelay /* ms */ , WAIT_EVENT_BGWRITER_MAIN);
     314             : 
     315             :         /*
     316             :          * If no latch event and BgBufferSync says nothing's happening, extend
     317             :          * the sleep in "hibernation" mode, where we sleep for much longer
     318             :          * than bgwriter_delay says.  Fewer wakeups save electricity.  When a
     319             :          * backend starts using buffers again, it will wake us up by setting
     320             :          * our latch.  Because the extra sleep will persist only as long as no
     321             :          * buffer allocations happen, this should not distort the behavior of
     322             :          * BgBufferSync's control loop too badly; essentially, it will think
     323             :          * that the system-wide idle interval didn't exist.
     324             :          *
     325             :          * There is a race condition here, in that a backend might allocate a
     326             :          * buffer between the time BgBufferSync saw the alloc count as zero
     327             :          * and the time we call StrategyNotifyBgWriter.  While it's not
     328             :          * critical that we not hibernate anyway, we try to reduce the odds of
     329             :          * that by only hibernating when BgBufferSync says nothing's happening
     330             :          * for two consecutive cycles.  Also, we mitigate any possible
     331             :          * consequences of a missed wakeup by not hibernating forever.
     332             :          */
     333        9840 :         if (rc == WL_TIMEOUT && can_hibernate && prev_hibernate)
     334             :         {
     335             :             /* Ask for notification at next buffer allocation */
     336         158 :             StrategyNotifyBgWriter(MyProc->pgprocno);
     337             :             /* Sleep ... */
     338         158 :             (void) WaitLatch(MyLatch,
     339             :                              WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH,
     340         158 :                              BgWriterDelay * HIBERNATE_FACTOR,
     341             :                              WAIT_EVENT_BGWRITER_HIBERNATE);
     342             :             /* Reset the notification request in case we timed out */
     343         158 :             StrategyNotifyBgWriter(-1);
     344             :         }
     345             : 
     346        9840 :         prev_hibernate = can_hibernate;
     347             :     }
     348             : }

Generated by: LCOV version 1.14