LCOV - code coverage report
Current view: top level - src/backend/postmaster - bgwriter.c (source / functions) Hit Total Coverage
Test: PostgreSQL 13beta1 Lines: 41 61 67.2 %
Date: 2020-06-03 08:06:29 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             :  * The bgwriter is started by the postmaster as soon as the startup subprocess
      16             :  * finishes, or as soon as recovery begins if we are doing archive recovery.
      17             :  * It remains alive until the postmaster commands it to terminate.
      18             :  * Normal termination is by SIGTERM, which instructs the bgwriter to exit(0).
      19             :  * Emergency termination is by SIGQUIT; like any backend, the bgwriter will
      20             :  * simply abort and exit on SIGQUIT.
      21             :  *
      22             :  * If the bgwriter exits unexpectedly, the postmaster treats that the same
      23             :  * as a backend crash: shared memory may be corrupted, so remaining backends
      24             :  * should be killed by SIGQUIT and then a recovery cycle started.
      25             :  *
      26             :  *
      27             :  * Portions Copyright (c) 1996-2020, PostgreSQL Global Development Group
      28             :  *
      29             :  *
      30             :  * IDENTIFICATION
      31             :  *    src/backend/postmaster/bgwriter.c
      32             :  *
      33             :  *-------------------------------------------------------------------------
      34             :  */
      35             : #include "postgres.h"
      36             : 
      37             : #include "access/xlog.h"
      38             : #include "access/xlog_internal.h"
      39             : #include "libpq/pqsignal.h"
      40             : #include "miscadmin.h"
      41             : #include "pgstat.h"
      42             : #include "postmaster/bgwriter.h"
      43             : #include "postmaster/interrupt.h"
      44             : #include "storage/buf_internals.h"
      45             : #include "storage/bufmgr.h"
      46             : #include "storage/condition_variable.h"
      47             : #include "storage/fd.h"
      48             : #include "storage/ipc.h"
      49             : #include "storage/lwlock.h"
      50             : #include "storage/proc.h"
      51             : #include "storage/procsignal.h"
      52             : #include "storage/shmem.h"
      53             : #include "storage/smgr.h"
      54             : #include "storage/spin.h"
      55             : #include "storage/standby.h"
      56             : #include "utils/guc.h"
      57             : #include "utils/memutils.h"
      58             : #include "utils/resowner.h"
      59             : #include "utils/timestamp.h"
      60             : 
      61             : /*
      62             :  * GUC parameters
      63             :  */
      64             : int         BgWriterDelay = 200;
      65             : 
      66             : /*
      67             :  * Multiplier to apply to BgWriterDelay when we decide to hibernate.
      68             :  * (Perhaps this needs to be configurable?)
      69             :  */
      70             : #define HIBERNATE_FACTOR            50
      71             : 
      72             : /*
      73             :  * Interval in which standby snapshots are logged into the WAL stream, in
      74             :  * milliseconds.
      75             :  */
      76             : #define LOG_SNAPSHOT_INTERVAL_MS 15000
      77             : 
      78             : /*
      79             :  * LSN and timestamp at which we last issued a LogStandbySnapshot(), to avoid
      80             :  * doing so too often or repeatedly if there has been no other write activity
      81             :  * in the system.
      82             :  */
      83             : static TimestampTz last_snapshot_ts;
      84             : static XLogRecPtr last_snapshot_lsn = InvalidXLogRecPtr;
      85             : 
      86             : 
      87             : /*
      88             :  * Main entry point for bgwriter process
      89             :  *
      90             :  * This is invoked from AuxiliaryProcessMain, which has already created the
      91             :  * basic execution environment, but not enabled signals yet.
      92             :  */
      93             : void
      94         398 : BackgroundWriterMain(void)
      95             : {
      96             :     sigjmp_buf  local_sigjmp_buf;
      97             :     MemoryContext bgwriter_context;
      98             :     bool        prev_hibernate;
      99             :     WritebackContext wb_context;
     100             : 
     101             :     /*
     102             :      * Properly accept or ignore signals that might be sent to us.
     103             :      */
     104         398 :     pqsignal(SIGHUP, SignalHandlerForConfigReload);
     105         398 :     pqsignal(SIGINT, SIG_IGN);
     106         398 :     pqsignal(SIGTERM, SignalHandlerForShutdownRequest);
     107         398 :     pqsignal(SIGQUIT, SignalHandlerForCrashExit);
     108         398 :     pqsignal(SIGALRM, SIG_IGN);
     109         398 :     pqsignal(SIGPIPE, SIG_IGN);
     110         398 :     pqsignal(SIGUSR1, procsignal_sigusr1_handler);
     111         398 :     pqsignal(SIGUSR2, SIG_IGN);
     112             : 
     113             :     /*
     114             :      * Reset some signals that are accepted by postmaster but not here
     115             :      */
     116         398 :     pqsignal(SIGCHLD, SIG_DFL);
     117             : 
     118             :     /* We allow SIGQUIT (quickdie) at all times */
     119         398 :     sigdelset(&BlockSig, SIGQUIT);
     120             : 
     121             :     /*
     122             :      * We just started, assume there has been either a shutdown or
     123             :      * end-of-recovery snapshot.
     124             :      */
     125         398 :     last_snapshot_ts = GetCurrentTimestamp();
     126             : 
     127             :     /*
     128             :      * Create a memory context that we will do all our work in.  We do this so
     129             :      * that we can reset the context during error recovery and thereby avoid
     130             :      * possible memory leaks.  Formerly this code just ran in
     131             :      * TopMemoryContext, but resetting that would be a really bad idea.
     132             :      */
     133         398 :     bgwriter_context = AllocSetContextCreate(TopMemoryContext,
     134             :                                              "Background Writer",
     135             :                                              ALLOCSET_DEFAULT_SIZES);
     136         398 :     MemoryContextSwitchTo(bgwriter_context);
     137             : 
     138         398 :     WritebackContextInit(&wb_context, &bgwriter_flush_after);
     139             : 
     140             :     /*
     141             :      * If an exception is encountered, processing resumes here.
     142             :      *
     143             :      * See notes in postgres.c about the design of this coding.
     144             :      */
     145         398 :     if (sigsetjmp(local_sigjmp_buf, 1) != 0)
     146             :     {
     147             :         /* Since not using PG_TRY, must reset error stack by hand */
     148           0 :         error_context_stack = NULL;
     149             : 
     150             :         /* Prevent interrupts while cleaning up */
     151           0 :         HOLD_INTERRUPTS();
     152             : 
     153             :         /* Report the error to the server log */
     154           0 :         EmitErrorReport();
     155             : 
     156             :         /*
     157             :          * These operations are really just a minimal subset of
     158             :          * AbortTransaction().  We don't have very many resources to worry
     159             :          * about in bgwriter, but we do have LWLocks, buffers, and temp files.
     160             :          */
     161           0 :         LWLockReleaseAll();
     162           0 :         ConditionVariableCancelSleep();
     163           0 :         AbortBufferIO();
     164           0 :         UnlockBuffers();
     165           0 :         ReleaseAuxProcessResources(false);
     166           0 :         AtEOXact_Buffers(false);
     167           0 :         AtEOXact_SMgr();
     168           0 :         AtEOXact_Files(false);
     169           0 :         AtEOXact_HashTables(false);
     170             : 
     171             :         /*
     172             :          * Now return to normal top-level context and clear ErrorContext for
     173             :          * next time.
     174             :          */
     175           0 :         MemoryContextSwitchTo(bgwriter_context);
     176           0 :         FlushErrorState();
     177             : 
     178             :         /* Flush any leaked data in the top-level context */
     179           0 :         MemoryContextResetAndDeleteChildren(bgwriter_context);
     180             : 
     181             :         /* re-initialize to avoid repeated errors causing problems */
     182           0 :         WritebackContextInit(&wb_context, &bgwriter_flush_after);
     183             : 
     184             :         /* Now we can allow interrupts again */
     185           0 :         RESUME_INTERRUPTS();
     186             : 
     187             :         /*
     188             :          * Sleep at least 1 second after any error.  A write error is likely
     189             :          * to be repeated, and we don't want to be filling the error logs as
     190             :          * fast as we can.
     191             :          */
     192           0 :         pg_usleep(1000000L);
     193             : 
     194             :         /*
     195             :          * Close all open files after any error.  This is helpful on Windows,
     196             :          * where holding deleted files open causes various strange errors.
     197             :          * It's not clear we need it elsewhere, but shouldn't hurt.
     198             :          */
     199           0 :         smgrcloseall();
     200             : 
     201             :         /* Report wait end here, when there is no further possibility of wait */
     202           0 :         pgstat_report_wait_end();
     203             :     }
     204             : 
     205             :     /* We can now handle ereport(ERROR) */
     206         398 :     PG_exception_stack = &local_sigjmp_buf;
     207             : 
     208             :     /*
     209             :      * Unblock signals (they were blocked when the postmaster forked us)
     210             :      */
     211         398 :     PG_SETMASK(&UnBlockSig);
     212             : 
     213             :     /*
     214             :      * Reset hibernation state after any error.
     215             :      */
     216         398 :     prev_hibernate = false;
     217             : 
     218             :     /*
     219             :      * Loop forever
     220             :      */
     221             :     for (;;)
     222        9808 :     {
     223             :         bool        can_hibernate;
     224             :         int         rc;
     225             : 
     226             :         /* Clear any already-pending wakeups */
     227       10206 :         ResetLatch(MyLatch);
     228             : 
     229       10206 :         HandleMainLoopInterrupts();
     230             : 
     231             :         /*
     232             :          * Do one cycle of dirty-buffer writing.
     233             :          */
     234        9814 :         can_hibernate = BgBufferSync(&wb_context);
     235             : 
     236             :         /*
     237             :          * Send off activity statistics to the stats collector
     238             :          */
     239        9814 :         pgstat_send_bgwriter();
     240             : 
     241        9814 :         if (FirstCallSinceLastCheckpoint())
     242             :         {
     243             :             /*
     244             :              * After any checkpoint, close all smgr files.  This is so we
     245             :              * won't hang onto smgr references to deleted files indefinitely.
     246             :              */
     247         306 :             smgrcloseall();
     248             :         }
     249             : 
     250             :         /*
     251             :          * Log a new xl_running_xacts every now and then so replication can
     252             :          * get into a consistent state faster (think of suboverflowed
     253             :          * snapshots) and clean up resources (locks, KnownXids*) more
     254             :          * frequently. The costs of this are relatively low, so doing it 4
     255             :          * times (LOG_SNAPSHOT_INTERVAL_MS) a minute seems fine.
     256             :          *
     257             :          * We assume the interval for writing xl_running_xacts is
     258             :          * significantly bigger than BgWriterDelay, so we don't complicate the
     259             :          * overall timeout handling but just assume we're going to get called
     260             :          * often enough even if hibernation mode is active. It's not that
     261             :          * important that LOG_SNAPSHOT_INTERVAL_MS is met strictly. To make
     262             :          * sure we're not waking the disk up unnecessarily on an idle system
     263             :          * we check whether there has been any WAL inserted since the last
     264             :          * time we've logged a running xacts.
     265             :          *
     266             :          * We do this logging in the bgwriter as it is the only process that
     267             :          * is run regularly and returns to its mainloop all the time. E.g.
     268             :          * Checkpointer, when active, is barely ever in its mainloop and thus
     269             :          * makes it hard to log regularly.
     270             :          */
     271        9814 :         if (XLogStandbyInfoActive() && !RecoveryInProgress())
     272             :         {
     273        8192 :             TimestampTz timeout = 0;
     274        8192 :             TimestampTz now = GetCurrentTimestamp();
     275             : 
     276        8192 :             timeout = TimestampTzPlusMilliseconds(last_snapshot_ts,
     277             :                                                   LOG_SNAPSHOT_INTERVAL_MS);
     278             : 
     279             :             /*
     280             :              * Only log if enough time has passed and interesting records have
     281             :              * been inserted since the last snapshot.  Have to compare with <=
     282             :              * instead of < because GetLastImportantRecPtr() points at the
     283             :              * start of a record, whereas last_snapshot_lsn points just past
     284             :              * the end of the record.
     285             :              */
     286        8192 :             if (now >= timeout &&
     287          60 :                 last_snapshot_lsn <= GetLastImportantRecPtr())
     288             :             {
     289          60 :                 last_snapshot_lsn = LogStandbySnapshot();
     290          60 :                 last_snapshot_ts = now;
     291             :             }
     292             :         }
     293             : 
     294             :         /*
     295             :          * Sleep until we are signaled or BgWriterDelay has elapsed.
     296             :          *
     297             :          * Note: the feedback control loop in BgBufferSync() expects that we
     298             :          * will call it every BgWriterDelay msec.  While it's not critical for
     299             :          * correctness that that be exact, the feedback loop might misbehave
     300             :          * if we stray too far from that.  Hence, avoid loading this process
     301             :          * down with latch events that are likely to happen frequently during
     302             :          * normal operation.
     303             :          */
     304        9814 :         rc = WaitLatch(MyLatch,
     305             :                        WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH,
     306             :                        BgWriterDelay /* ms */ , WAIT_EVENT_BGWRITER_MAIN);
     307             : 
     308             :         /*
     309             :          * If no latch event and BgBufferSync says nothing's happening, extend
     310             :          * the sleep in "hibernation" mode, where we sleep for much longer
     311             :          * than bgwriter_delay says.  Fewer wakeups save electricity.  When a
     312             :          * backend starts using buffers again, it will wake us up by setting
     313             :          * our latch.  Because the extra sleep will persist only as long as no
     314             :          * buffer allocations happen, this should not distort the behavior of
     315             :          * BgBufferSync's control loop too badly; essentially, it will think
     316             :          * that the system-wide idle interval didn't exist.
     317             :          *
     318             :          * There is a race condition here, in that a backend might allocate a
     319             :          * buffer between the time BgBufferSync saw the alloc count as zero
     320             :          * and the time we call StrategyNotifyBgWriter.  While it's not
     321             :          * critical that we not hibernate anyway, we try to reduce the odds of
     322             :          * that by only hibernating when BgBufferSync says nothing's happening
     323             :          * for two consecutive cycles.  Also, we mitigate any possible
     324             :          * consequences of a missed wakeup by not hibernating forever.
     325             :          */
     326        9808 :         if (rc == WL_TIMEOUT && can_hibernate && prev_hibernate)
     327             :         {
     328             :             /* Ask for notification at next buffer allocation */
     329         166 :             StrategyNotifyBgWriter(MyProc->pgprocno);
     330             :             /* Sleep ... */
     331         166 :             (void) WaitLatch(MyLatch,
     332             :                              WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH,
     333         166 :                              BgWriterDelay * HIBERNATE_FACTOR,
     334             :                              WAIT_EVENT_BGWRITER_HIBERNATE);
     335             :             /* Reset the notification request in case we timed out */
     336         166 :             StrategyNotifyBgWriter(-1);
     337             :         }
     338             : 
     339        9808 :         prev_hibernate = can_hibernate;
     340             :     }
     341             : }

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