LCOV - code coverage report
Current view: top level - src/backend/postmaster - bgwriter.c (source / functions) Hit Total Coverage
Test: PostgreSQL 13devel Lines: 63 85 74.1 %
Date: 2019-09-19 23:07:04 Functions: 4 5 80.0 %
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) 1996-2019, PostgreSQL Global Development Group
      28             :  *
      29             :  *
      30             :  * IDENTIFICATION
      31             :  *    src/backend/postmaster/bgwriter.c
      32             :  *
      33             :  *-------------------------------------------------------------------------
      34             :  */
      35             : #include "postgres.h"
      36             : 
      37             : #include <signal.h>
      38             : #include <sys/time.h>
      39             : #include <unistd.h>
      40             : 
      41             : #include "access/xlog.h"
      42             : #include "access/xlog_internal.h"
      43             : #include "libpq/pqsignal.h"
      44             : #include "miscadmin.h"
      45             : #include "pgstat.h"
      46             : #include "postmaster/bgwriter.h"
      47             : #include "storage/bufmgr.h"
      48             : #include "storage/buf_internals.h"
      49             : #include "storage/condition_variable.h"
      50             : #include "storage/fd.h"
      51             : #include "storage/ipc.h"
      52             : #include "storage/lwlock.h"
      53             : #include "storage/proc.h"
      54             : #include "storage/shmem.h"
      55             : #include "storage/smgr.h"
      56             : #include "storage/spin.h"
      57             : #include "storage/standby.h"
      58             : #include "utils/guc.h"
      59             : #include "utils/memutils.h"
      60             : #include "utils/resowner.h"
      61             : #include "utils/timestamp.h"
      62             : 
      63             : 
      64             : /*
      65             :  * GUC parameters
      66             :  */
      67             : int         BgWriterDelay = 200;
      68             : 
      69             : /*
      70             :  * Multiplier to apply to BgWriterDelay when we decide to hibernate.
      71             :  * (Perhaps this needs to be configurable?)
      72             :  */
      73             : #define HIBERNATE_FACTOR            50
      74             : 
      75             : /*
      76             :  * Interval in which standby snapshots are logged into the WAL stream, in
      77             :  * milliseconds.
      78             :  */
      79             : #define LOG_SNAPSHOT_INTERVAL_MS 15000
      80             : 
      81             : /*
      82             :  * LSN and timestamp at which we last issued a LogStandbySnapshot(), to avoid
      83             :  * doing so too often or repeatedly if there has been no other write activity
      84             :  * in the system.
      85             :  */
      86             : static TimestampTz last_snapshot_ts;
      87             : static XLogRecPtr last_snapshot_lsn = InvalidXLogRecPtr;
      88             : 
      89             : /*
      90             :  * Flags set by interrupt handlers for later service in the main loop.
      91             :  */
      92             : static volatile sig_atomic_t got_SIGHUP = false;
      93             : static volatile sig_atomic_t shutdown_requested = false;
      94             : 
      95             : /* Signal handlers */
      96             : 
      97             : static void bg_quickdie(SIGNAL_ARGS);
      98             : static void BgSigHupHandler(SIGNAL_ARGS);
      99             : static void ReqShutdownHandler(SIGNAL_ARGS);
     100             : static void bgwriter_sigusr1_handler(SIGNAL_ARGS);
     101             : 
     102             : 
     103             : /*
     104             :  * Main entry point for bgwriter process
     105             :  *
     106             :  * This is invoked from AuxiliaryProcessMain, which has already created the
     107             :  * basic execution environment, but not enabled signals yet.
     108             :  */
     109             : void
     110         380 : BackgroundWriterMain(void)
     111             : {
     112             :     sigjmp_buf  local_sigjmp_buf;
     113             :     MemoryContext bgwriter_context;
     114             :     bool        prev_hibernate;
     115             :     WritebackContext wb_context;
     116             : 
     117             :     /*
     118             :      * Properly accept or ignore signals the postmaster might send us.
     119             :      *
     120             :      * bgwriter doesn't participate in ProcSignal signalling, but a SIGUSR1
     121             :      * handler is still needed for latch wakeups.
     122             :      */
     123         380 :     pqsignal(SIGHUP, BgSigHupHandler);  /* set flag to read config file */
     124         380 :     pqsignal(SIGINT, SIG_IGN);
     125         380 :     pqsignal(SIGTERM, ReqShutdownHandler);  /* shutdown */
     126         380 :     pqsignal(SIGQUIT, bg_quickdie); /* hard crash time */
     127         380 :     pqsignal(SIGALRM, SIG_IGN);
     128         380 :     pqsignal(SIGPIPE, SIG_IGN);
     129         380 :     pqsignal(SIGUSR1, bgwriter_sigusr1_handler);
     130         380 :     pqsignal(SIGUSR2, SIG_IGN);
     131             : 
     132             :     /*
     133             :      * Reset some signals that are accepted by postmaster but not here
     134             :      */
     135         380 :     pqsignal(SIGCHLD, SIG_DFL);
     136             : 
     137             :     /* We allow SIGQUIT (quickdie) at all times */
     138         380 :     sigdelset(&BlockSig, SIGQUIT);
     139             : 
     140             :     /*
     141             :      * We just started, assume there has been either a shutdown or
     142             :      * end-of-recovery snapshot.
     143             :      */
     144         380 :     last_snapshot_ts = GetCurrentTimestamp();
     145             : 
     146             :     /*
     147             :      * Create a memory context that we will do all our work in.  We do this so
     148             :      * that we can reset the context during error recovery and thereby avoid
     149             :      * possible memory leaks.  Formerly this code just ran in
     150             :      * TopMemoryContext, but resetting that would be a really bad idea.
     151             :      */
     152         380 :     bgwriter_context = AllocSetContextCreate(TopMemoryContext,
     153             :                                              "Background Writer",
     154             :                                              ALLOCSET_DEFAULT_SIZES);
     155         380 :     MemoryContextSwitchTo(bgwriter_context);
     156             : 
     157         380 :     WritebackContextInit(&wb_context, &bgwriter_flush_after);
     158             : 
     159             :     /*
     160             :      * If an exception is encountered, processing resumes here.
     161             :      *
     162             :      * See notes in postgres.c about the design of this coding.
     163             :      */
     164         380 :     if (sigsetjmp(local_sigjmp_buf, 1) != 0)
     165             :     {
     166             :         /* Since not using PG_TRY, must reset error stack by hand */
     167           0 :         error_context_stack = NULL;
     168             : 
     169             :         /* Prevent interrupts while cleaning up */
     170           0 :         HOLD_INTERRUPTS();
     171             : 
     172             :         /* Report the error to the server log */
     173           0 :         EmitErrorReport();
     174             : 
     175             :         /*
     176             :          * These operations are really just a minimal subset of
     177             :          * AbortTransaction().  We don't have very many resources to worry
     178             :          * about in bgwriter, but we do have LWLocks, buffers, and temp files.
     179             :          */
     180           0 :         LWLockReleaseAll();
     181           0 :         ConditionVariableCancelSleep();
     182           0 :         AbortBufferIO();
     183           0 :         UnlockBuffers();
     184           0 :         ReleaseAuxProcessResources(false);
     185           0 :         AtEOXact_Buffers(false);
     186           0 :         AtEOXact_SMgr();
     187           0 :         AtEOXact_Files(false);
     188           0 :         AtEOXact_HashTables(false);
     189             : 
     190             :         /*
     191             :          * Now return to normal top-level context and clear ErrorContext for
     192             :          * next time.
     193             :          */
     194           0 :         MemoryContextSwitchTo(bgwriter_context);
     195           0 :         FlushErrorState();
     196             : 
     197             :         /* Flush any leaked data in the top-level context */
     198           0 :         MemoryContextResetAndDeleteChildren(bgwriter_context);
     199             : 
     200             :         /* re-initialize to avoid repeated errors causing problems */
     201           0 :         WritebackContextInit(&wb_context, &bgwriter_flush_after);
     202             : 
     203             :         /* Now we can allow interrupts again */
     204           0 :         RESUME_INTERRUPTS();
     205             : 
     206             :         /*
     207             :          * Sleep at least 1 second after any error.  A write error is likely
     208             :          * to be repeated, and we don't want to be filling the error logs as
     209             :          * fast as we can.
     210             :          */
     211           0 :         pg_usleep(1000000L);
     212             : 
     213             :         /*
     214             :          * Close all open files after any error.  This is helpful on Windows,
     215             :          * where holding deleted files open causes various strange errors.
     216             :          * It's not clear we need it elsewhere, but shouldn't hurt.
     217             :          */
     218           0 :         smgrcloseall();
     219             : 
     220             :         /* Report wait end here, when there is no further possibility of wait */
     221           0 :         pgstat_report_wait_end();
     222             :     }
     223             : 
     224             :     /* We can now handle ereport(ERROR) */
     225         380 :     PG_exception_stack = &local_sigjmp_buf;
     226             : 
     227             :     /*
     228             :      * Unblock signals (they were blocked when the postmaster forked us)
     229             :      */
     230         380 :     PG_SETMASK(&UnBlockSig);
     231             : 
     232             :     /*
     233             :      * Reset hibernation state after any error.
     234             :      */
     235         380 :     prev_hibernate = false;
     236             : 
     237             :     /*
     238             :      * Loop forever
     239             :      */
     240             :     for (;;)
     241        8456 :     {
     242             :         bool        can_hibernate;
     243             :         int         rc;
     244             : 
     245             :         /* Clear any already-pending wakeups */
     246        8836 :         ResetLatch(MyLatch);
     247             : 
     248        8836 :         if (got_SIGHUP)
     249             :         {
     250          10 :             got_SIGHUP = false;
     251          10 :             ProcessConfigFile(PGC_SIGHUP);
     252             :         }
     253        8836 :         if (shutdown_requested)
     254             :         {
     255             :             /*
     256             :              * From here on, elog(ERROR) should end with exit(1), not send
     257             :              * control back to the sigsetjmp block above
     258             :              */
     259         374 :             ExitOnAnyError = true;
     260             :             /* Normal exit from the bgwriter is here */
     261         374 :             proc_exit(0);       /* done */
     262             :         }
     263             : 
     264             :         /*
     265             :          * Do one cycle of dirty-buffer writing.
     266             :          */
     267        8462 :         can_hibernate = BgBufferSync(&wb_context);
     268             : 
     269             :         /*
     270             :          * Send off activity statistics to the stats collector
     271             :          */
     272        8462 :         pgstat_send_bgwriter();
     273             : 
     274        8462 :         if (FirstCallSinceLastCheckpoint())
     275             :         {
     276             :             /*
     277             :              * After any checkpoint, close all smgr files.  This is so we
     278             :              * won't hang onto smgr references to deleted files indefinitely.
     279             :              */
     280         322 :             smgrcloseall();
     281             :         }
     282             : 
     283             :         /*
     284             :          * Log a new xl_running_xacts every now and then so replication can
     285             :          * get into a consistent state faster (think of suboverflowed
     286             :          * snapshots) and clean up resources (locks, KnownXids*) more
     287             :          * frequently. The costs of this are relatively low, so doing it 4
     288             :          * times (LOG_SNAPSHOT_INTERVAL_MS) a minute seems fine.
     289             :          *
     290             :          * We assume the interval for writing xl_running_xacts is
     291             :          * significantly bigger than BgWriterDelay, so we don't complicate the
     292             :          * overall timeout handling but just assume we're going to get called
     293             :          * often enough even if hibernation mode is active. It's not that
     294             :          * important that LOG_SNAPSHOT_INTERVAL_MS is met strictly. To make
     295             :          * sure we're not waking the disk up unnecessarily on an idle system
     296             :          * we check whether there has been any WAL inserted since the last
     297             :          * time we've logged a running xacts.
     298             :          *
     299             :          * We do this logging in the bgwriter as it is the only process that
     300             :          * is run regularly and returns to its mainloop all the time. E.g.
     301             :          * Checkpointer, when active, is barely ever in its mainloop and thus
     302             :          * makes it hard to log regularly.
     303             :          */
     304        8462 :         if (XLogStandbyInfoActive() && !RecoveryInProgress())
     305             :         {
     306        7150 :             TimestampTz timeout = 0;
     307        7150 :             TimestampTz now = GetCurrentTimestamp();
     308             : 
     309        7150 :             timeout = TimestampTzPlusMilliseconds(last_snapshot_ts,
     310             :                                                   LOG_SNAPSHOT_INTERVAL_MS);
     311             : 
     312             :             /*
     313             :              * Only log if enough time has passed and interesting records have
     314             :              * been inserted since the last snapshot.  Have to compare with <=
     315             :              * instead of < because GetLastImportantRecPtr() points at the
     316             :              * start of a record, whereas last_snapshot_lsn points just past
     317             :              * the end of the record.
     318             :              */
     319        7198 :             if (now >= timeout &&
     320          48 :                 last_snapshot_lsn <= GetLastImportantRecPtr())
     321             :             {
     322          48 :                 last_snapshot_lsn = LogStandbySnapshot();
     323          48 :                 last_snapshot_ts = now;
     324             :             }
     325             :         }
     326             : 
     327             :         /*
     328             :          * Sleep until we are signaled or BgWriterDelay has elapsed.
     329             :          *
     330             :          * Note: the feedback control loop in BgBufferSync() expects that we
     331             :          * will call it every BgWriterDelay msec.  While it's not critical for
     332             :          * correctness that that be exact, the feedback loop might misbehave
     333             :          * if we stray too far from that.  Hence, avoid loading this process
     334             :          * down with latch events that are likely to happen frequently during
     335             :          * normal operation.
     336             :          */
     337        8462 :         rc = WaitLatch(MyLatch,
     338             :                        WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH,
     339             :                        BgWriterDelay /* ms */ , WAIT_EVENT_BGWRITER_MAIN);
     340             : 
     341             :         /*
     342             :          * If no latch event and BgBufferSync says nothing's happening, extend
     343             :          * the sleep in "hibernation" mode, where we sleep for much longer
     344             :          * than bgwriter_delay says.  Fewer wakeups save electricity.  When a
     345             :          * backend starts using buffers again, it will wake us up by setting
     346             :          * our latch.  Because the extra sleep will persist only as long as no
     347             :          * buffer allocations happen, this should not distort the behavior of
     348             :          * BgBufferSync's control loop too badly; essentially, it will think
     349             :          * that the system-wide idle interval didn't exist.
     350             :          *
     351             :          * There is a race condition here, in that a backend might allocate a
     352             :          * buffer between the time BgBufferSync saw the alloc count as zero
     353             :          * and the time we call StrategyNotifyBgWriter.  While it's not
     354             :          * critical that we not hibernate anyway, we try to reduce the odds of
     355             :          * that by only hibernating when BgBufferSync says nothing's happening
     356             :          * for two consecutive cycles.  Also, we mitigate any possible
     357             :          * consequences of a missed wakeup by not hibernating forever.
     358             :          */
     359        8456 :         if (rc == WL_TIMEOUT && can_hibernate && prev_hibernate)
     360             :         {
     361             :             /* Ask for notification at next buffer allocation */
     362         126 :             StrategyNotifyBgWriter(MyProc->pgprocno);
     363             :             /* Sleep ... */
     364         126 :             (void) WaitLatch(MyLatch,
     365             :                              WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH,
     366         126 :                              BgWriterDelay * HIBERNATE_FACTOR,
     367             :                              WAIT_EVENT_BGWRITER_HIBERNATE);
     368             :             /* Reset the notification request in case we timed out */
     369         126 :             StrategyNotifyBgWriter(-1);
     370             :         }
     371             : 
     372        8456 :         prev_hibernate = can_hibernate;
     373             :     }
     374             : }
     375             : 
     376             : 
     377             : /* --------------------------------
     378             :  *      signal handler routines
     379             :  * --------------------------------
     380             :  */
     381             : 
     382             : /*
     383             :  * bg_quickdie() occurs when signalled SIGQUIT by the postmaster.
     384             :  *
     385             :  * Some backend has bought the farm,
     386             :  * so we need to stop what we're doing and exit.
     387             :  */
     388             : static void
     389           0 : bg_quickdie(SIGNAL_ARGS)
     390             : {
     391             :     /*
     392             :      * We DO NOT want to run proc_exit() or atexit() callbacks -- we're here
     393             :      * because shared memory may be corrupted, so we don't want to try to
     394             :      * clean up our transaction.  Just nail the windows shut and get out of
     395             :      * town.  The callbacks wouldn't be safe to run from a signal handler,
     396             :      * anyway.
     397             :      *
     398             :      * Note we do _exit(2) not _exit(0).  This is to force the postmaster into
     399             :      * a system reset cycle if someone sends a manual SIGQUIT to a random
     400             :      * backend.  This is necessary precisely because we don't clean up our
     401             :      * shared memory state.  (The "dead man switch" mechanism in pmsignal.c
     402             :      * should ensure the postmaster sees this as a crash, too, but no harm in
     403             :      * being doubly sure.)
     404             :      */
     405           0 :     _exit(2);
     406             : }
     407             : 
     408             : /* SIGHUP: set flag to re-read config file at next convenient time */
     409             : static void
     410          10 : BgSigHupHandler(SIGNAL_ARGS)
     411             : {
     412          10 :     int         save_errno = errno;
     413             : 
     414          10 :     got_SIGHUP = true;
     415          10 :     SetLatch(MyLatch);
     416             : 
     417          10 :     errno = save_errno;
     418          10 : }
     419             : 
     420             : /* SIGTERM: set flag to shutdown and exit */
     421             : static void
     422         374 : ReqShutdownHandler(SIGNAL_ARGS)
     423             : {
     424         374 :     int         save_errno = errno;
     425             : 
     426         374 :     shutdown_requested = true;
     427         374 :     SetLatch(MyLatch);
     428             : 
     429         374 :     errno = save_errno;
     430         374 : }
     431             : 
     432             : /* SIGUSR1: used for latch wakeups */
     433             : static void
     434         110 : bgwriter_sigusr1_handler(SIGNAL_ARGS)
     435             : {
     436         110 :     int         save_errno = errno;
     437             : 
     438         110 :     latch_sigusr1_handler();
     439             : 
     440         110 :     errno = save_errno;
     441         110 : }

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