PostgreSQL Source Code  git master
bgwriter.c
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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"
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  */
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);
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
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  pqsignal(SIGHUP, BgSigHupHandler); /* set flag to read config file */
124  pqsignal(SIGINT, SIG_IGN);
125  pqsignal(SIGTERM, ReqShutdownHandler); /* shutdown */
126  pqsignal(SIGQUIT, bg_quickdie); /* hard crash time */
131 
132  /*
133  * Reset some signals that are accepted by postmaster but not here
134  */
136 
137  /* We allow SIGQUIT (quickdie) at all times */
138  sigdelset(&BlockSig, SIGQUIT);
139 
140  /*
141  * We just started, assume there has been either a shutdown or
142  * end-of-recovery snapshot.
143  */
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  bgwriter_context = AllocSetContextCreate(TopMemoryContext,
153  "Background Writer",
155  MemoryContextSwitchTo(bgwriter_context);
156 
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  if (sigsetjmp(local_sigjmp_buf, 1) != 0)
165  {
166  /* Since not using PG_TRY, must reset error stack by hand */
167  error_context_stack = NULL;
168 
169  /* Prevent interrupts while cleaning up */
170  HOLD_INTERRUPTS();
171 
172  /* Report the error to the server log */
173  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  */
182  AbortBufferIO();
183  UnlockBuffers();
185  AtEOXact_Buffers(false);
186  AtEOXact_SMgr();
187  AtEOXact_Files(false);
188  AtEOXact_HashTables(false);
189 
190  /*
191  * Now return to normal top-level context and clear ErrorContext for
192  * next time.
193  */
194  MemoryContextSwitchTo(bgwriter_context);
195  FlushErrorState();
196 
197  /* Flush any leaked data in the top-level context */
198  MemoryContextResetAndDeleteChildren(bgwriter_context);
199 
200  /* re-initialize to avoid repeated errors causing problems */
202 
203  /* Now we can allow interrupts again */
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  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  smgrcloseall();
219 
220  /* Report wait end here, when there is no further possibility of wait */
222  }
223 
224  /* We can now handle ereport(ERROR) */
225  PG_exception_stack = &local_sigjmp_buf;
226 
227  /*
228  * Unblock signals (they were blocked when the postmaster forked us)
229  */
231 
232  /*
233  * Reset hibernation state after any error.
234  */
235  prev_hibernate = false;
236 
237  /*
238  * Loop forever
239  */
240  for (;;)
241  {
242  bool can_hibernate;
243  int rc;
244 
245  /* Clear any already-pending wakeups */
247 
248  if (got_SIGHUP)
249  {
250  got_SIGHUP = false;
252  }
253  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  ExitOnAnyError = true;
260  /* Normal exit from the bgwriter is here */
261  proc_exit(0); /* done */
262  }
263 
264  /*
265  * Do one cycle of dirty-buffer writing.
266  */
267  can_hibernate = BgBufferSync(&wb_context);
268 
269  /*
270  * Send off activity statistics to the stats collector
271  */
273 
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  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  */
305  {
306  TimestampTz timeout = 0;
308 
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  if (now >= timeout &&
321  {
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  rc = WaitLatch(MyLatch,
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  if (rc == WL_TIMEOUT && can_hibernate && prev_hibernate)
360  {
361  /* Ask for notification at next buffer allocation */
363  /* Sleep ... */
364  (void) WaitLatch(MyLatch,
368  /* Reset the notification request in case we timed out */
370  }
371 
372  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
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  _exit(2);
406 }
407 
408 /* SIGHUP: set flag to re-read config file at next convenient time */
409 static void
411 {
412  int save_errno = errno;
413 
414  got_SIGHUP = true;
415  SetLatch(MyLatch);
416 
417  errno = save_errno;
418 }
419 
420 /* SIGTERM: set flag to shutdown and exit */
421 static void
423 {
424  int save_errno = errno;
425 
426  shutdown_requested = true;
427  SetLatch(MyLatch);
428 
429  errno = save_errno;
430 }
431 
432 /* SIGUSR1: used for latch wakeups */
433 static void
435 {
436  int save_errno = errno;
437 
439 
440  errno = save_errno;
441 }
XLogRecPtr GetLastImportantRecPtr(void)
Definition: xlog.c:8267
#define SIGQUIT
Definition: win32_port.h:155
#define InvalidXLogRecPtr
Definition: xlogdefs.h:28
static volatile sig_atomic_t got_SIGHUP
Definition: bgwriter.c:92
#define AllocSetContextCreate
Definition: memutils.h:170
bool BgBufferSync(WritebackContext *wb_context)
Definition: bufmgr.c:2053
int BgWriterDelay
Definition: bgwriter.c:67
static volatile sig_atomic_t shutdown_requested
Definition: bgwriter.c:93
#define WL_TIMEOUT
Definition: latch.h:127
void ProcessConfigFile(GucContext context)
static void BgSigHupHandler(SIGNAL_ARGS)
Definition: bgwriter.c:410
#define SIGUSR1
Definition: win32_port.h:166
TimestampTz GetCurrentTimestamp(void)
Definition: timestamp.c:1583
#define SIGCHLD
Definition: win32_port.h:164
PGPROC * MyProc
Definition: proc.c:68
int64 TimestampTz
Definition: timestamp.h:39
void AtEOXact_Buffers(bool isCommit)
Definition: bufmgr.c:2423
void ReleaseAuxProcessResources(bool isCommit)
Definition: resowner.c:855
static MemoryContext MemoryContextSwitchTo(MemoryContext context)
Definition: palloc.h:109
void proc_exit(int code)
Definition: ipc.c:104
#define SIGPIPE
Definition: win32_port.h:159
#define SIGUSR2
Definition: win32_port.h:167
bool RecoveryInProgress(void)
Definition: xlog.c:7917
void SetLatch(Latch *latch)
Definition: latch.c:436
void FlushErrorState(void)
Definition: elog.c:1574
void ResetLatch(Latch *latch)
Definition: latch.c:519
#define PG_SETMASK(mask)
Definition: pqsignal.h:19
void AtEOXact_Files(bool isCommit)
Definition: fd.c:2781
void WritebackContextInit(WritebackContext *context, int *max_pending)
Definition: bufmgr.c:4288
int WaitLatch(Latch *latch, int wakeEvents, long timeout, uint32 wait_event_info)
Definition: latch.c:344
int bgwriter_flush_after
Definition: bufmgr.c:120
void smgrcloseall(void)
Definition: smgr.c:286
#define RESUME_INTERRUPTS()
Definition: miscadmin.h:118
ErrorContextCallback * error_context_stack
Definition: elog.c:88
#define HIBERNATE_FACTOR
Definition: bgwriter.c:73
void pg_usleep(long microsec)
Definition: signal.c:53
void AtEOXact_SMgr(void)
Definition: smgr.c:655
void ConditionVariableCancelSleep(void)
XLogRecPtr LogStandbySnapshot(void)
Definition: standby.c:901
#define ALLOCSET_DEFAULT_SIZES
Definition: memutils.h:192
#define LOG_SNAPSHOT_INTERVAL_MS
Definition: bgwriter.c:79
#define SIGHUP
Definition: win32_port.h:154
static void bg_quickdie(SIGNAL_ARGS)
Definition: bgwriter.c:389
sigset_t UnBlockSig
Definition: pqsignal.c:22
static void pgstat_report_wait_end(void)
Definition: pgstat.h:1342
MemoryContext TopMemoryContext
Definition: mcxt.c:44
Definition: guc.h:72
#define SIG_IGN
Definition: win32_port.h:151
void UnlockBuffers(void)
Definition: bufmgr.c:3574
#define MemoryContextResetAndDeleteChildren(ctx)
Definition: memutils.h:67
bool ExitOnAnyError
Definition: globals.c:113
sigset_t BlockSig
Definition: pqsignal.c:22
bool FirstCallSinceLastCheckpoint(void)
#define XLogStandbyInfoActive()
Definition: xlog.h:195
void EmitErrorReport(void)
Definition: elog.c:1433
void pgstat_send_bgwriter(void)
Definition: pgstat.c:4392
#define TimestampTzPlusMilliseconds(tz, ms)
Definition: timestamp.h:56
pqsigfunc pqsignal(int signum, pqsigfunc handler)
Definition: signal.c:170
#define SIG_DFL
Definition: win32_port.h:149
#define SIGNAL_ARGS
Definition: c.h:1259
uint64 XLogRecPtr
Definition: xlogdefs.h:21
#define SIGALRM
Definition: win32_port.h:160
void AbortBufferIO(void)
Definition: bufmgr.c:4030
sigjmp_buf * PG_exception_stack
Definition: elog.c:90
void BackgroundWriterMain(void)
Definition: bgwriter.c:110
int pgprocno
Definition: proc.h:110
void StrategyNotifyBgWriter(int bgwprocno)
Definition: freelist.c:432
#define HOLD_INTERRUPTS()
Definition: miscadmin.h:116
static TimestampTz last_snapshot_ts
Definition: bgwriter.c:86
struct Latch * MyLatch
Definition: globals.c:54
void LWLockReleaseAll(void)
Definition: lwlock.c:1825
static XLogRecPtr last_snapshot_lsn
Definition: bgwriter.c:87
void latch_sigusr1_handler(void)
Definition: latch.c:1515
void AtEOXact_HashTables(bool isCommit)
Definition: dynahash.c:1834
#define WL_LATCH_SET
Definition: latch.h:124
Datum now(PG_FUNCTION_ARGS)
Definition: timestamp.c:1547
static void ReqShutdownHandler(SIGNAL_ARGS)
Definition: bgwriter.c:422
static void bgwriter_sigusr1_handler(SIGNAL_ARGS)
Definition: bgwriter.c:434
#define WL_EXIT_ON_PM_DEATH
Definition: latch.h:129