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  * 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) 1996-2023, 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"
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  */
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
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  */
102  pqsignal(SIGINT, SIG_IGN);
104  /* SIGQUIT handler was already set up by InitPostmasterChild */
109 
110  /*
111  * Reset some signals that are accepted by postmaster but not here
112  */
114 
115  /*
116  * We just started, assume there has been either a shutdown or
117  * end-of-recovery snapshot.
118  */
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  bgwriter_context = AllocSetContextCreate(TopMemoryContext,
128  "Background Writer",
130  MemoryContextSwitchTo(bgwriter_context);
131 
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  if (sigsetjmp(local_sigjmp_buf, 1) != 0)
153  {
154  /* Since not using PG_TRY, must reset error stack by hand */
155  error_context_stack = NULL;
156 
157  /* Prevent interrupts while cleaning up */
158  HOLD_INTERRUPTS();
159 
160  /* Report the error to the server log */
161  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  */
170  UnlockBuffers();
172  AtEOXact_Buffers(false);
173  AtEOXact_SMgr();
174  AtEOXact_Files(false);
175  AtEOXact_HashTables(false);
176 
177  /*
178  * Now return to normal top-level context and clear ErrorContext for
179  * next time.
180  */
181  MemoryContextSwitchTo(bgwriter_context);
182  FlushErrorState();
183 
184  /* Flush any leaked data in the top-level context */
185  MemoryContextResetAndDeleteChildren(bgwriter_context);
186 
187  /* re-initialize to avoid repeated errors causing problems */
189 
190  /* Now we can allow interrupts again */
192 
193  /*
194  * Sleep at least 1 second after any error. A write error is likely
195  * to be repeated, and we don't want to be filling the error logs as
196  * fast as we can.
197  */
198  pg_usleep(1000000L);
199 
200  /*
201  * Close all open files after any error. This is helpful on Windows,
202  * where holding deleted files open causes various strange errors.
203  * It's not clear we need it elsewhere, but shouldn't hurt.
204  */
205  smgrcloseall();
206 
207  /* Report wait end here, when there is no further possibility of wait */
209  }
210 
211  /* We can now handle ereport(ERROR) */
212  PG_exception_stack = &local_sigjmp_buf;
213 
214  /*
215  * Unblock signals (they were blocked when the postmaster forked us)
216  */
217  sigprocmask(SIG_SETMASK, &UnBlockSig, NULL);
218 
219  /*
220  * Reset hibernation state after any error.
221  */
222  prev_hibernate = false;
223 
224  /*
225  * Loop forever
226  */
227  for (;;)
228  {
229  bool can_hibernate;
230  int rc;
231 
232  /* Clear any already-pending wakeups */
234 
236 
237  /*
238  * Do one cycle of dirty-buffer writing.
239  */
240  can_hibernate = BgBufferSync(&wb_context);
241 
242  /* Report pending statistics to the cumulative stats system */
244 
246  {
247  /*
248  * After any checkpoint, close all smgr files. This is so we
249  * won't hang onto smgr references to deleted files indefinitely.
250  */
251  smgrcloseall();
252  }
253 
254  /*
255  * Log a new xl_running_xacts every now and then so replication can
256  * get into a consistent state faster (think of suboverflowed
257  * snapshots) and clean up resources (locks, KnownXids*) more
258  * frequently. The costs of this are relatively low, so doing it 4
259  * times (LOG_SNAPSHOT_INTERVAL_MS) a minute seems fine.
260  *
261  * We assume the interval for writing xl_running_xacts is
262  * significantly bigger than BgWriterDelay, so we don't complicate the
263  * overall timeout handling but just assume we're going to get called
264  * often enough even if hibernation mode is active. It's not that
265  * important that LOG_SNAPSHOT_INTERVAL_MS is met strictly. To make
266  * sure we're not waking the disk up unnecessarily on an idle system
267  * we check whether there has been any WAL inserted since the last
268  * time we've logged a running xacts.
269  *
270  * We do this logging in the bgwriter as it is the only process that
271  * is run regularly and returns to its mainloop all the time. E.g.
272  * Checkpointer, when active, is barely ever in its mainloop and thus
273  * makes it hard to log regularly.
274  */
276  {
277  TimestampTz timeout = 0;
279 
282 
283  /*
284  * Only log if enough time has passed and interesting records have
285  * been inserted since the last snapshot. Have to compare with <=
286  * instead of < because GetLastImportantRecPtr() points at the
287  * start of a record, whereas last_snapshot_lsn points just past
288  * the end of the record.
289  */
290  if (now >= timeout &&
292  {
295  }
296  }
297 
298  /*
299  * Sleep until we are signaled or BgWriterDelay has elapsed.
300  *
301  * Note: the feedback control loop in BgBufferSync() expects that we
302  * will call it every BgWriterDelay msec. While it's not critical for
303  * correctness that that be exact, the feedback loop might misbehave
304  * if we stray too far from that. Hence, avoid loading this process
305  * down with latch events that are likely to happen frequently during
306  * normal operation.
307  */
308  rc = WaitLatch(MyLatch,
310  BgWriterDelay /* ms */ , WAIT_EVENT_BGWRITER_MAIN);
311 
312  /*
313  * If no latch event and BgBufferSync says nothing's happening, extend
314  * the sleep in "hibernation" mode, where we sleep for much longer
315  * than bgwriter_delay says. Fewer wakeups save electricity. When a
316  * backend starts using buffers again, it will wake us up by setting
317  * our latch. Because the extra sleep will persist only as long as no
318  * buffer allocations happen, this should not distort the behavior of
319  * BgBufferSync's control loop too badly; essentially, it will think
320  * that the system-wide idle interval didn't exist.
321  *
322  * There is a race condition here, in that a backend might allocate a
323  * buffer between the time BgBufferSync saw the alloc count as zero
324  * and the time we call StrategyNotifyBgWriter. While it's not
325  * critical that we not hibernate anyway, we try to reduce the odds of
326  * that by only hibernating when BgBufferSync says nothing's happening
327  * for two consecutive cycles. Also, we mitigate any possible
328  * consequences of a missed wakeup by not hibernating forever.
329  */
330  if (rc == WL_TIMEOUT && can_hibernate && prev_hibernate)
331  {
332  /* Ask for notification at next buffer allocation */
334  /* Sleep ... */
335  (void) WaitLatch(MyLatch,
338  WAIT_EVENT_BGWRITER_HIBERNATE);
339  /* Reset the notification request in case we timed out */
341  }
342 
343  prev_hibernate = can_hibernate;
344  }
345 }
sigset_t UnBlockSig
Definition: pqsignal.c:22
TimestampTz GetCurrentTimestamp(void)
Definition: timestamp.c:1583
Datum now(PG_FUNCTION_ARGS)
Definition: timestamp.c:1547
static XLogRecPtr last_snapshot_lsn
Definition: bgwriter.c:81
static TimestampTz last_snapshot_ts
Definition: bgwriter.c:80
void BackgroundWriterMain(void)
Definition: bgwriter.c:91
int BgWriterDelay
Definition: bgwriter.c:61
#define HIBERNATE_FACTOR
Definition: bgwriter.c:67
#define LOG_SNAPSHOT_INTERVAL_MS
Definition: bgwriter.c:73
void AtEOXact_Buffers(bool isCommit)
Definition: bufmgr.c:3132
bool BgBufferSync(WritebackContext *wb_context)
Definition: bufmgr.c:2758
void UnlockBuffers(void)
Definition: bufmgr.c:4687
int bgwriter_flush_after
Definition: bufmgr.c:160
void WritebackContextInit(WritebackContext *context, int *max_pending)
Definition: bufmgr.c:5442
bool FirstCallSinceLastCheckpoint(void)
bool ConditionVariableCancelSleep(void)
int64 TimestampTz
Definition: timestamp.h:39
void AtEOXact_HashTables(bool isCommit)
Definition: dynahash.c:1878
void EmitErrorReport(void)
Definition: elog.c:1669
ErrorContextCallback * error_context_stack
Definition: elog.c:95
void FlushErrorState(void)
Definition: elog.c:1825
sigjmp_buf * PG_exception_stack
Definition: elog.c:97
void AtEOXact_Files(bool isCommit)
Definition: fd.c:3110
void StrategyNotifyBgWriter(int bgwprocno)
Definition: freelist.c:431
struct Latch * MyLatch
Definition: globals.c:58
void SignalHandlerForShutdownRequest(SIGNAL_ARGS)
Definition: interrupt.c:109
void SignalHandlerForConfigReload(SIGNAL_ARGS)
Definition: interrupt.c:61
void HandleMainLoopInterrupts(void)
Definition: interrupt.c:34
void ResetLatch(Latch *latch)
Definition: latch.c:697
int WaitLatch(Latch *latch, int wakeEvents, long timeout, uint32 wait_event_info)
Definition: latch.c:490
#define WL_TIMEOUT
Definition: latch.h:128
#define WL_EXIT_ON_PM_DEATH
Definition: latch.h:130
#define WL_LATCH_SET
Definition: latch.h:125
void LWLockReleaseAll(void)
Definition: lwlock.c:1903
MemoryContext TopMemoryContext
Definition: mcxt.c:141
#define AllocSetContextCreate
Definition: memutils.h:129
#define ALLOCSET_DEFAULT_SIZES
Definition: memutils.h:153
#define MemoryContextResetAndDeleteChildren(ctx)
Definition: memutils.h:70
#define RESUME_INTERRUPTS()
Definition: miscadmin.h:134
#define HOLD_INTERRUPTS()
Definition: miscadmin.h:132
static MemoryContext MemoryContextSwitchTo(MemoryContext context)
Definition: palloc.h:138
void pgstat_report_bgwriter(void)
pqsigfunc pqsignal(int signo, pqsigfunc func)
void procsignal_sigusr1_handler(SIGNAL_ARGS)
Definition: procsignal.c:639
void ReleaseAuxProcessResources(bool isCommit)
Definition: resowner.c:934
void pg_usleep(long microsec)
Definition: signal.c:53
void smgrcloseall(void)
Definition: smgr.c:327
void AtEOXact_SMgr(void)
Definition: smgr.c:739
PGPROC * MyProc
Definition: proc.c:66
XLogRecPtr LogStandbySnapshot(void)
Definition: standby.c:1287
int pgprocno
Definition: proc.h:191
#define TimestampTzPlusMilliseconds(tz, ms)
Definition: timestamp.h:85
static void pgstat_report_wait_end(void)
Definition: wait_event.h:104
#define SIGCHLD
Definition: win32_port.h:178
#define SIGHUP
Definition: win32_port.h:168
#define SIG_DFL
Definition: win32_port.h:163
#define SIGPIPE
Definition: win32_port.h:173
#define SIGUSR1
Definition: win32_port.h:180
#define SIGALRM
Definition: win32_port.h:174
#define SIGUSR2
Definition: win32_port.h:181
#define SIG_IGN
Definition: win32_port.h:165
bool RecoveryInProgress(void)
Definition: xlog.c:5948
XLogRecPtr GetLastImportantRecPtr(void)
Definition: xlog.c:6153
#define XLogStandbyInfoActive()
Definition: xlog.h:118
uint64 XLogRecPtr
Definition: xlogdefs.h:21
#define InvalidXLogRecPtr
Definition: xlogdefs.h:28