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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) 1996-2021, 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"
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  AbortBufferIO();
171  UnlockBuffers();
173  AtEOXact_Buffers(false);
174  AtEOXact_SMgr();
175  AtEOXact_Files(false);
176  AtEOXact_HashTables(false);
177 
178  /*
179  * Now return to normal top-level context and clear ErrorContext for
180  * next time.
181  */
182  MemoryContextSwitchTo(bgwriter_context);
183  FlushErrorState();
184 
185  /* Flush any leaked data in the top-level context */
186  MemoryContextResetAndDeleteChildren(bgwriter_context);
187 
188  /* re-initialize to avoid repeated errors causing problems */
190 
191  /* Now we can allow interrupts again */
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  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  smgrcloseall();
207 
208  /* Report wait end here, when there is no further possibility of wait */
210  }
211 
212  /* We can now handle ereport(ERROR) */
213  PG_exception_stack = &local_sigjmp_buf;
214 
215  /*
216  * Unblock signals (they were blocked when the postmaster forked us)
217  */
219 
220  /*
221  * Reset hibernation state after any error.
222  */
223  prev_hibernate = false;
224 
225  /*
226  * Loop forever
227  */
228  for (;;)
229  {
230  bool can_hibernate;
231  int rc;
232 
233  /* Clear any already-pending wakeups */
235 
237 
238  /*
239  * Do one cycle of dirty-buffer writing.
240  */
241  can_hibernate = BgBufferSync(&wb_context);
242 
243  /*
244  * Send off activity statistics to the stats collector
245  */
247 
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  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  */
279  {
280  TimestampTz timeout = 0;
282 
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  if (now >= timeout &&
295  {
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  rc = WaitLatch(MyLatch,
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  if (rc == WL_TIMEOUT && can_hibernate && prev_hibernate)
334  {
335  /* Ask for notification at next buffer allocation */
337  /* Sleep ... */
338  (void) WaitLatch(MyLatch,
342  /* Reset the notification request in case we timed out */
344  }
345 
346  prev_hibernate = can_hibernate;
347  }
348 }
XLogRecPtr GetLastImportantRecPtr(void)
Definition: xlog.c:8590
#define InvalidXLogRecPtr
Definition: xlogdefs.h:28
#define AllocSetContextCreate
Definition: memutils.h:173
static void pgstat_report_wait_end(void)
Definition: wait_event.h:278
bool BgBufferSync(WritebackContext *wb_context)
Definition: bufmgr.c:2209
int BgWriterDelay
Definition: bgwriter.c:61
#define WL_TIMEOUT
Definition: latch.h:128
void HandleMainLoopInterrupts(void)
Definition: interrupt.c:33
#define SIGUSR1
Definition: win32_port.h:171
TimestampTz GetCurrentTimestamp(void)
Definition: timestamp.c:1580
#define SIGCHLD
Definition: win32_port.h:169
PGPROC * MyProc
Definition: proc.c:68
int64 TimestampTz
Definition: timestamp.h:39
void AtEOXact_Buffers(bool isCommit)
Definition: bufmgr.c:2579
void SignalHandlerForConfigReload(SIGNAL_ARGS)
Definition: interrupt.c:56
void ReleaseAuxProcessResources(bool isCommit)
Definition: resowner.c:908
static MemoryContext MemoryContextSwitchTo(MemoryContext context)
Definition: palloc.h:109
#define SIGPIPE
Definition: win32_port.h:164
#define SIGUSR2
Definition: win32_port.h:172
bool RecoveryInProgress(void)
Definition: xlog.c:8220
void FlushErrorState(void)
Definition: elog.c:1654
void ResetLatch(Latch *latch)
Definition: latch.c:660
#define PG_SETMASK(mask)
Definition: pqsignal.h:19
void AtEOXact_Files(bool isCommit)
Definition: fd.c:3041
void WritebackContextInit(WritebackContext *context, int *max_pending)
Definition: bufmgr.c:4722
int WaitLatch(Latch *latch, int wakeEvents, long timeout, uint32 wait_event_info)
Definition: latch.c:452
int bgwriter_flush_after
Definition: bufmgr.c:157
void smgrcloseall(void)
Definition: smgr.c:286
#define RESUME_INTERRUPTS()
Definition: miscadmin.h:133
ErrorContextCallback * error_context_stack
Definition: elog.c:93
#define HIBERNATE_FACTOR
Definition: bgwriter.c:67
void pg_usleep(long microsec)
Definition: signal.c:53
void AtEOXact_SMgr(void)
Definition: smgr.c:678
void ConditionVariableCancelSleep(void)
XLogRecPtr LogStandbySnapshot(void)
Definition: standby.c:1220
#define ALLOCSET_DEFAULT_SIZES
Definition: memutils.h:195
#define LOG_SNAPSHOT_INTERVAL_MS
Definition: bgwriter.c:73
#define SIGHUP
Definition: win32_port.h:159
sigset_t UnBlockSig
Definition: pqsignal.c:22
MemoryContext TopMemoryContext
Definition: mcxt.c:48
void SignalHandlerForShutdownRequest(SIGNAL_ARGS)
Definition: interrupt.c:104
#define SIG_IGN
Definition: win32_port.h:156
void UnlockBuffers(void)
Definition: bufmgr.c:3979
#define MemoryContextResetAndDeleteChildren(ctx)
Definition: memutils.h:67
bool FirstCallSinceLastCheckpoint(void)
#define XLogStandbyInfoActive()
Definition: xlog.h:180
void EmitErrorReport(void)
Definition: elog.c:1509
void pgstat_send_bgwriter(void)
Definition: pgstat.c:3124
#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:154
uint64 XLogRecPtr
Definition: xlogdefs.h:21
#define SIGALRM
Definition: win32_port.h:165
void AbortBufferIO(void)
Definition: bufmgr.c:4477
sigjmp_buf * PG_exception_stack
Definition: elog.c:95
void BackgroundWriterMain(void)
Definition: bgwriter.c:91
int pgprocno
Definition: proc.h:150
void StrategyNotifyBgWriter(int bgwprocno)
Definition: freelist.c:432
#define HOLD_INTERRUPTS()
Definition: miscadmin.h:131
static TimestampTz last_snapshot_ts
Definition: bgwriter.c:80
struct Latch * MyLatch
Definition: globals.c:57
void LWLockReleaseAll(void)
Definition: lwlock.c:1902
static XLogRecPtr last_snapshot_lsn
Definition: bgwriter.c:81
void procsignal_sigusr1_handler(SIGNAL_ARGS)
Definition: procsignal.c:642
void AtEOXact_HashTables(bool isCommit)
Definition: dynahash.c:1883
#define WL_LATCH_SET
Definition: latch.h:125
Datum now(PG_FUNCTION_ARGS)
Definition: timestamp.c:1544
#define WL_EXIT_ON_PM_DEATH
Definition: latch.h:130