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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) 1996-2026, 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 "libpq/pqsignal.h"
36 : : #include "miscadmin.h"
37 : : #include "pgstat.h"
38 : : #include "postmaster/auxprocess.h"
39 : : #include "postmaster/bgwriter.h"
40 : : #include "postmaster/interrupt.h"
41 : : #include "storage/aio_subsys.h"
42 : : #include "storage/buf_internals.h"
43 : : #include "storage/bufmgr.h"
44 : : #include "storage/condition_variable.h"
45 : : #include "storage/fd.h"
46 : : #include "storage/lwlock.h"
47 : : #include "storage/proc.h"
48 : : #include "storage/procsignal.h"
49 : : #include "storage/smgr.h"
50 : : #include "storage/standby.h"
51 : : #include "utils/memutils.h"
52 : : #include "utils/resowner.h"
53 : : #include "utils/timestamp.h"
54 : :
55 : : /*
56 : : * GUC parameters
57 : : */
58 : : int BgWriterDelay = 200;
59 : :
60 : : /*
61 : : * Multiplier to apply to BgWriterDelay when we decide to hibernate.
62 : : * (Perhaps this needs to be configurable?)
63 : : */
64 : : #define HIBERNATE_FACTOR 50
65 : :
66 : : /*
67 : : * Interval in which standby snapshots are logged into the WAL stream, in
68 : : * milliseconds.
69 : : */
70 : : #define LOG_SNAPSHOT_INTERVAL_MS 15000
71 : :
72 : : /*
73 : : * LSN and timestamp at which we last issued a LogStandbySnapshot(), to avoid
74 : : * doing so too often or repeatedly if there has been no other write activity
75 : : * in the system.
76 : : */
77 : : static TimestampTz last_snapshot_ts;
78 : : static XLogRecPtr last_snapshot_lsn = InvalidXLogRecPtr;
79 : :
80 : :
81 : : /*
82 : : * Main entry point for bgwriter process
83 : : *
84 : : * This is invoked from AuxiliaryProcessMain, which has already created the
85 : : * basic execution environment, but not enabled signals yet.
86 : : */
87 : : void
88 : 0 : BackgroundWriterMain(const void *startup_data, size_t startup_data_len)
89 : : {
90 : 0 : sigjmp_buf local_sigjmp_buf;
91 : 0 : MemoryContext bgwriter_context;
92 : 0 : bool prev_hibernate;
93 : 0 : WritebackContext wb_context;
94 : :
95 [ # # ]: 0 : Assert(startup_data_len == 0);
96 : :
97 : 0 : MyBackendType = B_BG_WRITER;
98 : 0 : AuxiliaryProcessMainCommon();
99 : :
100 : : /*
101 : : * Properly accept or ignore signals that might be sent to us.
102 : : */
103 : 0 : pqsignal(SIGHUP, SignalHandlerForConfigReload);
104 : 0 : pqsignal(SIGINT, SIG_IGN);
105 : 0 : pqsignal(SIGTERM, SignalHandlerForShutdownRequest);
106 : : /* SIGQUIT handler was already set up by InitPostmasterChild */
107 : 0 : pqsignal(SIGALRM, SIG_IGN);
108 : 0 : pqsignal(SIGPIPE, SIG_IGN);
109 : 0 : pqsignal(SIGUSR1, procsignal_sigusr1_handler);
110 : 0 : pqsignal(SIGUSR2, SIG_IGN);
111 : :
112 : : /*
113 : : * Reset some signals that are accepted by postmaster but not here
114 : : */
115 : 0 : pqsignal(SIGCHLD, SIG_DFL);
116 : :
117 : : /*
118 : : * We just started, assume there has been either a shutdown or
119 : : * end-of-recovery snapshot.
120 : : */
121 : 0 : last_snapshot_ts = GetCurrentTimestamp();
122 : :
123 : : /*
124 : : * Create a memory context that we will do all our work in. We do this so
125 : : * that we can reset the context during error recovery and thereby avoid
126 : : * possible memory leaks. Formerly this code just ran in
127 : : * TopMemoryContext, but resetting that would be a really bad idea.
128 : : */
129 : 0 : bgwriter_context = AllocSetContextCreate(TopMemoryContext,
130 : : "Background Writer",
131 : : ALLOCSET_DEFAULT_SIZES);
132 : 0 : MemoryContextSwitchTo(bgwriter_context);
133 : :
134 : 0 : WritebackContextInit(&wb_context, &bgwriter_flush_after);
135 : :
136 : : /*
137 : : * If an exception is encountered, processing resumes here.
138 : : *
139 : : * You might wonder why this isn't coded as an infinite loop around a
140 : : * PG_TRY construct. The reason is that this is the bottom of the
141 : : * exception stack, and so with PG_TRY there would be no exception handler
142 : : * in force at all during the CATCH part. By leaving the outermost setjmp
143 : : * always active, we have at least some chance of recovering from an error
144 : : * during error recovery. (If we get into an infinite loop thereby, it
145 : : * will soon be stopped by overflow of elog.c's internal state stack.)
146 : : *
147 : : * Note that we use sigsetjmp(..., 1), so that the prevailing signal mask
148 : : * (to wit, BlockSig) will be restored when longjmp'ing to here. Thus,
149 : : * signals other than SIGQUIT will be blocked until we complete error
150 : : * recovery. It might seem that this policy makes the HOLD_INTERRUPTS()
151 : : * call redundant, but it is not since InterruptPending might be set
152 : : * already.
153 : : */
154 [ # # ]: 0 : if (sigsetjmp(local_sigjmp_buf, 1) != 0)
155 : : {
156 : : /* Since not using PG_TRY, must reset error stack by hand */
157 : 0 : error_context_stack = NULL;
158 : :
159 : : /* Prevent interrupts while cleaning up */
160 : 0 : HOLD_INTERRUPTS();
161 : :
162 : : /* Report the error to the server log */
163 : 0 : EmitErrorReport();
164 : :
165 : : /*
166 : : * These operations are really just a minimal subset of
167 : : * AbortTransaction(). We don't have very many resources to worry
168 : : * about in bgwriter, but we do have LWLocks, buffers, and temp files.
169 : : */
170 : 0 : LWLockReleaseAll();
171 : 0 : ConditionVariableCancelSleep();
172 : 0 : pgaio_error_cleanup();
173 : 0 : UnlockBuffers();
174 : 0 : ReleaseAuxProcessResources(false);
175 : 0 : AtEOXact_Buffers(false);
176 : 0 : AtEOXact_SMgr();
177 : 0 : AtEOXact_Files(false);
178 : 0 : AtEOXact_HashTables(false);
179 : :
180 : : /*
181 : : * Now return to normal top-level context and clear ErrorContext for
182 : : * next time.
183 : : */
184 : 0 : MemoryContextSwitchTo(bgwriter_context);
185 : 0 : FlushErrorState();
186 : :
187 : : /* Flush any leaked data in the top-level context */
188 : 0 : MemoryContextReset(bgwriter_context);
189 : :
190 : : /* re-initialize to avoid repeated errors causing problems */
191 : 0 : WritebackContextInit(&wb_context, &bgwriter_flush_after);
192 : :
193 : : /* Now we can allow interrupts again */
194 [ # # ]: 0 : RESUME_INTERRUPTS();
195 : :
196 : : /*
197 : : * Sleep at least 1 second after any error. A write error is likely
198 : : * to be repeated, and we don't want to be filling the error logs as
199 : : * fast as we can.
200 : : */
201 : 0 : pg_usleep(1000000L);
202 : :
203 : : /* Report wait end here, when there is no further possibility of wait */
204 : 0 : pgstat_report_wait_end();
205 : 0 : }
206 : :
207 : : /* We can now handle ereport(ERROR) */
208 : 0 : PG_exception_stack = &local_sigjmp_buf;
209 : :
210 : : /*
211 : : * Unblock signals (they were blocked when the postmaster forked us)
212 : : */
213 : 0 : sigprocmask(SIG_SETMASK, &UnBlockSig, NULL);
214 : :
215 : : /*
216 : : * Reset hibernation state after any error.
217 : : */
218 : 0 : prev_hibernate = false;
219 : :
220 : : /*
221 : : * Loop forever
222 : : */
223 : 24 : for (;;)
224 : : {
225 : 24 : bool can_hibernate;
226 : 24 : int rc;
227 : :
228 : : /* Clear any already-pending wakeups */
229 : 24 : ResetLatch(MyLatch);
230 : :
231 : 24 : ProcessMainLoopInterrupts();
232 : :
233 : : /*
234 : : * Do one cycle of dirty-buffer writing.
235 : : */
236 : 24 : can_hibernate = BgBufferSync(&wb_context);
237 : :
238 : : /* Report pending statistics to the cumulative stats system */
239 : 24 : pgstat_report_bgwriter();
240 : 24 : pgstat_report_wal(true);
241 : :
242 [ + - ]: 24 : if (FirstCallSinceLastCheckpoint())
243 : : {
244 : : /*
245 : : * After any checkpoint, free all smgr objects. Otherwise we
246 : : * would never do so for dropped relations, as the bgwriter does
247 : : * not process shared invalidation messages or call
248 : : * AtEOXact_SMgr().
249 : : */
250 : 0 : smgrdestroyall();
251 : 0 : }
252 : :
253 : : /*
254 : : * Log a new xl_running_xacts every now and then so replication can
255 : : * get into a consistent state faster (think of suboverflowed
256 : : * snapshots) and clean up resources (locks, KnownXids*) more
257 : : * frequently. The costs of this are relatively low, so doing it 4
258 : : * times (LOG_SNAPSHOT_INTERVAL_MS) a minute seems fine.
259 : : *
260 : : * We assume the interval for writing xl_running_xacts is
261 : : * significantly bigger than BgWriterDelay, so we don't complicate the
262 : : * overall timeout handling but just assume we're going to get called
263 : : * often enough even if hibernation mode is active. It's not that
264 : : * important that LOG_SNAPSHOT_INTERVAL_MS is met strictly. To make
265 : : * sure we're not waking the disk up unnecessarily on an idle system
266 : : * we check whether there has been any WAL inserted since the last
267 : : * time we've logged a running xacts.
268 : : *
269 : : * We do this logging in the bgwriter as it is the only process that
270 : : * is run regularly and returns to its mainloop all the time. E.g.
271 : : * Checkpointer, when active, is barely ever in its mainloop and thus
272 : : * makes it hard to log regularly.
273 : : */
274 [ + - + + ]: 24 : if (XLogStandbyInfoActive() && !RecoveryInProgress())
275 : : {
276 : 22 : TimestampTz timeout = 0;
277 : 22 : TimestampTz now = GetCurrentTimestamp();
278 : :
279 : 22 : timeout = TimestampTzPlusMilliseconds(last_snapshot_ts,
280 : : LOG_SNAPSHOT_INTERVAL_MS);
281 : :
282 : : /*
283 : : * Only log if enough time has passed and interesting records have
284 : : * been inserted since the last snapshot. Have to compare with <=
285 : : * instead of < because GetLastImportantRecPtr() points at the
286 : : * start of a record, whereas last_snapshot_lsn points just past
287 : : * the end of the record.
288 : : */
289 [ - + # # ]: 22 : if (now >= timeout &&
290 : 0 : last_snapshot_lsn <= GetLastImportantRecPtr())
291 : : {
292 : 0 : last_snapshot_lsn = LogStandbySnapshot();
293 : 0 : last_snapshot_ts = now;
294 : 0 : }
295 : 22 : }
296 : :
297 : : /*
298 : : * Sleep until we are signaled or BgWriterDelay has elapsed.
299 : : *
300 : : * Note: the feedback control loop in BgBufferSync() expects that we
301 : : * will call it every BgWriterDelay msec. While it's not critical for
302 : : * correctness that that be exact, the feedback loop might misbehave
303 : : * if we stray too far from that. Hence, avoid loading this process
304 : : * down with latch events that are likely to happen frequently during
305 : : * normal operation.
306 : : */
307 : 48 : rc = WaitLatch(MyLatch,
308 : : WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH,
309 : 24 : BgWriterDelay /* ms */ , WAIT_EVENT_BGWRITER_MAIN);
310 : :
311 : : /*
312 : : * If no latch event and BgBufferSync says nothing's happening, extend
313 : : * the sleep in "hibernation" mode, where we sleep for much longer
314 : : * than bgwriter_delay says. Fewer wakeups save electricity. When a
315 : : * backend starts using buffers again, it will wake us up by setting
316 : : * our latch. Because the extra sleep will persist only as long as no
317 : : * buffer allocations happen, this should not distort the behavior of
318 : : * BgBufferSync's control loop too badly; essentially, it will think
319 : : * that the system-wide idle interval didn't exist.
320 : : *
321 : : * There is a race condition here, in that a backend might allocate a
322 : : * buffer between the time BgBufferSync saw the alloc count as zero
323 : : * and the time we call StrategyNotifyBgWriter. While it's not
324 : : * critical that we not hibernate anyway, we try to reduce the odds of
325 : : * that by only hibernating when BgBufferSync says nothing's happening
326 : : * for two consecutive cycles. Also, we mitigate any possible
327 : : * consequences of a missed wakeup by not hibernating forever.
328 : : */
329 [ + + + + : 24 : if (rc == WL_TIMEOUT && can_hibernate && prev_hibernate)
+ + ]
330 : : {
331 : : /* Ask for notification at next buffer allocation */
332 : 3 : StrategyNotifyBgWriter(MyProcNumber);
333 : : /* Sleep ... */
334 : 6 : (void) WaitLatch(MyLatch,
335 : : WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH,
336 : 3 : BgWriterDelay * HIBERNATE_FACTOR,
337 : : WAIT_EVENT_BGWRITER_HIBERNATE);
338 : : /* Reset the notification request in case we timed out */
339 : 3 : StrategyNotifyBgWriter(-1);
340 : 3 : }
341 : :
342 : 24 : prev_hibernate = can_hibernate;
343 : 24 : }
344 : : }
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