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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) 1996-2024, 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"
39#include "postmaster/bgwriter.h"
42#include "storage/bufmgr.h"
44#include "storage/fd.h"
45#include "storage/lwlock.h"
46#include "storage/proc.h"
47#include "storage/procsignal.h"
48#include "storage/smgr.h"
49#include "storage/standby.h"
50#include "utils/memutils.h"
51#include "utils/resowner.h"
52#include "utils/timestamp.h"
53
54/*
55 * GUC parameters
56 */
57int BgWriterDelay = 200;
58
59/*
60 * Multiplier to apply to BgWriterDelay when we decide to hibernate.
61 * (Perhaps this needs to be configurable?)
62 */
63#define HIBERNATE_FACTOR 50
64
65/*
66 * Interval in which standby snapshots are logged into the WAL stream, in
67 * milliseconds.
68 */
69#define LOG_SNAPSHOT_INTERVAL_MS 15000
70
71/*
72 * LSN and timestamp at which we last issued a LogStandbySnapshot(), to avoid
73 * doing so too often or repeatedly if there has been no other write activity
74 * in the system.
75 */
78
79
80/*
81 * Main entry point for bgwriter process
82 *
83 * This is invoked from AuxiliaryProcessMain, which has already created the
84 * basic execution environment, but not enabled signals yet.
85 */
86void
87BackgroundWriterMain(char *startup_data, size_t startup_data_len)
88{
89 sigjmp_buf local_sigjmp_buf;
90 MemoryContext bgwriter_context;
91 bool prev_hibernate;
92 WritebackContext wb_context;
93
94 Assert(startup_data_len == 0);
95
98
99 /*
100 * Properly accept or ignore signals that might be sent to us.
101 */
103 pqsignal(SIGINT, SIG_IGN);
105 /* SIGQUIT handler was already set up by InitPostmasterChild */
110
111 /*
112 * Reset some signals that are accepted by postmaster but not here
113 */
115
116 /*
117 * We just started, assume there has been either a shutdown or
118 * end-of-recovery snapshot.
119 */
121
122 /*
123 * Create a memory context that we will do all our work in. We do this so
124 * that we can reset the context during error recovery and thereby avoid
125 * possible memory leaks. Formerly this code just ran in
126 * TopMemoryContext, but resetting that would be a really bad idea.
127 */
128 bgwriter_context = AllocSetContextCreate(TopMemoryContext,
129 "Background Writer",
131 MemoryContextSwitchTo(bgwriter_context);
132
134
135 /*
136 * If an exception is encountered, processing resumes here.
137 *
138 * You might wonder why this isn't coded as an infinite loop around a
139 * PG_TRY construct. The reason is that this is the bottom of the
140 * exception stack, and so with PG_TRY there would be no exception handler
141 * in force at all during the CATCH part. By leaving the outermost setjmp
142 * always active, we have at least some chance of recovering from an error
143 * during error recovery. (If we get into an infinite loop thereby, it
144 * will soon be stopped by overflow of elog.c's internal state stack.)
145 *
146 * Note that we use sigsetjmp(..., 1), so that the prevailing signal mask
147 * (to wit, BlockSig) will be restored when longjmp'ing to here. Thus,
148 * signals other than SIGQUIT will be blocked until we complete error
149 * recovery. It might seem that this policy makes the HOLD_INTERRUPTS()
150 * call redundant, but it is not since InterruptPending might be set
151 * already.
152 */
153 if (sigsetjmp(local_sigjmp_buf, 1) != 0)
154 {
155 /* Since not using PG_TRY, must reset error stack by hand */
156 error_context_stack = NULL;
157
158 /* Prevent interrupts while cleaning up */
160
161 /* Report the error to the server log */
163
164 /*
165 * These operations are really just a minimal subset of
166 * AbortTransaction(). We don't have very many resources to worry
167 * about in bgwriter, but we do have LWLocks, buffers, and temp files.
168 */
173 AtEOXact_Buffers(false);
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);
184
185 /* Flush any leaked data in the top-level context */
186 MemoryContextReset(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 /* Report wait end here, when there is no further possibility of wait */
203 }
204
205 /* We can now handle ereport(ERROR) */
206 PG_exception_stack = &local_sigjmp_buf;
207
208 /*
209 * Unblock signals (they were blocked when the postmaster forked us)
210 */
211 sigprocmask(SIG_SETMASK, &UnBlockSig, NULL);
212
213 /*
214 * Reset hibernation state after any error.
215 */
216 prev_hibernate = false;
217
218 /*
219 * Loop forever
220 */
221 for (;;)
222 {
223 bool can_hibernate;
224 int rc;
225
226 /* Clear any already-pending wakeups */
228
230
231 /*
232 * Do one cycle of dirty-buffer writing.
233 */
234 can_hibernate = BgBufferSync(&wb_context);
235
236 /* Report pending statistics to the cumulative stats system */
238 pgstat_report_wal(true);
239
241 {
242 /*
243 * After any checkpoint, free all smgr objects. Otherwise we
244 * would never do so for dropped relations, as the bgwriter does
245 * not process shared invalidation messages or call
246 * AtEOXact_SMgr().
247 */
249 }
250
251 /*
252 * Log a new xl_running_xacts every now and then so replication can
253 * get into a consistent state faster (think of suboverflowed
254 * snapshots) and clean up resources (locks, KnownXids*) more
255 * frequently. The costs of this are relatively low, so doing it 4
256 * times (LOG_SNAPSHOT_INTERVAL_MS) a minute seems fine.
257 *
258 * We assume the interval for writing xl_running_xacts is
259 * significantly bigger than BgWriterDelay, so we don't complicate the
260 * overall timeout handling but just assume we're going to get called
261 * often enough even if hibernation mode is active. It's not that
262 * important that LOG_SNAPSHOT_INTERVAL_MS is met strictly. To make
263 * sure we're not waking the disk up unnecessarily on an idle system
264 * we check whether there has been any WAL inserted since the last
265 * time we've logged a running xacts.
266 *
267 * We do this logging in the bgwriter as it is the only process that
268 * is run regularly and returns to its mainloop all the time. E.g.
269 * Checkpointer, when active, is barely ever in its mainloop and thus
270 * makes it hard to log regularly.
271 */
273 {
274 TimestampTz timeout = 0;
276
279
280 /*
281 * Only log if enough time has passed and interesting records have
282 * been inserted since the last snapshot. Have to compare with <=
283 * instead of < because GetLastImportantRecPtr() points at the
284 * start of a record, whereas last_snapshot_lsn points just past
285 * the end of the record.
286 */
287 if (now >= timeout &&
289 {
292 }
293 }
294
295 /*
296 * Sleep until we are signaled or BgWriterDelay has elapsed.
297 *
298 * Note: the feedback control loop in BgBufferSync() expects that we
299 * will call it every BgWriterDelay msec. While it's not critical for
300 * correctness that that be exact, the feedback loop might misbehave
301 * if we stray too far from that. Hence, avoid loading this process
302 * down with latch events that are likely to happen frequently during
303 * normal operation.
304 */
305 rc = WaitLatch(MyLatch,
307 BgWriterDelay /* ms */ , WAIT_EVENT_BGWRITER_MAIN);
308
309 /*
310 * If no latch event and BgBufferSync says nothing's happening, extend
311 * the sleep in "hibernation" mode, where we sleep for much longer
312 * than bgwriter_delay says. Fewer wakeups save electricity. When a
313 * backend starts using buffers again, it will wake us up by setting
314 * our latch. Because the extra sleep will persist only as long as no
315 * buffer allocations happen, this should not distort the behavior of
316 * BgBufferSync's control loop too badly; essentially, it will think
317 * that the system-wide idle interval didn't exist.
318 *
319 * There is a race condition here, in that a backend might allocate a
320 * buffer between the time BgBufferSync saw the alloc count as zero
321 * and the time we call StrategyNotifyBgWriter. While it's not
322 * critical that we not hibernate anyway, we try to reduce the odds of
323 * that by only hibernating when BgBufferSync says nothing's happening
324 * for two consecutive cycles. Also, we mitigate any possible
325 * consequences of a missed wakeup by not hibernating forever.
326 */
327 if (rc == WL_TIMEOUT && can_hibernate && prev_hibernate)
328 {
329 /* Ask for notification at next buffer allocation */
331 /* Sleep ... */
332 (void) WaitLatch(MyLatch,
335 WAIT_EVENT_BGWRITER_HIBERNATE);
336 /* Reset the notification request in case we timed out */
338 }
339
340 prev_hibernate = can_hibernate;
341 }
342}
void AuxiliaryProcessMainCommon(void)
Definition: auxprocess.c:39
sigset_t UnBlockSig
Definition: pqsignal.c:22
TimestampTz GetCurrentTimestamp(void)
Definition: timestamp.c:1644
Datum now(PG_FUNCTION_ARGS)
Definition: timestamp.c:1608
static XLogRecPtr last_snapshot_lsn
Definition: bgwriter.c:77
static TimestampTz last_snapshot_ts
Definition: bgwriter.c:76
void BackgroundWriterMain(char *startup_data, size_t startup_data_len)
Definition: bgwriter.c:87
int BgWriterDelay
Definition: bgwriter.c:57
#define HIBERNATE_FACTOR
Definition: bgwriter.c:63
#define LOG_SNAPSHOT_INTERVAL_MS
Definition: bgwriter.c:69
void AtEOXact_Buffers(bool isCommit)
Definition: bufmgr.c:3559
bool BgBufferSync(WritebackContext *wb_context)
Definition: bufmgr.c:3188
void UnlockBuffers(void)
Definition: bufmgr.c:5130
int bgwriter_flush_after
Definition: bufmgr.c:172
void WritebackContextInit(WritebackContext *context, int *max_pending)
Definition: bufmgr.c:5903
#define Assert(condition)
Definition: c.h:812
bool FirstCallSinceLastCheckpoint(void)
bool ConditionVariableCancelSleep(void)
int64 TimestampTz
Definition: timestamp.h:39
void AtEOXact_HashTables(bool isCommit)
Definition: dynahash.c:1912
void EmitErrorReport(void)
Definition: elog.c:1687
ErrorContextCallback * error_context_stack
Definition: elog.c:94
void FlushErrorState(void)
Definition: elog.c:1867
sigjmp_buf * PG_exception_stack
Definition: elog.c:96
void AtEOXact_Files(bool isCommit)
Definition: fd.c:3187
void StrategyNotifyBgWriter(int bgwprocno)
Definition: freelist.c:431
ProcNumber MyProcNumber
Definition: globals.c:89
struct Latch * MyLatch
Definition: globals.c:62
void SignalHandlerForShutdownRequest(SIGNAL_ARGS)
Definition: interrupt.c:105
void SignalHandlerForConfigReload(SIGNAL_ARGS)
Definition: interrupt.c:61
void HandleMainLoopInterrupts(void)
Definition: interrupt.c:34
void ResetLatch(Latch *latch)
Definition: latch.c:724
int WaitLatch(Latch *latch, int wakeEvents, long timeout, uint32 wait_event_info)
Definition: latch.c:517
#define WL_TIMEOUT
Definition: latch.h:130
#define WL_EXIT_ON_PM_DEATH
Definition: latch.h:132
#define WL_LATCH_SET
Definition: latch.h:127
void LWLockReleaseAll(void)
Definition: lwlock.c:1876
void MemoryContextReset(MemoryContext context)
Definition: mcxt.c:383
MemoryContext TopMemoryContext
Definition: mcxt.c:149
#define AllocSetContextCreate
Definition: memutils.h:129
#define ALLOCSET_DEFAULT_SIZES
Definition: memutils.h:160
#define RESUME_INTERRUPTS()
Definition: miscadmin.h:135
#define HOLD_INTERRUPTS()
Definition: miscadmin.h:133
@ B_BG_WRITER
Definition: miscadmin.h:361
BackendType MyBackendType
Definition: miscinit.c:64
void pgstat_report_bgwriter(void)
void pgstat_report_wal(bool force)
Definition: pgstat_wal.c:48
pqsigfunc pqsignal(int signo, pqsigfunc func)
void procsignal_sigusr1_handler(SIGNAL_ARGS)
Definition: procsignal.c:671
MemoryContextSwitchTo(old_ctx)
void ReleaseAuxProcessResources(bool isCommit)
Definition: resowner.c:1002
void pg_usleep(long microsec)
Definition: signal.c:53
void smgrdestroyall(void)
Definition: smgr.c:335
void AtEOXact_SMgr(void)
Definition: smgr.c:835
XLogRecPtr LogStandbySnapshot(void)
Definition: standby.c:1281
#define TimestampTzPlusMilliseconds(tz, ms)
Definition: timestamp.h:85
static void pgstat_report_wait_end(void)
Definition: wait_event.h:101
#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:6334
XLogRecPtr GetLastImportantRecPtr(void)
Definition: xlog.c:6556
#define XLogStandbyInfoActive()
Definition: xlog.h:123
uint64 XLogRecPtr
Definition: xlogdefs.h:21
#define InvalidXLogRecPtr
Definition: xlogdefs.h:28