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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  * 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) 1996-2017, 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  */
140 
141  /* We allow SIGQUIT (quickdie) at all times */
142  sigdelset(&BlockSig, SIGQUIT);
143 
144  /*
145  * Create a resource owner to keep track of our resources (currently only
146  * buffer pins).
147  */
148  CurrentResourceOwner = ResourceOwnerCreate(NULL, "Background Writer");
149 
150  /*
151  * We just started, assume there has been either a shutdown or
152  * end-of-recovery snapshot.
153  */
155 
156  /*
157  * Create a memory context that we will do all our work in. We do this so
158  * that we can reset the context during error recovery and thereby avoid
159  * possible memory leaks. Formerly this code just ran in
160  * TopMemoryContext, but resetting that would be a really bad idea.
161  */
162  bgwriter_context = AllocSetContextCreate(TopMemoryContext,
163  "Background Writer",
165  MemoryContextSwitchTo(bgwriter_context);
166 
168 
169  /*
170  * If an exception is encountered, processing resumes here.
171  *
172  * See notes in postgres.c about the design of this coding.
173  */
174  if (sigsetjmp(local_sigjmp_buf, 1) != 0)
175  {
176  /* Since not using PG_TRY, must reset error stack by hand */
178 
179  /* Prevent interrupts while cleaning up */
180  HOLD_INTERRUPTS();
181 
182  /* Report the error to the server log */
183  EmitErrorReport();
184 
185  /*
186  * These operations are really just a minimal subset of
187  * AbortTransaction(). We don't have very many resources to worry
188  * about in bgwriter, but we do have LWLocks, buffers, and temp files.
189  */
192  AbortBufferIO();
193  UnlockBuffers();
194  /* buffer pins are released here: */
197  false, true);
198  /* we needn't bother with the other ResourceOwnerRelease phases */
199  AtEOXact_Buffers(false);
200  AtEOXact_SMgr();
201  AtEOXact_Files();
202  AtEOXact_HashTables(false);
203 
204  /*
205  * Now return to normal top-level context and clear ErrorContext for
206  * next time.
207  */
208  MemoryContextSwitchTo(bgwriter_context);
209  FlushErrorState();
210 
211  /* Flush any leaked data in the top-level context */
212  MemoryContextResetAndDeleteChildren(bgwriter_context);
213 
214  /* re-initialize to avoid repeated errors causing problems */
216 
217  /* Now we can allow interrupts again */
219 
220  /*
221  * Sleep at least 1 second after any error. A write error is likely
222  * to be repeated, and we don't want to be filling the error logs as
223  * fast as we can.
224  */
225  pg_usleep(1000000L);
226 
227  /*
228  * Close all open files after any error. This is helpful on Windows,
229  * where holding deleted files open causes various strange errors.
230  * It's not clear we need it elsewhere, but shouldn't hurt.
231  */
232  smgrcloseall();
233 
234  /* Report wait end here, when there is no further possibility of wait */
236  }
237 
238  /* We can now handle ereport(ERROR) */
239  PG_exception_stack = &local_sigjmp_buf;
240 
241  /*
242  * Unblock signals (they were blocked when the postmaster forked us)
243  */
245 
246  /*
247  * Reset hibernation state after any error.
248  */
249  prev_hibernate = false;
250 
251  /*
252  * Loop forever
253  */
254  for (;;)
255  {
256  bool can_hibernate;
257  int rc;
258 
259  /* Clear any already-pending wakeups */
261 
262  if (got_SIGHUP)
263  {
264  got_SIGHUP = false;
266  }
267  if (shutdown_requested)
268  {
269  /*
270  * From here on, elog(ERROR) should end with exit(1), not send
271  * control back to the sigsetjmp block above
272  */
273  ExitOnAnyError = true;
274  /* Normal exit from the bgwriter is here */
275  proc_exit(0); /* done */
276  }
277 
278  /*
279  * Do one cycle of dirty-buffer writing.
280  */
281  can_hibernate = BgBufferSync(&wb_context);
282 
283  /*
284  * Send off activity statistics to the stats collector
285  */
287 
289  {
290  /*
291  * After any checkpoint, close all smgr files. This is so we
292  * won't hang onto smgr references to deleted files indefinitely.
293  */
294  smgrcloseall();
295  }
296 
297  /*
298  * Log a new xl_running_xacts every now and then so replication can
299  * get into a consistent state faster (think of suboverflowed
300  * snapshots) and clean up resources (locks, KnownXids*) more
301  * frequently. The costs of this are relatively low, so doing it 4
302  * times (LOG_SNAPSHOT_INTERVAL_MS) a minute seems fine.
303  *
304  * We assume the interval for writing xl_running_xacts is
305  * significantly bigger than BgWriterDelay, so we don't complicate the
306  * overall timeout handling but just assume we're going to get called
307  * often enough even if hibernation mode is active. It's not that
308  * important that log_snap_interval_ms is met strictly. To make sure
309  * we're not waking the disk up unnecessarily on an idle system we
310  * check whether there has been any WAL inserted since the last time
311  * we've logged a running xacts.
312  *
313  * We do this logging in the bgwriter as it is the only process that is
314  * run regularly and returns to its mainloop all the time. E.g.
315  * Checkpointer, when active, is barely ever in its mainloop and thus
316  * makes it hard to log regularly.
317  */
319  {
320  TimestampTz timeout = 0;
322 
325 
326  /*
327  * Only log if enough time has passed and interesting records have
328  * been inserted since the last snapshot.
329  */
330  if (now >= timeout &&
332  {
335  }
336  }
337 
338  /*
339  * Sleep until we are signaled or BgWriterDelay has elapsed.
340  *
341  * Note: the feedback control loop in BgBufferSync() expects that we
342  * will call it every BgWriterDelay msec. While it's not critical for
343  * correctness that that be exact, the feedback loop might misbehave
344  * if we stray too far from that. Hence, avoid loading this process
345  * down with latch events that are likely to happen frequently during
346  * normal operation.
347  */
348  rc = WaitLatch(MyLatch,
351 
352  /*
353  * If no latch event and BgBufferSync says nothing's happening, extend
354  * the sleep in "hibernation" mode, where we sleep for much longer
355  * than bgwriter_delay says. Fewer wakeups save electricity. When a
356  * backend starts using buffers again, it will wake us up by setting
357  * our latch. Because the extra sleep will persist only as long as no
358  * buffer allocations happen, this should not distort the behavior of
359  * BgBufferSync's control loop too badly; essentially, it will think
360  * that the system-wide idle interval didn't exist.
361  *
362  * There is a race condition here, in that a backend might allocate a
363  * buffer between the time BgBufferSync saw the alloc count as zero
364  * and the time we call StrategyNotifyBgWriter. While it's not
365  * critical that we not hibernate anyway, we try to reduce the odds of
366  * that by only hibernating when BgBufferSync says nothing's happening
367  * for two consecutive cycles. Also, we mitigate any possible
368  * consequences of a missed wakeup by not hibernating forever.
369  */
370  if (rc == WL_TIMEOUT && can_hibernate && prev_hibernate)
371  {
372  /* Ask for notification at next buffer allocation */
374  /* Sleep ... */
375  rc = WaitLatch(MyLatch,
379  /* Reset the notification request in case we timed out */
381  }
382 
383  /*
384  * Emergency bailout if postmaster has died. This is to avoid the
385  * necessity for manual cleanup of all postmaster children.
386  */
387  if (rc & WL_POSTMASTER_DEATH)
388  exit(1);
389 
390  prev_hibernate = can_hibernate;
391  }
392 }
393 
394 
395 /* --------------------------------
396  * signal handler routines
397  * --------------------------------
398  */
399 
400 /*
401  * bg_quickdie() occurs when signalled SIGQUIT by the postmaster.
402  *
403  * Some backend has bought the farm,
404  * so we need to stop what we're doing and exit.
405  */
406 static void
408 {
410 
411  /*
412  * We DO NOT want to run proc_exit() callbacks -- we're here because
413  * shared memory may be corrupted, so we don't want to try to clean up our
414  * transaction. Just nail the windows shut and get out of town. Now that
415  * there's an atexit callback to prevent third-party code from breaking
416  * things by calling exit() directly, we have to reset the callbacks
417  * explicitly to make this work as intended.
418  */
419  on_exit_reset();
420 
421  /*
422  * Note we do exit(2) not exit(0). This is to force the postmaster into a
423  * system reset cycle if some idiot DBA sends a manual SIGQUIT to a random
424  * backend. This is necessary precisely because we don't clean up our
425  * shared memory state. (The "dead man switch" mechanism in pmsignal.c
426  * should ensure the postmaster sees this as a crash, too, but no harm in
427  * being doubly sure.)
428  */
429  exit(2);
430 }
431 
432 /* SIGHUP: set flag to re-read config file at next convenient time */
433 static void
435 {
436  int save_errno = errno;
437 
438  got_SIGHUP = true;
439  SetLatch(MyLatch);
440 
441  errno = save_errno;
442 }
443 
444 /* SIGTERM: set flag to shutdown and exit */
445 static void
447 {
448  int save_errno = errno;
449 
450  shutdown_requested = true;
451  SetLatch(MyLatch);
452 
453  errno = save_errno;
454 }
455 
456 /* SIGUSR1: used for latch wakeups */
457 static void
459 {
460  int save_errno = errno;
461 
463 
464  errno = save_errno;
465 }
XLogRecPtr GetLastImportantRecPtr(void)
Definition: xlog.c:8222
#define SIGUSR1
Definition: win32.h:211
#define InvalidXLogRecPtr
Definition: xlogdefs.h:28
static volatile sig_atomic_t got_SIGHUP
Definition: bgwriter.c:92
#define SIGCONT
Definition: win32.h:205
bool BgBufferSync(WritebackContext *wb_context)
Definition: bufmgr.c:2045
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:434
TimestampTz GetCurrentTimestamp(void)
Definition: timestamp.c:1569
PGPROC * MyProc
Definition: proc.c:67
int64 TimestampTz
Definition: timestamp.h:39
#define SIGWINCH
Definition: win32.h:209
void AtEOXact_Buffers(bool isCommit)
Definition: bufmgr.c:2415
ResourceOwner CurrentResourceOwner
Definition: resowner.c:138
#define SIGTTIN
Definition: win32.h:207
static MemoryContext MemoryContextSwitchTo(MemoryContext context)
Definition: palloc.h:109
void proc_exit(int code)
Definition: ipc.c:99
void ResetLatch(volatile Latch *latch)
Definition: latch.c:461
bool RecoveryInProgress(void)
Definition: xlog.c:7855
#define SIGQUIT
Definition: win32.h:197
void FlushErrorState(void)
Definition: elog.c:1587
#define PG_SETMASK(mask)
Definition: pqsignal.h:19
void WritebackContextInit(WritebackContext *context, int *max_pending)
Definition: bufmgr.c:4230
int bgwriter_flush_after
Definition: bufmgr.c:119
void smgrcloseall(void)
Definition: smgr.c:326
#define RESUME_INTERRUPTS()
Definition: miscadmin.h:116
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:798
int WaitLatch(volatile Latch *latch, int wakeEvents, long timeout, uint32 wait_event_info)
Definition: latch.c:300
#define SIG_IGN
Definition: win32.h:193
void ConditionVariableCancelSleep(void)
XLogRecPtr LogStandbySnapshot(void)
Definition: standby.c:909
void on_exit_reset(void)
Definition: ipc.c:396
void AtEOXact_Files(void)
Definition: fd.c:2580
#define ALLOCSET_DEFAULT_SIZES
Definition: memutils.h:165
#define LOG_SNAPSHOT_INTERVAL_MS
Definition: bgwriter.c:79
static void bg_quickdie(SIGNAL_ARGS)
Definition: bgwriter.c:407
sigset_t UnBlockSig
Definition: pqsignal.c:22
static void pgstat_report_wait_end(void)
Definition: pgstat.h:1203
MemoryContext TopMemoryContext
Definition: mcxt.c:43
Definition: guc.h:72
void UnlockBuffers(void)
Definition: bufmgr.c:3518
#define MemoryContextResetAndDeleteChildren(ctx)
Definition: memutils.h:67
bool ExitOnAnyError
Definition: globals.c:104
sigset_t BlockSig
Definition: pqsignal.c:22
#define WL_POSTMASTER_DEATH
Definition: latch.h:128
MemoryContext AllocSetContextCreate(MemoryContext parent, const char *name, Size minContextSize, Size initBlockSize, Size maxBlockSize)
Definition: aset.c:322
bool FirstCallSinceLastCheckpoint(void)
#define XLogStandbyInfoActive()
Definition: xlog.h:159
void EmitErrorReport(void)
Definition: elog.c:1446
void pgstat_send_bgwriter(void)
Definition: pgstat.c:3904
#define SIGPIPE
Definition: win32.h:201
#define SIGHUP
Definition: win32.h:196
#define TimestampTzPlusMilliseconds(tz, ms)
Definition: timestamp.h:56
#define SIG_DFL
Definition: win32.h:191
pqsigfunc pqsignal(int signum, pqsigfunc handler)
Definition: signal.c:168
void SetLatch(volatile Latch *latch)
Definition: latch.c:379
#define SIGNAL_ARGS
Definition: c.h:1079
#define NULL
Definition: c.h:229
uint64 XLogRecPtr
Definition: xlogdefs.h:21
void ResourceOwnerRelease(ResourceOwner owner, ResourceReleasePhase phase, bool isCommit, bool isTopLevel)
Definition: resowner.c:471
void AbortBufferIO(void)
Definition: bufmgr.c:3974
sigjmp_buf * PG_exception_stack
Definition: elog.c:90
void BackgroundWriterMain(void)
Definition: bgwriter.c:110
#define SIGTTOU
Definition: win32.h:208
int pgprocno
Definition: proc.h:104
void StrategyNotifyBgWriter(int bgwprocno)
Definition: freelist.c:415
#define HOLD_INTERRUPTS()
Definition: miscadmin.h:114
static TimestampTz last_snapshot_ts
Definition: bgwriter.c:86
struct Latch * MyLatch
Definition: globals.c:51
void LWLockReleaseAll(void)
Definition: lwlock.c:1814
static XLogRecPtr last_snapshot_lsn
Definition: bgwriter.c:87
void latch_sigusr1_handler(void)
Definition: latch.c:1572
void AtEOXact_HashTables(bool isCommit)
Definition: dynahash.c:1798
#define SIGCHLD
Definition: win32.h:206
#define WL_LATCH_SET
Definition: latch.h:124
#define SIGALRM
Definition: win32.h:202
Datum now(PG_FUNCTION_ARGS)
Definition: timestamp.c:1533
static void ReqShutdownHandler(SIGNAL_ARGS)
Definition: bgwriter.c:446
#define SIGUSR2
Definition: win32.h:212
static void bgwriter_sigusr1_handler(SIGNAL_ARGS)
Definition: bgwriter.c:458
ResourceOwner ResourceOwnerCreate(ResourceOwner parent, const char *name)
Definition: resowner.c:416