PostgreSQL Source Code  git master
bgwriter.c File Reference
#include "postgres.h"
#include "access/xlog.h"
#include "libpq/pqsignal.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "postmaster/auxprocess.h"
#include "postmaster/bgwriter.h"
#include "postmaster/interrupt.h"
#include "storage/buf_internals.h"
#include "storage/bufmgr.h"
#include "storage/condition_variable.h"
#include "storage/fd.h"
#include "storage/lwlock.h"
#include "storage/proc.h"
#include "storage/procsignal.h"
#include "storage/smgr.h"
#include "storage/standby.h"
#include "utils/memutils.h"
#include "utils/resowner.h"
#include "utils/timestamp.h"
Include dependency graph for bgwriter.c:

Go to the source code of this file.

Macros

#define HIBERNATE_FACTOR   50
 
#define LOG_SNAPSHOT_INTERVAL_MS   15000
 

Functions

void BackgroundWriterMain (char *startup_data, size_t startup_data_len)
 

Variables

int BgWriterDelay = 200
 
static TimestampTz last_snapshot_ts
 
static XLogRecPtr last_snapshot_lsn = InvalidXLogRecPtr
 

Macro Definition Documentation

◆ HIBERNATE_FACTOR

#define HIBERNATE_FACTOR   50

Definition at line 63 of file bgwriter.c.

◆ LOG_SNAPSHOT_INTERVAL_MS

#define LOG_SNAPSHOT_INTERVAL_MS   15000

Definition at line 69 of file bgwriter.c.

Function Documentation

◆ BackgroundWriterMain()

void BackgroundWriterMain ( char *  startup_data,
size_t  startup_data_len 
)

Definition at line 87 of file bgwriter.c.

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 */
159  HOLD_INTERRUPTS();
160 
161  /* Report the error to the server log */
162  EmitErrorReport();
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  */
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  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  */
248  smgrdestroyall();
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:44
sigset_t UnBlockSig
Definition: pqsignal.c:22
TimestampTz GetCurrentTimestamp(void)
Definition: timestamp.c:1654
Datum now(PG_FUNCTION_ARGS)
Definition: timestamp.c:1618
static XLogRecPtr last_snapshot_lsn
Definition: bgwriter.c:77
static TimestampTz last_snapshot_ts
Definition: bgwriter.c:76
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:3502
bool BgBufferSync(WritebackContext *wb_context)
Definition: bufmgr.c:3131
void UnlockBuffers(void)
Definition: bufmgr.c:5057
int bgwriter_flush_after
Definition: bufmgr.c:171
void WritebackContextInit(WritebackContext *context, int *max_pending)
Definition: bufmgr.c:5830
#define Assert(condition)
Definition: c.h:858
bool FirstCallSinceLastCheckpoint(void)
bool ConditionVariableCancelSleep(void)
int64 TimestampTz
Definition: timestamp.h:39
void AtEOXact_HashTables(bool isCommit)
Definition: dynahash.c:1869
void EmitErrorReport(void)
Definition: elog.c:1670
ErrorContextCallback * error_context_stack
Definition: elog.c:94
void FlushErrorState(void)
Definition: elog.c:1834
sigjmp_buf * PG_exception_stack
Definition: elog.c:96
void AtEOXact_Files(bool isCommit)
Definition: fd.c:3165
void StrategyNotifyBgWriter(int bgwprocno)
Definition: freelist.c:431
ProcNumber MyProcNumber
Definition: globals.c:87
struct Latch * MyLatch
Definition: globals.c:60
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:1878
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:356
BackendType MyBackendType
Definition: miscinit.c:63
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:635
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:332
void AtEOXact_SMgr(void)
Definition: smgr.c:807
XLogRecPtr LogStandbySnapshot(void)
Definition: standby.c:1285
#define TimestampTzPlusMilliseconds(tz, ms)
Definition: timestamp.h:85
static void pgstat_report_wait_end(void)
Definition: wait_event.h:98
#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:6290
XLogRecPtr GetLastImportantRecPtr(void)
Definition: xlog.c:6493
#define XLogStandbyInfoActive()
Definition: xlog.h:121

References ALLOCSET_DEFAULT_SIZES, AllocSetContextCreate, Assert, AtEOXact_Buffers(), AtEOXact_Files(), AtEOXact_HashTables(), AtEOXact_SMgr(), AuxiliaryProcessMainCommon(), B_BG_WRITER, BgBufferSync(), bgwriter_flush_after, BgWriterDelay, ConditionVariableCancelSleep(), EmitErrorReport(), error_context_stack, FirstCallSinceLastCheckpoint(), FlushErrorState(), GetCurrentTimestamp(), GetLastImportantRecPtr(), HandleMainLoopInterrupts(), HIBERNATE_FACTOR, HOLD_INTERRUPTS, last_snapshot_lsn, last_snapshot_ts, LOG_SNAPSHOT_INTERVAL_MS, LogStandbySnapshot(), LWLockReleaseAll(), MemoryContextReset(), MemoryContextSwitchTo(), MyBackendType, MyLatch, MyProcNumber, now(), PG_exception_stack, pg_usleep(), pgstat_report_bgwriter(), pgstat_report_wait_end(), pgstat_report_wal(), pqsignal(), procsignal_sigusr1_handler(), RecoveryInProgress(), ReleaseAuxProcessResources(), ResetLatch(), RESUME_INTERRUPTS, SIG_DFL, SIG_IGN, SIGALRM, SIGCHLD, SIGHUP, SignalHandlerForConfigReload(), SignalHandlerForShutdownRequest(), SIGPIPE, SIGUSR1, SIGUSR2, smgrdestroyall(), StrategyNotifyBgWriter(), TimestampTzPlusMilliseconds, TopMemoryContext, UnBlockSig, UnlockBuffers(), WaitLatch(), WL_EXIT_ON_PM_DEATH, WL_LATCH_SET, WL_TIMEOUT, WritebackContextInit(), and XLogStandbyInfoActive.

Variable Documentation

◆ BgWriterDelay

int BgWriterDelay = 200

Definition at line 57 of file bgwriter.c.

Referenced by BackgroundWriterMain(), and BgBufferSync().

◆ last_snapshot_lsn

XLogRecPtr last_snapshot_lsn = InvalidXLogRecPtr
static

Definition at line 77 of file bgwriter.c.

Referenced by BackgroundWriterMain().

◆ last_snapshot_ts

TimestampTz last_snapshot_ts
static

Definition at line 76 of file bgwriter.c.

Referenced by BackgroundWriterMain().