PostgreSQL Source Code  git master
bgwriter.c File Reference
#include "postgres.h"
#include "access/xlog.h"
#include "access/xlog_internal.h"
#include "libpq/pqsignal.h"
#include "miscadmin.h"
#include "pgstat.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/ipc.h"
#include "storage/lwlock.h"
#include "storage/proc.h"
#include "storage/procsignal.h"
#include "storage/shmem.h"
#include "storage/smgr.h"
#include "storage/spin.h"
#include "storage/standby.h"
#include "utils/guc.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 (void)
 

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 70 of file bgwriter.c.

Referenced by BackgroundWriterMain().

◆ LOG_SNAPSHOT_INTERVAL_MS

#define LOG_SNAPSHOT_INTERVAL_MS   15000

Definition at line 76 of file bgwriter.c.

Referenced by BackgroundWriterMain().

Function Documentation

◆ BackgroundWriterMain()

void BackgroundWriterMain ( void  )

Definition at line 94 of file bgwriter.c.

References AbortBufferIO(), ALLOCSET_DEFAULT_SIZES, AllocSetContextCreate, AtEOXact_Buffers(), AtEOXact_Files(), AtEOXact_HashTables(), AtEOXact_SMgr(), 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(), MemoryContextResetAndDeleteChildren, MemoryContextSwitchTo(), MyLatch, MyProc, now(), PG_exception_stack, PG_SETMASK, pg_usleep(), PGPROC::pgprocno, pgstat_report_wait_end(), pgstat_send_bgwriter(), pqsignal(), procsignal_sigusr1_handler(), RecoveryInProgress(), ReleaseAuxProcessResources(), ResetLatch(), RESUME_INTERRUPTS, SIG_DFL, SIG_IGN, SIGALRM, SIGCHLD, SIGHUP, SignalHandlerForConfigReload(), SignalHandlerForShutdownRequest(), SIGPIPE, SIGUSR1, SIGUSR2, smgrcloseall(), StrategyNotifyBgWriter(), TimestampTzPlusMilliseconds, TopMemoryContext, UnBlockSig, UnlockBuffers(), WAIT_EVENT_BGWRITER_HIBERNATE, WAIT_EVENT_BGWRITER_MAIN, WaitLatch(), WL_EXIT_ON_PM_DEATH, WL_LATCH_SET, WL_TIMEOUT, WritebackContextInit(), and XLogStandbyInfoActive.

Referenced by AuxiliaryProcessMain().

95 {
96  sigjmp_buf local_sigjmp_buf;
97  MemoryContext bgwriter_context;
98  bool prev_hibernate;
99  WritebackContext wb_context;
100 
101  /*
102  * Properly accept or ignore signals that might be sent to us.
103  */
105  pqsignal(SIGINT, SIG_IGN);
107  /* SIGQUIT handler was already set up by InitPostmasterChild */
112 
113  /*
114  * Reset some signals that are accepted by postmaster but not here
115  */
117 
118  /*
119  * We just started, assume there has been either a shutdown or
120  * end-of-recovery snapshot.
121  */
123 
124  /*
125  * Create a memory context that we will do all our work in. We do this so
126  * that we can reset the context during error recovery and thereby avoid
127  * possible memory leaks. Formerly this code just ran in
128  * TopMemoryContext, but resetting that would be a really bad idea.
129  */
130  bgwriter_context = AllocSetContextCreate(TopMemoryContext,
131  "Background Writer",
133  MemoryContextSwitchTo(bgwriter_context);
134 
136 
137  /*
138  * If an exception is encountered, processing resumes here.
139  *
140  * You might wonder why this isn't coded as an infinite loop around a
141  * PG_TRY construct. The reason is that this is the bottom of the
142  * exception stack, and so with PG_TRY there would be no exception handler
143  * in force at all during the CATCH part. By leaving the outermost setjmp
144  * always active, we have at least some chance of recovering from an error
145  * during error recovery. (If we get into an infinite loop thereby, it
146  * will soon be stopped by overflow of elog.c's internal state stack.)
147  *
148  * Note that we use sigsetjmp(..., 1), so that the prevailing signal mask
149  * (to wit, BlockSig) will be restored when longjmp'ing to here. Thus,
150  * signals other than SIGQUIT will be blocked until we complete error
151  * recovery. It might seem that this policy makes the HOLD_INTERRUPTS()
152  * call redundant, but it is not since InterruptPending might be set
153  * already.
154  */
155  if (sigsetjmp(local_sigjmp_buf, 1) != 0)
156  {
157  /* Since not using PG_TRY, must reset error stack by hand */
158  error_context_stack = NULL;
159 
160  /* Prevent interrupts while cleaning up */
161  HOLD_INTERRUPTS();
162 
163  /* Report the error to the server log */
164  EmitErrorReport();
165 
166  /*
167  * These operations are really just a minimal subset of
168  * AbortTransaction(). We don't have very many resources to worry
169  * about in bgwriter, but we do have LWLocks, buffers, and temp files.
170  */
173  AbortBufferIO();
174  UnlockBuffers();
176  AtEOXact_Buffers(false);
177  AtEOXact_SMgr();
178  AtEOXact_Files(false);
179  AtEOXact_HashTables(false);
180 
181  /*
182  * Now return to normal top-level context and clear ErrorContext for
183  * next time.
184  */
185  MemoryContextSwitchTo(bgwriter_context);
186  FlushErrorState();
187 
188  /* Flush any leaked data in the top-level context */
189  MemoryContextResetAndDeleteChildren(bgwriter_context);
190 
191  /* re-initialize to avoid repeated errors causing problems */
193 
194  /* Now we can allow interrupts again */
196 
197  /*
198  * Sleep at least 1 second after any error. A write error is likely
199  * to be repeated, and we don't want to be filling the error logs as
200  * fast as we can.
201  */
202  pg_usleep(1000000L);
203 
204  /*
205  * Close all open files after any error. This is helpful on Windows,
206  * where holding deleted files open causes various strange errors.
207  * It's not clear we need it elsewhere, but shouldn't hurt.
208  */
209  smgrcloseall();
210 
211  /* Report wait end here, when there is no further possibility of wait */
213  }
214 
215  /* We can now handle ereport(ERROR) */
216  PG_exception_stack = &local_sigjmp_buf;
217 
218  /*
219  * Unblock signals (they were blocked when the postmaster forked us)
220  */
222 
223  /*
224  * Reset hibernation state after any error.
225  */
226  prev_hibernate = false;
227 
228  /*
229  * Loop forever
230  */
231  for (;;)
232  {
233  bool can_hibernate;
234  int rc;
235 
236  /* Clear any already-pending wakeups */
238 
240 
241  /*
242  * Do one cycle of dirty-buffer writing.
243  */
244  can_hibernate = BgBufferSync(&wb_context);
245 
246  /*
247  * Send off activity statistics to the stats collector
248  */
250 
252  {
253  /*
254  * After any checkpoint, close all smgr files. This is so we
255  * won't hang onto smgr references to deleted files indefinitely.
256  */
257  smgrcloseall();
258  }
259 
260  /*
261  * Log a new xl_running_xacts every now and then so replication can
262  * get into a consistent state faster (think of suboverflowed
263  * snapshots) and clean up resources (locks, KnownXids*) more
264  * frequently. The costs of this are relatively low, so doing it 4
265  * times (LOG_SNAPSHOT_INTERVAL_MS) a minute seems fine.
266  *
267  * We assume the interval for writing xl_running_xacts is
268  * significantly bigger than BgWriterDelay, so we don't complicate the
269  * overall timeout handling but just assume we're going to get called
270  * often enough even if hibernation mode is active. It's not that
271  * important that LOG_SNAPSHOT_INTERVAL_MS is met strictly. To make
272  * sure we're not waking the disk up unnecessarily on an idle system
273  * we check whether there has been any WAL inserted since the last
274  * time we've logged a running xacts.
275  *
276  * We do this logging in the bgwriter as it is the only process that
277  * is run regularly and returns to its mainloop all the time. E.g.
278  * Checkpointer, when active, is barely ever in its mainloop and thus
279  * makes it hard to log regularly.
280  */
282  {
283  TimestampTz timeout = 0;
285 
288 
289  /*
290  * Only log if enough time has passed and interesting records have
291  * been inserted since the last snapshot. Have to compare with <=
292  * instead of < because GetLastImportantRecPtr() points at the
293  * start of a record, whereas last_snapshot_lsn points just past
294  * the end of the record.
295  */
296  if (now >= timeout &&
298  {
301  }
302  }
303 
304  /*
305  * Sleep until we are signaled or BgWriterDelay has elapsed.
306  *
307  * Note: the feedback control loop in BgBufferSync() expects that we
308  * will call it every BgWriterDelay msec. While it's not critical for
309  * correctness that that be exact, the feedback loop might misbehave
310  * if we stray too far from that. Hence, avoid loading this process
311  * down with latch events that are likely to happen frequently during
312  * normal operation.
313  */
314  rc = WaitLatch(MyLatch,
317 
318  /*
319  * If no latch event and BgBufferSync says nothing's happening, extend
320  * the sleep in "hibernation" mode, where we sleep for much longer
321  * than bgwriter_delay says. Fewer wakeups save electricity. When a
322  * backend starts using buffers again, it will wake us up by setting
323  * our latch. Because the extra sleep will persist only as long as no
324  * buffer allocations happen, this should not distort the behavior of
325  * BgBufferSync's control loop too badly; essentially, it will think
326  * that the system-wide idle interval didn't exist.
327  *
328  * There is a race condition here, in that a backend might allocate a
329  * buffer between the time BgBufferSync saw the alloc count as zero
330  * and the time we call StrategyNotifyBgWriter. While it's not
331  * critical that we not hibernate anyway, we try to reduce the odds of
332  * that by only hibernating when BgBufferSync says nothing's happening
333  * for two consecutive cycles. Also, we mitigate any possible
334  * consequences of a missed wakeup by not hibernating forever.
335  */
336  if (rc == WL_TIMEOUT && can_hibernate && prev_hibernate)
337  {
338  /* Ask for notification at next buffer allocation */
340  /* Sleep ... */
341  (void) WaitLatch(MyLatch,
345  /* Reset the notification request in case we timed out */
347  }
348 
349  prev_hibernate = can_hibernate;
350  }
351 }
XLogRecPtr GetLastImportantRecPtr(void)
Definition: xlog.c:8446
#define AllocSetContextCreate
Definition: memutils.h:170
bool BgBufferSync(WritebackContext *wb_context)
Definition: bufmgr.c:2110
int BgWriterDelay
Definition: bgwriter.c:64
#define WL_TIMEOUT
Definition: latch.h:127
void HandleMainLoopInterrupts(void)
Definition: interrupt.c:33
#define SIGUSR1
Definition: win32_port.h:171
TimestampTz GetCurrentTimestamp(void)
Definition: timestamp.c:1578
#define SIGCHLD
Definition: win32_port.h:169
PGPROC * MyProc
Definition: proc.c:67
int64 TimestampTz
Definition: timestamp.h:39
void AtEOXact_Buffers(bool isCommit)
Definition: bufmgr.c:2480
void SignalHandlerForConfigReload(SIGNAL_ARGS)
Definition: interrupt.c:56
void ReleaseAuxProcessResources(bool isCommit)
Definition: resowner.c:879
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:8076
void FlushErrorState(void)
Definition: elog.c:1565
void ResetLatch(Latch *latch)
Definition: latch.c:588
#define PG_SETMASK(mask)
Definition: pqsignal.h:19
void AtEOXact_Files(bool isCommit)
Definition: fd.c:2904
void WritebackContextInit(WritebackContext *context, int *max_pending)
Definition: bufmgr.c:4460
int WaitLatch(Latch *latch, int wakeEvents, long timeout, uint32 wait_event_info)
Definition: latch.c:390
int bgwriter_flush_after
Definition: bufmgr.c:149
void smgrcloseall(void)
Definition: smgr.c:287
#define RESUME_INTERRUPTS()
Definition: miscadmin.h:118
ErrorContextCallback * error_context_stack
Definition: elog.c:92
#define HIBERNATE_FACTOR
Definition: bgwriter.c:70
void pg_usleep(long microsec)
Definition: signal.c:53
void AtEOXact_SMgr(void)
Definition: smgr.c:661
void ConditionVariableCancelSleep(void)
XLogRecPtr LogStandbySnapshot(void)
Definition: standby.c:923
#define ALLOCSET_DEFAULT_SIZES
Definition: memutils.h:192
#define LOG_SNAPSHOT_INTERVAL_MS
Definition: bgwriter.c:76
#define SIGHUP
Definition: win32_port.h:159
sigset_t UnBlockSig
Definition: pqsignal.c:22
static void pgstat_report_wait_end(void)
Definition: pgstat.h:1466
MemoryContext TopMemoryContext
Definition: mcxt.c:44
void SignalHandlerForShutdownRequest(SIGNAL_ARGS)
Definition: interrupt.c:104
#define SIG_IGN
Definition: win32_port.h:156
void UnlockBuffers(void)
Definition: bufmgr.c:3724
#define MemoryContextResetAndDeleteChildren(ctx)
Definition: memutils.h:67
bool FirstCallSinceLastCheckpoint(void)
#define XLogStandbyInfoActive()
Definition: xlog.h:205
void EmitErrorReport(void)
Definition: elog.c:1420
void pgstat_send_bgwriter(void)
Definition: pgstat.c:4543
#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
#define SIGALRM
Definition: win32_port.h:165
void AbortBufferIO(void)
Definition: bufmgr.c:4202
sigjmp_buf * PG_exception_stack
Definition: elog.c:94
int pgprocno
Definition: proc.h:141
void StrategyNotifyBgWriter(int bgwprocno)
Definition: freelist.c:432
#define HOLD_INTERRUPTS()
Definition: miscadmin.h:116
static TimestampTz last_snapshot_ts
Definition: bgwriter.c:83
struct Latch * MyLatch
Definition: globals.c:54
void LWLockReleaseAll(void)
Definition: lwlock.c:1911
static XLogRecPtr last_snapshot_lsn
Definition: bgwriter.c:84
void procsignal_sigusr1_handler(SIGNAL_ARGS)
Definition: procsignal.c:551
void AtEOXact_HashTables(bool isCommit)
Definition: dynahash.c:1848
#define WL_LATCH_SET
Definition: latch.h:124
Datum now(PG_FUNCTION_ARGS)
Definition: timestamp.c:1542
#define WL_EXIT_ON_PM_DEATH
Definition: latch.h:129

Variable Documentation

◆ BgWriterDelay

int BgWriterDelay = 200

Definition at line 64 of file bgwriter.c.

Referenced by BackgroundWriterMain(), and BgBufferSync().

◆ last_snapshot_lsn

XLogRecPtr last_snapshot_lsn = InvalidXLogRecPtr
static

Definition at line 84 of file bgwriter.c.

Referenced by BackgroundWriterMain().

◆ last_snapshot_ts

TimestampTz last_snapshot_ts
static

Definition at line 83 of file bgwriter.c.

Referenced by BackgroundWriterMain().