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walwriter.c
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1 /*-------------------------------------------------------------------------
2  *
3  * walwriter.c
4  *
5  * The WAL writer background process is new as of Postgres 8.3. It attempts
6  * to keep regular backends from having to write out (and fsync) WAL pages.
7  * Also, it guarantees that transaction commit records that weren't synced
8  * to disk immediately upon commit (ie, were "asynchronously committed")
9  * will reach disk within a knowable time --- which, as it happens, is at
10  * most three times the wal_writer_delay cycle time.
11  *
12  * Note that as with the bgwriter for shared buffers, regular backends are
13  * still empowered to issue WAL writes and fsyncs when the walwriter doesn't
14  * keep up. This means that the WALWriter is not an essential process and
15  * can shutdown quickly when requested.
16  *
17  * Because the walwriter's cycle is directly linked to the maximum delay
18  * before async-commit transactions are guaranteed committed, it's probably
19  * unwise to load additional functionality onto it. For instance, if you've
20  * got a yen to create xlog segments further in advance, that'd be better done
21  * in bgwriter than in walwriter.
22  *
23  * The walwriter is started by the postmaster as soon as the startup subprocess
24  * finishes. It remains alive until the postmaster commands it to terminate.
25  * Normal termination is by SIGTERM, which instructs the walwriter to exit(0).
26  * Emergency termination is by SIGQUIT; like any backend, the walwriter will
27  * simply abort and exit on SIGQUIT.
28  *
29  * If the walwriter exits unexpectedly, the postmaster treats that the same
30  * as a backend crash: shared memory may be corrupted, so remaining backends
31  * should be killed by SIGQUIT and then a recovery cycle started.
32  *
33  *
34  * Portions Copyright (c) 1996-2020, PostgreSQL Global Development Group
35  *
36  *
37  * IDENTIFICATION
38  * src/backend/postmaster/walwriter.c
39  *
40  *-------------------------------------------------------------------------
41  */
42 #include "postgres.h"
43 
44 #include <signal.h>
45 #include <unistd.h>
46 
47 #include "access/xlog.h"
48 #include "libpq/pqsignal.h"
49 #include "miscadmin.h"
50 #include "pgstat.h"
51 #include "postmaster/interrupt.h"
52 #include "postmaster/walwriter.h"
53 #include "storage/bufmgr.h"
55 #include "storage/fd.h"
56 #include "storage/ipc.h"
57 #include "storage/lwlock.h"
58 #include "storage/proc.h"
59 #include "storage/procsignal.h"
60 #include "storage/smgr.h"
61 #include "utils/guc.h"
62 #include "utils/hsearch.h"
63 #include "utils/memutils.h"
64 #include "utils/resowner.h"
65 
66 
67 /*
68  * GUC parameters
69  */
70 int WalWriterDelay = 200;
72 
73 /*
74  * Number of do-nothing loops before lengthening the delay time, and the
75  * multiplier to apply to WalWriterDelay when we do decide to hibernate.
76  * (Perhaps these need to be configurable?)
77  */
78 #define LOOPS_UNTIL_HIBERNATE 50
79 #define HIBERNATE_FACTOR 25
80 
81 /*
82  * Main entry point for walwriter 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
89 {
90  sigjmp_buf local_sigjmp_buf;
91  MemoryContext walwriter_context;
92  int left_till_hibernate;
93  bool hibernating;
94 
95  /*
96  * Properly accept or ignore signals the postmaster might send us
97  *
98  * We have no particular use for SIGINT at the moment, but seems
99  * reasonable to treat like SIGTERM.
100  */
104  /* SIGQUIT handler was already set up by InitPostmasterChild */
108  pqsignal(SIGUSR2, SIG_IGN); /* not used */
109 
110  /*
111  * Reset some signals that are accepted by postmaster but not here
112  */
114 
115  /*
116  * Create a memory context that we will do all our work in. We do this so
117  * that we can reset the context during error recovery and thereby avoid
118  * possible memory leaks. Formerly this code just ran in
119  * TopMemoryContext, but resetting that would be a really bad idea.
120  */
121  walwriter_context = AllocSetContextCreate(TopMemoryContext,
122  "Wal Writer",
124  MemoryContextSwitchTo(walwriter_context);
125 
126  /*
127  * If an exception is encountered, processing resumes here.
128  *
129  * You might wonder why this isn't coded as an infinite loop around a
130  * PG_TRY construct. The reason is that this is the bottom of the
131  * exception stack, and so with PG_TRY there would be no exception handler
132  * in force at all during the CATCH part. By leaving the outermost setjmp
133  * always active, we have at least some chance of recovering from an error
134  * during error recovery. (If we get into an infinite loop thereby, it
135  * will soon be stopped by overflow of elog.c's internal state stack.)
136  *
137  * Note that we use sigsetjmp(..., 1), so that the prevailing signal mask
138  * (to wit, BlockSig) will be restored when longjmp'ing to here. Thus,
139  * signals other than SIGQUIT will be blocked until we complete error
140  * recovery. It might seem that this policy makes the HOLD_INTERRUPTS()
141  * call redundant, but it is not since InterruptPending might be set
142  * already.
143  */
144  if (sigsetjmp(local_sigjmp_buf, 1) != 0)
145  {
146  /* Since not using PG_TRY, must reset error stack by hand */
147  error_context_stack = NULL;
148 
149  /* Prevent interrupts while cleaning up */
150  HOLD_INTERRUPTS();
151 
152  /* Report the error to the server log */
153  EmitErrorReport();
154 
155  /*
156  * These operations are really just a minimal subset of
157  * AbortTransaction(). We don't have very many resources to worry
158  * about in walwriter, but we do have LWLocks, and perhaps buffers?
159  */
163  AbortBufferIO();
164  UnlockBuffers();
166  AtEOXact_Buffers(false);
167  AtEOXact_SMgr();
168  AtEOXact_Files(false);
169  AtEOXact_HashTables(false);
170 
171  /*
172  * Now return to normal top-level context and clear ErrorContext for
173  * next time.
174  */
175  MemoryContextSwitchTo(walwriter_context);
176  FlushErrorState();
177 
178  /* Flush any leaked data in the top-level context */
179  MemoryContextResetAndDeleteChildren(walwriter_context);
180 
181  /* Now we can allow interrupts again */
183 
184  /*
185  * Sleep at least 1 second after any error. A write error is likely
186  * to be repeated, and we don't want to be filling the error logs as
187  * fast as we can.
188  */
189  pg_usleep(1000000L);
190 
191  /*
192  * Close all open files after any error. This is helpful on Windows,
193  * where holding deleted files open causes various strange errors.
194  * It's not clear we need it elsewhere, but shouldn't hurt.
195  */
196  smgrcloseall();
197  }
198 
199  /* We can now handle ereport(ERROR) */
200  PG_exception_stack = &local_sigjmp_buf;
201 
202  /*
203  * Unblock signals (they were blocked when the postmaster forked us)
204  */
206 
207  /*
208  * Reset hibernation state after any error.
209  */
210  left_till_hibernate = LOOPS_UNTIL_HIBERNATE;
211  hibernating = false;
212  SetWalWriterSleeping(false);
213 
214  /*
215  * Advertise our latch that backends can use to wake us up while we're
216  * sleeping.
217  */
219 
220  /*
221  * Loop forever
222  */
223  for (;;)
224  {
225  long cur_timeout;
226 
227  /*
228  * Advertise whether we might hibernate in this cycle. We do this
229  * before resetting the latch to ensure that any async commits will
230  * see the flag set if they might possibly need to wake us up, and
231  * that we won't miss any signal they send us. (If we discover work
232  * to do in the last cycle before we would hibernate, the global flag
233  * will be set unnecessarily, but little harm is done.) But avoid
234  * touching the global flag if it doesn't need to change.
235  */
236  if (hibernating != (left_till_hibernate <= 1))
237  {
238  hibernating = (left_till_hibernate <= 1);
239  SetWalWriterSleeping(hibernating);
240  }
241 
242  /* Clear any already-pending wakeups */
244 
246 
247  /*
248  * Do what we're here for; then, if XLogBackgroundFlush() found useful
249  * work to do, reset hibernation counter.
250  */
251  if (XLogBackgroundFlush())
252  left_till_hibernate = LOOPS_UNTIL_HIBERNATE;
253  else if (left_till_hibernate > 0)
254  left_till_hibernate--;
255 
256  /*
257  * Sleep until we are signaled or WalWriterDelay has elapsed. If we
258  * haven't done anything useful for quite some time, lengthen the
259  * sleep time so as to reduce the server's idle power consumption.
260  */
261  if (left_till_hibernate > 0)
262  cur_timeout = WalWriterDelay; /* in ms */
263  else
264  cur_timeout = WalWriterDelay * HIBERNATE_FACTOR;
265 
266  (void) WaitLatch(MyLatch,
268  cur_timeout,
270  }
271 }
#define AllocSetContextCreate
Definition: memutils.h:170
void WalWriterMain(void)
Definition: walwriter.c:88
#define WL_TIMEOUT
Definition: latch.h:127
void HandleMainLoopInterrupts(void)
Definition: interrupt.c:33
#define SIGUSR1
Definition: win32_port.h:171
#define SIGCHLD
Definition: win32_port.h:169
PGPROC * MyProc
Definition: proc.c:67
void AtEOXact_Buffers(bool isCommit)
Definition: bufmgr.c:2478
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 LOOPS_UNTIL_HIBERNATE
Definition: walwriter.c:78
PROC_HDR * ProcGlobal
Definition: proc.c:79
#define SIGPIPE
Definition: win32_port.h:164
#define SIGUSR2
Definition: win32_port.h:172
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
Latch procLatch
Definition: proc.h:121
int WaitLatch(Latch *latch, int wakeEvents, long timeout, uint32 wait_event_info)
Definition: latch.c:390
void smgrcloseall(void)
Definition: smgr.c:287
#define RESUME_INTERRUPTS()
Definition: miscadmin.h:118
ErrorContextCallback * error_context_stack
Definition: elog.c:92
void pg_usleep(long microsec)
Definition: signal.c:53
void AtEOXact_SMgr(void)
Definition: smgr.c:661
Latch * walwriterLatch
Definition: proc.h:338
void ConditionVariableCancelSleep(void)
#define ALLOCSET_DEFAULT_SIZES
Definition: memutils.h:192
bool XLogBackgroundFlush(void)
Definition: xlog.c:3036
#define SIGHUP
Definition: win32_port.h:159
sigset_t UnBlockSig
Definition: pqsignal.c:22
static void pgstat_report_wait_end(void)
Definition: pgstat.h:1460
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:3722
#define MemoryContextResetAndDeleteChildren(ctx)
Definition: memutils.h:67
void EmitErrorReport(void)
Definition: elog.c:1420
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:4200
void SetWalWriterSleeping(bool sleeping)
Definition: xlog.c:12684
int WalWriterDelay
Definition: walwriter.c:70
int WalWriterFlushAfter
Definition: walwriter.c:71
sigjmp_buf * PG_exception_stack
Definition: elog.c:94
#define HOLD_INTERRUPTS()
Definition: miscadmin.h:116
struct Latch * MyLatch
Definition: globals.c:54
void LWLockReleaseAll(void)
Definition: lwlock.c:1911
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
#define WL_EXIT_ON_PM_DEATH
Definition: latch.h:129
#define HIBERNATE_FACTOR
Definition: walwriter.c:79