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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) 1996-2024, 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"
54#include "storage/bufmgr.h"
56#include "storage/fd.h"
57#include "storage/lwlock.h"
58#include "storage/proc.h"
59#include "storage/procsignal.h"
60#include "storage/smgr.h"
61#include "utils/hsearch.h"
62#include "utils/memutils.h"
63#include "utils/resowner.h"
64
65
66/*
67 * GUC parameters
68 */
71
72/*
73 * Number of do-nothing loops before lengthening the delay time, and the
74 * multiplier to apply to WalWriterDelay when we do decide to hibernate.
75 * (Perhaps these need to be configurable?)
76 */
77#define LOOPS_UNTIL_HIBERNATE 50
78#define HIBERNATE_FACTOR 25
79
80/*
81 * Main entry point for walwriter process
82 *
83 * This is invoked from AuxiliaryProcessMain, which has already created the
84 * basic execution environment, but not enabled signals yet.
85 */
86void
87WalWriterMain(char *startup_data, size_t startup_data_len)
88{
89 sigjmp_buf local_sigjmp_buf;
90 MemoryContext walwriter_context;
91 int left_till_hibernate;
92 bool hibernating;
93
94 Assert(startup_data_len == 0);
95
98
99 /*
100 * Properly accept or ignore signals the postmaster might send us
101 *
102 * We have no particular use for SIGINT at the moment, but seems
103 * reasonable to treat like SIGTERM.
104 */
108 /* SIGQUIT handler was already set up by InitPostmasterChild */
112 pqsignal(SIGUSR2, SIG_IGN); /* not used */
113
114 /*
115 * Reset some signals that are accepted by postmaster but not here
116 */
118
119 /*
120 * Create a memory context that we will do all our work in. We do this so
121 * that we can reset the context during error recovery and thereby avoid
122 * possible memory leaks. Formerly this code just ran in
123 * TopMemoryContext, but resetting that would be a really bad idea.
124 */
125 walwriter_context = AllocSetContextCreate(TopMemoryContext,
126 "Wal Writer",
128 MemoryContextSwitchTo(walwriter_context);
129
130 /*
131 * If an exception is encountered, processing resumes here.
132 *
133 * You might wonder why this isn't coded as an infinite loop around a
134 * PG_TRY construct. The reason is that this is the bottom of the
135 * exception stack, and so with PG_TRY there would be no exception handler
136 * in force at all during the CATCH part. By leaving the outermost setjmp
137 * always active, we have at least some chance of recovering from an error
138 * during error recovery. (If we get into an infinite loop thereby, it
139 * will soon be stopped by overflow of elog.c's internal state stack.)
140 *
141 * Note that we use sigsetjmp(..., 1), so that the prevailing signal mask
142 * (to wit, BlockSig) will be restored when longjmp'ing to here. Thus,
143 * signals other than SIGQUIT will be blocked until we complete error
144 * recovery. It might seem that this policy makes the HOLD_INTERRUPTS()
145 * call redundant, but it is not since InterruptPending might be set
146 * already.
147 */
148 if (sigsetjmp(local_sigjmp_buf, 1) != 0)
149 {
150 /* Since not using PG_TRY, must reset error stack by hand */
151 error_context_stack = NULL;
152
153 /* Prevent interrupts while cleaning up */
155
156 /* Report the error to the server log */
158
159 /*
160 * These operations are really just a minimal subset of
161 * AbortTransaction(). We don't have very many resources to worry
162 * about in walwriter, but we do have LWLocks, and perhaps buffers?
163 */
169 AtEOXact_Buffers(false);
171 AtEOXact_Files(false);
172 AtEOXact_HashTables(false);
173
174 /*
175 * Now return to normal top-level context and clear ErrorContext for
176 * next time.
177 */
178 MemoryContextSwitchTo(walwriter_context);
180
181 /* Flush any leaked data in the top-level context */
182 MemoryContextReset(walwriter_context);
183
184 /* Now we can allow interrupts again */
186
187 /*
188 * Sleep at least 1 second after any error. A write error is likely
189 * to be repeated, and we don't want to be filling the error logs as
190 * fast as we can.
191 */
192 pg_usleep(1000000L);
193 }
194
195 /* We can now handle ereport(ERROR) */
196 PG_exception_stack = &local_sigjmp_buf;
197
198 /*
199 * Unblock signals (they were blocked when the postmaster forked us)
200 */
201 sigprocmask(SIG_SETMASK, &UnBlockSig, NULL);
202
203 /*
204 * Reset hibernation state after any error.
205 */
206 left_till_hibernate = LOOPS_UNTIL_HIBERNATE;
207 hibernating = false;
209
210 /*
211 * Advertise our proc number that backends can use to wake us up while
212 * we're sleeping.
213 */
215
216 /*
217 * Loop forever
218 */
219 for (;;)
220 {
221 long cur_timeout;
222
223 /*
224 * Advertise whether we might hibernate in this cycle. We do this
225 * before resetting the latch to ensure that any async commits will
226 * see the flag set if they might possibly need to wake us up, and
227 * that we won't miss any signal they send us. (If we discover work
228 * to do in the last cycle before we would hibernate, the global flag
229 * will be set unnecessarily, but little harm is done.) But avoid
230 * touching the global flag if it doesn't need to change.
231 */
232 if (hibernating != (left_till_hibernate <= 1))
233 {
234 hibernating = (left_till_hibernate <= 1);
235 SetWalWriterSleeping(hibernating);
236 }
237
238 /* Clear any already-pending wakeups */
240
241 /* Process any signals received recently */
243
244 /*
245 * Do what we're here for; then, if XLogBackgroundFlush() found useful
246 * work to do, reset hibernation counter.
247 */
249 left_till_hibernate = LOOPS_UNTIL_HIBERNATE;
250 else if (left_till_hibernate > 0)
251 left_till_hibernate--;
252
253 /* report pending statistics to the cumulative stats system */
254 pgstat_report_wal(false);
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,
269 WAIT_EVENT_WAL_WRITER_MAIN);
270 }
271}
void AuxiliaryProcessMainCommon(void)
Definition: auxprocess.c:39
sigset_t UnBlockSig
Definition: pqsignal.c:22
void AtEOXact_Buffers(bool isCommit)
Definition: bufmgr.c:3559
void UnlockBuffers(void)
Definition: bufmgr.c:5130
#define Assert(condition)
Definition: c.h:812
bool ConditionVariableCancelSleep(void)
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
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_WAL_WRITER
Definition: miscadmin.h:366
BackendType MyBackendType
Definition: miscinit.c:64
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 AtEOXact_SMgr(void)
Definition: smgr.c:835
PROC_HDR * ProcGlobal
Definition: proc.c:78
ProcNumber walwriterProc
Definition: proc.h:420
static void pgstat_report_wait_end(void)
Definition: wait_event.h:101
int WalWriterFlushAfter
Definition: walwriter.c:70
#define HIBERNATE_FACTOR
Definition: walwriter.c:78
#define LOOPS_UNTIL_HIBERNATE
Definition: walwriter.c:77
void WalWriterMain(char *startup_data, size_t startup_data_len)
Definition: walwriter.c:87
int WalWriterDelay
Definition: walwriter.c:69
#define DEFAULT_WAL_WRITER_FLUSH_AFTER
Definition: walwriter.h:15
#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
void SetWalWriterSleeping(bool sleeping)
Definition: xlog.c:9507
bool XLogBackgroundFlush(void)
Definition: xlog.c:2990