Line data Source code
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) 1996-2025, 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/auxprocess.h"
52 : #include "postmaster/interrupt.h"
53 : #include "postmaster/walwriter.h"
54 : #include "storage/aio_subsys.h"
55 : #include "storage/bufmgr.h"
56 : #include "storage/condition_variable.h"
57 : #include "storage/fd.h"
58 : #include "storage/lwlock.h"
59 : #include "storage/proc.h"
60 : #include "storage/procsignal.h"
61 : #include "storage/smgr.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;
71 : int WalWriterFlushAfter = DEFAULT_WAL_WRITER_FLUSH_AFTER;
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
88 896 : WalWriterMain(const void *startup_data, size_t startup_data_len)
89 : {
90 : sigjmp_buf local_sigjmp_buf;
91 : MemoryContext walwriter_context;
92 : int left_till_hibernate;
93 : bool hibernating;
94 :
95 : Assert(startup_data_len == 0);
96 :
97 896 : MyBackendType = B_WAL_WRITER;
98 896 : AuxiliaryProcessMainCommon();
99 :
100 : /*
101 : * Properly accept or ignore signals the postmaster might send us
102 : *
103 : * We have no particular use for SIGINT at the moment, but seems
104 : * reasonable to treat like SIGTERM.
105 : */
106 896 : pqsignal(SIGHUP, SignalHandlerForConfigReload);
107 896 : pqsignal(SIGINT, SignalHandlerForShutdownRequest);
108 896 : pqsignal(SIGTERM, SignalHandlerForShutdownRequest);
109 : /* SIGQUIT handler was already set up by InitPostmasterChild */
110 896 : pqsignal(SIGALRM, SIG_IGN);
111 896 : pqsignal(SIGPIPE, SIG_IGN);
112 896 : pqsignal(SIGUSR1, procsignal_sigusr1_handler);
113 896 : pqsignal(SIGUSR2, SIG_IGN); /* not used */
114 :
115 : /*
116 : * Reset some signals that are accepted by postmaster but not here
117 : */
118 896 : pqsignal(SIGCHLD, SIG_DFL);
119 :
120 : /*
121 : * Create a memory context that we will do all our work in. We do this so
122 : * that we can reset the context during error recovery and thereby avoid
123 : * possible memory leaks. Formerly this code just ran in
124 : * TopMemoryContext, but resetting that would be a really bad idea.
125 : */
126 896 : walwriter_context = AllocSetContextCreate(TopMemoryContext,
127 : "Wal Writer",
128 : ALLOCSET_DEFAULT_SIZES);
129 896 : MemoryContextSwitchTo(walwriter_context);
130 :
131 : /*
132 : * If an exception is encountered, processing resumes here.
133 : *
134 : * You might wonder why this isn't coded as an infinite loop around a
135 : * PG_TRY construct. The reason is that this is the bottom of the
136 : * exception stack, and so with PG_TRY there would be no exception handler
137 : * in force at all during the CATCH part. By leaving the outermost setjmp
138 : * always active, we have at least some chance of recovering from an error
139 : * during error recovery. (If we get into an infinite loop thereby, it
140 : * will soon be stopped by overflow of elog.c's internal state stack.)
141 : *
142 : * Note that we use sigsetjmp(..., 1), so that the prevailing signal mask
143 : * (to wit, BlockSig) will be restored when longjmp'ing to here. Thus,
144 : * signals other than SIGQUIT will be blocked until we complete error
145 : * recovery. It might seem that this policy makes the HOLD_INTERRUPTS()
146 : * call redundant, but it is not since InterruptPending might be set
147 : * already.
148 : */
149 896 : if (sigsetjmp(local_sigjmp_buf, 1) != 0)
150 : {
151 : /* Since not using PG_TRY, must reset error stack by hand */
152 0 : error_context_stack = NULL;
153 :
154 : /* Prevent interrupts while cleaning up */
155 0 : HOLD_INTERRUPTS();
156 :
157 : /* Report the error to the server log */
158 0 : EmitErrorReport();
159 :
160 : /*
161 : * These operations are really just a minimal subset of
162 : * AbortTransaction(). We don't have very many resources to worry
163 : * about in walwriter, but we do have LWLocks, and perhaps buffers?
164 : */
165 0 : LWLockReleaseAll();
166 0 : ConditionVariableCancelSleep();
167 0 : pgstat_report_wait_end();
168 0 : pgaio_error_cleanup();
169 0 : UnlockBuffers();
170 0 : ReleaseAuxProcessResources(false);
171 0 : AtEOXact_Buffers(false);
172 0 : AtEOXact_SMgr();
173 0 : AtEOXact_Files(false);
174 0 : AtEOXact_HashTables(false);
175 :
176 : /*
177 : * Now return to normal top-level context and clear ErrorContext for
178 : * next time.
179 : */
180 0 : MemoryContextSwitchTo(walwriter_context);
181 0 : FlushErrorState();
182 :
183 : /* Flush any leaked data in the top-level context */
184 0 : MemoryContextReset(walwriter_context);
185 :
186 : /* Now we can allow interrupts again */
187 0 : RESUME_INTERRUPTS();
188 :
189 : /*
190 : * Sleep at least 1 second after any error. A write error is likely
191 : * to be repeated, and we don't want to be filling the error logs as
192 : * fast as we can.
193 : */
194 0 : pg_usleep(1000000L);
195 : }
196 :
197 : /* We can now handle ereport(ERROR) */
198 896 : PG_exception_stack = &local_sigjmp_buf;
199 :
200 : /*
201 : * Unblock signals (they were blocked when the postmaster forked us)
202 : */
203 896 : sigprocmask(SIG_SETMASK, &UnBlockSig, NULL);
204 :
205 : /*
206 : * Reset hibernation state after any error.
207 : */
208 896 : left_till_hibernate = LOOPS_UNTIL_HIBERNATE;
209 896 : hibernating = false;
210 896 : SetWalWriterSleeping(false);
211 :
212 : /*
213 : * Advertise our proc number that backends can use to wake us up while
214 : * we're sleeping.
215 : */
216 896 : ProcGlobal->walwriterProc = MyProcNumber;
217 :
218 : /*
219 : * Loop forever
220 : */
221 : for (;;)
222 20816 : {
223 : long cur_timeout;
224 :
225 : /*
226 : * Advertise whether we might hibernate in this cycle. We do this
227 : * before resetting the latch to ensure that any async commits will
228 : * see the flag set if they might possibly need to wake us up, and
229 : * that we won't miss any signal they send us. (If we discover work
230 : * to do in the last cycle before we would hibernate, the global flag
231 : * will be set unnecessarily, but little harm is done.) But avoid
232 : * touching the global flag if it doesn't need to change.
233 : */
234 21712 : if (hibernating != (left_till_hibernate <= 1))
235 : {
236 28 : hibernating = (left_till_hibernate <= 1);
237 28 : SetWalWriterSleeping(hibernating);
238 : }
239 :
240 : /* Clear any already-pending wakeups */
241 21712 : ResetLatch(MyLatch);
242 :
243 : /* Process any signals received recently */
244 21712 : ProcessMainLoopInterrupts();
245 :
246 : /*
247 : * Do what we're here for; then, if XLogBackgroundFlush() found useful
248 : * work to do, reset hibernation counter.
249 : */
250 20822 : if (XLogBackgroundFlush())
251 7608 : left_till_hibernate = LOOPS_UNTIL_HIBERNATE;
252 13214 : else if (left_till_hibernate > 0)
253 13190 : left_till_hibernate--;
254 :
255 : /* report pending statistics to the cumulative stats system */
256 20822 : pgstat_report_wal(false);
257 :
258 : /*
259 : * Sleep until we are signaled or WalWriterDelay has elapsed. If we
260 : * haven't done anything useful for quite some time, lengthen the
261 : * sleep time so as to reduce the server's idle power consumption.
262 : */
263 20822 : if (left_till_hibernate > 0)
264 20780 : cur_timeout = WalWriterDelay; /* in ms */
265 : else
266 42 : cur_timeout = WalWriterDelay * HIBERNATE_FACTOR;
267 :
268 20822 : (void) WaitLatch(MyLatch,
269 : WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH,
270 : cur_timeout,
271 : WAIT_EVENT_WAL_WRITER_MAIN);
272 : }
273 : }
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