343 lines
11 KiB
C
343 lines
11 KiB
C
/*-------------------------------------------------------------------------
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*
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* bgwriter.c
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*
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* The background writer (bgwriter) is new as of Postgres 8.0. It attempts
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* to keep regular backends from having to write out dirty shared buffers
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* (which they would only do when needing to free a shared buffer to read in
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* another page). In the best scenario all writes from shared buffers will
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* be issued by the background writer process. However, regular backends are
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* still empowered to issue writes if the bgwriter fails to maintain enough
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* clean shared buffers.
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*
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* As of Postgres 9.2 the bgwriter no longer handles checkpoints.
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*
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* Normal termination is by SIGTERM, which instructs the bgwriter to exit(0).
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* Emergency termination is by SIGQUIT; like any backend, the bgwriter will
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* simply abort and exit on SIGQUIT.
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*
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* If the bgwriter exits unexpectedly, the postmaster treats that the same
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* as a backend crash: shared memory may be corrupted, so remaining backends
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* should be killed by SIGQUIT and then a recovery cycle started.
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*
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*
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* Portions Copyright (c) 1996-2024, PostgreSQL Global Development Group
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*
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*
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* IDENTIFICATION
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* src/backend/postmaster/bgwriter.c
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*
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*-------------------------------------------------------------------------
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*/
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#include "postgres.h"
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#include "access/xlog.h"
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#include "libpq/pqsignal.h"
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#include "miscadmin.h"
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#include "pgstat.h"
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#include "postmaster/auxprocess.h"
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#include "postmaster/bgwriter.h"
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#include "postmaster/interrupt.h"
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#include "storage/buf_internals.h"
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#include "storage/bufmgr.h"
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#include "storage/condition_variable.h"
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#include "storage/fd.h"
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#include "storage/lwlock.h"
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#include "storage/proc.h"
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#include "storage/procsignal.h"
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#include "storage/smgr.h"
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#include "storage/standby.h"
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#include "utils/memutils.h"
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#include "utils/resowner.h"
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#include "utils/timestamp.h"
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/*
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* GUC parameters
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*/
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int BgWriterDelay = 200;
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/*
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* Multiplier to apply to BgWriterDelay when we decide to hibernate.
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* (Perhaps this needs to be configurable?)
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*/
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#define HIBERNATE_FACTOR 50
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/*
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* Interval in which standby snapshots are logged into the WAL stream, in
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* milliseconds.
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*/
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#define LOG_SNAPSHOT_INTERVAL_MS 15000
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/*
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* LSN and timestamp at which we last issued a LogStandbySnapshot(), to avoid
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* doing so too often or repeatedly if there has been no other write activity
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* in the system.
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*/
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static TimestampTz last_snapshot_ts;
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static XLogRecPtr last_snapshot_lsn = InvalidXLogRecPtr;
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/*
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* Main entry point for bgwriter process
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*
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* This is invoked from AuxiliaryProcessMain, which has already created the
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* basic execution environment, but not enabled signals yet.
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*/
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void
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BackgroundWriterMain(char *startup_data, size_t startup_data_len)
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{
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sigjmp_buf local_sigjmp_buf;
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MemoryContext bgwriter_context;
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bool prev_hibernate;
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WritebackContext wb_context;
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Assert(startup_data_len == 0);
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MyBackendType = B_BG_WRITER;
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AuxiliaryProcessMainCommon();
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/*
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* Properly accept or ignore signals that might be sent to us.
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*/
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pqsignal(SIGHUP, SignalHandlerForConfigReload);
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pqsignal(SIGINT, SIG_IGN);
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pqsignal(SIGTERM, SignalHandlerForShutdownRequest);
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/* SIGQUIT handler was already set up by InitPostmasterChild */
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pqsignal(SIGALRM, SIG_IGN);
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pqsignal(SIGPIPE, SIG_IGN);
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pqsignal(SIGUSR1, procsignal_sigusr1_handler);
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pqsignal(SIGUSR2, SIG_IGN);
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/*
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* Reset some signals that are accepted by postmaster but not here
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*/
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pqsignal(SIGCHLD, SIG_DFL);
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/*
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* We just started, assume there has been either a shutdown or
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* end-of-recovery snapshot.
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*/
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last_snapshot_ts = GetCurrentTimestamp();
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/*
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* Create a memory context that we will do all our work in. We do this so
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* that we can reset the context during error recovery and thereby avoid
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* possible memory leaks. Formerly this code just ran in
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* TopMemoryContext, but resetting that would be a really bad idea.
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*/
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bgwriter_context = AllocSetContextCreate(TopMemoryContext,
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"Background Writer",
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ALLOCSET_DEFAULT_SIZES);
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MemoryContextSwitchTo(bgwriter_context);
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WritebackContextInit(&wb_context, &bgwriter_flush_after);
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/*
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* If an exception is encountered, processing resumes here.
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*
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* You might wonder why this isn't coded as an infinite loop around a
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* PG_TRY construct. The reason is that this is the bottom of the
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* exception stack, and so with PG_TRY there would be no exception handler
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* in force at all during the CATCH part. By leaving the outermost setjmp
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* always active, we have at least some chance of recovering from an error
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* during error recovery. (If we get into an infinite loop thereby, it
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* will soon be stopped by overflow of elog.c's internal state stack.)
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*
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* Note that we use sigsetjmp(..., 1), so that the prevailing signal mask
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* (to wit, BlockSig) will be restored when longjmp'ing to here. Thus,
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* signals other than SIGQUIT will be blocked until we complete error
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* recovery. It might seem that this policy makes the HOLD_INTERRUPTS()
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* call redundant, but it is not since InterruptPending might be set
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* already.
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*/
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if (sigsetjmp(local_sigjmp_buf, 1) != 0)
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{
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/* Since not using PG_TRY, must reset error stack by hand */
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error_context_stack = NULL;
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/* Prevent interrupts while cleaning up */
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HOLD_INTERRUPTS();
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/* Report the error to the server log */
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EmitErrorReport();
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/*
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* These operations are really just a minimal subset of
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* AbortTransaction(). We don't have very many resources to worry
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* about in bgwriter, but we do have LWLocks, buffers, and temp files.
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*/
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LWLockReleaseAll();
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ConditionVariableCancelSleep();
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UnlockBuffers();
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ReleaseAuxProcessResources(false);
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AtEOXact_Buffers(false);
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AtEOXact_SMgr();
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AtEOXact_Files(false);
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AtEOXact_HashTables(false);
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/*
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* Now return to normal top-level context and clear ErrorContext for
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* next time.
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*/
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MemoryContextSwitchTo(bgwriter_context);
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FlushErrorState();
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/* Flush any leaked data in the top-level context */
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MemoryContextReset(bgwriter_context);
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/* re-initialize to avoid repeated errors causing problems */
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WritebackContextInit(&wb_context, &bgwriter_flush_after);
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/* Now we can allow interrupts again */
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RESUME_INTERRUPTS();
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/*
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* Sleep at least 1 second after any error. A write error is likely
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* to be repeated, and we don't want to be filling the error logs as
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* fast as we can.
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*/
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pg_usleep(1000000L);
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/* Report wait end here, when there is no further possibility of wait */
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pgstat_report_wait_end();
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}
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/* We can now handle ereport(ERROR) */
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PG_exception_stack = &local_sigjmp_buf;
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/*
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* Unblock signals (they were blocked when the postmaster forked us)
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*/
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sigprocmask(SIG_SETMASK, &UnBlockSig, NULL);
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/*
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* Reset hibernation state after any error.
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*/
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prev_hibernate = false;
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/*
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* Loop forever
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*/
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for (;;)
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{
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bool can_hibernate;
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int rc;
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/* Clear any already-pending wakeups */
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ResetLatch(MyLatch);
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HandleMainLoopInterrupts();
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/*
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* Do one cycle of dirty-buffer writing.
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*/
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can_hibernate = BgBufferSync(&wb_context);
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/* Report pending statistics to the cumulative stats system */
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pgstat_report_bgwriter();
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pgstat_report_wal(true);
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if (FirstCallSinceLastCheckpoint())
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{
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/*
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* After any checkpoint, free all smgr objects. Otherwise we
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* would never do so for dropped relations, as the bgwriter does
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* not process shared invalidation messages or call
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* AtEOXact_SMgr().
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*/
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smgrdestroyall();
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}
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/*
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* Log a new xl_running_xacts every now and then so replication can
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* get into a consistent state faster (think of suboverflowed
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* snapshots) and clean up resources (locks, KnownXids*) more
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* frequently. The costs of this are relatively low, so doing it 4
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* times (LOG_SNAPSHOT_INTERVAL_MS) a minute seems fine.
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*
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* We assume the interval for writing xl_running_xacts is
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* significantly bigger than BgWriterDelay, so we don't complicate the
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* overall timeout handling but just assume we're going to get called
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* often enough even if hibernation mode is active. It's not that
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* important that LOG_SNAPSHOT_INTERVAL_MS is met strictly. To make
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* sure we're not waking the disk up unnecessarily on an idle system
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* we check whether there has been any WAL inserted since the last
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* time we've logged a running xacts.
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*
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* We do this logging in the bgwriter as it is the only process that
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* is run regularly and returns to its mainloop all the time. E.g.
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* Checkpointer, when active, is barely ever in its mainloop and thus
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* makes it hard to log regularly.
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*/
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if (XLogStandbyInfoActive() && !RecoveryInProgress())
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{
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TimestampTz timeout = 0;
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TimestampTz now = GetCurrentTimestamp();
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timeout = TimestampTzPlusMilliseconds(last_snapshot_ts,
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LOG_SNAPSHOT_INTERVAL_MS);
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/*
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* Only log if enough time has passed and interesting records have
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* been inserted since the last snapshot. Have to compare with <=
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* instead of < because GetLastImportantRecPtr() points at the
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* start of a record, whereas last_snapshot_lsn points just past
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* the end of the record.
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*/
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if (now >= timeout &&
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last_snapshot_lsn <= GetLastImportantRecPtr())
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{
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last_snapshot_lsn = LogStandbySnapshot();
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last_snapshot_ts = now;
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}
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}
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/*
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* Sleep until we are signaled or BgWriterDelay has elapsed.
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*
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* Note: the feedback control loop in BgBufferSync() expects that we
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* will call it every BgWriterDelay msec. While it's not critical for
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* correctness that that be exact, the feedback loop might misbehave
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* if we stray too far from that. Hence, avoid loading this process
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* down with latch events that are likely to happen frequently during
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* normal operation.
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*/
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rc = WaitLatch(MyLatch,
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WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH,
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BgWriterDelay /* ms */ , WAIT_EVENT_BGWRITER_MAIN);
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/*
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* If no latch event and BgBufferSync says nothing's happening, extend
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* the sleep in "hibernation" mode, where we sleep for much longer
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* than bgwriter_delay says. Fewer wakeups save electricity. When a
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* backend starts using buffers again, it will wake us up by setting
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* our latch. Because the extra sleep will persist only as long as no
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* buffer allocations happen, this should not distort the behavior of
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* BgBufferSync's control loop too badly; essentially, it will think
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* that the system-wide idle interval didn't exist.
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*
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* There is a race condition here, in that a backend might allocate a
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* buffer between the time BgBufferSync saw the alloc count as zero
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* and the time we call StrategyNotifyBgWriter. While it's not
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* critical that we not hibernate anyway, we try to reduce the odds of
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* that by only hibernating when BgBufferSync says nothing's happening
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* for two consecutive cycles. Also, we mitigate any possible
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* consequences of a missed wakeup by not hibernating forever.
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*/
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if (rc == WL_TIMEOUT && can_hibernate && prev_hibernate)
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{
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/* Ask for notification at next buffer allocation */
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StrategyNotifyBgWriter(MyProcNumber);
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/* Sleep ... */
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(void) WaitLatch(MyLatch,
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WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH,
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BgWriterDelay * HIBERNATE_FACTOR,
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WAIT_EVENT_BGWRITER_HIBERNATE);
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/* Reset the notification request in case we timed out */
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StrategyNotifyBgWriter(-1);
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}
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prev_hibernate = can_hibernate;
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}
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}
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