greenplumn bgwriter 源码
greenplumn bgwriter 代码
文件路径：/src/backend/postmaster/bgwriter.c
/*-------------------------------------------------------------------------
*
* bgwriter.c
*
* The background writer (bgwriter) is new as of Postgres 8.0. It attempts
* to keep regular backends from having to write out dirty shared buffers
* (which they would only do when needing to free a shared buffer to read in
* another page). In the best scenario all writes from shared buffers will
* be issued by the background writer process. However, regular backends are
* still empowered to issue writes if the bgwriter fails to maintain enough
* clean shared buffers.
*
* As of Postgres 9.2 the bgwriter no longer handles checkpoints.
*
* The bgwriter is started by the postmaster as soon as the startup subprocess
* finishes, or as soon as recovery begins if we are doing archive recovery.
* It remains alive until the postmaster commands it to terminate.
* Normal termination is by SIGTERM, which instructs the bgwriter to exit(0).
* Emergency termination is by SIGQUIT; like any backend, the bgwriter will
* simply abort and exit on SIGQUIT.
*
* If the bgwriter exits unexpectedly, the postmaster treats that the same
* as a backend crash: shared memory may be corrupted, so remaining backends
* should be killed by SIGQUIT and then a recovery cycle started.
*
*
* Portions Copyright (c) 1996-2019, PostgreSQL Global Development Group
*
*
* IDENTIFICATION
* src/backend/postmaster/bgwriter.c
*
*-------------------------------------------------------------------------
*/
#include "postgres.h"
#include <signal.h>
#include <sys/time.h>
#include <unistd.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 "storage/bufmgr.h"
#include "storage/buf_internals.h"
#include "storage/condition_variable.h"
#include "storage/fd.h"
#include "storage/ipc.h"
#include "storage/lwlock.h"
#include "storage/proc.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/faultinjector.h"
#include "utils/timestamp.h"
/*
* GUC parameters
*/
int BgWriterDelay = 200;
/*
* Multiplier to apply to BgWriterDelay when we decide to hibernate.
* (Perhaps this needs to be configurable?)
*/
#define HIBERNATE_FACTOR 50
/*
* Interval in which standby snapshots are logged into the WAL stream, in
* milliseconds.
*/
#define LOG_SNAPSHOT_INTERVAL_MS 15000
/*
* LSN and timestamp at which we last issued a LogStandbySnapshot(), to avoid
* doing so too often or repeatedly if there has been no other write activity
* in the system.
*/
static TimestampTz last_snapshot_ts;
static XLogRecPtr last_snapshot_lsn = InvalidXLogRecPtr;
/*
* Flags set by interrupt handlers for later service in the main loop.
*/
static volatile sig_atomic_t got_SIGHUP = false;
static volatile sig_atomic_t shutdown_requested = false;
/* Signal handlers */
static void bg_quickdie(SIGNAL_ARGS);
static void BgSigHupHandler(SIGNAL_ARGS);
static void ReqShutdownHandler(SIGNAL_ARGS);
static void bgwriter_sigusr1_handler(SIGNAL_ARGS);
/*
* Main entry point for bgwriter process
*
* This is invoked from AuxiliaryProcessMain, which has already created the
* basic execution environment, but not enabled signals yet.
*/
void
BackgroundWriterMain(void)
{
sigjmp_buf local_sigjmp_buf;
MemoryContext bgwriter_context;
bool prev_hibernate;
WritebackContext wb_context;
/*
* Properly accept or ignore signals the postmaster might send us.
*
* bgwriter doesn't participate in ProcSignal signalling, but a SIGUSR1
* handler is still needed for latch wakeups.
*/
pqsignal(SIGHUP, BgSigHupHandler); /* set flag to read config file */
pqsignal(SIGINT, SIG_IGN);
pqsignal(SIGTERM, ReqShutdownHandler); /* shutdown */
pqsignal(SIGQUIT, bg_quickdie); /* hard crash time */
pqsignal(SIGALRM, SIG_IGN);
pqsignal(SIGPIPE, SIG_IGN);
pqsignal(SIGUSR1, bgwriter_sigusr1_handler);
pqsignal(SIGUSR2, SIG_IGN);
/*
* Reset some signals that are accepted by postmaster but not here
*/
pqsignal(SIGCHLD, SIG_DFL);
/* We allow SIGQUIT (quickdie) at all times */
sigdelset(&BlockSig, SIGQUIT);
/*
* We just started, assume there has been either a shutdown or
* end-of-recovery snapshot.
*/
last_snapshot_ts = GetCurrentTimestamp();
/*
* Create a memory context that we will do all our work in. We do this so
* that we can reset the context during error recovery and thereby avoid
* possible memory leaks. Formerly this code just ran in
* TopMemoryContext, but resetting that would be a really bad idea.
*/
bgwriter_context = AllocSetContextCreate(TopMemoryContext,
"Background Writer",
ALLOCSET_DEFAULT_SIZES);
MemoryContextSwitchTo(bgwriter_context);
WritebackContextInit(&wb_context, &bgwriter_flush_after);
/*
* If an exception is encountered, processing resumes here.
*
* See notes in postgres.c about the design of this coding.
*/
if (sigsetjmp(local_sigjmp_buf, 1) != 0)
{
/* Since not using PG_TRY, must reset error stack by hand */
error_context_stack = NULL;
/* Prevent interrupts while cleaning up */
HOLD_INTERRUPTS();
/* Report the error to the server log */
EmitErrorReport();
/*
* These operations are really just a minimal subset of
* AbortTransaction(). We don't have very many resources to worry
* about in bgwriter, but we do have LWLocks, buffers, and temp files.
*/
LWLockReleaseAll();
ConditionVariableCancelSleep();
AbortBufferIO();
UnlockBuffers();
ReleaseAuxProcessResources(false);
AtEOXact_Buffers(false);
AtEOXact_SMgr();
AtEOXact_Files(false);
AtEOXact_HashTables(false);
/*
* Now return to normal top-level context and clear ErrorContext for
* next time.
*/
MemoryContextSwitchTo(bgwriter_context);
FlushErrorState();
/* Flush any leaked data in the top-level context */
MemoryContextResetAndDeleteChildren(bgwriter_context);
/* re-initialize to avoid repeated errors causing problems */
WritebackContextInit(&wb_context, &bgwriter_flush_after);
/* Now we can allow interrupts again */
RESUME_INTERRUPTS();
/*
* Sleep at least 1 second after any error. A write error is likely
* to be repeated, and we don't want to be filling the error logs as
* fast as we can.
*/
pg_usleep(1000000L);
/*
* Close all open files after any error. This is helpful on Windows,
* where holding deleted files open causes various strange errors.
* It's not clear we need it elsewhere, but shouldn't hurt.
*/
smgrcloseall();
/* Report wait end here, when there is no further possibility of wait */
pgstat_report_wait_end();
}
/* We can now handle ereport(ERROR) */
PG_exception_stack = &local_sigjmp_buf;
/*
* Unblock signals (they were blocked when the postmaster forked us)
*/
PG_SETMASK(&UnBlockSig);
/*
* Reset hibernation state after any error.
*/
prev_hibernate = false;
/*
* Loop forever
*/
for (;;)
{
bool can_hibernate;
int rc;
SIMPLE_FAULT_INJECTOR("fault_in_background_writer_main");
/* Clear any already-pending wakeups */
ResetLatch(MyLatch);
if (got_SIGHUP)
{
got_SIGHUP = false;
ProcessConfigFile(PGC_SIGHUP);
}
if (shutdown_requested)
{
/*
* From here on, elog(ERROR) should end with exit(1), not send
* control back to the sigsetjmp block above
*/
ExitOnAnyError = true;
/* Normal exit from the bgwriter is here */
proc_exit(0); /* done */
}
/*
* Do one cycle of dirty-buffer writing.
*/
can_hibernate = BgBufferSync(&wb_context);
/*
* Send off activity statistics to the stats collector
*/
pgstat_send_bgwriter();
if (FirstCallSinceLastCheckpoint())
{
/*
* After any checkpoint, close all smgr files. This is so we
* won't hang onto smgr references to deleted files indefinitely.
*/
smgrcloseall();
}
/*
* Log a new xl_running_xacts every now and then so replication can
* get into a consistent state faster (think of suboverflowed
* snapshots) and clean up resources (locks, KnownXids*) more
* frequently. The costs of this are relatively low, so doing it 4
* times (LOG_SNAPSHOT_INTERVAL_MS) a minute seems fine.
*
* We assume the interval for writing xl_running_xacts is
* significantly bigger than BgWriterDelay, so we don't complicate the
* overall timeout handling but just assume we're going to get called
* often enough even if hibernation mode is active. It's not that
* important that log_snap_interval_ms is met strictly. To make sure
* we're not waking the disk up unnecessarily on an idle system we
* check whether there has been any WAL inserted since the last time
* we've logged a running xacts.
*
* We do this logging in the bgwriter as it is the only process that
* is run regularly and returns to its mainloop all the time. E.g.
* Checkpointer, when active, is barely ever in its mainloop and thus
* makes it hard to log regularly.
*/
if (XLogStandbyInfoActive() && !RecoveryInProgress())
{
TimestampTz timeout = 0;
TimestampTz now = GetCurrentTimestamp();
timeout = TimestampTzPlusMilliseconds(last_snapshot_ts,
LOG_SNAPSHOT_INTERVAL_MS);
/*
* Only log if enough time has passed and interesting records have
* been inserted since the last snapshot. Have to compare with <=
* instead of < because GetLastImportantRecPtr() points at the
* start of a record, whereas last_snapshot_lsn points just past
* the end of the record.
*/
if (now >= timeout &&
last_snapshot_lsn <= GetLastImportantRecPtr())
{
last_snapshot_lsn = LogStandbySnapshot();
last_snapshot_ts = now;
}
}
/*
* Sleep until we are signaled or BgWriterDelay has elapsed.
*
* Note: the feedback control loop in BgBufferSync() expects that we
* will call it every BgWriterDelay msec. While it's not critical for
* correctness that that be exact, the feedback loop might misbehave
* if we stray too far from that. Hence, avoid loading this process
* down with latch events that are likely to happen frequently during
* normal operation.
*/
rc = WaitLatch(MyLatch,
WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH,
BgWriterDelay /* ms */ , WAIT_EVENT_BGWRITER_MAIN);
/*
* If no latch event and BgBufferSync says nothing's happening, extend
* the sleep in "hibernation" mode, where we sleep for much longer
* than bgwriter_delay says. Fewer wakeups save electricity. When a
* backend starts using buffers again, it will wake us up by setting
* our latch. Because the extra sleep will persist only as long as no
* buffer allocations happen, this should not distort the behavior of
* BgBufferSync's control loop too badly; essentially, it will think
* that the system-wide idle interval didn't exist.
*
* There is a race condition here, in that a backend might allocate a
* buffer between the time BgBufferSync saw the alloc count as zero
* and the time we call StrategyNotifyBgWriter. While it's not
* critical that we not hibernate anyway, we try to reduce the odds of
* that by only hibernating when BgBufferSync says nothing's happening
* for two consecutive cycles. Also, we mitigate any possible
* consequences of a missed wakeup by not hibernating forever.
*/
if (rc == WL_TIMEOUT && can_hibernate && prev_hibernate)
{
/* Ask for notification at next buffer allocation */
StrategyNotifyBgWriter(MyProc->pgprocno);
/* Sleep ... */
(void) WaitLatch(MyLatch,
WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH,
BgWriterDelay * HIBERNATE_FACTOR,
WAIT_EVENT_BGWRITER_HIBERNATE);
/* Reset the notification request in case we timed out */
StrategyNotifyBgWriter(-1);
}
prev_hibernate = can_hibernate;
}
}
/* --------------------------------
* signal handler routines
* --------------------------------
*/
/*
* bg_quickdie() occurs when signalled SIGQUIT by the postmaster.
*
* Some backend has bought the farm,
* so we need to stop what we're doing and exit.
*/
static void
bg_quickdie(SIGNAL_ARGS)
{
SIMPLE_FAULT_INJECTOR("fault_in_background_writer_quickdie");
/*
* We DO NOT want to run proc_exit() or atexit() callbacks -- we're here
* because shared memory may be corrupted, so we don't want to try to
* clean up our transaction. Just nail the windows shut and get out of
* town. The callbacks wouldn't be safe to run from a signal handler,
* anyway.
*
* Note we do _exit(2) not _exit(0). This is to force the postmaster into
* a system reset cycle if someone sends a manual SIGQUIT to a random
* backend. This is necessary precisely because we don't clean up our
* shared memory state. (The "dead man switch" mechanism in pmsignal.c
* should ensure the postmaster sees this as a crash, too, but no harm in
* being doubly sure.)
*/
_exit(2);
}
/* SIGHUP: set flag to re-read config file at next convenient time */
static void
BgSigHupHandler(SIGNAL_ARGS)
{
int save_errno = errno;
got_SIGHUP = true;
SetLatch(MyLatch);
errno = save_errno;
}
/* SIGTERM: set flag to shutdown and exit */
static void
ReqShutdownHandler(SIGNAL_ARGS)
{
int save_errno = errno;
shutdown_requested = true;
SetLatch(MyLatch);
errno = save_errno;
}
/* SIGUSR1: used for latch wakeups */
static void
bgwriter_sigusr1_handler(SIGNAL_ARGS)
{
int save_errno = errno;
latch_sigusr1_handler();
errno = save_errno;
}
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