/*------------------------------------------------------------------------- * * bgwriter.c * * The background writer (bgwriter) is new in 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. * * The bgwriter is also charged with handling all checkpoints. It will * automatically dispatch a checkpoint after a certain amount of time has * elapsed since the last one, and it can be signaled to perform requested * checkpoints as well. (The GUC parameter that mandates a checkpoint every * so many WAL segments is implemented by having backends signal the bgwriter * when they fill WAL segments; the bgwriter itself doesn't watch for the * condition.) * * The bgwriter is started by the postmaster as soon as the startup subprocess * finishes. It remains alive until the postmaster commands it to terminate. * Normal termination is by SIGUSR2, which instructs the bgwriter to execute * a shutdown checkpoint and then exit(0). (All backends must be stopped * before SIGUSR2 is issued!) 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. (Even if * shared memory isn't corrupted, we have lost information about which * files need to be fsync'd for the next checkpoint, and so a system * restart needs to be forced.) * * * Portions Copyright (c) 1996-2005, PostgreSQL Global Development Group * * * IDENTIFICATION * $PostgreSQL: pgsql/src/backend/postmaster/bgwriter.c,v 1.16 2005/05/28 17:21:32 tgl Exp $ * *------------------------------------------------------------------------- */ #include "postgres.h" #include #include #include "access/xlog.h" #include "libpq/pqsignal.h" #include "miscadmin.h" #include "postmaster/bgwriter.h" #include "storage/bufmgr.h" #include "storage/freespace.h" #include "storage/ipc.h" #include "storage/pmsignal.h" #include "storage/smgr.h" #include "tcop/tcopprot.h" #include "utils/guc.h" #include "utils/memutils.h" /*---------- * Shared memory area for communication between bgwriter and backends * * The ckpt counters allow backends to watch for completion of a checkpoint * request they send. Here's how it works: * * At start of a checkpoint, bgwriter increments ckpt_started. * * On completion of a checkpoint, bgwriter sets ckpt_done to * equal ckpt_started. * * On failure of a checkpoint, bgwrite first increments ckpt_failed, * then sets ckpt_done to equal ckpt_started. * All three fields are declared sig_atomic_t to ensure they can be read * and written without explicit locking. The algorithm for backends is: * 1. Record current values of ckpt_failed and ckpt_started (in that * order!). * 2. Send signal to request checkpoint. * 3. Sleep until ckpt_started changes. Now you know a checkpoint has * begun since you started this algorithm (although *not* that it was * specifically initiated by your signal). * 4. Record new value of ckpt_started. * 5. Sleep until ckpt_done >= saved value of ckpt_started. (Use modulo * arithmetic here in case counters wrap around.) Now you know a * checkpoint has started and completed, but not whether it was * successful. * 6. If ckpt_failed is different from the originally saved value, * assume request failed; otherwise it was definitely successful. * * The requests array holds fsync requests sent by backends and not yet * absorbed by the bgwriter. *---------- */ typedef struct { RelFileNode rnode; BlockNumber segno; /* might add a request-type field later */ } BgWriterRequest; typedef struct { pid_t bgwriter_pid; /* PID of bgwriter (0 if not started) */ sig_atomic_t ckpt_started; /* advances when checkpoint starts */ sig_atomic_t ckpt_done; /* advances when checkpoint done */ sig_atomic_t ckpt_failed; /* advances when checkpoint fails */ int num_requests; /* current # of requests */ int max_requests; /* allocated array size */ BgWriterRequest requests[1]; /* VARIABLE LENGTH ARRAY */ } BgWriterShmemStruct; static BgWriterShmemStruct *BgWriterShmem; /* * GUC parameters */ int BgWriterDelay = 200; int CheckPointTimeout = 300; int CheckPointWarning = 30; /* * 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 checkpoint_requested = false; static volatile sig_atomic_t shutdown_requested = false; /* * Private state */ static bool am_bg_writer = false; static bool ckpt_active = false; static time_t last_checkpoint_time; static void bg_quickdie(SIGNAL_ARGS); static void BgSigHupHandler(SIGNAL_ARGS); static void ReqCheckpointHandler(SIGNAL_ARGS); static void ReqShutdownHandler(SIGNAL_ARGS); /* * Main entry point for bgwriter process * * This is invoked from BootstrapMain, which has already created the basic * execution environment, but not enabled signals yet. */ void BackgroundWriterMain(void) { sigjmp_buf local_sigjmp_buf; Assert(BgWriterShmem != NULL); BgWriterShmem->bgwriter_pid = MyProcPid; am_bg_writer = true; /* * Properly accept or ignore signals the postmaster might send us * * Note: we deliberately ignore SIGTERM, because during a standard Unix * system shutdown cycle, init will SIGTERM all processes at once. We * want to wait for the backends to exit, whereupon the postmaster * will tell us it's okay to shut down (via SIGUSR2). * * SIGUSR1 is presently unused; keep it spare in case someday we want * this process to participate in sinval messaging. */ pqsignal(SIGHUP, BgSigHupHandler); /* set flag to read config file */ pqsignal(SIGINT, ReqCheckpointHandler); /* request checkpoint */ pqsignal(SIGTERM, SIG_IGN); /* ignore SIGTERM */ pqsignal(SIGQUIT, bg_quickdie); /* hard crash time */ pqsignal(SIGALRM, SIG_IGN); pqsignal(SIGPIPE, SIG_IGN); pqsignal(SIGUSR1, SIG_IGN); /* reserve for sinval */ pqsignal(SIGUSR2, ReqShutdownHandler); /* request shutdown */ /* * Reset some signals that are accepted by postmaster but not here */ pqsignal(SIGCHLD, SIG_DFL); pqsignal(SIGTTIN, SIG_DFL); pqsignal(SIGTTOU, SIG_DFL); pqsignal(SIGCONT, SIG_DFL); pqsignal(SIGWINCH, SIG_DFL); /* We allow SIGQUIT (quickdie) at all times */ #ifdef HAVE_SIGPROCMASK sigdelset(&BlockSig, SIGQUIT); #else BlockSig &= ~(sigmask(SIGQUIT)); #endif /* * Initialize so that first time-driven checkpoint happens at the * correct time. */ last_checkpoint_time = time(NULL); /* * 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 and buffers. */ LWLockReleaseAll(); AbortBufferIO(); UnlockBuffers(); /* Warn any waiting backends that the checkpoint failed. */ if (ckpt_active) { /* use volatile pointer to prevent code rearrangement */ volatile BgWriterShmemStruct *bgs = BgWriterShmem; bgs->ckpt_failed++; bgs->ckpt_done = bgs->ckpt_started; ckpt_active = false; } /* * Now return to normal top-level context and clear ErrorContext * for next time. */ MemoryContextSwitchTo(TopMemoryContext); FlushErrorState(); /* 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. (XXX think about ways to make * progress when the LRU dirty buffer cannot be written...) */ pg_usleep(1000000L); } /* 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); /* * Loop forever */ for (;;) { bool do_checkpoint = false; bool force_checkpoint = false; time_t now; int elapsed_secs; long udelay; /* * Emergency bailout if postmaster has died. This is to avoid the * necessity for manual cleanup of all postmaster children. */ if (!PostmasterIsAlive(true)) exit(1); /* * Process any requests or signals received recently. */ AbsorbFsyncRequests(); if (got_SIGHUP) { got_SIGHUP = false; ProcessConfigFile(PGC_SIGHUP); } if (checkpoint_requested) { checkpoint_requested = false; do_checkpoint = true; force_checkpoint = true; } if (shutdown_requested) { ShutdownXLOG(0, 0); DumpFreeSpaceMap(0, 0); /* Normal exit from the bgwriter is here */ proc_exit(0); /* done */ } /* * Do an unforced checkpoint if too much time has elapsed since * the last one. */ now = time(NULL); elapsed_secs = now - last_checkpoint_time; if (elapsed_secs >= CheckPointTimeout) do_checkpoint = true; /* * Do a checkpoint if requested, otherwise do one cycle of * dirty-buffer writing. */ if (do_checkpoint) { if (CheckPointWarning != 0) { /* * Ideally we should only warn if this checkpoint was * requested due to running out of segment files, and not * if it was manually requested. However we can't tell * the difference with the current signalling mechanism. */ if (elapsed_secs < CheckPointWarning) ereport(LOG, (errmsg("checkpoints are occurring too frequently (%d seconds apart)", elapsed_secs), errhint("Consider increasing the configuration parameter \"checkpoint_segments\"."))); } /* * Indicate checkpoint start to any waiting backends. */ ckpt_active = true; BgWriterShmem->ckpt_started++; CreateCheckPoint(false, force_checkpoint); /* * After any checkpoint, close all smgr files. This is so we * won't hang onto smgr references to deleted files * indefinitely. */ smgrcloseall(); /* * Indicate checkpoint completion to any waiting backends. */ BgWriterShmem->ckpt_done = BgWriterShmem->ckpt_started; ckpt_active = false; /* * Note we record the checkpoint start time not end time as * last_checkpoint_time. This is so that time-driven * checkpoints happen at a predictable spacing. */ last_checkpoint_time = now; } else BgBufferSync(); /* * Nap for the configured time, or sleep for 10 seconds if there * is no bgwriter activity configured. * * On some platforms, signals won't interrupt the sleep. To ensure * we respond reasonably promptly when someone signals us, break * down the sleep into 1-second increments, and check for * interrupts after each nap. * * We absorb pending requests after each short sleep. */ if ((bgwriter_all_percent > 0.0 && bgwriter_all_maxpages > 0) || (bgwriter_lru_percent > 0.0 && bgwriter_lru_maxpages > 0)) udelay = BgWriterDelay * 1000L; else udelay = 10000000L; while (udelay > 1000000L) { if (got_SIGHUP || checkpoint_requested || shutdown_requested) break; pg_usleep(1000000L); AbsorbFsyncRequests(); udelay -= 1000000L; } if (!(got_SIGHUP || checkpoint_requested || shutdown_requested)) pg_usleep(udelay); } } /* -------------------------------- * 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) { PG_SETMASK(&BlockSig); /* * DO NOT proc_exit() -- 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. * * Note we do exit(1) not exit(0). This is to force the postmaster into * a system reset cycle if some idiot DBA sends a manual SIGQUIT to a * random backend. This is necessary precisely because we don't clean * up our shared memory state. */ exit(1); } /* SIGHUP: set flag to re-read config file at next convenient time */ static void BgSigHupHandler(SIGNAL_ARGS) { got_SIGHUP = true; } /* SIGINT: set flag to run a normal checkpoint right away */ static void ReqCheckpointHandler(SIGNAL_ARGS) { checkpoint_requested = true; } /* SIGUSR2: set flag to run a shutdown checkpoint and exit */ static void ReqShutdownHandler(SIGNAL_ARGS) { shutdown_requested = true; } /* -------------------------------- * communication with backends * -------------------------------- */ /* * BgWriterShmemSize * Compute space needed for bgwriter-related shared memory */ int BgWriterShmemSize(void) { /* * Currently, the size of the requests[] array is arbitrarily set * equal to NBuffers. This may prove too large or small ... */ return MAXALIGN(sizeof(BgWriterShmemStruct) + (NBuffers - 1) *sizeof(BgWriterRequest)); } /* * BgWriterShmemInit * Allocate and initialize bgwriter-related shared memory */ void BgWriterShmemInit(void) { bool found; BgWriterShmem = (BgWriterShmemStruct *) ShmemInitStruct("Background Writer Data", BgWriterShmemSize(), &found); if (BgWriterShmem == NULL) ereport(FATAL, (errcode(ERRCODE_OUT_OF_MEMORY), errmsg("not enough shared memory for background writer"))); if (found) return; /* already initialized */ MemSet(BgWriterShmem, 0, sizeof(BgWriterShmemStruct)); BgWriterShmem->max_requests = NBuffers; } /* * RequestCheckpoint * Called in backend processes to request an immediate checkpoint * * If waitforit is true, wait until the checkpoint is completed * before returning; otherwise, just signal the request and return * immediately. */ void RequestCheckpoint(bool waitforit) { /* use volatile pointer to prevent code rearrangement */ volatile BgWriterShmemStruct *bgs = BgWriterShmem; sig_atomic_t old_failed = bgs->ckpt_failed; sig_atomic_t old_started = bgs->ckpt_started; /* * If in a standalone backend, just do it ourselves. */ if (!IsPostmasterEnvironment) { CreateCheckPoint(false, true); /* * After any checkpoint, close all smgr files. This is so we * won't hang onto smgr references to deleted files * indefinitely. */ smgrcloseall(); return; } /* * Send signal to request checkpoint. When waitforit is false, we * consider failure to send the signal to be nonfatal. */ if (BgWriterShmem->bgwriter_pid == 0) elog(waitforit ? ERROR : LOG, "could not request checkpoint because bgwriter not running"); if (kill(BgWriterShmem->bgwriter_pid, SIGINT) != 0) elog(waitforit ? ERROR : LOG, "could not signal for checkpoint: %m"); /* * If requested, wait for completion. We detect completion according * to the algorithm given above. */ if (waitforit) { while (bgs->ckpt_started == old_started) { CHECK_FOR_INTERRUPTS(); pg_usleep(100000L); } old_started = bgs->ckpt_started; /* * We are waiting for ckpt_done >= old_started, in a modulo sense. * This is a little tricky since we don't know the width or * signedness of sig_atomic_t. We make the lowest common * denominator assumption that it is only as wide as "char". This * means that this algorithm will cope correctly as long as we * don't sleep for more than 127 completed checkpoints. (If we * do, we will get another chance to exit after 128 more * checkpoints...) */ while (((signed char) (bgs->ckpt_done - old_started)) < 0) { CHECK_FOR_INTERRUPTS(); pg_usleep(100000L); } if (bgs->ckpt_failed != old_failed) ereport(ERROR, (errmsg("checkpoint request failed"), errhint("Consult the server log for details."))); } } /* * ForwardFsyncRequest * Forward a file-fsync request from a backend to the bgwriter * * Whenever a backend is compelled to write directly to a relation * (which should be seldom, if the bgwriter is getting its job done), * the backend calls this routine to pass over knowledge that the relation * is dirty and must be fsync'd before next checkpoint. * * If we are unable to pass over the request (at present, this can happen * if the shared memory queue is full), we return false. That forces * the backend to do its own fsync. We hope that will be even more seldom. * * Note: we presently make no attempt to eliminate duplicate requests * in the requests[] queue. The bgwriter will have to eliminate dups * internally anyway, so we may as well avoid holding the lock longer * than we have to here. */ bool ForwardFsyncRequest(RelFileNode rnode, BlockNumber segno) { BgWriterRequest *request; if (!IsUnderPostmaster) return false; /* probably shouldn't even get here */ Assert(BgWriterShmem != NULL); LWLockAcquire(BgWriterCommLock, LW_EXCLUSIVE); if (BgWriterShmem->bgwriter_pid == 0 || BgWriterShmem->num_requests >= BgWriterShmem->max_requests) { LWLockRelease(BgWriterCommLock); return false; } request = &BgWriterShmem->requests[BgWriterShmem->num_requests++]; request->rnode = rnode; request->segno = segno; LWLockRelease(BgWriterCommLock); return true; } /* * AbsorbFsyncRequests * Retrieve queued fsync requests and pass them to local smgr. * * This is exported because it must be called during CreateCheckPoint; * we have to be sure we have accepted all pending requests *after* we * establish the checkpoint REDO pointer. Since CreateCheckPoint * sometimes runs in non-bgwriter processes, do nothing if not bgwriter. */ void AbsorbFsyncRequests(void) { BgWriterRequest *requests = NULL; BgWriterRequest *request; int n; if (!am_bg_writer) return; /* * We have to PANIC if we fail to absorb all the pending requests * (eg, because our hashtable runs out of memory). This is because * the system cannot run safely if we are unable to fsync what we * have been told to fsync. Fortunately, the hashtable is so small * that the problem is quite unlikely to arise in practice. */ START_CRIT_SECTION(); /* * We try to avoid holding the lock for a long time by copying the * request array. */ LWLockAcquire(BgWriterCommLock, LW_EXCLUSIVE); n = BgWriterShmem->num_requests; if (n > 0) { requests = (BgWriterRequest *) palloc(n * sizeof(BgWriterRequest)); memcpy(requests, BgWriterShmem->requests, n * sizeof(BgWriterRequest)); } BgWriterShmem->num_requests = 0; LWLockRelease(BgWriterCommLock); for (request = requests; n > 0; request++, n--) RememberFsyncRequest(request->rnode, request->segno); if (requests) pfree(requests); END_CRIT_SECTION(); }