pgstat_backend.c

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/* -------------------------------------------------------------------------
 *
 * pgstat_backend.c
 *    Implementation of backend statistics.
 *
 * This file contains the implementation of backend statistics.  It is kept
 * separate from pgstat.c to enforce the line between the statistics access /
 * storage implementation and the details about individual types of
 * statistics.
 *
 * This statistics kind uses a proc number as object ID for the hash table
 * of pgstats.  Entries are created each time a process is spawned, and are
 * dropped when the process exits.  These are not written to the pgstats file
 * on disk.  Pending statistics are managed without direct interactions with
 * PgStat_EntryRef->pending, relying on PendingBackendStats instead so as it
 * is possible to report data within critical sections.
 *
 * Copyright (c) 2001-2026, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *    src/backend/utils/activity/pgstat_backend.c
 * -------------------------------------------------------------------------
 */
 
#include "postgres.h"
 
#include "access/xlog.h"
#include "executor/instrument.h"
#include "storage/bufmgr.h"
#include "storage/proc.h"
#include "storage/procarray.h"
#include "utils/memutils.h"
#include "utils/pgstat_internal.h"
 
/*
 * Backend statistics counts waiting to be flushed out. These counters may be
 * reported within critical sections so we use static memory in order to avoid
 * memory allocation.
 */
static PgStat_BackendPending PendingBackendStats;
static bool backend_has_iostats = false;
 
/*
 * WAL usage counters saved from pgWalUsage at the previous call to
 * pgstat_flush_backend().  This is used to calculate how much WAL usage
 * happens between pgstat_flush_backend() calls, by subtracting the
 * previous counters from the current ones.
 */
static WalUsage prevBackendWalUsage;
 
/*
 * Utility routines to report I/O stats for backends, kept here to avoid
 * exposing PendingBackendStats to the outside world.
 */
void
pgstat_count_backend_io_op_time(IOObject io_object, IOContext io_context,
                                IOOp io_op, instr_time io_time)
{
    Assert(track_io_timing || track_wal_io_timing);
 
    if (!pgstat_tracks_backend_bktype(MyBackendType))
        return;
 
    Assert(pgstat_tracks_io_op(MyBackendType, io_object, io_context, io_op));
 
    INSTR_TIME_ADD(PendingBackendStats.pending_io.pending_times[io_object][io_context][io_op],
                   io_time);
 
    backend_has_iostats = true;
    pgstat_report_fixed = true;
}
 
void
pgstat_count_backend_io_op(IOObject io_object, IOContext io_context,
                           IOOp io_op, uint32 cnt, uint64 bytes)
{
    if (!pgstat_tracks_backend_bktype(MyBackendType))
        return;
 
    Assert(pgstat_tracks_io_op(MyBackendType, io_object, io_context, io_op));
 
    PendingBackendStats.pending_io.counts[io_object][io_context][io_op] += cnt;
    PendingBackendStats.pending_io.bytes[io_object][io_context][io_op] += bytes;
 
    backend_has_iostats = true;
    pgstat_report_fixed = true;
}
 
/*
 * Returns statistics of a backend by proc number.
 */
PgStat_Backend *
pgstat_fetch_stat_backend(ProcNumber procNumber)
{
    PgStat_Backend *backend_entry;
 
    backend_entry = (PgStat_Backend *) pgstat_fetch_entry(PGSTAT_KIND_BACKEND,
                                                          InvalidOid, procNumber);
 
    return backend_entry;
}
 
/*
 * Returns statistics of a backend by pid.
 *
 * This routine includes sanity checks to ensure that the backend exists and
 * is running.  "bktype" can be optionally defined to return the BackendType
 * of the backend whose statistics are returned.
 */
PgStat_Backend *
pgstat_fetch_stat_backend_by_pid(int pid, BackendType *bktype)
{
    PGPROC     *proc;
    PgBackendStatus *beentry;
    ProcNumber  procNumber;
    PgStat_Backend *backend_stats;
 
    proc = BackendPidGetProc(pid);
    if (bktype)
        *bktype = B_INVALID;
 
    /* this could be an auxiliary process */
    if (!proc)
        proc = AuxiliaryPidGetProc(pid);
 
    if (!proc)
        return NULL;
 
    procNumber = GetNumberFromPGProc(proc);
 
    beentry = pgstat_get_beentry_by_proc_number(procNumber);
    if (!beentry)
        return NULL;
 
    /* check if the backend type tracks statistics */
    if (!pgstat_tracks_backend_bktype(beentry->st_backendType))
        return NULL;
 
    /* if PID does not match, leave */
    if (beentry->st_procpid != pid)
        return NULL;
 
    if (bktype)
        *bktype = beentry->st_backendType;
 
    /*
     * Retrieve the entry.  Note that "beentry" may be freed depending on the
     * value of stats_fetch_consistency, so do not access it from this point.
     */
    backend_stats = pgstat_fetch_stat_backend(procNumber);
    if (!backend_stats)
    {
        if (bktype)
            *bktype = B_INVALID;
        return NULL;
    }
 
    return backend_stats;
}
 
/*
 * Flush out locally pending backend IO statistics.  Locking is managed
 * by the caller.
 */
static void
pgstat_flush_backend_entry_io(PgStat_EntryRef *entry_ref)
{
    PgStatShared_Backend *shbackendent;
    PgStat_BktypeIO *bktype_shstats;
    PgStat_PendingIO pending_io;
 
    /*
     * This function can be called even if nothing at all has happened for IO
     * statistics.  In this case, avoid unnecessarily modifying the stats
     * entry.
     */
    if (!backend_has_iostats)
        return;
 
    shbackendent = (PgStatShared_Backend *) entry_ref->shared_stats;
    bktype_shstats = &shbackendent->stats.io_stats;
    pending_io = PendingBackendStats.pending_io;
 
    for (int io_object = 0; io_object < IOOBJECT_NUM_TYPES; io_object++)
    {
        for (int io_context = 0; io_context < IOCONTEXT_NUM_TYPES; io_context++)
        {
            for (int io_op = 0; io_op < IOOP_NUM_TYPES; io_op++)
            {
                instr_time  time;
 
                bktype_shstats->counts[io_object][io_context][io_op] +=
                    pending_io.counts[io_object][io_context][io_op];
                bktype_shstats->bytes[io_object][io_context][io_op] +=
                    pending_io.bytes[io_object][io_context][io_op];
                time = pending_io.pending_times[io_object][io_context][io_op];
 
                bktype_shstats->times[io_object][io_context][io_op] +=
                    INSTR_TIME_GET_MICROSEC(time);
            }
        }
    }
 
    /*
     * Clear out the statistics buffer, so it can be re-used.
     */
    MemSet(&PendingBackendStats.pending_io, 0, sizeof(PgStat_PendingIO));
 
    backend_has_iostats = false;
}
 
/*
 * To determine whether WAL usage happened.
 */
static inline bool
pgstat_backend_wal_have_pending(void)
{
    return (pgWalUsage.wal_records != prevBackendWalUsage.wal_records);
}
 
/*
 * Flush out locally pending backend WAL statistics.  Locking is managed
 * by the caller.
 */
static void
pgstat_flush_backend_entry_wal(PgStat_EntryRef *entry_ref)
{
    PgStatShared_Backend *shbackendent;
    PgStat_WalCounters *bktype_shstats;
    WalUsage    wal_usage_diff = {0};
 
    /*
     * This function can be called even if nothing at all has happened for WAL
     * statistics.  In this case, avoid unnecessarily modifying the stats
     * entry.
     */
    if (!pgstat_backend_wal_have_pending())
        return;
 
    shbackendent = (PgStatShared_Backend *) entry_ref->shared_stats;
    bktype_shstats = &shbackendent->stats.wal_counters;
 
    /*
     * Calculate how much WAL usage counters were increased by subtracting the
     * previous counters from the current ones.
     */
    WalUsageAccumDiff(&wal_usage_diff, &pgWalUsage, &prevBackendWalUsage);
 
#define WALSTAT_ACC(fld, var_to_add) \
    (bktype_shstats->fld += var_to_add.fld)
    WALSTAT_ACC(wal_buffers_full, wal_usage_diff);
    WALSTAT_ACC(wal_records, wal_usage_diff);
    WALSTAT_ACC(wal_fpi, wal_usage_diff);
    WALSTAT_ACC(wal_bytes, wal_usage_diff);
    WALSTAT_ACC(wal_fpi_bytes, wal_usage_diff);
#undef WALSTAT_ACC
 
    /*
     * Save the current counters for the subsequent calculation of WAL usage.
     */
    prevBackendWalUsage = pgWalUsage;
}
 
/*
 * Flush out locally pending backend statistics
 *
 * "flags" parameter controls which statistics to flush.  Returns true
 * if some statistics could not be flushed due to lock contention.
 */
bool
pgstat_flush_backend(bool nowait, bits32 flags)
{
    PgStat_EntryRef *entry_ref;
    bool        has_pending_data = false;
 
    if (!pgstat_tracks_backend_bktype(MyBackendType))
        return false;
 
    /* Some IO data pending? */
    if ((flags & PGSTAT_BACKEND_FLUSH_IO) && backend_has_iostats)
        has_pending_data = true;
 
    /* Some WAL data pending? */
    if ((flags & PGSTAT_BACKEND_FLUSH_WAL) &&
        pgstat_backend_wal_have_pending())
        has_pending_data = true;
 
    if (!has_pending_data)
        return false;
 
    entry_ref = pgstat_get_entry_ref_locked(PGSTAT_KIND_BACKEND, InvalidOid,
                                            MyProcNumber, nowait);
    if (!entry_ref)
        return true;
 
    /* Flush requested statistics */
    if (flags & PGSTAT_BACKEND_FLUSH_IO)
        pgstat_flush_backend_entry_io(entry_ref);
 
    if (flags & PGSTAT_BACKEND_FLUSH_WAL)
        pgstat_flush_backend_entry_wal(entry_ref);
 
    pgstat_unlock_entry(entry_ref);
 
    return false;
}
 
/*
 * Callback to flush out locally pending backend statistics.
 *
 * If some stats could not be flushed due to lock contention, return true.
 */
bool
pgstat_backend_flush_cb(bool nowait)
{
    return pgstat_flush_backend(nowait, PGSTAT_BACKEND_FLUSH_ALL);
}
 
/*
 * Create backend statistics entry for proc number.
 */
void
pgstat_create_backend(ProcNumber procnum)
{
    PgStat_EntryRef *entry_ref;
    PgStatShared_Backend *shstatent;
 
    entry_ref = pgstat_get_entry_ref_locked(PGSTAT_KIND_BACKEND, InvalidOid,
                                            MyProcNumber, false);
    shstatent = (PgStatShared_Backend *) entry_ref->shared_stats;
 
    /*
     * NB: need to accept that there might be stats from an older backend,
     * e.g. if we previously used this proc number.
     */
    memset(&shstatent->stats, 0, sizeof(shstatent->stats));
    pgstat_unlock_entry(entry_ref);
 
    MemSet(&PendingBackendStats, 0, sizeof(PgStat_BackendPending));
    backend_has_iostats = false;
 
    /*
     * Initialize prevBackendWalUsage with pgWalUsage so that
     * pgstat_backend_flush_cb() can calculate how much pgWalUsage counters
     * are increased by subtracting prevBackendWalUsage from pgWalUsage.
     */
    prevBackendWalUsage = pgWalUsage;
}
 
/*
 * Backend statistics are not collected for all BackendTypes.
 *
 * The following BackendTypes do not participate in the backend stats
 * subsystem:
 * - The same and for the same reasons as in pgstat_tracks_io_bktype().
 * - B_BG_WRITER, B_CHECKPOINTER, B_STARTUP and B_AUTOVAC_LAUNCHER because their
 * I/O stats are already visible in pg_stat_io and there is only one of those.
 *
 * Function returns true if BackendType participates in the backend stats
 * subsystem and false if it does not.
 *
 * When adding a new BackendType, also consider adding relevant restrictions to
 * pgstat_tracks_io_object() and pgstat_tracks_io_op().
 */
bool
pgstat_tracks_backend_bktype(BackendType bktype)
{
    /*
     * List every type so that new backend types trigger a warning about
     * needing to adjust this switch.
     */
    switch (bktype)
    {
        case B_INVALID:
        case B_AUTOVAC_LAUNCHER:
        case B_DEAD_END_BACKEND:
        case B_ARCHIVER:
        case B_LOGGER:
        case B_BG_WRITER:
        case B_CHECKPOINTER:
        case B_IO_WORKER:
        case B_STARTUP:
            return false;
 
        case B_AUTOVAC_WORKER:
        case B_BACKEND:
        case B_BG_WORKER:
        case B_STANDALONE_BACKEND:
        case B_SLOTSYNC_WORKER:
        case B_WAL_RECEIVER:
        case B_WAL_SENDER:
        case B_WAL_SUMMARIZER:
        case B_WAL_WRITER:
            return true;
    }
 
    return false;
}
 
void
pgstat_backend_reset_timestamp_cb(PgStatShared_Common *header, TimestampTz ts)
{
    ((PgStatShared_Backend *) header)->stats.stat_reset_timestamp = ts;
}