pgstat_bgwriter.c

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/* -------------------------------------------------------------------------
 *
 * pgstat_bgwriter.c
 *    Implementation of bgwriter statistics.
 *
 * This file contains the implementation of bgwriter 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.
 *
 * Copyright (c) 2001-2025, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *    src/backend/utils/activity/pgstat_bgwriter.c
 * -------------------------------------------------------------------------
 */
 
#include "postgres.h"
 
#include "utils/memutils.h"
#include "utils/pgstat_internal.h"
 
 
PgStat_BgWriterStats PendingBgWriterStats = {0};
 
 
/*
 * Report bgwriter and IO statistics
 */
void
pgstat_report_bgwriter(void)
{
    PgStatShared_BgWriter *stats_shmem = &pgStatLocal.shmem->bgwriter;
 
    Assert(!pgStatLocal.shmem->is_shutdown);
    pgstat_assert_is_up();
 
    /*
     * This function can be called even if nothing at all has happened. In
     * this case, avoid unnecessarily modifying the stats entry.
     */
    if (pg_memory_is_all_zeros(&PendingBgWriterStats,
                               sizeof(struct PgStat_BgWriterStats)))
        return;
 
    pgstat_begin_changecount_write(&stats_shmem->changecount);
 
#define BGWRITER_ACC(fld) stats_shmem->stats.fld += PendingBgWriterStats.fld
    BGWRITER_ACC(buf_written_clean);
    BGWRITER_ACC(maxwritten_clean);
    BGWRITER_ACC(buf_alloc);
#undef BGWRITER_ACC
 
    pgstat_end_changecount_write(&stats_shmem->changecount);
 
    /*
     * Clear out the statistics buffer, so it can be re-used.
     */
    MemSet(&PendingBgWriterStats, 0, sizeof(PendingBgWriterStats));
 
    /*
     * Report IO statistics
     */
    pgstat_flush_io(false);
}
 
/*
 * Support function for the SQL-callable pgstat* functions. Returns
 * a pointer to the bgwriter statistics struct.
 */
PgStat_BgWriterStats *
pgstat_fetch_stat_bgwriter(void)
{
    pgstat_snapshot_fixed(PGSTAT_KIND_BGWRITER);
 
    return &pgStatLocal.snapshot.bgwriter;
}
 
void
pgstat_bgwriter_init_shmem_cb(void *stats)
{
    PgStatShared_BgWriter *stats_shmem = (PgStatShared_BgWriter *) stats;
 
    LWLockInitialize(&stats_shmem->lock, LWTRANCHE_PGSTATS_DATA);
}
 
void
pgstat_bgwriter_reset_all_cb(TimestampTz ts)
{
    PgStatShared_BgWriter *stats_shmem = &pgStatLocal.shmem->bgwriter;
 
    /* see explanation above PgStatShared_BgWriter for the reset protocol */
    LWLockAcquire(&stats_shmem->lock, LW_EXCLUSIVE);
    pgstat_copy_changecounted_stats(&stats_shmem->reset_offset,
                                    &stats_shmem->stats,
                                    sizeof(stats_shmem->stats),
                                    &stats_shmem->changecount);
    stats_shmem->stats.stat_reset_timestamp = ts;
    LWLockRelease(&stats_shmem->lock);
}
 
void
pgstat_bgwriter_snapshot_cb(void)
{
    PgStatShared_BgWriter *stats_shmem = &pgStatLocal.shmem->bgwriter;
    PgStat_BgWriterStats *reset_offset = &stats_shmem->reset_offset;
    PgStat_BgWriterStats reset;
 
    pgstat_copy_changecounted_stats(&pgStatLocal.snapshot.bgwriter,
                                    &stats_shmem->stats,
                                    sizeof(stats_shmem->stats),
                                    &stats_shmem->changecount);
 
    LWLockAcquire(&stats_shmem->lock, LW_SHARED);
    memcpy(&reset, reset_offset, sizeof(stats_shmem->stats));
    LWLockRelease(&stats_shmem->lock);
 
    /* compensate by reset offsets */
#define BGWRITER_COMP(fld) pgStatLocal.snapshot.bgwriter.fld -= reset.fld;
    BGWRITER_COMP(buf_written_clean);
    BGWRITER_COMP(maxwritten_clean);
    BGWRITER_COMP(buf_alloc);
#undef BGWRITER_COMP
}