vacuumparallel.c

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/*-------------------------------------------------------------------------
 *
 * vacuumparallel.c
 *    Support routines for parallel vacuum execution.
 *
 * This file contains routines that are intended to support setting up, using,
 * and tearing down a ParallelVacuumState.
 *
 * In a parallel vacuum, we perform both index bulk deletion and index cleanup
 * with parallel worker processes.  Individual indexes are processed by one
 * vacuum process.  ParallelVacuumState contains shared information as well as
 * the memory space for storing dead items allocated in the DSA area.  We
 * launch parallel worker processes at the start of parallel index
 * bulk-deletion and index cleanup and once all indexes are processed, the
 * parallel worker processes exit.  Each time we process indexes in parallel,
 * the parallel context is re-initialized so that the same DSM can be used for
 * multiple passes of index bulk-deletion and index cleanup.
 *
 * Portions Copyright (c) 1996-2024, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *    src/backend/commands/vacuumparallel.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"
 
#include "access/amapi.h"
#include "access/table.h"
#include "access/xact.h"
#include "commands/progress.h"
#include "commands/vacuum.h"
#include "executor/instrument.h"
#include "optimizer/paths.h"
#include "pgstat.h"
#include "storage/bufmgr.h"
#include "tcop/tcopprot.h"
#include "utils/lsyscache.h"
#include "utils/rel.h"
 
/*
 * DSM keys for parallel vacuum.  Unlike other parallel execution code, since
 * we don't need to worry about DSM keys conflicting with plan_node_id we can
 * use small integers.
 */
#define PARALLEL_VACUUM_KEY_SHARED          1
#define PARALLEL_VACUUM_KEY_QUERY_TEXT      2
#define PARALLEL_VACUUM_KEY_BUFFER_USAGE    3
#define PARALLEL_VACUUM_KEY_WAL_USAGE       4
#define PARALLEL_VACUUM_KEY_INDEX_STATS     5
 
/*
 * Shared information among parallel workers.  So this is allocated in the DSM
 * segment.
 */
typedef struct PVShared
{
    /*
     * Target table relid and log level (for messages about parallel workers
     * launched during VACUUM VERBOSE).  These fields are not modified during
     * the parallel vacuum.
     */
    Oid         relid;
    int         elevel;
 
    /*
     * Fields for both index vacuum and cleanup.
     *
     * reltuples is the total number of input heap tuples.  We set either old
     * live tuples in the index vacuum case or the new live tuples in the
     * index cleanup case.
     *
     * estimated_count is true if reltuples is an estimated value.  (Note that
     * reltuples could be -1 in this case, indicating we have no idea.)
     */
    double      reltuples;
    bool        estimated_count;
 
    /*
     * In single process vacuum we could consume more memory during index
     * vacuuming or cleanup apart from the memory for heap scanning.  In
     * parallel vacuum, since individual vacuum workers can consume memory
     * equal to maintenance_work_mem, the new maintenance_work_mem for each
     * worker is set such that the parallel operation doesn't consume more
     * memory than single process vacuum.
     */
    int         maintenance_work_mem_worker;
 
    /*
     * The number of buffers each worker's Buffer Access Strategy ring should
     * contain.
     */
    int         ring_nbuffers;
 
    /*
     * Shared vacuum cost balance.  During parallel vacuum,
     * VacuumSharedCostBalance points to this value and it accumulates the
     * balance of each parallel vacuum worker.
     */
    pg_atomic_uint32 cost_balance;
 
    /*
     * Number of active parallel workers.  This is used for computing the
     * minimum threshold of the vacuum cost balance before a worker sleeps for
     * cost-based delay.
     */
    pg_atomic_uint32 active_nworkers;
 
    /* Counter for vacuuming and cleanup */
    pg_atomic_uint32 idx;
 
    /* DSA handle where the TidStore lives */
    dsa_handle  dead_items_dsa_handle;
 
    /* DSA pointer to the shared TidStore */
    dsa_pointer dead_items_handle;
 
    /* Statistics of shared dead items */
    VacDeadItemsInfo dead_items_info;
} PVShared;
 
/* Status used during parallel index vacuum or cleanup */
typedef enum PVIndVacStatus
{
    PARALLEL_INDVAC_STATUS_INITIAL = 0,
    PARALLEL_INDVAC_STATUS_NEED_BULKDELETE,
    PARALLEL_INDVAC_STATUS_NEED_CLEANUP,
    PARALLEL_INDVAC_STATUS_COMPLETED,
} PVIndVacStatus;
 
/*
 * Struct for index vacuum statistics of an index that is used for parallel vacuum.
 * This includes the status of parallel index vacuum as well as index statistics.
 */
typedef struct PVIndStats
{
    /*
     * The following two fields are set by leader process before executing
     * parallel index vacuum or parallel index cleanup.  These fields are not
     * fixed for the entire VACUUM operation.  They are only fixed for an
     * individual parallel index vacuum and cleanup.
     *
     * parallel_workers_can_process is true if both leader and worker can
     * process the index, otherwise only leader can process it.
     */
    PVIndVacStatus status;
    bool        parallel_workers_can_process;
 
    /*
     * Individual worker or leader stores the result of index vacuum or
     * cleanup.
     */
    bool        istat_updated;  /* are the stats updated? */
    IndexBulkDeleteResult istat;
} PVIndStats;
 
/*
 * Struct for maintaining a parallel vacuum state. typedef appears in vacuum.h.
 */
struct ParallelVacuumState
{
    /* NULL for worker processes */
    ParallelContext *pcxt;
 
    /* Parent Heap Relation */
    Relation    heaprel;
 
    /* Target indexes */
    Relation   *indrels;
    int         nindexes;
 
    /* Shared information among parallel vacuum workers */
    PVShared   *shared;
 
    /*
     * Shared index statistics among parallel vacuum workers. The array
     * element is allocated for every index, even those indexes where parallel
     * index vacuuming is unsafe or not worthwhile (e.g.,
     * will_parallel_vacuum[] is false).  During parallel vacuum,
     * IndexBulkDeleteResult of each index is kept in DSM and is copied into
     * local memory at the end of parallel vacuum.
     */
    PVIndStats *indstats;
 
    /* Shared dead items space among parallel vacuum workers */
    TidStore   *dead_items;
 
    /* Points to buffer usage area in DSM */
    BufferUsage *buffer_usage;
 
    /* Points to WAL usage area in DSM */
    WalUsage   *wal_usage;
 
    /*
     * False if the index is totally unsuitable target for all parallel
     * processing. For example, the index could be <
     * min_parallel_index_scan_size cutoff.
     */
    bool       *will_parallel_vacuum;
 
    /*
     * The number of indexes that support parallel index bulk-deletion and
     * parallel index cleanup respectively.
     */
    int         nindexes_parallel_bulkdel;
    int         nindexes_parallel_cleanup;
    int         nindexes_parallel_condcleanup;
 
    /* Buffer access strategy used by leader process */
    BufferAccessStrategy bstrategy;
 
    /*
     * Error reporting state.  The error callback is set only for workers
     * processes during parallel index vacuum.
     */
    char       *relnamespace;
    char       *relname;
    char       *indname;
    PVIndVacStatus status;
};
 
static int  parallel_vacuum_compute_workers(Relation *indrels, int nindexes, int nrequested,
                                            bool *will_parallel_vacuum);
static void parallel_vacuum_process_all_indexes(ParallelVacuumState *pvs, int num_index_scans,
                                                bool vacuum);
static void parallel_vacuum_process_safe_indexes(ParallelVacuumState *pvs);
static void parallel_vacuum_process_unsafe_indexes(ParallelVacuumState *pvs);
static void parallel_vacuum_process_one_index(ParallelVacuumState *pvs, Relation indrel,
                                              PVIndStats *indstats);
static bool parallel_vacuum_index_is_parallel_safe(Relation indrel, int num_index_scans,
                                                   bool vacuum);
static void parallel_vacuum_error_callback(void *arg);
 
/*
 * Try to enter parallel mode and create a parallel context.  Then initialize
 * shared memory state.
 *
 * On success, return parallel vacuum state.  Otherwise return NULL.
 */
ParallelVacuumState *
parallel_vacuum_init(Relation rel, Relation *indrels, int nindexes,
                     int nrequested_workers, int vac_work_mem,
                     int elevel, BufferAccessStrategy bstrategy)
{
    ParallelVacuumState *pvs;
    ParallelContext *pcxt;
    PVShared   *shared;
    TidStore   *dead_items;
    PVIndStats *indstats;
    BufferUsage *buffer_usage;
    WalUsage   *wal_usage;
    bool       *will_parallel_vacuum;
    Size        est_indstats_len;
    Size        est_shared_len;
    int         nindexes_mwm = 0;
    int         parallel_workers = 0;
    int         querylen;
 
    /*
     * A parallel vacuum must be requested and there must be indexes on the
     * relation
     */
    Assert(nrequested_workers >= 0);
    Assert(nindexes > 0);
 
    /*
     * Compute the number of parallel vacuum workers to launch
     */
    will_parallel_vacuum = (bool *) palloc0(sizeof(bool) * nindexes);
    parallel_workers = parallel_vacuum_compute_workers(indrels, nindexes,
                                                       nrequested_workers,
                                                       will_parallel_vacuum);
    if (parallel_workers <= 0)
    {
        /* Can't perform vacuum in parallel -- return NULL */
        pfree(will_parallel_vacuum);
        return NULL;
    }
 
    pvs = (ParallelVacuumState *) palloc0(sizeof(ParallelVacuumState));
    pvs->indrels = indrels;
    pvs->nindexes = nindexes;
    pvs->will_parallel_vacuum = will_parallel_vacuum;
    pvs->bstrategy = bstrategy;
    pvs->heaprel = rel;
 
    EnterParallelMode();
    pcxt = CreateParallelContext("postgres", "parallel_vacuum_main",
                                 parallel_workers);
    Assert(pcxt->nworkers > 0);
    pvs->pcxt = pcxt;
 
    /* Estimate size for index vacuum stats -- PARALLEL_VACUUM_KEY_INDEX_STATS */
    est_indstats_len = mul_size(sizeof(PVIndStats), nindexes);
    shm_toc_estimate_chunk(&pcxt->estimator, est_indstats_len);
    shm_toc_estimate_keys(&pcxt->estimator, 1);
 
    /* Estimate size for shared information -- PARALLEL_VACUUM_KEY_SHARED */
    est_shared_len = sizeof(PVShared);
    shm_toc_estimate_chunk(&pcxt->estimator, est_shared_len);
    shm_toc_estimate_keys(&pcxt->estimator, 1);
 
    /*
     * Estimate space for BufferUsage and WalUsage --
     * PARALLEL_VACUUM_KEY_BUFFER_USAGE and PARALLEL_VACUUM_KEY_WAL_USAGE.
     *
     * If there are no extensions loaded that care, we could skip this.  We
     * have no way of knowing whether anyone's looking at pgBufferUsage or
     * pgWalUsage, so do it unconditionally.
     */
    shm_toc_estimate_chunk(&pcxt->estimator,
                           mul_size(sizeof(BufferUsage), pcxt->nworkers));
    shm_toc_estimate_keys(&pcxt->estimator, 1);
    shm_toc_estimate_chunk(&pcxt->estimator,
                           mul_size(sizeof(WalUsage), pcxt->nworkers));
    shm_toc_estimate_keys(&pcxt->estimator, 1);
 
    /* Finally, estimate PARALLEL_VACUUM_KEY_QUERY_TEXT space */
    if (debug_query_string)
    {
        querylen = strlen(debug_query_string);
        shm_toc_estimate_chunk(&pcxt->estimator, querylen + 1);
        shm_toc_estimate_keys(&pcxt->estimator, 1);
    }
    else
        querylen = 0;           /* keep compiler quiet */
 
    InitializeParallelDSM(pcxt);
 
    /* Prepare index vacuum stats */
    indstats = (PVIndStats *) shm_toc_allocate(pcxt->toc, est_indstats_len);
    MemSet(indstats, 0, est_indstats_len);
    for (int i = 0; i < nindexes; i++)
    {
        Relation    indrel = indrels[i];
        uint8       vacoptions = indrel->rd_indam->amparallelvacuumoptions;
 
        /*
         * Cleanup option should be either disabled, always performing in
         * parallel or conditionally performing in parallel.
         */
        Assert(((vacoptions & VACUUM_OPTION_PARALLEL_CLEANUP) == 0) ||
               ((vacoptions & VACUUM_OPTION_PARALLEL_COND_CLEANUP) == 0));
        Assert(vacoptions <= VACUUM_OPTION_MAX_VALID_VALUE);
 
        if (!will_parallel_vacuum[i])
            continue;
 
        if (indrel->rd_indam->amusemaintenanceworkmem)
            nindexes_mwm++;
 
        /*
         * Remember the number of indexes that support parallel operation for
         * each phase.
         */
        if ((vacoptions & VACUUM_OPTION_PARALLEL_BULKDEL) != 0)
            pvs->nindexes_parallel_bulkdel++;
        if ((vacoptions & VACUUM_OPTION_PARALLEL_CLEANUP) != 0)
            pvs->nindexes_parallel_cleanup++;
        if ((vacoptions & VACUUM_OPTION_PARALLEL_COND_CLEANUP) != 0)
            pvs->nindexes_parallel_condcleanup++;
    }
    shm_toc_insert(pcxt->toc, PARALLEL_VACUUM_KEY_INDEX_STATS, indstats);
    pvs->indstats = indstats;
 
    /* Prepare shared information */
    shared = (PVShared *) shm_toc_allocate(pcxt->toc, est_shared_len);
    MemSet(shared, 0, est_shared_len);
    shared->relid = RelationGetRelid(rel);
    shared->elevel = elevel;
    shared->maintenance_work_mem_worker =
        (nindexes_mwm > 0) ?
        maintenance_work_mem / Min(parallel_workers, nindexes_mwm) :
        maintenance_work_mem;
    shared->dead_items_info.max_bytes = vac_work_mem * 1024L;
 
    /* Prepare DSA space for dead items */
    dead_items = TidStoreCreateShared(shared->dead_items_info.max_bytes,
                                      LWTRANCHE_PARALLEL_VACUUM_DSA);
    pvs->dead_items = dead_items;
    shared->dead_items_handle = TidStoreGetHandle(dead_items);
    shared->dead_items_dsa_handle = dsa_get_handle(TidStoreGetDSA(dead_items));
 
    /* Use the same buffer size for all workers */
    shared->ring_nbuffers = GetAccessStrategyBufferCount(bstrategy);
 
    pg_atomic_init_u32(&(shared->cost_balance), 0);
    pg_atomic_init_u32(&(shared->active_nworkers), 0);
    pg_atomic_init_u32(&(shared->idx), 0);
 
    shm_toc_insert(pcxt->toc, PARALLEL_VACUUM_KEY_SHARED, shared);
    pvs->shared = shared;
 
    /*
     * Allocate space for each worker's BufferUsage and WalUsage; no need to
     * initialize
     */
    buffer_usage = shm_toc_allocate(pcxt->toc,
                                    mul_size(sizeof(BufferUsage), pcxt->nworkers));
    shm_toc_insert(pcxt->toc, PARALLEL_VACUUM_KEY_BUFFER_USAGE, buffer_usage);
    pvs->buffer_usage = buffer_usage;
    wal_usage = shm_toc_allocate(pcxt->toc,
                                 mul_size(sizeof(WalUsage), pcxt->nworkers));
    shm_toc_insert(pcxt->toc, PARALLEL_VACUUM_KEY_WAL_USAGE, wal_usage);
    pvs->wal_usage = wal_usage;
 
    /* Store query string for workers */
    if (debug_query_string)
    {
        char       *sharedquery;
 
        sharedquery = (char *) shm_toc_allocate(pcxt->toc, querylen + 1);
        memcpy(sharedquery, debug_query_string, querylen + 1);
        sharedquery[querylen] = '\0';
        shm_toc_insert(pcxt->toc,
                       PARALLEL_VACUUM_KEY_QUERY_TEXT, sharedquery);
    }
 
    /* Success -- return parallel vacuum state */
    return pvs;
}
 
/*
 * Destroy the parallel context, and end parallel mode.
 *
 * Since writes are not allowed during parallel mode, copy the
 * updated index statistics from DSM into local memory and then later use that
 * to update the index statistics.  One might think that we can exit from
 * parallel mode, update the index statistics and then destroy parallel
 * context, but that won't be safe (see ExitParallelMode).
 */
void
parallel_vacuum_end(ParallelVacuumState *pvs, IndexBulkDeleteResult **istats)
{
    Assert(!IsParallelWorker());
 
    /* Copy the updated statistics */
    for (int i = 0; i < pvs->nindexes; i++)
    {
        PVIndStats *indstats = &(pvs->indstats[i]);
 
        if (indstats->istat_updated)
        {
            istats[i] = (IndexBulkDeleteResult *) palloc0(sizeof(IndexBulkDeleteResult));
            memcpy(istats[i], &indstats->istat, sizeof(IndexBulkDeleteResult));
        }
        else
            istats[i] = NULL;
    }
 
    TidStoreDestroy(pvs->dead_items);
 
    DestroyParallelContext(pvs->pcxt);
    ExitParallelMode();
 
    pfree(pvs->will_parallel_vacuum);
    pfree(pvs);
}
 
/*
 * Returns the dead items space and dead items information.
 */
TidStore *
parallel_vacuum_get_dead_items(ParallelVacuumState *pvs, VacDeadItemsInfo **dead_items_info_p)
{
    *dead_items_info_p = &(pvs->shared->dead_items_info);
    return pvs->dead_items;
}
 
/* Forget all items in dead_items */
void
parallel_vacuum_reset_dead_items(ParallelVacuumState *pvs)
{
    TidStore   *dead_items = pvs->dead_items;
    VacDeadItemsInfo *dead_items_info = &(pvs->shared->dead_items_info);
 
    /*
     * Free the current tidstore and return allocated DSA segments to the
     * operating system. Then we recreate the tidstore with the same max_bytes
     * limitation we just used.
     */
    TidStoreDestroy(dead_items);
    pvs->dead_items = TidStoreCreateShared(dead_items_info->max_bytes,
                                           LWTRANCHE_PARALLEL_VACUUM_DSA);
 
    /* Update the DSA pointer for dead_items to the new one */
    pvs->shared->dead_items_dsa_handle = dsa_get_handle(TidStoreGetDSA(dead_items));
    pvs->shared->dead_items_handle = TidStoreGetHandle(dead_items);
 
    /* Reset the counter */
    dead_items_info->num_items = 0;
}
 
/*
 * Do parallel index bulk-deletion with parallel workers.
 */
void
parallel_vacuum_bulkdel_all_indexes(ParallelVacuumState *pvs, long num_table_tuples,
                                    int num_index_scans)
{
    Assert(!IsParallelWorker());
 
    /*
     * We can only provide an approximate value of num_heap_tuples, at least
     * for now.
     */
    pvs->shared->reltuples = num_table_tuples;
    pvs->shared->estimated_count = true;
 
    parallel_vacuum_process_all_indexes(pvs, num_index_scans, true);
}
 
/*
 * Do parallel index cleanup with parallel workers.
 */
void
parallel_vacuum_cleanup_all_indexes(ParallelVacuumState *pvs, long num_table_tuples,
                                    int num_index_scans, bool estimated_count)
{
    Assert(!IsParallelWorker());
 
    /*
     * We can provide a better estimate of total number of surviving tuples
     * (we assume indexes are more interested in that than in the number of
     * nominally live tuples).
     */
    pvs->shared->reltuples = num_table_tuples;
    pvs->shared->estimated_count = estimated_count;
 
    parallel_vacuum_process_all_indexes(pvs, num_index_scans, false);
}
 
/*
 * Compute the number of parallel worker processes to request.  Both index
 * vacuum and index cleanup can be executed with parallel workers.
 * The index is eligible for parallel vacuum iff its size is greater than
 * min_parallel_index_scan_size as invoking workers for very small indexes
 * can hurt performance.
 *
 * nrequested is the number of parallel workers that user requested.  If
 * nrequested is 0, we compute the parallel degree based on nindexes, that is
 * the number of indexes that support parallel vacuum.  This function also
 * sets will_parallel_vacuum to remember indexes that participate in parallel
 * vacuum.
 */
static int
parallel_vacuum_compute_workers(Relation *indrels, int nindexes, int nrequested,
                                bool *will_parallel_vacuum)
{
    int         nindexes_parallel = 0;
    int         nindexes_parallel_bulkdel = 0;
    int         nindexes_parallel_cleanup = 0;
    int         parallel_workers;
 
    /*
     * We don't allow performing parallel operation in standalone backend or
     * when parallelism is disabled.
     */
    if (!IsUnderPostmaster || max_parallel_maintenance_workers == 0)
        return 0;
 
    /*
     * Compute the number of indexes that can participate in parallel vacuum.
     */
    for (int i = 0; i < nindexes; i++)
    {
        Relation    indrel = indrels[i];
        uint8       vacoptions = indrel->rd_indam->amparallelvacuumoptions;
 
        /* Skip index that is not a suitable target for parallel index vacuum */
        if (vacoptions == VACUUM_OPTION_NO_PARALLEL ||
            RelationGetNumberOfBlocks(indrel) < min_parallel_index_scan_size)
            continue;
 
        will_parallel_vacuum[i] = true;
 
        if ((vacoptions & VACUUM_OPTION_PARALLEL_BULKDEL) != 0)
            nindexes_parallel_bulkdel++;
        if (((vacoptions & VACUUM_OPTION_PARALLEL_CLEANUP) != 0) ||
            ((vacoptions & VACUUM_OPTION_PARALLEL_COND_CLEANUP) != 0))
            nindexes_parallel_cleanup++;
    }
 
    nindexes_parallel = Max(nindexes_parallel_bulkdel,
                            nindexes_parallel_cleanup);
 
    /* The leader process takes one index */
    nindexes_parallel--;
 
    /* No index supports parallel vacuum */
    if (nindexes_parallel <= 0)
        return 0;
 
    /* Compute the parallel degree */
    parallel_workers = (nrequested > 0) ?
        Min(nrequested, nindexes_parallel) : nindexes_parallel;
 
    /* Cap by max_parallel_maintenance_workers */
    parallel_workers = Min(parallel_workers, max_parallel_maintenance_workers);
 
    return parallel_workers;
}
 
/*
 * Perform index vacuum or index cleanup with parallel workers.  This function
 * must be used by the parallel vacuum leader process.
 */
static void
parallel_vacuum_process_all_indexes(ParallelVacuumState *pvs, int num_index_scans,
                                    bool vacuum)
{
    int         nworkers;
    PVIndVacStatus new_status;
 
    Assert(!IsParallelWorker());
 
    if (vacuum)
    {
        new_status = PARALLEL_INDVAC_STATUS_NEED_BULKDELETE;
 
        /* Determine the number of parallel workers to launch */
        nworkers = pvs->nindexes_parallel_bulkdel;
    }
    else
    {
        new_status = PARALLEL_INDVAC_STATUS_NEED_CLEANUP;
 
        /* Determine the number of parallel workers to launch */
        nworkers = pvs->nindexes_parallel_cleanup;
 
        /* Add conditionally parallel-aware indexes if in the first time call */
        if (num_index_scans == 0)
            nworkers += pvs->nindexes_parallel_condcleanup;
    }
 
    /* The leader process will participate */
    nworkers--;
 
    /*
     * It is possible that parallel context is initialized with fewer workers
     * than the number of indexes that need a separate worker in the current
     * phase, so we need to consider it.  See
     * parallel_vacuum_compute_workers().
     */
    nworkers = Min(nworkers, pvs->pcxt->nworkers);
 
    /*
     * Set index vacuum status and mark whether parallel vacuum worker can
     * process it.
     */
    for (int i = 0; i < pvs->nindexes; i++)
    {
        PVIndStats *indstats = &(pvs->indstats[i]);
 
        Assert(indstats->status == PARALLEL_INDVAC_STATUS_INITIAL);
        indstats->status = new_status;
        indstats->parallel_workers_can_process =
            (pvs->will_parallel_vacuum[i] &&
             parallel_vacuum_index_is_parallel_safe(pvs->indrels[i],
                                                    num_index_scans,
                                                    vacuum));
    }
 
    /* Reset the parallel index processing and progress counters */
    pg_atomic_write_u32(&(pvs->shared->idx), 0);
 
    /* Setup the shared cost-based vacuum delay and launch workers */
    if (nworkers > 0)
    {
        /* Reinitialize parallel context to relaunch parallel workers */
        if (num_index_scans > 0)
            ReinitializeParallelDSM(pvs->pcxt);
 
        /*
         * Set up shared cost balance and the number of active workers for
         * vacuum delay.  We need to do this before launching workers as
         * otherwise, they might not see the updated values for these
         * parameters.
         */
        pg_atomic_write_u32(&(pvs->shared->cost_balance), VacuumCostBalance);
        pg_atomic_write_u32(&(pvs->shared->active_nworkers), 0);
 
        /*
         * The number of workers can vary between bulkdelete and cleanup
         * phase.
         */
        ReinitializeParallelWorkers(pvs->pcxt, nworkers);
 
        LaunchParallelWorkers(pvs->pcxt);
 
        if (pvs->pcxt->nworkers_launched > 0)
        {
            /*
             * Reset the local cost values for leader backend as we have
             * already accumulated the remaining balance of heap.
             */
            VacuumCostBalance = 0;
            VacuumCostBalanceLocal = 0;
 
            /* Enable shared cost balance for leader backend */
            VacuumSharedCostBalance = &(pvs->shared->cost_balance);
            VacuumActiveNWorkers = &(pvs->shared->active_nworkers);
        }
 
        if (vacuum)
            ereport(pvs->shared->elevel,
                    (errmsg(ngettext("launched %d parallel vacuum worker for index vacuuming (planned: %d)",
                                     "launched %d parallel vacuum workers for index vacuuming (planned: %d)",
                                     pvs->pcxt->nworkers_launched),
                            pvs->pcxt->nworkers_launched, nworkers)));
        else
            ereport(pvs->shared->elevel,
                    (errmsg(ngettext("launched %d parallel vacuum worker for index cleanup (planned: %d)",
                                     "launched %d parallel vacuum workers for index cleanup (planned: %d)",
                                     pvs->pcxt->nworkers_launched),
                            pvs->pcxt->nworkers_launched, nworkers)));
    }
 
    /* Vacuum the indexes that can be processed by only leader process */
    parallel_vacuum_process_unsafe_indexes(pvs);
 
    /*
     * Join as a parallel worker.  The leader vacuums alone processes all
     * parallel-safe indexes in the case where no workers are launched.
     */
    parallel_vacuum_process_safe_indexes(pvs);
 
    /*
     * Next, accumulate buffer and WAL usage.  (This must wait for the workers
     * to finish, or we might get incomplete data.)
     */
    if (nworkers > 0)
    {
        /* Wait for all vacuum workers to finish */
        WaitForParallelWorkersToFinish(pvs->pcxt);
 
        for (int i = 0; i < pvs->pcxt->nworkers_launched; i++)
            InstrAccumParallelQuery(&pvs->buffer_usage[i], &pvs->wal_usage[i]);
    }
 
    /*
     * Reset all index status back to initial (while checking that we have
     * vacuumed all indexes).
     */
    for (int i = 0; i < pvs->nindexes; i++)
    {
        PVIndStats *indstats = &(pvs->indstats[i]);
 
        if (indstats->status != PARALLEL_INDVAC_STATUS_COMPLETED)
            elog(ERROR, "parallel index vacuum on index \"%s\" is not completed",
                 RelationGetRelationName(pvs->indrels[i]));
 
        indstats->status = PARALLEL_INDVAC_STATUS_INITIAL;
    }
 
    /*
     * Carry the shared balance value to heap scan and disable shared costing
     */
    if (VacuumSharedCostBalance)
    {
        VacuumCostBalance = pg_atomic_read_u32(VacuumSharedCostBalance);
        VacuumSharedCostBalance = NULL;
        VacuumActiveNWorkers = NULL;
    }
}
 
/*
 * Index vacuum/cleanup routine used by the leader process and parallel
 * vacuum worker processes to vacuum the indexes in parallel.
 */
static void
parallel_vacuum_process_safe_indexes(ParallelVacuumState *pvs)
{
    /*
     * Increment the active worker count if we are able to launch any worker.
     */
    if (VacuumActiveNWorkers)
        pg_atomic_add_fetch_u32(VacuumActiveNWorkers, 1);
 
    /* Loop until all indexes are vacuumed */
    for (;;)
    {
        int         idx;
        PVIndStats *indstats;
 
        /* Get an index number to process */
        idx = pg_atomic_fetch_add_u32(&(pvs->shared->idx), 1);
 
        /* Done for all indexes? */
        if (idx >= pvs->nindexes)
            break;
 
        indstats = &(pvs->indstats[idx]);
 
        /*
         * Skip vacuuming index that is unsafe for workers or has an
         * unsuitable target for parallel index vacuum (this is vacuumed in
         * parallel_vacuum_process_unsafe_indexes() by the leader).
         */
        if (!indstats->parallel_workers_can_process)
            continue;
 
        /* Do vacuum or cleanup of the index */
        parallel_vacuum_process_one_index(pvs, pvs->indrels[idx], indstats);
    }
 
    /*
     * We have completed the index vacuum so decrement the active worker
     * count.
     */
    if (VacuumActiveNWorkers)
        pg_atomic_sub_fetch_u32(VacuumActiveNWorkers, 1);
}
 
/*
 * Perform parallel vacuuming of indexes in leader process.
 *
 * Handles index vacuuming (or index cleanup) for indexes that are not
 * parallel safe.  It's possible that this will vary for a given index, based
 * on details like whether we're performing index cleanup right now.
 *
 * Also performs vacuuming of smaller indexes that fell under the size cutoff
 * enforced by parallel_vacuum_compute_workers().
 */
static void
parallel_vacuum_process_unsafe_indexes(ParallelVacuumState *pvs)
{
    Assert(!IsParallelWorker());
 
    /*
     * Increment the active worker count if we are able to launch any worker.
     */
    if (VacuumActiveNWorkers)
        pg_atomic_add_fetch_u32(VacuumActiveNWorkers, 1);
 
    for (int i = 0; i < pvs->nindexes; i++)
    {
        PVIndStats *indstats = &(pvs->indstats[i]);
 
        /* Skip, indexes that are safe for workers */
        if (indstats->parallel_workers_can_process)
            continue;
 
        /* Do vacuum or cleanup of the index */
        parallel_vacuum_process_one_index(pvs, pvs->indrels[i], indstats);
    }
 
    /*
     * We have completed the index vacuum so decrement the active worker
     * count.
     */
    if (VacuumActiveNWorkers)
        pg_atomic_sub_fetch_u32(VacuumActiveNWorkers, 1);
}
 
/*
 * Vacuum or cleanup index either by leader process or by one of the worker
 * process.  After vacuuming the index this function copies the index
 * statistics returned from ambulkdelete and amvacuumcleanup to the DSM
 * segment.
 */
static void
parallel_vacuum_process_one_index(ParallelVacuumState *pvs, Relation indrel,
                                  PVIndStats *indstats)
{
    IndexBulkDeleteResult *istat = NULL;
    IndexBulkDeleteResult *istat_res;
    IndexVacuumInfo ivinfo;
 
    /*
     * Update the pointer to the corresponding bulk-deletion result if someone
     * has already updated it
     */
    if (indstats->istat_updated)
        istat = &(indstats->istat);
 
    ivinfo.index = indrel;
    ivinfo.heaprel = pvs->heaprel;
    ivinfo.analyze_only = false;
    ivinfo.report_progress = false;
    ivinfo.message_level = DEBUG2;
    ivinfo.estimated_count = pvs->shared->estimated_count;
    ivinfo.num_heap_tuples = pvs->shared->reltuples;
    ivinfo.strategy = pvs->bstrategy;
 
    /* Update error traceback information */
    pvs->indname = pstrdup(RelationGetRelationName(indrel));
    pvs->status = indstats->status;
 
    switch (indstats->status)
    {
        case PARALLEL_INDVAC_STATUS_NEED_BULKDELETE:
            istat_res = vac_bulkdel_one_index(&ivinfo, istat, pvs->dead_items,
                                              &pvs->shared->dead_items_info);
            break;
        case PARALLEL_INDVAC_STATUS_NEED_CLEANUP:
            istat_res = vac_cleanup_one_index(&ivinfo, istat);
            break;
        default:
            elog(ERROR, "unexpected parallel vacuum index status %d for index \"%s\"",
                 indstats->status,
                 RelationGetRelationName(indrel));
    }
 
    /*
     * Copy the index bulk-deletion result returned from ambulkdelete and
     * amvacuumcleanup to the DSM segment if it's the first cycle because they
     * allocate locally and it's possible that an index will be vacuumed by a
     * different vacuum process the next cycle.  Copying the result normally
     * happens only the first time an index is vacuumed.  For any additional
     * vacuum pass, we directly point to the result on the DSM segment and
     * pass it to vacuum index APIs so that workers can update it directly.
     *
     * Since all vacuum workers write the bulk-deletion result at different
     * slots we can write them without locking.
     */
    if (!indstats->istat_updated && istat_res != NULL)
    {
        memcpy(&(indstats->istat), istat_res, sizeof(IndexBulkDeleteResult));
        indstats->istat_updated = true;
 
        /* Free the locally-allocated bulk-deletion result */
        pfree(istat_res);
    }
 
    /*
     * Update the status to completed. No need to lock here since each worker
     * touches different indexes.
     */
    indstats->status = PARALLEL_INDVAC_STATUS_COMPLETED;
 
    /* Reset error traceback information */
    pvs->status = PARALLEL_INDVAC_STATUS_COMPLETED;
    pfree(pvs->indname);
    pvs->indname = NULL;
 
    /*
     * Call the parallel variant of pgstat_progress_incr_param so workers can
     * report progress of index vacuum to the leader.
     */
    pgstat_progress_parallel_incr_param(PROGRESS_VACUUM_INDEXES_PROCESSED, 1);
}
 
/*
 * Returns false, if the given index can't participate in the next execution of
 * parallel index vacuum or parallel index cleanup.
 */
static bool
parallel_vacuum_index_is_parallel_safe(Relation indrel, int num_index_scans,
                                       bool vacuum)
{
    uint8       vacoptions;
 
    vacoptions = indrel->rd_indam->amparallelvacuumoptions;
 
    /* In parallel vacuum case, check if it supports parallel bulk-deletion */
    if (vacuum)
        return ((vacoptions & VACUUM_OPTION_PARALLEL_BULKDEL) != 0);
 
    /* Not safe, if the index does not support parallel cleanup */
    if (((vacoptions & VACUUM_OPTION_PARALLEL_CLEANUP) == 0) &&
        ((vacoptions & VACUUM_OPTION_PARALLEL_COND_CLEANUP) == 0))
        return false;
 
    /*
     * Not safe, if the index supports parallel cleanup conditionally, but we
     * have already processed the index (for bulkdelete).  We do this to avoid
     * the need to invoke workers when parallel index cleanup doesn't need to
     * scan the index.  See the comments for option
     * VACUUM_OPTION_PARALLEL_COND_CLEANUP to know when indexes support
     * parallel cleanup conditionally.
     */
    if (num_index_scans > 0 &&
        ((vacoptions & VACUUM_OPTION_PARALLEL_COND_CLEANUP) != 0))
        return false;
 
    return true;
}
 
/*
 * Perform work within a launched parallel process.
 *
 * Since parallel vacuum workers perform only index vacuum or index cleanup,
 * we don't need to report progress information.
 */
void
parallel_vacuum_main(dsm_segment *seg, shm_toc *toc)
{
    ParallelVacuumState pvs;
    Relation    rel;
    Relation   *indrels;
    PVIndStats *indstats;
    PVShared   *shared;
    TidStore   *dead_items;
    BufferUsage *buffer_usage;
    WalUsage   *wal_usage;
    int         nindexes;
    char       *sharedquery;
    ErrorContextCallback errcallback;
 
    /*
     * A parallel vacuum worker must have only PROC_IN_VACUUM flag since we
     * don't support parallel vacuum for autovacuum as of now.
     */
    Assert(MyProc->statusFlags == PROC_IN_VACUUM);
 
    elog(DEBUG1, "starting parallel vacuum worker");
 
    shared = (PVShared *) shm_toc_lookup(toc, PARALLEL_VACUUM_KEY_SHARED, false);
 
    /* Set debug_query_string for individual workers */
    sharedquery = shm_toc_lookup(toc, PARALLEL_VACUUM_KEY_QUERY_TEXT, true);
    debug_query_string = sharedquery;
    pgstat_report_activity(STATE_RUNNING, debug_query_string);
 
    /*
     * Open table.  The lock mode is the same as the leader process.  It's
     * okay because the lock mode does not conflict among the parallel
     * workers.
     */
    rel = table_open(shared->relid, ShareUpdateExclusiveLock);
 
    /*
     * Open all indexes. indrels are sorted in order by OID, which should be
     * matched to the leader's one.
     */
    vac_open_indexes(rel, RowExclusiveLock, &nindexes, &indrels);
    Assert(nindexes > 0);
 
    if (shared->maintenance_work_mem_worker > 0)
        maintenance_work_mem = shared->maintenance_work_mem_worker;
 
    /* Set index statistics */
    indstats = (PVIndStats *) shm_toc_lookup(toc,
                                             PARALLEL_VACUUM_KEY_INDEX_STATS,
                                             false);
 
    /* Find dead_items in shared memory */
    dead_items = TidStoreAttach(shared->dead_items_dsa_handle,
                                shared->dead_items_handle);
 
    /* Set cost-based vacuum delay */
    VacuumUpdateCosts();
    VacuumCostBalance = 0;
    VacuumPageHit = 0;
    VacuumPageMiss = 0;
    VacuumPageDirty = 0;
    VacuumCostBalanceLocal = 0;
    VacuumSharedCostBalance = &(shared->cost_balance);
    VacuumActiveNWorkers = &(shared->active_nworkers);
 
    /* Set parallel vacuum state */
    pvs.indrels = indrels;
    pvs.nindexes = nindexes;
    pvs.indstats = indstats;
    pvs.shared = shared;
    pvs.dead_items = dead_items;
    pvs.relnamespace = get_namespace_name(RelationGetNamespace(rel));
    pvs.relname = pstrdup(RelationGetRelationName(rel));
    pvs.heaprel = rel;
 
    /* These fields will be filled during index vacuum or cleanup */
    pvs.indname = NULL;
    pvs.status = PARALLEL_INDVAC_STATUS_INITIAL;
 
    /* Each parallel VACUUM worker gets its own access strategy. */
    pvs.bstrategy = GetAccessStrategyWithSize(BAS_VACUUM,
                                              shared->ring_nbuffers * (BLCKSZ / 1024));
 
    /* Setup error traceback support for ereport() */
    errcallback.callback = parallel_vacuum_error_callback;
    errcallback.arg = &pvs;
    errcallback.previous = error_context_stack;
    error_context_stack = &errcallback;
 
    /* Prepare to track buffer usage during parallel execution */
    InstrStartParallelQuery();
 
    /* Process indexes to perform vacuum/cleanup */
    parallel_vacuum_process_safe_indexes(&pvs);
 
    /* Report buffer/WAL usage during parallel execution */
    buffer_usage = shm_toc_lookup(toc, PARALLEL_VACUUM_KEY_BUFFER_USAGE, false);
    wal_usage = shm_toc_lookup(toc, PARALLEL_VACUUM_KEY_WAL_USAGE, false);
    InstrEndParallelQuery(&buffer_usage[ParallelWorkerNumber],
                          &wal_usage[ParallelWorkerNumber]);
 
    TidStoreDetach(dead_items);
 
    /* Pop the error context stack */
    error_context_stack = errcallback.previous;
 
    vac_close_indexes(nindexes, indrels, RowExclusiveLock);
    table_close(rel, ShareUpdateExclusiveLock);
    FreeAccessStrategy(pvs.bstrategy);
}
 
/*
 * Error context callback for errors occurring during parallel index vacuum.
 * The error context messages should match the messages set in the lazy vacuum
 * error context.  If you change this function, change vacuum_error_callback()
 * as well.
 */
static void
parallel_vacuum_error_callback(void *arg)
{
    ParallelVacuumState *errinfo = arg;
 
    switch (errinfo->status)
    {
        case PARALLEL_INDVAC_STATUS_NEED_BULKDELETE:
            errcontext("while vacuuming index \"%s\" of relation \"%s.%s\"",
                       errinfo->indname,
                       errinfo->relnamespace,
                       errinfo->relname);
            break;
        case PARALLEL_INDVAC_STATUS_NEED_CLEANUP:
            errcontext("while cleaning up index \"%s\" of relation \"%s.%s\"",
                       errinfo->indname,
                       errinfo->relnamespace,
                       errinfo->relname);
            break;
        case PARALLEL_INDVAC_STATUS_INITIAL:
        case PARALLEL_INDVAC_STATUS_COMPLETED:
        default:
            return;
    }
}