Update copyright for 2022

[pgsql.git] / src / backend / access / common / syncscan.c

/*-------------------------------------------------------------------------
 *
 * syncscan.c
 *        scan synchronization support
 *
 * When multiple backends run a sequential scan on the same table, we try
 * to keep them synchronized to reduce the overall I/O needed.  The goal is
 * to read each page into shared buffer cache only once, and let all backends
 * that take part in the shared scan process the page before it falls out of
 * the cache.
 *
 * Since the "leader" in a pack of backends doing a seqscan will have to wait
 * for I/O, while the "followers" don't, there is a strong self-synchronizing
 * effect once we can get the backends examining approximately the same part
 * of the table at the same time.  Hence all that is really needed is to get
 * a new backend beginning a seqscan to begin it close to where other backends
 * are reading.  We can scan the table circularly, from block X up to the
 * end and then from block 0 to X-1, to ensure we visit all rows while still
 * participating in the common scan.
 *
 * To accomplish that, we keep track of the scan position of each table, and
 * start new scans close to where the previous scan(s) are.  We don't try to
 * do any extra synchronization to keep the scans together afterwards; some
 * scans might progress much more slowly than others, for example if the
 * results need to be transferred to the client over a slow network, and we
 * don't want such queries to slow down others.
 *
 * There can realistically only be a few large sequential scans on different
 * tables in progress at any time.  Therefore we just keep the scan positions
 * in a small LRU list which we scan every time we need to look up or update a
 * scan position.  The whole mechanism is only applied for tables exceeding
 * a threshold size (but that is not the concern of this module).
 *
 * INTERFACE ROUTINES
 *              ss_get_location         - return current scan location of a relation
 *              ss_report_location      - update current scan location
 *
 *
 * Portions Copyright (c) 1996-2022, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *        src/backend/access/common/syncscan.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "access/syncscan.h"
#include "miscadmin.h"
#include "storage/lwlock.h"
#include "storage/shmem.h"
#include "utils/rel.h"


/* GUC variables */
#ifdef TRACE_SYNCSCAN
bool            trace_syncscan = false;
#endif


/*
 * Size of the LRU list.
 *
 * Note: the code assumes that SYNC_SCAN_NELEM > 1.
 *
 * XXX: What's a good value? It should be large enough to hold the
 * maximum number of large tables scanned simultaneously.  But a larger value
 * means more traversing of the LRU list when starting a new scan.
 */
#define SYNC_SCAN_NELEM 20

/*
 * Interval between reports of the location of the current scan, in pages.
 *
 * Note: This should be smaller than the ring size (see buffer/freelist.c)
 * we use for bulk reads.  Otherwise a scan joining other scans might start
 * from a page that's no longer in the buffer cache.  This is a bit fuzzy;
 * there's no guarantee that the new scan will read the page before it leaves
 * the buffer cache anyway, and on the other hand the page is most likely
 * still in the OS cache.
 */
#define SYNC_SCAN_REPORT_INTERVAL (128 * 1024 / BLCKSZ)


/*
 * The scan locations structure is essentially a doubly-linked LRU with head
 * and tail pointer, but designed to hold a fixed maximum number of elements in
 * fixed-size shared memory.
 */
typedef struct ss_scan_location_t
{
        RelFileNode relfilenode;        /* identity of a relation */
        BlockNumber location;           /* last-reported location in the relation */
} ss_scan_location_t;

typedef struct ss_lru_item_t
{
        struct ss_lru_item_t *prev;
        struct ss_lru_item_t *next;
        ss_scan_location_t location;
} ss_lru_item_t;

typedef struct ss_scan_locations_t
{
        ss_lru_item_t *head;
        ss_lru_item_t *tail;
        ss_lru_item_t items[FLEXIBLE_ARRAY_MEMBER]; /* SYNC_SCAN_NELEM items */
} ss_scan_locations_t;

#define SizeOfScanLocations(N) \
        (offsetof(ss_scan_locations_t, items) + (N) * sizeof(ss_lru_item_t))

/* Pointer to struct in shared memory */
static ss_scan_locations_t *scan_locations;

/* prototypes for internal functions */
static BlockNumber ss_search(RelFileNode relfilenode,
                                                         BlockNumber location, bool set);


/*
 * SyncScanShmemSize --- report amount of shared memory space needed
 */
Size
SyncScanShmemSize(void)
{
        return SizeOfScanLocations(SYNC_SCAN_NELEM);
}

/*
 * SyncScanShmemInit --- initialize this module's shared memory
 */
void
SyncScanShmemInit(void)
{
        int                     i;
        bool            found;

        scan_locations = (ss_scan_locations_t *)
                ShmemInitStruct("Sync Scan Locations List",
                                                SizeOfScanLocations(SYNC_SCAN_NELEM),
                                                &found);

        if (!IsUnderPostmaster)
        {
                /* Initialize shared memory area */
                Assert(!found);

                scan_locations->head = &scan_locations->items[0];
                scan_locations->tail = &scan_locations->items[SYNC_SCAN_NELEM - 1];

                for (i = 0; i < SYNC_SCAN_NELEM; i++)
                {
                        ss_lru_item_t *item = &scan_locations->items[i];

                        /*
                         * Initialize all slots with invalid values. As scans are started,
                         * these invalid entries will fall off the LRU list and get
                         * replaced with real entries.
                         */
                        item->location.relfilenode.spcNode = InvalidOid;
                        item->location.relfilenode.dbNode = InvalidOid;
                        item->location.relfilenode.relNode = InvalidOid;
                        item->location.location = InvalidBlockNumber;

                        item->prev = (i > 0) ?
                                (&scan_locations->items[i - 1]) : NULL;
                        item->next = (i < SYNC_SCAN_NELEM - 1) ?
                                (&scan_locations->items[i + 1]) : NULL;
                }
        }
        else
                Assert(found);
}

/*
 * ss_search --- search the scan_locations structure for an entry with the
 *              given relfilenode.
 *
 * If "set" is true, the location is updated to the given location.  If no
 * entry for the given relfilenode is found, it will be created at the head
 * of the list with the given location, even if "set" is false.
 *
 * In any case, the location after possible update is returned.
 *
 * Caller is responsible for having acquired suitable lock on the shared
 * data structure.
 */
static BlockNumber
ss_search(RelFileNode relfilenode, BlockNumber location, bool set)
{
        ss_lru_item_t *item;

        item = scan_locations->head;
        for (;;)
        {
                bool            match;

                match = RelFileNodeEquals(item->location.relfilenode, relfilenode);

                if (match || item->next == NULL)
                {
                        /*
                         * If we reached the end of list and no match was found, take over
                         * the last entry
                         */
                        if (!match)
                        {
                                item->location.relfilenode = relfilenode;
                                item->location.location = location;
                        }
                        else if (set)
                                item->location.location = location;

                        /* Move the entry to the front of the LRU list */
                        if (item != scan_locations->head)
                        {
                                /* unlink */
                                if (item == scan_locations->tail)
                                        scan_locations->tail = item->prev;
                                item->prev->next = item->next;
                                if (item->next)
                                        item->next->prev = item->prev;

                                /* link */
                                item->prev = NULL;
                                item->next = scan_locations->head;
                                scan_locations->head->prev = item;
                                scan_locations->head = item;
                        }

                        return item->location.location;
                }

                item = item->next;
        }

        /* not reached */
}

/*
 * ss_get_location --- get the optimal starting location for scan
 *
 * Returns the last-reported location of a sequential scan on the
 * relation, or 0 if no valid location is found.
 *
 * We expect the caller has just done RelationGetNumberOfBlocks(), and
 * so that number is passed in rather than computing it again.  The result
 * is guaranteed less than relnblocks (assuming that's > 0).
 */
BlockNumber
ss_get_location(Relation rel, BlockNumber relnblocks)
{
        BlockNumber startloc;

        LWLockAcquire(SyncScanLock, LW_EXCLUSIVE);
        startloc = ss_search(rel->rd_node, 0, false);
        LWLockRelease(SyncScanLock);

        /*
         * If the location is not a valid block number for this scan, start at 0.
         *
         * This can happen if for instance a VACUUM truncated the table since the
         * location was saved.
         */
        if (startloc >= relnblocks)
                startloc = 0;

#ifdef TRACE_SYNCSCAN
        if (trace_syncscan)
                elog(LOG,
                         "SYNC_SCAN: start \"%s\" (size %u) at %u",
                         RelationGetRelationName(rel), relnblocks, startloc);
#endif

        return startloc;
}

/*
 * ss_report_location --- update the current scan location
 *
 * Writes an entry into the shared Sync Scan state of the form
 * (relfilenode, blocknumber), overwriting any existing entry for the
 * same relfilenode.
 */
void
ss_report_location(Relation rel, BlockNumber location)
{
#ifdef TRACE_SYNCSCAN
        if (trace_syncscan)
        {
                if ((location % 1024) == 0)
                        elog(LOG,
                                 "SYNC_SCAN: scanning \"%s\" at %u",
                                 RelationGetRelationName(rel), location);
        }
#endif

        /*
         * To reduce lock contention, only report scan progress every N pages. For
         * the same reason, don't block if the lock isn't immediately available.
         * Missing a few updates isn't critical, it just means that a new scan
         * that wants to join the pack will start a little bit behind the head of
         * the scan.  Hopefully the pages are still in OS cache and the scan
         * catches up quickly.
         */
        if ((location % SYNC_SCAN_REPORT_INTERVAL) == 0)
        {
                if (LWLockConditionalAcquire(SyncScanLock, LW_EXCLUSIVE))
                {
                        (void) ss_search(rel->rd_node, location, true);
                        LWLockRelease(SyncScanLock);
                }
#ifdef TRACE_SYNCSCAN
                else if (trace_syncscan)
                        elog(LOG,
                                 "SYNC_SCAN: missed update for \"%s\" at %u",
                                 RelationGetRelationName(rel), location);
#endif
        }
}