slru.h

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/*-------------------------------------------------------------------------
 *
 * slru.h
 *      Simple LRU buffering for transaction status logfiles
 *
 * Portions Copyright (c) 1996-2025, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * src/include/access/slru.h
 *
 *-------------------------------------------------------------------------
 */
#ifndef SLRU_H
#define SLRU_H
 
#include "access/xlogdefs.h"
#include "storage/lwlock.h"
#include "storage/sync.h"
 
/*
 * To avoid overflowing internal arithmetic and the size_t data type, the
 * number of buffers must not exceed this number.
 */
#define SLRU_MAX_ALLOWED_BUFFERS ((1024 * 1024 * 1024) / BLCKSZ)
 
/*
 * Define SLRU segment size.  A page is the same BLCKSZ as is used everywhere
 * else in Postgres.  The segment size can be chosen somewhat arbitrarily;
 * we make it 32 pages by default, or 256Kb, i.e. 1M transactions for CLOG
 * or 64K transactions for SUBTRANS.
 *
 * Note: because TransactionIds are 32 bits and wrap around at 0xFFFFFFFF,
 * page numbering also wraps around at 0xFFFFFFFF/xxxx_XACTS_PER_PAGE (where
 * xxxx is CLOG or SUBTRANS, respectively), and segment numbering at
 * 0xFFFFFFFF/xxxx_XACTS_PER_PAGE/SLRU_PAGES_PER_SEGMENT.  We need
 * take no explicit notice of that fact in slru.c, except when comparing
 * segment and page numbers in SimpleLruTruncate (see PagePrecedes()).
 */
#define SLRU_PAGES_PER_SEGMENT  32
 
/*
 * Page status codes.  Note that these do not include the "dirty" bit.
 * page_dirty can be true only in the VALID or WRITE_IN_PROGRESS states;
 * in the latter case it implies that the page has been re-dirtied since
 * the write started.
 */
typedef enum
{
    SLRU_PAGE_EMPTY,            /* buffer is not in use */
    SLRU_PAGE_READ_IN_PROGRESS, /* page is being read in */
    SLRU_PAGE_VALID,            /* page is valid and not being written */
    SLRU_PAGE_WRITE_IN_PROGRESS,    /* page is being written out */
} SlruPageStatus;
 
/*
 * Shared-memory state
 *
 * ControlLock is used to protect access to the other fields, except
 * latest_page_number, which uses atomics; see comment in slru.c.
 */
typedef struct SlruSharedData
{
    /* Number of buffers managed by this SLRU structure */
    int         num_slots;
 
    /*
     * Arrays holding info for each buffer slot.  Page number is undefined
     * when status is EMPTY, as is page_lru_count.
     */
    char      **page_buffer;
    SlruPageStatus *page_status;
    bool       *page_dirty;
    int64      *page_number;
    int        *page_lru_count;
 
    /* The buffer_locks protects the I/O on each buffer slots */
    LWLockPadded *buffer_locks;
 
    /* Locks to protect the in memory buffer slot access in SLRU bank. */
    LWLockPadded *bank_locks;
 
    /*----------
     * A bank-wise LRU counter is maintained because we do a victim buffer
     * search within a bank. Furthermore, manipulating an individual bank
     * counter avoids frequent cache invalidation since we update it every time
     * we access the page.
     *
     * We mark a page "most recently used" by setting
     *      page_lru_count[slotno] = ++bank_cur_lru_count[bankno];
     * The oldest page in the bank is therefore the one with the highest value
     * of
     *      bank_cur_lru_count[bankno] - page_lru_count[slotno]
     * The counts will eventually wrap around, but this calculation still
     * works as long as no page's age exceeds INT_MAX counts.
     *----------
     */
    int        *bank_cur_lru_count;
 
    /*
     * Optional array of WAL flush LSNs associated with entries in the SLRU
     * pages.  If not zero/NULL, we must flush WAL before writing pages (true
     * for pg_xact, false for everything else).  group_lsn[] has
     * lsn_groups_per_page entries per buffer slot, each containing the
     * highest LSN known for a contiguous group of SLRU entries on that slot's
     * page.
     */
    XLogRecPtr *group_lsn;
    int         lsn_groups_per_page;
 
    /*
     * latest_page_number is the page number of the current end of the log;
     * this is not critical data, since we use it only to avoid swapping out
     * the latest page.
     */
    pg_atomic_uint64 latest_page_number;
 
    /* SLRU's index for statistics purposes (might not be unique) */
    int         slru_stats_idx;
} SlruSharedData;
 
typedef SlruSharedData *SlruShared;
 
/*
 * SlruCtlData is an unshared structure that points to the active information
 * in shared memory.
 */
typedef struct SlruCtlData
{
    SlruShared  shared;
 
    /* Number of banks in this SLRU. */
    uint16      nbanks;
 
    /*
     * If true, use long segment file names.  Otherwise, use short file names.
     *
     * For details about the file name format, see SlruFileName().
     */
    bool        long_segment_names;
 
    /*
     * Which sync handler function to use when handing sync requests over to
     * the checkpointer.  SYNC_HANDLER_NONE to disable fsync (eg pg_notify).
     */
    SyncRequestHandler sync_handler;
 
    /*
     * Decide whether a page is "older" for truncation and as a hint for
     * evicting pages in LRU order.  Return true if every entry of the first
     * argument is older than every entry of the second argument.  Note that
     * !PagePrecedes(a,b) && !PagePrecedes(b,a) need not imply a==b; it also
     * arises when some entries are older and some are not.  For SLRUs using
     * SimpleLruTruncate(), this must use modular arithmetic.  (For others,
     * the behavior of this callback has no functional implications.)  Use
     * SlruPagePrecedesUnitTests() in SLRUs meeting its criteria.
     */
    bool        (*PagePrecedes) (int64, int64);
 
    /*
     * Dir is set during SimpleLruInit and does not change thereafter. Since
     * it's always the same, it doesn't need to be in shared memory.
     */
    char        Dir[64];
} SlruCtlData;
 
typedef SlruCtlData *SlruCtl;
 
/*
 * Get the SLRU bank lock for given SlruCtl and the pageno.
 *
 * This lock needs to be acquired to access the slru buffer slots in the
 * respective bank.
 */
static inline LWLock *
SimpleLruGetBankLock(SlruCtl ctl, int64 pageno)
{
    int         bankno;
 
    bankno = pageno % ctl->nbanks;
    return &(ctl->shared->bank_locks[bankno].lock);
}
 
extern Size SimpleLruShmemSize(int nslots, int nlsns);
extern int  SimpleLruAutotuneBuffers(int divisor, int max);
extern void SimpleLruInit(SlruCtl ctl, const char *name, int nslots, int nlsns,
                          const char *subdir, int buffer_tranche_id,
                          int bank_tranche_id, SyncRequestHandler sync_handler,
                          bool long_segment_names);
extern int  SimpleLruZeroPage(SlruCtl ctl, int64 pageno);
extern int  SimpleLruReadPage(SlruCtl ctl, int64 pageno, bool write_ok,
                              TransactionId xid);
extern int  SimpleLruReadPage_ReadOnly(SlruCtl ctl, int64 pageno,
                                       TransactionId xid);
extern void SimpleLruWritePage(SlruCtl ctl, int slotno);
extern void SimpleLruWriteAll(SlruCtl ctl, bool allow_redirtied);
#ifdef USE_ASSERT_CHECKING
extern void SlruPagePrecedesUnitTests(SlruCtl ctl, int per_page);
#else
#define SlruPagePrecedesUnitTests(ctl, per_page) do {} while (0)
#endif
extern void SimpleLruTruncate(SlruCtl ctl, int64 cutoffPage);
extern bool SimpleLruDoesPhysicalPageExist(SlruCtl ctl, int64 pageno);
 
typedef bool (*SlruScanCallback) (SlruCtl ctl, char *filename, int64 segpage,
                                  void *data);
extern bool SlruScanDirectory(SlruCtl ctl, SlruScanCallback callback, void *data);
extern void SlruDeleteSegment(SlruCtl ctl, int64 segno);
 
extern int  SlruSyncFileTag(SlruCtl ctl, const FileTag *ftag, char *path);
 
/* SlruScanDirectory public callbacks */
extern bool SlruScanDirCbReportPresence(SlruCtl ctl, char *filename,
                                        int64 segpage, void *data);
extern bool SlruScanDirCbDeleteAll(SlruCtl ctl, char *filename, int64 segpage,
                                   void *data);
extern bool check_slru_buffers(const char *name, int *newval);
 
#endif                          /* SLRU_H */