multixact_internal.h

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/*
 * multixact_internal.h
 *
 * PostgreSQL multi-transaction-log manager internal declarations
 *
 * These functions and definitions are for dealing with pg_multixact SLRU
 * pages.  They are internal to multixact.c, but they are exported here to
 * allow pg_upgrade to write pg_multixact files directly.
 *
 * Portions Copyright (c) 1996-2025, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * src/include/access/multixact_internal.h
 */
#ifndef MULTIXACT_INTERNAL_H
 
/*
 * Note: This is not only to prevent including this file twice.
 * MULTIXACT_INTERNAL_H is checked explicitly in multixact_read_v18.c.
 */
#define MULTIXACT_INTERNAL_H
 
#include "access/multixact.h"
 
 
/*
 * Defines for MultiXactOffset page sizes.  A page is the same BLCKSZ as is
 * used everywhere else in Postgres.
 */
 
/* We need 8 bytes per offset */
#define MULTIXACT_OFFSETS_PER_PAGE (BLCKSZ / sizeof(MultiXactOffset))
 
static inline int64
MultiXactIdToOffsetPage(MultiXactId multi)
{
    return multi / MULTIXACT_OFFSETS_PER_PAGE;
}
 
static inline int
MultiXactIdToOffsetEntry(MultiXactId multi)
{
    return multi % MULTIXACT_OFFSETS_PER_PAGE;
}
 
static inline int64
MultiXactIdToOffsetSegment(MultiXactId multi)
{
    return MultiXactIdToOffsetPage(multi) / SLRU_PAGES_PER_SEGMENT;
}
 
/*
 * The situation for members is a bit more complex: we store one byte of
 * additional flag bits for each TransactionId.  To do this without getting
 * into alignment issues, we store four bytes of flags, and then the
 * corresponding 4 Xids.  Each such 5-word (20-byte) set we call a "group", and
 * are stored as a whole in pages.  Thus, with 8kB BLCKSZ, we keep 409 groups
 * per page.  This wastes 12 bytes per page, but that's OK -- simplicity (and
 * performance) trumps space efficiency here.
 *
 * Note that the "offset" macros work with byte offset, not array indexes, so
 * arithmetic must be done using "char *" pointers.
 */
/* We need eight bits per xact, so one xact fits in a byte */
#define MXACT_MEMBER_BITS_PER_XACT          8
#define MXACT_MEMBER_FLAGS_PER_BYTE         1
#define MXACT_MEMBER_XACT_BITMASK   ((1 << MXACT_MEMBER_BITS_PER_XACT) - 1)
 
/* how many full bytes of flags are there in a group? */
#define MULTIXACT_FLAGBYTES_PER_GROUP       4
#define MULTIXACT_MEMBERS_PER_MEMBERGROUP   \
    (MULTIXACT_FLAGBYTES_PER_GROUP * MXACT_MEMBER_FLAGS_PER_BYTE)
/* size in bytes of a complete group */
#define MULTIXACT_MEMBERGROUP_SIZE \
    (sizeof(TransactionId) * MULTIXACT_MEMBERS_PER_MEMBERGROUP + MULTIXACT_FLAGBYTES_PER_GROUP)
#define MULTIXACT_MEMBERGROUPS_PER_PAGE (BLCKSZ / MULTIXACT_MEMBERGROUP_SIZE)
#define MULTIXACT_MEMBERS_PER_PAGE  \
    (MULTIXACT_MEMBERGROUPS_PER_PAGE * MULTIXACT_MEMBERS_PER_MEMBERGROUP)
 
/* page in which a member is to be found */
static inline int64
MXOffsetToMemberPage(MultiXactOffset offset)
{
    return offset / MULTIXACT_MEMBERS_PER_PAGE;
}
 
static inline int64
MXOffsetToMemberSegment(MultiXactOffset offset)
{
    return MXOffsetToMemberPage(offset) / SLRU_PAGES_PER_SEGMENT;
}
 
/* Location (byte offset within page) of flag word for a given member */
static inline int
MXOffsetToFlagsOffset(MultiXactOffset offset)
{
    MultiXactOffset group = offset / MULTIXACT_MEMBERS_PER_MEMBERGROUP;
    int         grouponpg = group % MULTIXACT_MEMBERGROUPS_PER_PAGE;
    int         byteoff = grouponpg * MULTIXACT_MEMBERGROUP_SIZE;
 
    return byteoff;
}
 
static inline int
MXOffsetToFlagsBitShift(MultiXactOffset offset)
{
    int         member_in_group = offset % MULTIXACT_MEMBERS_PER_MEMBERGROUP;
    int         bshift = member_in_group * MXACT_MEMBER_BITS_PER_XACT;
 
    return bshift;
}
 
/* Location (byte offset within page) of TransactionId of given member */
static inline int
MXOffsetToMemberOffset(MultiXactOffset offset)
{
    int         member_in_group = offset % MULTIXACT_MEMBERS_PER_MEMBERGROUP;
 
    return MXOffsetToFlagsOffset(offset) +
        MULTIXACT_FLAGBYTES_PER_GROUP +
        member_in_group * sizeof(TransactionId);
}
 
#endif                          /* MULTIXACT_INTERNAL_H */