相关变量、结构定义和初始化完成后数据结构图在下面。

static MT_LOCAL SlruCtlData MultiXactOffsetCtlData;

static MT_LOCAL SlruCtlData MultiXactMemberCtlData;

#define MultiXactOffsetCtl (&MultiXactOffsetCtlData)

#define MultiXactMemberCtl (&MultiXactMemberCtlData)

typedef struct SlruCtlData

{

SlruShared shared;

/*

* This flag tells whether to fsync writes(true for pg_clog, false for

* pg_subtrans).

*/

bool do_fsync;

/*

* Decide which of two page numbers is"older" for truncation purposes. We

* need to use comparison of TransactionIdshere in order to do the right

* thing with wraparound XID arithmetic.

*/

bool (*PagePrecedes)(int, int);

/*

* Dir is set during SimpleLruInit and does notchange thereafter. Since

* it's always the same, it doesn't need to bein shared memory.

*/

char Dir[64];

} SlruCtlData;

typedef SlruCtlData *SlruCtl;

/*

* Shared-memorystate

*/

typedef struct SlruSharedData

{

LWLockId ControlLock;

/* Number of buffers managed by this SLRU structure */

int num_slots;

/*

* Arrays holding info for each bufferslot. Page number is undefined

* when status is EMPTY, as is page_lru_count.

*/

char **page_buffer;

SlruPageStatus*page_status;

bool *page_dirty;

int *page_number;

int *page_lru_count;

LWLockId *buffer_locks;

/*----------

* We mark a page "most recentlyused" by setting

* page_lru_count[slotno]= ++cur_lru_count;

* The oldest page is therefore the one withthe highest value of

* cur_lru_count- page_lru_count[slotno]

* The counts will eventually wrap around, butthis calculation still

* works as long as no page's age exceedsINT_MAX counts.

*----------

*/

int cur_lru_count;

/*

* latest_page_number is the page number of thecurrent end of the log;

* this is not critical data, since we use itonly to avoid swapping out

* the latest page.

*/

int latest_page_number;

} SlruSharedData;

typedef SlruSharedData *SlruShared;

static MultiXactStateData *MultiXactState;

typedef structMultiXactStateData

{

/* next-to-be-assigned MultiXactId */

MultiXactIdnextMXact;

/* next-to-be-assigned offset */

MultiXactOffsetnextOffset;

/* the Offset SLRU area was last truncated at thisMultiXactId */

MultiXactIdlastTruncationPoint;

/*

* Per-backend data starts here. We have two arrays stored in the area

* immediately following the MultiXactStateDatastruct. Each is indexed by

* BackendId. (Note: valid BackendIds run from 1 to MaxBackends; element

* zero of each array is never used.)

*

* OldestMemberMXactId[k] is the oldestMultiXactId each backend's current

* transaction(s) could possibly be a memberof, or InvalidMultiXactId

* when the backend has no live transactionthat could possibly be a

* member of a MultiXact. Each backend sets its entry to the current

* nextMXact counter just before firstacquiring a shared lock in a given