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+/*-------------------------------------------------------------------------
+ *
+ * buf_internals.h
+ *	  Internal definitions for buffer manager and the buffer replacement
+ *	  strategy.
+ *
+ *
+ * Portions Copyright (c) 1996-2020, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * src/include/storage/buf_internals.h
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef BUFMGR_INTERNALS_H
+#define BUFMGR_INTERNALS_H
+
+#include "port/atomics.h"
+#include "storage/buf.h"
+#include "storage/bufmgr.h"
+#include "storage/latch.h"
+#include "storage/lwlock.h"
+#include "storage/shmem.h"
+#include "storage/smgr.h"
+#include "storage/spin.h"
+#include "utils/relcache.h"
+
+/*
+ * Buffer state is a single 32-bit variable where following data is combined.
+ *
+ * - 18 bits refcount
+ * - 4 bits usage count
+ * - 10 bits of flags
+ *
+ * Combining these values allows to perform some operations without locking
+ * the buffer header, by modifying them together with a CAS loop.
+ *
+ * The definition of buffer state components is below.
+ */
+#define BUF_REFCOUNT_ONE 1
+#define BUF_REFCOUNT_MASK ((1U << 18) - 1)
+#define BUF_USAGECOUNT_MASK 0x003C0000U
+#define BUF_USAGECOUNT_ONE (1U << 18)
+#define BUF_USAGECOUNT_SHIFT 18
+#define BUF_FLAG_MASK 0xFFC00000U
+
+/* Get refcount and usagecount from buffer state */
+#define BUF_STATE_GET_REFCOUNT(state) ((state) & BUF_REFCOUNT_MASK)
+#define BUF_STATE_GET_USAGECOUNT(state) (((state) & BUF_USAGECOUNT_MASK) >> BUF_USAGECOUNT_SHIFT)
+
+/*
+ * Flags for buffer descriptors
+ *
+ * Note: BM_TAG_VALID essentially means that there is a buffer hashtable
+ * entry associated with the buffer's tag.
+ */
+#define BM_LOCKED				(1U << 22)	/* buffer header is locked */
+#define BM_DIRTY				(1U << 23)	/* data needs writing */
+#define BM_VALID				(1U << 24)	/* data is valid */
+#define BM_TAG_VALID			(1U << 25)	/* tag is assigned */
+#define BM_IO_IN_PROGRESS		(1U << 26)	/* read or write in progress */
+#define BM_IO_ERROR				(1U << 27)	/* previous I/O failed */
+#define BM_JUST_DIRTIED			(1U << 28)	/* dirtied since write started */
+#define BM_PIN_COUNT_WAITER		(1U << 29)	/* have waiter for sole pin */
+#define BM_CHECKPOINT_NEEDED	(1U << 30)	/* must write for checkpoint */
+#define BM_PERMANENT			(1U << 31)	/* permanent buffer (not unlogged,
+											 * or init fork) */
+/*
+ * The maximum allowed value of usage_count represents a tradeoff between
+ * accuracy and speed of the clock-sweep buffer management algorithm.  A
+ * large value (comparable to NBuffers) would approximate LRU semantics.
+ * But it can take as many as BM_MAX_USAGE_COUNT+1 complete cycles of
+ * clock sweeps to find a free buffer, so in practice we don't want the
+ * value to be very large.
+ */
+#define BM_MAX_USAGE_COUNT	5
+
+/*
+ * Buffer tag identifies which disk block the buffer contains.
+ *
+ * Note: the BufferTag data must be sufficient to determine where to write the
+ * block, without reference to pg_class or pg_tablespace entries.  It's
+ * possible that the backend flushing the buffer doesn't even believe the
+ * relation is visible yet (its xact may have started before the xact that
+ * created the rel).  The storage manager must be able to cope anyway.
+ *
+ * Note: if there's any pad bytes in the struct, INIT_BUFFERTAG will have
+ * to be fixed to zero them, since this struct is used as a hash key.
+ */
+typedef struct buftag
+{
+	RelFileNode rnode;			/* physical relation identifier */
+	ForkNumber	forkNum;
+	BlockNumber blockNum;		/* blknum relative to begin of reln */
+} BufferTag;
+
+#define CLEAR_BUFFERTAG(a) \
+( \
+	(a).rnode.spcNode = InvalidOid, \
+	(a).rnode.dbNode = InvalidOid, \
+	(a).rnode.relNode = InvalidOid, \
+	(a).forkNum = InvalidForkNumber, \
+	(a).blockNum = InvalidBlockNumber \
+)
+
+#define INIT_BUFFERTAG(a,xx_rnode,xx_forkNum,xx_blockNum) \
+( \
+	(a).rnode = (xx_rnode), \
+	(a).forkNum = (xx_forkNum), \
+	(a).blockNum = (xx_blockNum) \
+)
+
+#define BUFFERTAGS_EQUAL(a,b) \
+( \
+	RelFileNodeEquals((a).rnode, (b).rnode) && \
+	(a).blockNum == (b).blockNum && \
+	(a).forkNum == (b).forkNum \
+)
+
+/*
+ * The shared buffer mapping table is partitioned to reduce contention.
+ * To determine which partition lock a given tag requires, compute the tag's
+ * hash code with BufTableHashCode(), then apply BufMappingPartitionLock().
+ * NB: NUM_BUFFER_PARTITIONS must be a power of 2!
+ */
+#define BufTableHashPartition(hashcode) \
+	((hashcode) % NUM_BUFFER_PARTITIONS)
+#define BufMappingPartitionLock(hashcode) \
+	(&MainLWLockArray[BUFFER_MAPPING_LWLOCK_OFFSET + \
+		BufTableHashPartition(hashcode)].lock)
+#define BufMappingPartitionLockByIndex(i) \
+	(&MainLWLockArray[BUFFER_MAPPING_LWLOCK_OFFSET + (i)].lock)
+
+/*
+ *	BufferDesc -- shared descriptor/state data for a single shared buffer.
+ *
+ * Note: Buffer header lock (BM_LOCKED flag) must be held to examine or change
+ * the tag, state or wait_backend_pid fields.  In general, buffer header lock
+ * is a spinlock which is combined with flags, refcount and usagecount into
+ * single atomic variable.  This layout allow us to do some operations in a
+ * single atomic operation, without actually acquiring and releasing spinlock;
+ * for instance, increase or decrease refcount.  buf_id field never changes
+ * after initialization, so does not need locking.  freeNext is protected by
+ * the buffer_strategy_lock not buffer header lock.  The LWLock can take care
+ * of itself.  The buffer header lock is *not* used to control access to the
+ * data in the buffer!
+ *
+ * It's assumed that nobody changes the state field while buffer header lock
+ * is held.  Thus buffer header lock holder can do complex updates of the
+ * state variable in single write, simultaneously with lock release (cleaning
+ * BM_LOCKED flag).  On the other hand, updating of state without holding
+ * buffer header lock is restricted to CAS, which insure that BM_LOCKED flag
+ * is not set.  Atomic increment/decrement, OR/AND etc. are not allowed.
+ *
+ * An exception is that if we have the buffer pinned, its tag can't change
+ * underneath us, so we can examine the tag without locking the buffer header.
+ * Also, in places we do one-time reads of the flags without bothering to
+ * lock the buffer header; this is generally for situations where we don't
+ * expect the flag bit being tested to be changing.
+ *
+ * We can't physically remove items from a disk page if another backend has
+ * the buffer pinned.  Hence, a backend may need to wait for all other pins
+ * to go away.  This is signaled by storing its own PID into
+ * wait_backend_pid and setting flag bit BM_PIN_COUNT_WAITER.  At present,
+ * there can be only one such waiter per buffer.
+ *
+ * We use this same struct for local buffer headers, but the locks are not
+ * used and not all of the flag bits are useful either. To avoid unnecessary
+ * overhead, manipulations of the state field should be done without actual
+ * atomic operations (i.e. only pg_atomic_read_u32() and
+ * pg_atomic_unlocked_write_u32()).
+ *
+ * Be careful to avoid increasing the size of the struct when adding or
+ * reordering members.  Keeping it below 64 bytes (the most common CPU
+ * cache line size) is fairly important for performance.
+ */
+typedef struct BufferDesc
+{
+	BufferTag	tag;			/* ID of page contained in buffer */
+	int			buf_id;			/* buffer's index number (from 0) */
+
+	/* state of the tag, containing flags, refcount and usagecount */
+	pg_atomic_uint32 state;
+
+	int			wait_backend_pid;	/* backend PID of pin-count waiter */
+	int			freeNext;		/* link in freelist chain */
+
+	LWLock		content_lock;	/* to lock access to buffer contents */
+} BufferDesc;
+
+/*
+ * Concurrent access to buffer headers has proven to be more efficient if
+ * they're cache line aligned. So we force the start of the BufferDescriptors
+ * array to be on a cache line boundary and force the elements to be cache
+ * line sized.
+ *
+ * XXX: As this is primarily matters in highly concurrent workloads which
+ * probably all are 64bit these days, and the space wastage would be a bit
+ * more noticeable on 32bit systems, we don't force the stride to be cache
+ * line sized on those. If somebody does actual performance testing, we can
+ * reevaluate.
+ *
+ * Note that local buffer descriptors aren't forced to be aligned - as there's
+ * no concurrent access to those it's unlikely to be beneficial.
+ *
+ * We use a 64-byte cache line size here, because that's the most common
+ * size. Making it bigger would be a waste of memory. Even if running on a
+ * platform with either 32 or 128 byte line sizes, it's good to align to
+ * boundaries and avoid false sharing.
+ */
+#define BUFFERDESC_PAD_TO_SIZE	(SIZEOF_VOID_P == 8 ? 64 : 1)
+
+typedef union BufferDescPadded
+{
+	BufferDesc	bufferdesc;
+	char		pad[BUFFERDESC_PAD_TO_SIZE];
+} BufferDescPadded;
+
+#define GetBufferDescriptor(id) (&BufferDescriptors[(id)].bufferdesc)
+#define GetLocalBufferDescriptor(id) (&LocalBufferDescriptors[(id)])
+
+#define BufferDescriptorGetBuffer(bdesc) ((bdesc)->buf_id + 1)
+
+#define BufferDescriptorGetIOLock(bdesc) \
+	(&(BufferIOLWLockArray[(bdesc)->buf_id]).lock)
+#define BufferDescriptorGetContentLock(bdesc) \
+	((LWLock*) (&(bdesc)->content_lock))
+
+extern PGDLLIMPORT LWLockMinimallyPadded *BufferIOLWLockArray;
+
+/*
+ * The freeNext field is either the index of the next freelist entry,
+ * or one of these special values:
+ */
+#define FREENEXT_END_OF_LIST	(-1)
+#define FREENEXT_NOT_IN_LIST	(-2)
+
+/*
+ * Functions for acquiring/releasing a shared buffer header's spinlock.  Do
+ * not apply these to local buffers!
+ */
+extern uint32 LockBufHdr(BufferDesc *desc);
+#define UnlockBufHdr(desc, s)	\
+	do {	\
+		pg_write_barrier(); \
+		pg_atomic_write_u32(&(desc)->state, (s) & (~BM_LOCKED)); \
+	} while (0)
+
+
+/*
+ * The PendingWriteback & WritebackContext structure are used to keep
+ * information about pending flush requests to be issued to the OS.
+ */
+typedef struct PendingWriteback
+{
+	/* could store different types of pending flushes here */
+	BufferTag	tag;
+} PendingWriteback;
+
+/* struct forward declared in bufmgr.h */
+typedef struct WritebackContext
+{
+	/* pointer to the max number of writeback requests to coalesce */
+	int		   *max_pending;
+
+	/* current number of pending writeback requests */
+	int			nr_pending;
+
+	/* pending requests */
+	PendingWriteback pending_writebacks[WRITEBACK_MAX_PENDING_FLUSHES];
+} WritebackContext;
+
+/* in buf_init.c */
+extern PGDLLIMPORT BufferDescPadded *BufferDescriptors;
+extern PGDLLIMPORT WritebackContext BackendWritebackContext;
+
+/* in localbuf.c */
+extern BufferDesc *LocalBufferDescriptors;
+
+/* in bufmgr.c */
+
+/*
+ * Structure to sort buffers per file on checkpoints.
+ *
+ * This structure is allocated per buffer in shared memory, so it should be
+ * kept as small as possible.
+ */
+typedef struct CkptSortItem
+{
+	Oid			tsId;
+	Oid			relNode;
+	ForkNumber	forkNum;
+	BlockNumber blockNum;
+	int			buf_id;
+} CkptSortItem;
+
+extern CkptSortItem *CkptBufferIds;
+
+/*
+ * Internal buffer management routines
+ */
+/* bufmgr.c */
+extern void WritebackContextInit(WritebackContext *context, int *max_pending);
+extern void IssuePendingWritebacks(WritebackContext *context);
+extern void ScheduleBufferTagForWriteback(WritebackContext *context, BufferTag *tag);
+
+/* freelist.c */
+extern BufferDesc *StrategyGetBuffer(BufferAccessStrategy strategy,
+									 uint32 *buf_state);
+extern void StrategyFreeBuffer(BufferDesc *buf);
+extern bool StrategyRejectBuffer(BufferAccessStrategy strategy,
+								 BufferDesc *buf);
+
+extern int	StrategySyncStart(uint32 *complete_passes, uint32 *num_buf_alloc);
+extern void StrategyNotifyBgWriter(int bgwprocno);
+
+extern Size StrategyShmemSize(void);
+extern void StrategyInitialize(bool init);
+extern bool have_free_buffer(void);
+
+/* buf_table.c */
+extern Size BufTableShmemSize(int size);
+extern void InitBufTable(int size);
+extern uint32 BufTableHashCode(BufferTag *tagPtr);
+extern int	BufTableLookup(BufferTag *tagPtr, uint32 hashcode);
+extern int	BufTableInsert(BufferTag *tagPtr, uint32 hashcode, int buf_id);
+extern void BufTableDelete(BufferTag *tagPtr, uint32 hashcode);
+
+/* localbuf.c */
+extern PrefetchBufferResult PrefetchLocalBuffer(SMgrRelation smgr,
+												ForkNumber forkNum,
+												BlockNumber blockNum);
+extern BufferDesc *LocalBufferAlloc(SMgrRelation smgr, ForkNumber forkNum,
+									BlockNumber blockNum, bool *foundPtr);
+extern void MarkLocalBufferDirty(Buffer buffer);
+extern void DropRelFileNodeLocalBuffers(RelFileNode rnode, ForkNumber forkNum,
+										BlockNumber firstDelBlock);
+extern void DropRelFileNodeAllLocalBuffers(RelFileNode rnode);
+extern void AtEOXact_LocalBuffers(bool isCommit);
+
+#endif							/* BUFMGR_INTERNALS_H */