buf_internals.h

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/*-------------------------------------------------------------------------
 *
 * buf_internals.h
 *    Internal definitions for buffer manager and the buffer replacement
 *    strategy.
 *
 *
 * Portions Copyright (c) 1996-2025, 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 "pgstat.h"
#include "port/atomics.h"
#include "storage/buf.h"
#include "storage/bufmgr.h"
#include "storage/condition_variable.h"
#include "storage/lwlock.h"
#include "storage/procnumber.h"
#include "storage/shmem.h"
#include "storage/smgr.h"
#include "storage/spin.h"
#include "utils/relcache.h"
#include "utils/resowner.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_BITS 18
#define BUF_USAGECOUNT_BITS 4
#define BUF_FLAG_BITS 10
 
StaticAssertDecl(BUF_REFCOUNT_BITS + BUF_USAGECOUNT_BITS + BUF_FLAG_BITS == 32,
                 "parts of buffer state space need to equal 32");
 
#define BUF_REFCOUNT_ONE 1
#define BUF_REFCOUNT_MASK ((1U << BUF_REFCOUNT_BITS) - 1)
#define BUF_USAGECOUNT_MASK (((1U << BUF_USAGECOUNT_BITS) - 1) << (BUF_REFCOUNT_BITS))
#define BUF_USAGECOUNT_ONE (1U << BUF_REFCOUNT_BITS)
#define BUF_USAGECOUNT_SHIFT BUF_REFCOUNT_BITS
#define BUF_FLAG_MASK (((1U << BUF_FLAG_BITS) - 1) << (BUF_REFCOUNT_BITS + BUF_USAGECOUNT_BITS))
 
/* 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
 
StaticAssertDecl(BM_MAX_USAGE_COUNT < (1 << BUF_USAGECOUNT_BITS),
                 "BM_MAX_USAGE_COUNT doesn't fit in BUF_USAGECOUNT_BITS bits");
StaticAssertDecl(MAX_BACKENDS_BITS <= BUF_REFCOUNT_BITS,
                 "MAX_BACKENDS_BITS needs to be <= BUF_REFCOUNT_BITS");
 
/*
 * 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, InitBufferTag will have
 * to be fixed to zero them, since this struct is used as a hash key.
 */
typedef struct buftag
{
    Oid         spcOid;         /* tablespace oid */
    Oid         dbOid;          /* database oid */
    RelFileNumber relNumber;    /* relation file number */
    ForkNumber  forkNum;        /* fork number */
    BlockNumber blockNum;       /* blknum relative to begin of reln */
} BufferTag;
 
static inline RelFileNumber
BufTagGetRelNumber(const BufferTag *tag)
{
    return tag->relNumber;
}
 
static inline ForkNumber
BufTagGetForkNum(const BufferTag *tag)
{
    return tag->forkNum;
}
 
static inline void
BufTagSetRelForkDetails(BufferTag *tag, RelFileNumber relnumber,
                        ForkNumber forknum)
{
    tag->relNumber = relnumber;
    tag->forkNum = forknum;
}
 
static inline RelFileLocator
BufTagGetRelFileLocator(const BufferTag *tag)
{
    RelFileLocator rlocator;
 
    rlocator.spcOid = tag->spcOid;
    rlocator.dbOid = tag->dbOid;
    rlocator.relNumber = BufTagGetRelNumber(tag);
 
    return rlocator;
}
 
static inline void
ClearBufferTag(BufferTag *tag)
{
    tag->spcOid = InvalidOid;
    tag->dbOid = InvalidOid;
    BufTagSetRelForkDetails(tag, InvalidRelFileNumber, InvalidForkNumber);
    tag->blockNum = InvalidBlockNumber;
}
 
static inline void
InitBufferTag(BufferTag *tag, const RelFileLocator *rlocator,
              ForkNumber forkNum, BlockNumber blockNum)
{
    tag->spcOid = rlocator->spcOid;
    tag->dbOid = rlocator->dbOid;
    BufTagSetRelForkDetails(tag, rlocator->relNumber, forkNum);
    tag->blockNum = blockNum;
}
 
static inline bool
BufferTagsEqual(const BufferTag *tag1, const BufferTag *tag2)
{
    return (tag1->spcOid == tag2->spcOid) &&
        (tag1->dbOid == tag2->dbOid) &&
        (tag1->relNumber == tag2->relNumber) &&
        (tag1->blockNum == tag2->blockNum) &&
        (tag1->forkNum == tag2->forkNum);
}
 
static inline bool
BufTagMatchesRelFileLocator(const BufferTag *tag,
                            const RelFileLocator *rlocator)
{
    return (tag->spcOid == rlocator->spcOid) &&
        (tag->dbOid == rlocator->dbOid) &&
        (BufTagGetRelNumber(tag) == rlocator->relNumber);
}
 
 
/*
 * 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!
 */
static inline uint32
BufTableHashPartition(uint32 hashcode)
{
    return hashcode % NUM_BUFFER_PARTITIONS;
}
 
static inline LWLock *
BufMappingPartitionLock(uint32 hashcode)
{
    return &MainLWLockArray[BUFFER_MAPPING_LWLOCK_OFFSET +
                            BufTableHashPartition(hashcode)].lock;
}
 
static inline LWLock *
BufMappingPartitionLockByIndex(uint32 index)
{
    return &MainLWLockArray[BUFFER_MAPPING_LWLOCK_OFFSET + index].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
 * tag, state or wait_backend_pgprocno 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 ensures 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 pgprocno into
 * wait_backend_pgprocno 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.
 *
 * Per-buffer I/O condition variables are currently kept outside this struct in
 * a separate array.  They could be moved in here and still fit within that
 * limit on common systems, but for now that is not done.
 */
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_pgprocno;  /* backend 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;
 
/*
 * 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 ConditionVariableMinimallyPadded *BufferIOCVArray;
extern PGDLLIMPORT WritebackContext BackendWritebackContext;
 
/* in localbuf.c */
extern PGDLLIMPORT BufferDesc *LocalBufferDescriptors;
 
 
static inline BufferDesc *
GetBufferDescriptor(uint32 id)
{
    return &(BufferDescriptors[id]).bufferdesc;
}
 
static inline BufferDesc *
GetLocalBufferDescriptor(uint32 id)
{
    return &LocalBufferDescriptors[id];
}
 
static inline Buffer
BufferDescriptorGetBuffer(const BufferDesc *bdesc)
{
    return (Buffer) (bdesc->buf_id + 1);
}
 
static inline ConditionVariable *
BufferDescriptorGetIOCV(const BufferDesc *bdesc)
{
    return &(BufferIOCVArray[bdesc->buf_id]).cv;
}
 
static inline LWLock *
BufferDescriptorGetContentLock(const BufferDesc *bdesc)
{
    return (LWLock *) (&bdesc->content_lock);
}
 
/*
 * 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);
 
static inline void
UnlockBufHdr(BufferDesc *desc, uint32 buf_state)
{
    pg_write_barrier();
    pg_atomic_write_u32(&desc->state, buf_state & (~BM_LOCKED));
}
 
/* 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;
    RelFileNumber relNumber;
    ForkNumber  forkNum;
    BlockNumber blockNum;
    int         buf_id;
} CkptSortItem;
 
extern PGDLLIMPORT CkptSortItem *CkptBufferIds;
 
/* ResourceOwner callbacks to hold buffer I/Os and pins */
extern PGDLLIMPORT const ResourceOwnerDesc buffer_io_resowner_desc;
extern PGDLLIMPORT const ResourceOwnerDesc buffer_pin_resowner_desc;
 
/* Convenience wrappers over ResourceOwnerRemember/Forget */
static inline void
ResourceOwnerRememberBuffer(ResourceOwner owner, Buffer buffer)
{
    ResourceOwnerRemember(owner, Int32GetDatum(buffer), &buffer_pin_resowner_desc);
}
static inline void
ResourceOwnerForgetBuffer(ResourceOwner owner, Buffer buffer)
{
    ResourceOwnerForget(owner, Int32GetDatum(buffer), &buffer_pin_resowner_desc);
}
static inline void
ResourceOwnerRememberBufferIO(ResourceOwner owner, Buffer buffer)
{
    ResourceOwnerRemember(owner, Int32GetDatum(buffer), &buffer_io_resowner_desc);
}
static inline void
ResourceOwnerForgetBufferIO(ResourceOwner owner, Buffer buffer)
{
    ResourceOwnerForget(owner, Int32GetDatum(buffer), &buffer_io_resowner_desc);
}
 
/*
 * Internal buffer management routines
 */
/* bufmgr.c */
extern void WritebackContextInit(WritebackContext *context, int *max_pending);
extern void IssuePendingWritebacks(WritebackContext *wb_context, IOContext io_context);
extern void ScheduleBufferTagForWriteback(WritebackContext *wb_context,
                                          IOContext io_context, BufferTag *tag);
 
/* freelist.c */
extern IOContext IOContextForStrategy(BufferAccessStrategy strategy);
extern BufferDesc *StrategyGetBuffer(BufferAccessStrategy strategy,
                                     uint32 *buf_state, bool *from_ring);
extern void StrategyFreeBuffer(BufferDesc *buf);
extern bool StrategyRejectBuffer(BufferAccessStrategy strategy,
                                 BufferDesc *buf, bool from_ring);
 
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 bool PinLocalBuffer(BufferDesc *buf_hdr, bool adjust_usagecount);
extern void UnpinLocalBuffer(Buffer buffer);
extern void UnpinLocalBufferNoOwner(Buffer buffer);
extern PrefetchBufferResult PrefetchLocalBuffer(SMgrRelation smgr,
                                                ForkNumber forkNum,
                                                BlockNumber blockNum);
extern BufferDesc *LocalBufferAlloc(SMgrRelation smgr, ForkNumber forkNum,
                                    BlockNumber blockNum, bool *foundPtr);
extern BlockNumber ExtendBufferedRelLocal(BufferManagerRelation bmr,
                                          ForkNumber fork,
                                          uint32 flags,
                                          uint32 extend_by,
                                          BlockNumber extend_upto,
                                          Buffer *buffers,
                                          uint32 *extended_by);
extern void MarkLocalBufferDirty(Buffer buffer);
extern void TerminateLocalBufferIO(BufferDesc *bufHdr, bool clear_dirty,
                                   uint32 set_flag_bits);
extern bool StartLocalBufferIO(BufferDesc *bufHdr, bool forInput);
extern void FlushLocalBuffer(BufferDesc *bufHdr, SMgrRelation reln);
extern void DropRelationLocalBuffers(RelFileLocator rlocator,
                                     ForkNumber forkNum,
                                     BlockNumber firstDelBlock);
extern void DropRelationAllLocalBuffers(RelFileLocator rlocator);
extern void AtEOXact_LocalBuffers(bool isCommit);
 
#endif                          /* BUFMGR_INTERNALS_H */