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* buf_internals.h
* Internal definitions for buffer manager and the buffer replacement
* strategy.
* Portions Copyright (c) 1996-2019, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
* src/include/storage/buf_internals.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 "port/atomics.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_MASK ((1U << 18) - 1)
#define BUF_USAGECOUNT_MASK 0x003C0000U
#define BUF_USAGECOUNT_ONE (1U << 18)
#define BUF_FLAG_MASK 0xFFC00000U
/* Get refcount and usagecount from buffer state */
* Flags for buffer descriptors
* Note: 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.
* 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) \
#define BufMappingPartitionLock(hashcode) \
#define BufMappingPartitionLockByIndex(i) \
* 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 64bit as the 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;
} BufferDescPadded;
#define GetBufferDescriptor(id) (&BufferDescriptors[(id)].bufferdesc)
#define GetLocalBufferDescriptor(id) (&LocalBufferDescriptors[(id)])
#define BufferDescriptorGetBuffer(bdesc) ((bdesc)->buf_id + 1)
#define BufferDescriptorGetIOLock(bdesc) \
#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:
* 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 void LocalPrefetchBuffer(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);