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db_include/storage/buf_internals.h
Executable file
345
db_include/storage/buf_internals.h
Executable file
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/*-------------------------------------------------------------------------
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*
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* buf_internals.h
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* Internal definitions for buffer manager and the buffer replacement
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* strategy.
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*
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*
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* Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
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* Portions Copyright (c) 1994, Regents of the University of California
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*
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* src/include/storage/buf_internals.h
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*
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*-------------------------------------------------------------------------
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*/
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#ifndef BUFMGR_INTERNALS_H
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#define BUFMGR_INTERNALS_H
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#include "port/atomics.h"
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#include "storage/buf.h"
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#include "storage/bufmgr.h"
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#include "storage/condition_variable.h"
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#include "storage/latch.h"
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#include "storage/lwlock.h"
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#include "storage/shmem.h"
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#include "storage/smgr.h"
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#include "storage/spin.h"
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#include "utils/relcache.h"
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/*
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* Buffer state is a single 32-bit variable where following data is combined.
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*
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* - 18 bits refcount
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* - 4 bits usage count
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* - 10 bits of flags
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*
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* Combining these values allows to perform some operations without locking
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* the buffer header, by modifying them together with a CAS loop.
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*
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* The definition of buffer state components is below.
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*/
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#define BUF_REFCOUNT_ONE 1
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#define BUF_REFCOUNT_MASK ((1U << 18) - 1)
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#define BUF_USAGECOUNT_MASK 0x003C0000U
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#define BUF_USAGECOUNT_ONE (1U << 18)
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#define BUF_USAGECOUNT_SHIFT 18
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#define BUF_FLAG_MASK 0xFFC00000U
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/* Get refcount and usagecount from buffer state */
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#define BUF_STATE_GET_REFCOUNT(state) ((state) & BUF_REFCOUNT_MASK)
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#define BUF_STATE_GET_USAGECOUNT(state) (((state) & BUF_USAGECOUNT_MASK) >> BUF_USAGECOUNT_SHIFT)
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/*
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* Flags for buffer descriptors
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*
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* Note: BM_TAG_VALID essentially means that there is a buffer hashtable
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* entry associated with the buffer's tag.
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*/
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#define BM_LOCKED (1U << 22) /* buffer header is locked */
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#define BM_DIRTY (1U << 23) /* data needs writing */
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#define BM_VALID (1U << 24) /* data is valid */
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#define BM_TAG_VALID (1U << 25) /* tag is assigned */
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#define BM_IO_IN_PROGRESS (1U << 26) /* read or write in progress */
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#define BM_IO_ERROR (1U << 27) /* previous I/O failed */
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#define BM_JUST_DIRTIED (1U << 28) /* dirtied since write started */
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#define BM_PIN_COUNT_WAITER (1U << 29) /* have waiter for sole pin */
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#define BM_CHECKPOINT_NEEDED (1U << 30) /* must write for checkpoint */
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#define BM_PERMANENT (1U << 31) /* permanent buffer (not unlogged,
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* or init fork) */
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/*
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* The maximum allowed value of usage_count represents a tradeoff between
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* accuracy and speed of the clock-sweep buffer management algorithm. A
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* large value (comparable to NBuffers) would approximate LRU semantics.
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* But it can take as many as BM_MAX_USAGE_COUNT+1 complete cycles of
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* clock sweeps to find a free buffer, so in practice we don't want the
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* value to be very large.
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*/
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#define BM_MAX_USAGE_COUNT 5
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/*
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* Buffer tag identifies which disk block the buffer contains.
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*
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* Note: the BufferTag data must be sufficient to determine where to write the
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* block, without reference to pg_class or pg_tablespace entries. It's
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* possible that the backend flushing the buffer doesn't even believe the
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* relation is visible yet (its xact may have started before the xact that
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* created the rel). The storage manager must be able to cope anyway.
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*
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* Note: if there's any pad bytes in the struct, INIT_BUFFERTAG will have
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* to be fixed to zero them, since this struct is used as a hash key.
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*/
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typedef struct buftag
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{
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RelFileNode rnode; /* physical relation identifier */
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ForkNumber forkNum;
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BlockNumber blockNum; /* blknum relative to begin of reln */
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} BufferTag;
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#define CLEAR_BUFFERTAG(a) \
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( \
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(a).rnode.spcNode = InvalidOid, \
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(a).rnode.dbNode = InvalidOid, \
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(a).rnode.relNode = InvalidOid, \
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(a).forkNum = InvalidForkNumber, \
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(a).blockNum = InvalidBlockNumber \
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)
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#define INIT_BUFFERTAG(a,xx_rnode,xx_forkNum,xx_blockNum) \
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( \
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(a).rnode = (xx_rnode), \
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(a).forkNum = (xx_forkNum), \
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(a).blockNum = (xx_blockNum) \
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)
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#define BUFFERTAGS_EQUAL(a,b) \
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( \
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RelFileNodeEquals((a).rnode, (b).rnode) && \
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(a).blockNum == (b).blockNum && \
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(a).forkNum == (b).forkNum \
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)
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/*
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* The shared buffer mapping table is partitioned to reduce contention.
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* To determine which partition lock a given tag requires, compute the tag's
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* hash code with BufTableHashCode(), then apply BufMappingPartitionLock().
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* NB: NUM_BUFFER_PARTITIONS must be a power of 2!
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*/
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#define BufTableHashPartition(hashcode) \
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((hashcode) % NUM_BUFFER_PARTITIONS)
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#define BufMappingPartitionLock(hashcode) \
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(&MainLWLockArray[BUFFER_MAPPING_LWLOCK_OFFSET + \
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BufTableHashPartition(hashcode)].lock)
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#define BufMappingPartitionLockByIndex(i) \
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(&MainLWLockArray[BUFFER_MAPPING_LWLOCK_OFFSET + (i)].lock)
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/*
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* BufferDesc -- shared descriptor/state data for a single shared buffer.
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*
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* Note: Buffer header lock (BM_LOCKED flag) must be held to examine or change
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* the tag, state or wait_backend_pid fields. In general, buffer header lock
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* is a spinlock which is combined with flags, refcount and usagecount into
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* single atomic variable. This layout allow us to do some operations in a
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* single atomic operation, without actually acquiring and releasing spinlock;
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* for instance, increase or decrease refcount. buf_id field never changes
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* after initialization, so does not need locking. freeNext is protected by
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* the buffer_strategy_lock not buffer header lock. The LWLock can take care
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* of itself. The buffer header lock is *not* used to control access to the
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* data in the buffer!
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*
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* It's assumed that nobody changes the state field while buffer header lock
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* is held. Thus buffer header lock holder can do complex updates of the
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* state variable in single write, simultaneously with lock release (cleaning
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* BM_LOCKED flag). On the other hand, updating of state without holding
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* buffer header lock is restricted to CAS, which insure that BM_LOCKED flag
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* is not set. Atomic increment/decrement, OR/AND etc. are not allowed.
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*
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* An exception is that if we have the buffer pinned, its tag can't change
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* underneath us, so we can examine the tag without locking the buffer header.
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* Also, in places we do one-time reads of the flags without bothering to
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* lock the buffer header; this is generally for situations where we don't
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* expect the flag bit being tested to be changing.
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*
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* We can't physically remove items from a disk page if another backend has
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* the buffer pinned. Hence, a backend may need to wait for all other pins
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* to go away. This is signaled by storing its own PID into
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* wait_backend_pid and setting flag bit BM_PIN_COUNT_WAITER. At present,
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* there can be only one such waiter per buffer.
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*
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* We use this same struct for local buffer headers, but the locks are not
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* used and not all of the flag bits are useful either. To avoid unnecessary
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* overhead, manipulations of the state field should be done without actual
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* atomic operations (i.e. only pg_atomic_read_u32() and
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* pg_atomic_unlocked_write_u32()).
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*
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* Be careful to avoid increasing the size of the struct when adding or
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* reordering members. Keeping it below 64 bytes (the most common CPU
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* cache line size) is fairly important for performance.
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*
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* Per-buffer I/O condition variables are currently kept outside this struct in
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* a separate array. They could be moved in here and still fit within that
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* limit on common systems, but for now that is not done.
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*/
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typedef struct BufferDesc
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{
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BufferTag tag; /* ID of page contained in buffer */
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int buf_id; /* buffer's index number (from 0) */
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/* state of the tag, containing flags, refcount and usagecount */
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pg_atomic_uint32 state;
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int wait_backend_pid; /* backend PID of pin-count waiter */
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int freeNext; /* link in freelist chain */
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LWLock content_lock; /* to lock access to buffer contents */
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} BufferDesc;
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/*
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* Concurrent access to buffer headers has proven to be more efficient if
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* they're cache line aligned. So we force the start of the BufferDescriptors
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* array to be on a cache line boundary and force the elements to be cache
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* line sized.
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*
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* XXX: As this is primarily matters in highly concurrent workloads which
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* probably all are 64bit these days, and the space wastage would be a bit
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* more noticeable on 32bit systems, we don't force the stride to be cache
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* line sized on those. If somebody does actual performance testing, we can
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* reevaluate.
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*
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* Note that local buffer descriptors aren't forced to be aligned - as there's
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* no concurrent access to those it's unlikely to be beneficial.
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*
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* We use a 64-byte cache line size here, because that's the most common
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* size. Making it bigger would be a waste of memory. Even if running on a
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* platform with either 32 or 128 byte line sizes, it's good to align to
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* boundaries and avoid false sharing.
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*/
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#define BUFFERDESC_PAD_TO_SIZE (SIZEOF_VOID_P == 8 ? 64 : 1)
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typedef union BufferDescPadded
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{
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BufferDesc bufferdesc;
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char pad[BUFFERDESC_PAD_TO_SIZE];
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} BufferDescPadded;
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#define GetBufferDescriptor(id) (&BufferDescriptors[(id)].bufferdesc)
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#define GetLocalBufferDescriptor(id) (&LocalBufferDescriptors[(id)])
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#define BufferDescriptorGetBuffer(bdesc) ((bdesc)->buf_id + 1)
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#define BufferDescriptorGetIOCV(bdesc) \
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(&(BufferIOCVArray[(bdesc)->buf_id]).cv)
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#define BufferDescriptorGetContentLock(bdesc) \
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((LWLock*) (&(bdesc)->content_lock))
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extern PGDLLIMPORT ConditionVariableMinimallyPadded *BufferIOCVArray;
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/*
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* The freeNext field is either the index of the next freelist entry,
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* or one of these special values:
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*/
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#define FREENEXT_END_OF_LIST (-1)
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#define FREENEXT_NOT_IN_LIST (-2)
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/*
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* Functions for acquiring/releasing a shared buffer header's spinlock. Do
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* not apply these to local buffers!
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*/
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extern uint32 LockBufHdr(BufferDesc *desc);
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#define UnlockBufHdr(desc, s) \
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do { \
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pg_write_barrier(); \
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pg_atomic_write_u32(&(desc)->state, (s) & (~BM_LOCKED)); \
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} while (0)
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/*
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* The PendingWriteback & WritebackContext structure are used to keep
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* information about pending flush requests to be issued to the OS.
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*/
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typedef struct PendingWriteback
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{
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/* could store different types of pending flushes here */
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BufferTag tag;
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} PendingWriteback;
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/* struct forward declared in bufmgr.h */
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typedef struct WritebackContext
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{
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/* pointer to the max number of writeback requests to coalesce */
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int *max_pending;
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/* current number of pending writeback requests */
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int nr_pending;
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/* pending requests */
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PendingWriteback pending_writebacks[WRITEBACK_MAX_PENDING_FLUSHES];
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} WritebackContext;
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/* in buf_init.c */
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extern PGDLLIMPORT BufferDescPadded *BufferDescriptors;
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extern PGDLLIMPORT WritebackContext BackendWritebackContext;
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/* in localbuf.c */
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extern BufferDesc *LocalBufferDescriptors;
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/* in bufmgr.c */
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/*
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* Structure to sort buffers per file on checkpoints.
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*
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* This structure is allocated per buffer in shared memory, so it should be
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* kept as small as possible.
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*/
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typedef struct CkptSortItem
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{
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Oid tsId;
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Oid relNode;
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ForkNumber forkNum;
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BlockNumber blockNum;
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int buf_id;
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} CkptSortItem;
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extern CkptSortItem *CkptBufferIds;
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/*
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* Internal buffer management routines
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*/
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/* bufmgr.c */
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extern void WritebackContextInit(WritebackContext *context, int *max_pending);
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extern void IssuePendingWritebacks(WritebackContext *context);
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extern void ScheduleBufferTagForWriteback(WritebackContext *context, BufferTag *tag);
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/* freelist.c */
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extern BufferDesc *StrategyGetBuffer(BufferAccessStrategy strategy,
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uint32 *buf_state);
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extern void StrategyFreeBuffer(BufferDesc *buf);
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extern bool StrategyRejectBuffer(BufferAccessStrategy strategy,
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BufferDesc *buf);
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extern int StrategySyncStart(uint32 *complete_passes, uint32 *num_buf_alloc);
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extern void StrategyNotifyBgWriter(int bgwprocno);
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extern Size StrategyShmemSize(void);
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extern void StrategyInitialize(bool init);
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extern bool have_free_buffer(void);
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/* buf_table.c */
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extern Size BufTableShmemSize(int size);
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extern void InitBufTable(int size);
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extern uint32 BufTableHashCode(BufferTag *tagPtr);
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extern int BufTableLookup(BufferTag *tagPtr, uint32 hashcode);
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extern int BufTableInsert(BufferTag *tagPtr, uint32 hashcode, int buf_id);
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extern void BufTableDelete(BufferTag *tagPtr, uint32 hashcode);
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/* localbuf.c */
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extern PrefetchBufferResult PrefetchLocalBuffer(SMgrRelation smgr,
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ForkNumber forkNum,
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BlockNumber blockNum);
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extern BufferDesc *LocalBufferAlloc(SMgrRelation smgr, ForkNumber forkNum,
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BlockNumber blockNum, bool *foundPtr);
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extern void MarkLocalBufferDirty(Buffer buffer);
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extern void DropRelFileNodeLocalBuffers(RelFileNode rnode, ForkNumber forkNum,
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BlockNumber firstDelBlock);
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extern void DropRelFileNodeAllLocalBuffers(RelFileNode rnode);
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extern void AtEOXact_LocalBuffers(bool isCommit);
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#endif /* BUFMGR_INTERNALS_H */
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