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PostgreSQL Buffer Manager

• PostgreSQL Buffer Manager
• Clock-sweep Replacement Strategy
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❖ PostgreSQL Buffer Manager

PostgreSQL buffer manager:
• provides a shared pool of memory buffers for all backends
• all access methods get data from disk via buffer manager
Buffers are located in a large region of shared memory.
Definitions:  src/include/storage/buf*.h
Functions:  src/backend/storage/buffer/*.c


Buffer code is also used by backends who want a private buffer pool
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❖ PostgreSQL Buffer Manager (cont)

Buffer pool consists of:
BufferDescriptors
• shared fixed array (size NBuffers) of BufferDesc
BufferBlocks
• shared fixed array (size NBuffers) of 8KB frames
Buffer = index values in above arrays
• indexes: global buffers 1..NBuffers; local buffers negative
Size of buffer pool is set in postgresql.conf, e.g.

shared_buffers = 16MB # min 128KB, 16*8KB buffers

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❖ PostgreSQL Buffer Manager (cont)



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❖ PostgreSQL Buffer Manager (cont)

include/storage/buf.h
• basic buffer manager data types (e.g. Buffer)
include/storage/bufmgr.h
• definitions for buffer manager function interface 
(i.e. functions that other parts of the system call to use buffer manager)
include/storage/buf_internals.h
• definitions for buffer manager internals (e.g. BufferDesc)
Code: backend/storage/buffer/*.c
Commentary: backend/storage/buffer/README
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❖ PostgreSQL Buffer Manager (cont)

Definition of buffer descriptors simplified:

typedef struct BufferDesc
{
BufferTag tag; // ID of page contained in buffer
int buf_id; // buffer’s index number (from 0)

// state, containing flags, refcount and usagecount
pg_atomic_uint32 state;

int freeNext; // link in freelist chain
…
} BufferDesc;

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❖ Clock-sweep Replacement Strategy

PostgreSQL page replacement strategy: clock-sweep
• treat buffer pool as circular list of buffer slots
• NextVictimBuffer (NVB) holds index of next possible evictee
• if Buf[NVB] page is pinned or “popular”, leave it
◦ usage_count implements “popularity/recency” measure
◦ incremented on each access to buffer (up to small limit)
◦ decremented each time considered for eviction
• else if pin_count = 0 and usage_count = 0 then grab this buffer
• increment NextVictimBuffer and try again (wrap at end)
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❖ Clock-sweep Replacement Strategy (cont)

Action of clock-sweep:



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<< ∧ ❖ Clock-sweep Replacement Strategy (cont) For specialised kinds of access (e.g. sequential scan), • clock-sweep is not the best replacement strategy • can allocate a private "buffer ring" • use this buffer ring with alternative replacement strategy COMP9315 21T1 ♢ PG Buffers ♢ [8/8] Produced: 22 Feb 2021