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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
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Produced: 22 Feb 2021