• PostgreSQL Buffer Manager
• Clock-sweep Replacement Strategy

❖ 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

❖ 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

❖ PostgreSQL Buffer Manager (cont)

❖ 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

❖ 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;

❖ 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)

❖ Clock-sweep Replacement Strategy (cont)

Action of clock-sweep:

❖ 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