[SOLVED] CS chain PostgreSQL Buffer Manager

PostgreSQL Buffer Manager

>>
PostgreSQL Buffer Manager

PostgreSQL Buffer Manager

Clock-sweep Replacement Strategy

COMP9315 21T1 PG Buffers [0/8]

>>
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

COMP9315 21T1 PG Buffers [1/8]

<< >>
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

COMP9315 21T1 PG Buffers [2/8]

<< >>
PostgreSQL Buffer Manager (cont)

COMP9315 21T1 PG Buffers [3/8]

<< >>
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

COMP9315 21T1 PG Buffers [4/8]

<< >>
PostgreSQL Buffer Manager (cont)

Definition of buffer descriptors simplified:

typedef struct BufferDesc
{
BufferTagtag; // ID of page contained in buffer
intbuf_id;// buffers index number (from 0)

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

intfreeNext;// link in freelist chain

} BufferDesc;

COMP9315 21T1 PG Buffers [5/8]

<< >>
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)

COMP9315 21T1 PG Buffers [6/8]

<< >>
Clock-sweep Replacement Strategy (cont)

Action of clock-sweep:

COMP9315 21T1 PG Buffers [7/8]

<< 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 strategyCOMP9315 21T1 PG Buffers [8/8]Produced: 22 Feb 2021