cmu 15-445/645 database systems (fall 2018) :: buffer pools · database systems 15-445/15-645 fall...
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![Page 1: CMU 15-445/645 Database Systems (Fall 2018) :: Buffer Pools · Database Systems 15-445/15-645 Fall 2018 Andy Pavlo Computer Science AP Carnegie Mellon Univ. Lecture #05 Buffer Pools](https://reader030.vdocuments.us/reader030/viewer/2022021523/5d57e81088c993a1048bbe7c/html5/thumbnails/1.jpg)
Database Systems
15-445/15-645
Fall 2018
Andy PavloComputer Science Carnegie Mellon Univ.AP
Lecture #05
Buffer Pools
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CMU 15-445/645 (Fall 2018)
UPCOMING DATABASE EVENTS
Relational AI Talk→ Wednesday Sep 12th @ 4:00pm→ GHC 8102
MapD Talk→ Thursday Sept 20th @ 12pm→ CIC 4th Floor
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CMU 15-445/645 (Fall 2018)
DATABASE STORAGE
Problem #1: How the DBMS represents the database in files on disk.
Problem #2: How the DBMS manages its memory and move data back-and-forth from disk.
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CMU 15-445/645 (Fall 2018)
DATABASE STORAGE
Spatial Control:→ Where to write pages on disk.→ The goal is to keep pages that are used together often as
physically close together as possible on disk.
Temporal Control:→ When to read pages into memory, and when to write
them to disk.→ The goal is minimize the number of stalls from having to
read data from disk.
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CMU 15-445/645 (Fall 2018)
DISK-ORIENTED DBMS
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Disk
Memory
Da
tab
ase
File
1HeaderDirectory
2Header
3Header
… Pages
Bu
ffe
r P
oo
l
2Header
4Header
5Header
ExecutionEngine
Get page #2
Directory
Pointer to page #2
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CMU 15-445/645 (Fall 2018)
TODAY'S AGENDA
Buffer Pool Manager
Replacement Policies
Allocation Policies
Other Memory Pools
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CMU 15-445/645 (Fall 2018)
BUFFER POOL ORGANIZATION
Memory region organized as an array of fixed-size pages.An array entry is called a frame.
When the DBMS requests a page, an exact copy is placed into one of these frames.
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BufferPool
frame1
frame2
frame3
frame4
page1
page3
On-Disk File
page1 page2 page3 page4
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CMU 15-445/645 (Fall 2018)
BUFFER POOL META-DATA
The page table keeps track of pages that are currently in memory.
Also maintains additional meta-data per page:→ Dirty Flag→ Pin/Reference Counter
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page1 page2 page3 page4
On-Disk File
BufferPool
frame1
frame2
frame3
frame4
page1
page3
PageTable
page1
page3
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CMU 15-445/645 (Fall 2018)
BUFFER POOL META-DATA
The page table keeps track of pages that are currently in memory.
Also maintains additional meta-data per page:→ Dirty Flag→ Pin/Reference Counter
8
page1 page2 page3 page4
On-Disk File
BufferPool
frame1
frame2
frame3
frame4
page1
page3
PageTable
page1
page3
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CMU 15-445/645 (Fall 2018)
BUFFER POOL META-DATA
The page table keeps track of pages that are currently in memory.
Also maintains additional meta-data per page:→ Dirty Flag→ Pin/Reference Counter
8
page1 page2 page3 page4
On-Disk File
BufferPool
frame1
frame2
frame3
frame4
page1
page3
PageTable
page1
page3
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CMU 15-445/645 (Fall 2018)
BUFFER POOL META-DATA
The page table keeps track of pages that are currently in memory.
Also maintains additional meta-data per page:→ Dirty Flag→ Pin/Reference Counter
8
page1 page2 page3 page4
On-Disk File
BufferPool
frame1
frame2
frame3
frame4
page1
page3
PageTable
page1
page3
page2
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CMU 15-445/645 (Fall 2018)
BUFFER POOL META-DATA
The page table keeps track of pages that are currently in memory.
Also maintains additional meta-data per page:→ Dirty Flag→ Pin/Reference Counter
8
page1 page2 page3 page4
On-Disk File
BufferPool
frame1
frame2
frame3
frame4
page1
page3
PageTable
page1
page3
page2
page2
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CMU 15-445/645 (Fall 2018)
LOCKS VS. L ATCHES
Locks:→ Protects the database's logical contents from other
transactions.→ Held for transaction duration.→ Need to be able to rollback changes.
Latches:→ Protects the critical sections of the DBMS's internal data
structure from other threads.→ Held for operation duration.→ Do not need to be able to rollback changes.
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CMU 15-445/645 (Fall 2018)
PAGE TABLE VS. PAGE DIRECTORY
The page directory is the mapping from page ids to page locations in the database files.→ All changes must be recorded on disk to allow the DBMS
to find on restart.
The page table is the mapping from page ids to a copy of the page in buffer pool frames.→ This is an in-memory data structure that does not need to
be stored on disk.
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CMU 15-445/645 (Fall 2018)
MULTIPLE BUFFER POOLS
The DBMS does not always have a single buffer pool for the entire system.→ Multiple buffer pool instances→ Per-database buffer pool→ Per-page type buffer pool
Helps reduce latch contention and improve locality.
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CMU 15-445/645 (Fall 2018)
PRE-FETCHING
The DBMS can also prefetch pages based on a query plan.→ Sequential Scans→ Index Scans
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Buffer Pool
Disk Pages
page0
page1
page2
page3
page4
page5
Q1
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CMU 15-445/645 (Fall 2018)
PRE-FETCHING
The DBMS can also prefetch pages based on a query plan.→ Sequential Scans→ Index Scans
12
Buffer Pool
page0
Disk Pages
page0
page1
page2
page3
page4
page5
Q1
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CMU 15-445/645 (Fall 2018)
PRE-FETCHING
The DBMS can also prefetch pages based on a query plan.→ Sequential Scans→ Index Scans
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Buffer Pool
page0
page1
Disk Pages
page0
page1
page2
page3
page4
page5
Q1
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CMU 15-445/645 (Fall 2018)
PRE-FETCHING
The DBMS can also prefetch pages based on a query plan.→ Sequential Scans→ Index Scans
12
Buffer Pool
page0
page1
Disk Pages
page0
page1
page2
page3
page4
page5
Q1
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CMU 15-445/645 (Fall 2018)
PRE-FETCHING
The DBMS can also prefetch pages based on a query plan.→ Sequential Scans→ Index Scans
12
Buffer Pool
page1
page2
Disk Pages
page0
page1
page2
page3
page4
page5
Q1
page3
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CMU 15-445/645 (Fall 2018)
PRE-FETCHING
The DBMS can also prefetch pages based on a query plan.→ Sequential Scans→ Index Scans
12
Buffer Pool
page1
page2
Disk Pages
page0
page1
page2
page3
page4
page5
Q1
page3
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CMU 15-445/645 (Fall 2018)
PRE-FETCHING
The DBMS can also prefetch pages based on a query plan.→ Sequential Scans→ Index Scans
12
Buffer Pool
Disk Pages
page0
page1
page2
page3
page4
page5Q1
page3
page4
page5
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CMU 15-445/645 (Fall 2018)
PRE-FETCHING
13
Buffer Pool
Disk Pages
index-page0
index-page1
index-page2
index-page3
index-page4
index-page5
index-page0
index-page4index-page1
index-page2 index-page5index-page3 index-page6
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CMU 15-445/645 (Fall 2018)
PRE-FETCHING
13
Buffer Pool
index-page0
Disk Pages
index-page0
index-page1
index-page2
index-page3
index-page4
index-page5
Q1
index-page0
index-page4index-page1
index-page2 index-page5index-page3 index-page6
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CMU 15-445/645 (Fall 2018)
PRE-FETCHING
13
Buffer Pool
index-page0
index-page1
Disk Pages
index-page0
index-page1
index-page2
index-page3
index-page4
index-page5
Q1
index-page0
index-page4index-page1
index-page2 index-page5index-page3 index-page6
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CMU 15-445/645 (Fall 2018)
PRE-FETCHING
13
Buffer Pool
index-page0
index-page1
Disk Pages
index-page0
index-page1
index-page2
index-page3
index-page4
index-page5
Q1
index-page0
index-page4index-page1
index-page2 index-page5index-page3 index-page6
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CMU 15-445/645 (Fall 2018)
SCAN SHARING
Queries are able to reuse data retrieved from storage or operator computations.→ This is different from result caching.
Allow multiple queries to attach to a single cursor that scans a table.→ Queries do not have to be exactly the same.→ Can also share intermediate results.
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CMU 15-445/645 (Fall 2018)
SCAN SHARING
If a query starts a scan and if there one already doing this, then the DBMS will attach to the second query's cursor.→ The DBMS keeps track of where the second query joined
with the first so that it can finish the scan when it reaches the end of the data structure.
Fully supported in IBM DB2 and MSSQL.Oracle only supports cursor sharing for identical queries.
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CMU 15-445/645 (Fall 2018)
Buffer Pool
SCAN SHARING
16
Disk Pages
page0
page1
page2
page3
page4
page5
SELECT SUM(val) FROM AQ1Q1
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CMU 15-445/645 (Fall 2018)
Buffer Pool
page0
SCAN SHARING
16
Disk Pages
page0
page1
page2
page3
page4
page5
SELECT SUM(val) FROM AQ1Q1
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CMU 15-445/645 (Fall 2018)
Buffer Pool
page0
page1
page2
SCAN SHARING
16
Disk Pages
page0
page1
page2
page3
page4
page5
SELECT SUM(val) FROM AQ1
Q1
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CMU 15-445/645 (Fall 2018)
Buffer Pool
page0
page1
page2
SCAN SHARING
16
Disk Pages
page0
page1
page2
page3
page4
page5
SELECT SUM(val) FROM AQ1
Q1
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CMU 15-445/645 (Fall 2018)
Buffer Pool
page1
page2
SCAN SHARING
16
Disk Pages
page0
page1
page2
page3
page4
page5
SELECT SUM(val) FROM AQ1
Q1page3
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CMU 15-445/645 (Fall 2018)
Buffer Pool
page1
page2
SCAN SHARING
16
Disk Pages
page0
page1
page2
page3
page4
page5
SELECT SUM(val) FROM AQ1
SELECT AVG(val) FROM AQ2
Q1page3
Q2
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CMU 15-445/645 (Fall 2018)
Buffer Pool
page1
page2
SCAN SHARING
16
Disk Pages
page0
page1
page2
page3
page4
page5
SELECT SUM(val) FROM AQ1
SELECT AVG(val) FROM AQ2
Q1page3 Q2
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CMU 15-445/645 (Fall 2018)
Buffer Pool
SCAN SHARING
16
Disk Pages
page0
page1
page2
page3
page4
page5
SELECT SUM(val) FROM AQ1
SELECT AVG(val) FROM AQ2
Q1
page3
Q2
page4
page5
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CMU 15-445/645 (Fall 2018)
Buffer Pool
SCAN SHARING
16
Disk Pages
page0
page1
page2
page3
page4
page5
SELECT SUM(val) FROM AQ1
SELECT AVG(val) FROM AQ2
page3
Q2
page4
page5
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CMU 15-445/645 (Fall 2018)
Buffer Pool
page0
page1
page2
SCAN SHARING
16
Disk Pages
page0
page1
page2
page3
page4
page5
SELECT SUM(val) FROM AQ1
SELECT AVG(val) FROM AQ2
Q2
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CMU 15-445/645 (Fall 2018)
BUFFER POOL BYPASS
The sequential scan operator will not store fetched pages in the buffer pool to avoid overhead.→ Memory is local to running query.→ Works well if operator needs to read a large sequence of
pages that are contiguous on disk.
Called "Light Scans" in Informix.
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CMU 15-445/645 (Fall 2018)
OS PAGE CACHE
Most disk operations go through the OS API.
Unless you tell it not to, the OS maintains its own filesystem cache.
Most DBMSs use direct I/O (O_DIRECT)to bypass the OS's cache.→ Redundant copies of pages.→ Different eviction policies.
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CMU 15-445/645 (Fall 2018)
BUFFER REPL ACEMENT POLICIES
When the DBMS needs to free up a frame to make room for a new page, it must decide which page to evict from the buffer pool.
Goals:→ Correctness→ Accuracy→ Speed→ Meta-data overhead
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CMU 15-445/645 (Fall 2018)
LEAST-RECENTLY USED
Maintain a timestamp of when each page was last accessed.
When the DBMS needs to evict a page, select the one with the oldest timestamp.→ Keep the pages in sorted order to reduce the search time
on eviction.
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CMU 15-445/645 (Fall 2018)
CLOCK
Approximation of LRU without needing a separate timestamp per page.→ Each page has a reference bit.→ When a page is accessed, set to 1.
Organize the pages in a circular buffer with a "clock hand":→ Upon sweeping, check if a page's bit is set to 1.→ If yes, set to zero. If no, then evict.
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CMU 15-445/645 (Fall 2018)
CLOCK
Approximation of LRU without needing a separate timestamp per page.→ Each page has a reference bit.→ When a page is accessed, set to 1.
Organize the pages in a circular buffer with a "clock hand":→ Upon sweeping, check if a page's bit is set to 1.→ If yes, set to zero. If no, then evict.
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CMU 15-445/645 (Fall 2018)
CLOCK
Approximation of LRU without needing a separate timestamp per page.→ Each page has a reference bit.→ When a page is accessed, set to 1.
Organize the pages in a circular buffer with a "clock hand":→ Upon sweeping, check if a page's bit is set to 1.→ If yes, set to zero. If no, then evict.
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CMU 15-445/645 (Fall 2018)
CLOCK
Approximation of LRU without needing a separate timestamp per page.→ Each page has a reference bit.→ When a page is accessed, set to 1.
Organize the pages in a circular buffer with a "clock hand":→ Upon sweeping, check if a page's bit is set to 1.→ If yes, set to zero. If no, then evict.
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CMU 15-445/645 (Fall 2018)
CLOCK
Approximation of LRU without needing a separate timestamp per page.→ Each page has a reference bit.→ When a page is accessed, set to 1.
Organize the pages in a circular buffer with a "clock hand":→ Upon sweeping, check if a page's bit is set to 1.→ If yes, set to zero. If no, then evict.
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CMU 15-445/645 (Fall 2018)
CLOCK
Approximation of LRU without needing a separate timestamp per page.→ Each page has a reference bit.→ When a page is accessed, set to 1.
Organize the pages in a circular buffer with a "clock hand":→ Upon sweeping, check if a page's bit is set to 1.→ If yes, set to zero. If no, then evict.
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CMU 15-445/645 (Fall 2018)
CLOCK
Approximation of LRU without needing a separate timestamp per page.→ Each page has a reference bit.→ When a page is accessed, set to 1.
Organize the pages in a circular buffer with a "clock hand":→ Upon sweeping, check if a page's bit is set to 1.→ If yes, set to zero. If no, then evict.
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CMU 15-445/645 (Fall 2018)
CLOCK
Approximation of LRU without needing a separate timestamp per page.→ Each page has a reference bit.→ When a page is accessed, set to 1.
Organize the pages in a circular buffer with a "clock hand":→ Upon sweeping, check if a page's bit is set to 1.→ If yes, set to zero. If no, then evict.
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CMU 15-445/645 (Fall 2018)
PROBLEMS
LRU and CLOCK replacement policies are susceptible to sequential flooding.→ A query performs a sequential scan that reads every page.→ This pollutes the buffer pool with pages that are read
once and then never again.
The most recently used page is actually the most unneeded page.
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CMU 15-445/645 (Fall 2018)
Buffer Pool
page0
SEQUENTIAL FLOODING
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Disk Pages
page0
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SELECT * FROM A WHERE id = 1Q1
Q1
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CMU 15-445/645 (Fall 2018)
Buffer Pool
page0
SEQUENTIAL FLOODING
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Disk Pages
page0
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page2
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SELECT * FROM A WHERE id = 1Q1
SELECT AVG(val) FROM AQ2 Q2
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CMU 15-445/645 (Fall 2018)
Buffer Pool
page0
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page2
SEQUENTIAL FLOODING
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Disk Pages
page0
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SELECT * FROM A WHERE id = 1Q1
SELECT AVG(val) FROM AQ2
Q2
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CMU 15-445/645 (Fall 2018)
Buffer Pool
page1
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SEQUENTIAL FLOODING
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Disk Pages
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SELECT * FROM A WHERE id = 1Q1
SELECT AVG(val) FROM AQ2
page3 Q2
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CMU 15-445/645 (Fall 2018)
Buffer Pool
page1
page2
SEQUENTIAL FLOODING
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Disk Pages
page0
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SELECT * FROM A WHERE id = 1Q1
SELECT AVG(val) FROM AQ2
page3 Q2
SELECT * FROM A WHERE id = 1Q3
Q2
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CMU 15-445/645 (Fall 2018)
Buffer Pool
page1
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SEQUENTIAL FLOODING
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SELECT * FROM A WHERE id = 1Q1
SELECT AVG(val) FROM AQ2
page3 Q2
SELECT * FROM A WHERE id = 1Q3
Q2
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CMU 15-445/645 (Fall 2018)
BET TER POLICIES: LRU -K
Take into account history of the last K references as timestamps and compute the interval between subsequent accesses.
The DBMS then uses this history to estimate the next time that page is going to be accessed.
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CMU 15-445/645 (Fall 2018)
BET TER POLICIES: LOCALIZATION
The DBMS chooses which pages to evict on a per txn/query basis. This minimizes the pollution of the buffer pool from each query.→ Keep track of the pages that a query has accessed.
Example: Postgres maintains a small ring buffer that is private to the query.
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CMU 15-445/645 (Fall 2018)
BET TER POLICIES: PRIORIT Y HINTS
The DBMS knows what the context of each page during query execution.
It can provide hints to the buffer pool on whether a page is important or not.
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CMU 15-445/645 (Fall 2018)
BET TER POLICIES: PRIORIT Y HINTS
The DBMS knows what the context of each page during query execution.
It can provide hints to the buffer pool on whether a page is important or not.
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index-page0
index-page4index-page1
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INSERT INTO A VALUES (id++)Q1
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CMU 15-445/645 (Fall 2018)
BET TER POLICIES: PRIORIT Y HINTS
The DBMS knows what the context of each page during query execution.
It can provide hints to the buffer pool on whether a page is important or not.
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index-page0
index-page4index-page1
index-page2 index-page5index-page3 index-page6
INSERT INTO A VALUES (id++)Q1
MIN MAXid
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CMU 15-445/645 (Fall 2018)
BET TER POLICIES: PRIORIT Y HINTS
The DBMS knows what the context of each page during query execution.
It can provide hints to the buffer pool on whether a page is important or not.
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index-page0
index-page4index-page1
index-page2 index-page5index-page3 index-page6
SELECT * FROM A WHERE id = ?Q2
INSERT INTO A VALUES (id++)Q1
MIN MAXid
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CMU 15-445/645 (Fall 2018)
DIRT Y PAGES
FAST: If a page in the buffer pool is not dirty, then the DBMS can simply "drop" it.
SLOW: If a page is dirty, then the DBMS must write back to disk to ensure that its changes are persisted.
Trade-off between fast evictions versus dirty writing pages that will not be read again in the future.
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CMU 15-445/645 (Fall 2018)
BACKGROUND WRITING
The DBMS can periodically walk through the page table and write dirty pages to disk.
When a dirty page is safely written, the DBMS can either evict the page or just unset the dirty flag.
Need to be careful that we don’t write dirty pages before their log records have been written…
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CMU 15-445/645 (Fall 2018)
ALLOCATION POLICIES
Global Policies:→ Make decisions for all active txns.
Local Policies:→ Allocate frames to a specific txn without considering the
behavior of concurrent txns.→ Still need to support sharing pages.
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CMU 15-445/645 (Fall 2018)
OTHER MEMORY POOLS
The DBMS needs memory for things other than just tuples and indexes.
These other memory pools may not always backed by disk. Depends on implementation.→ Sorting + Join Buffers→ Query Caches→ Maintenance Buffers→ Log Buffers→ Dictionary Caches
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CMU 15-445/645 (Fall 2018)
CONCLUSION
The DBMS can manage that sweet, sweet memory better than the OS.
Leverage the semantics about the query plan to make better decisions:→ Evictions→ Allocations→ Pre-fetching
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CMU 15-445/645 (Fall 2018)
PROJECT #1
You will build the first component of your storage manager.→ Extendible Hash Table→ LRU Replacement Policy→ Buffer Pool Manager
All of the projects are based on SQLite, but you will not be able to use your storage manger just yet after this first project.
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Due Date:Wed Sept 26th @ 11:59pm
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CMU 15-445/645 (Fall 2018)
TASK #1 EXTENDIBLE HASH TABLE
Build a thread-safe extendible hash table.→ Use unordered buckets to store key/value
pairs.→ You must support growing table size.→ You do not need to support shrinking.
General Hints:→ You can use std::hash and std::mutex.
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CMU 15-445/645 (Fall 2018)
TASK #2 LRU REPL ACEMENT POLICY
Build a data structure that tracks the usage of Pageobjects in the buffer pool using the least-recently used policy.
General Hints:→ Your LRUReplacer does not need to worry about the
"pinned" status of a Page.
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CMU 15-445/645 (Fall 2018)
TASK #3 BUFFER POOL MANAGER
Combine your hash table and LRU replacer together to manage the allocation of pages.→ Need to maintain an internal data
structures of allocated + free pages.→ We will provide you components to
read/write data from disk.
General Hints:→ Make sure you get the order of operations
correct when pinning.
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Buffer Pool(In-Memory)
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Database(On-Disk)
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CMU 15-445/645 (Fall 2018)
GET TING STARTED
Download the source code from the project webpage.
Make sure you can build it on your machine.→ We've test it on Andrew machines, OSX, and Linux.→ It should compile on Windows 10 w/ Ubuntu, but we
haven't tried it.
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CMU 15-445/645 (Fall 2018)
THINGS TO NOTE
Do not change any file other than the six that you have to hand in.
The projects are cumulative.
We will not be providing solutions.
Post your questions on Piazza or come to our office hours. We will not help you debug.
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CMU 15-445/645 (Fall 2018)
PL AGIARISM WARNING
Your project implementation must be your own work.→ You may not copy source code from other
groups or the web.→ Do not publish your implementation on
Github.
Plagiarism will not be tolerated.See CMU's Policy on Academic Integrity for additional information.
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CMU 15-445/645 (Fall 2018)
NEXT CL ASS
HASH TABLES!
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