Wait! What’s going on inside my database?

Jeremy Schneider, Amazon RDS
Wait! What’s going on inside
my database?
PostgreSQL and the Science of Database Performance
Updated: Sep 19, 2019

About PostgreSQL
1970: Mathematician Edgar F. Codd, working as researcher
for IBM, publishes “A Relational Model of Data for Large
Shared Data Banks”
1973: Michael Stonebraker and Eugene Wong at University
of California Berkeley seek funding and begin development
of a relational database called INGRES
1986: Michael Stonebraker and Lawrence A. Rowe at
University of California Berkeley publish “The Design of
POSTGRES” – a new database that is the successor to INGRES
1994: Andrew Yu and Jolly Chen at University of California
Berkeley add support for the SQL language
1996: Transition to non-university core team of volunteers,
official release under new name POSTGRESQL 1985

About Database Performance
1990’s Manager:
“Dear DBA: Expert consultants
have taught us that if the Buffer
Cache Hit Ratio (BCHR) is below
90% then the system
immediately needs an expensive
tuning engagement.
Please report any databases that
have BCHR < 90%.”
Delfador Chibi by Peileppe
CC0

Nørgaard, Mogens et al. Oracle Insights:
Tales of the Oak Table. Berkeley, CA:
Apress/OakTable Press, 2004. p76-77.

Millsap, Cary V. Optimizing Oracle Performance.
Sebastopol, CA: OReilly, 2003. p225, 240, 258-259
R = S + W
“How long
the SQL
takes to run”
See also:
• Shallahamer, Craig.
Forecasting Oracle
Performance. Berkeley,
CA: Apress, 2007.

Active Session Sampling
(JB’s notebook, 2004)
Images & Quotes
Used With Permission

© 2019, Amazon Web Services, Inc. or its Affiliates. All rights reserved.
Mariinsky Theatre, St. Petersburg
by Sandra Cohen-Rose and Colin Rose (Montreal, Canada)
CC BY-SA

CC BY-SA
Wait Events

CC BY-SA
Wait Events
• 1990s: Database kernel instrumentation:
• Counters and tools to snapshot/compare them
• Events (log a message under certain circumstances)
• 1992: Unable to solve a performance problem, as a last resort,
engineers added event code in version 7.0.12 capable of emitting
log messages when the database waited for something
• First exposed in V$SESSION_WAIT and later in V$SESSION
(equivalent of pg_stat_activity)
• PostgreSQL built on concepts that had become standard across
the industry

CC BY-SA
Wait Events
Millsap, Cary V. Optimizing Oracle Performance.
Sebastopol, CA: OReilly, 2003. p225, 240, 258-259
R = S + W
“How long
the SQL
takes to run”
See also:
• Shallahamer, Craig.
Forecasting Oracle
Performance. Berkeley,
CA: Apress, 2007.

CC BY-SA
Wait Events
Active Session Sampling
(JB’s notebook, 2004)
Images & Quotes
Used With Permission

CC BY-SA
Wait Events
“But why are these events called wait events?
…
In short, when a session is not using the CPU, it may be
waiting for a resource, an action to complete, or simply
more work. Hence, events that are associated with all
such waits are known as wait events.”
Shee, Richmond, Kirtikumar Deshpande, and K. Gopalakrishnan. Oracle Wait Interface a Practical
Guide to Performance Diagnostics & Tuning. New York: London, 2004. p16

CC BY-SA
Wait Events
High-Level Idea:
Caveats:
• OS scheduling/runqueue
• Measurement overhead, OS kernel CPU time (e.g. I/O)
The database is WAITING any time when it’s not running on the CPU

CC BY-SA
Wait Events
psql> SELECT…
Idle
WaitingCpu

CC BY-SA
Wait Events
Significant Commits: Version 9.6
• Aa65de0 – 11 Sep 2015 – Autogenerate lwlocknames.[c|h]
• 53be0b1 – 10 Mar 2016 – Heavy/Lightweight Locks, Buffer Pins

CC BY-SA
Wait Events
Significant Commits: Version 9.6
• Aa65de0 – 11 Sep 2015 – Autogenerate lwlocknames.[c|h]
• 53be0b1 – 10 Mar 2016 – Heavy/Lightweight Locks, Buffer Pins
Version 10
• 6f3bd98 – 4 Oct 2016 – Latches & Sockets, Clients, Main Loops
• 249cf07 – 18 Mar 2017 – I/O
• Fc70a4b – 26 Mar 2017 – Background and Auxiliary Processes
Version 11
• 1804284 – 20 Dec 2017 – Parallel-Aware Hash Joins

CC BY-SA
Wait Events
src/include/pgstat.h

CC BY-SA
Wait Events
doxygen.postgresql.org

CC BY-SA
Wait Events
src/backend/postmaster/pgstat.c

CC BY-SA
Wait Events
Gaps after migrating to Open Source/Community PostgreSQL
• Wait Event Counters and Cumulative Times
• Wait Event Arguments (object, block, etc)
• Comprehensive tracking of CPU time (POSIX rusage)
• Ability to find previous SQL for COMMIT/ROLLBACK
• Needed to identify which transaction is committing. (Other databases do not
update SQL text for COMMIT statement)
• On-CPU State
• SQL Execution Stage (parse/plan/execute/fetch)
• SQL Execution Plan Identifier in pg_stat_statements
• Current plan node
• Progress on long operations (e.g. large seqscan)
• Better runtime visibility into PLs

I can haz Wait Events?
Solving Problems with Wait Events in PostgreSQL
By Antony Griffiths (Flickr), CC BY

Active Session Sampling of Wait Events on PostgreSQL:
• Performance Insights on Amazon RDS
• RDS PostgreSQL 10+
• Aurora PostgreSQL 9.6+ (v10 Wait Events were backported)
• pg_wait_sampling and PoWa
• pgSentinal
• pgCenter
• (what am I missing?)

SELECT sql_statement, count(*)
FROM pg_stat_activity_samples
WHERE date BETWEEN problem_start AND problem_end
GROUP BY sql_statement
ORDER BY count(*) DESC;
SELECT wait_event, count(*)
FROM pg_stat_activity_samples
WHERE sql_statement=top_problem_sql_statement
AND date BETWEEN problem_start AND problem_end
GROUP BY wait_event
ORDER BY count(*) DESC;

Active Session Summary (Performance Insights, etc)
Top SQL & Top Wait Events
EXPLAIN ANALYZE with Buffers, IO timing, etc
Investigate STEP & WAIT Taking The Most Time

Thank you!
aws.amazon.com/rds/postgresql

Stuffed Elephant Store
A unique service that produces on-
demand stuffed elephants
Multiple sizes with long or short
fur
Any color the customer wants, as
long as its blue
Breibeest (Flickr), CC BY

Performance Insights
See the query text
and the wait
events by query
Look back 7 days
or as much as 2
years to find
activity

explain.depesz.com

Execution Plans
Explains how
Postgres plans to
execute a query
Shows the type of
operation, the
estimated cost,
and the estimated
number of rows

System Catalogs
Contains the
structure of all
objects in the
database
Statistics views
shows usage of
the objects

Indexes
PostgreSQL has a
rich set of index
types
Base functionality
can be enhanced
by specialized
extensions

Performance Insights
Drill down into
time periods show
finer grain detail
The 3 minute view
shows 1 second
granularity

Locking
Locks are held for
the duration of
the transaction
Locks can be held
on a table, row or
other object such
as transaction IDs

It’s so simple!

Aurora PostgreSQL:
• AWS Documentation covers
Aurora-Specific Wait
Events
• Shares Code With
Community PostgreSQL
(and merges regularly)

DataFileRead, buffer_io
• I/O Read Path: Check SQL
execution plans, optimize
for fewer block reads.
XactSync, WALWrite
• I/O Write Path: Check
commit rate, volume of
change.
transactionid, relation, etc.
• Application Design: check
pg_locks during contention.
buffer_content
• Hot Block in Memory: check
foreign keys, optimize
contention (e.g. schema
redesign, fillfactor, etc).

Individual Named LWLocks
Tranches for SLRU
Tranches for Shared Buffers
Individual Named Tranches

Uppercase LWLocks: see lwlocknames.txt, search code directly
Lowercase LWLocks: tranches (arrays of locks for groups of objects)
• SLRUs – see SimpleLruInit() callers on doxygen
• Shared Buffers (buffer_content, buffer_io)
• Other Tranches – see RegisterLWLockTranches() in lwlock.c
(Heavyweight/SQL/Transaction) Locks: LockTagType enum in lock.h
• Strings come from matching structure LockTagTypeNames in lockfuncs.c
BufferPin: Vacuuming - see PG_WAIT_BUFFER_PIN refs on doxygen
Extension: FDWs, BG worker startup, etc - see PG_WAIT_EXTENSION refs
on doxygen
Activity, Client, IPC, Timeout and IO: enums, see pgstat.h

Multi-AZ [multiple physical locations]
Physical Backups
• Max allowed retention (35 days in RDS)
• Regular restore testing
Logical Backups
• Scheduled Exports/Dumps and Application Re-Drive
• Logical Replication
Huge Pages
Autovacuum Logging (RDS: need “force” setting)
• Logging Level = INFO
• Minimum duration = 10 seconds
PostgreSQL quarterly updates
• Stable minor releases for security and bug fixes (RDS)
• Some Aurora minors have new development work (Aurora)
• Remember to upgrade extensions; it’s not automatic
Connection Pooling
• Centralized and decentralized (app-tier) architectures exist
• Recycle server connections (e.g. server_lifetime)
Performance Insights [monitor active session waits]
• Keep the history
Enhanced Monitoring [OS monitoring]
• 10 second (or lower) granularity
Preload pg_stat_statements
Limit on temp usage by default (esp. Aurora)
• Log temp usage when close to the limit
Alarms
• Maximum used transaction IDs
• DBLoad [Average Active Sessions]
• Free disk space (RDS) / Free local storage (Aurora)
• Memory / swap
• Replica Lag (RDS)
PostgreSQL Happiness Hints version:
jer_s/2019-09-19
tinyurl.com/waitevents

244 GB DDR4 Memory
16 Physical Intel Xeon E5-2686 v4 (Broadwell) processors

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