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F.23. pgbench
pgbench is a simple program for running benchmark tests on PostgreSQL. It runs the same sequence of SQL commands over and over, possibly in multiple concurrent database sessions, and then calculates the average transaction rate (transactions per second). By default, pgbench tests a scenario that is loosely based on TPC-B, involving five SELECT, UPDATE, and INSERT commands per transaction. However, it is easy to test other cases by writing your own transaction script files.
Typical output from pgbench looks like:
transaction type: TPC-B (sort of) scaling factor: 10 query mode: simple number of clients: 10 number of threads: 1 number of transactions per client: 1000 number of transactions actually processed: 10000/10000 tps = 85.184871 (including connections establishing) tps = 85.296346 (excluding connections establishing)
The first six lines report some of the most important parameter settings. The next line reports the number of transactions completed and intended (the latter being just the product of number of clients and number of transactions per client); these will be equal unless the run failed before completion. The last two lines report the TPS rate, figured with and without counting the time to start database sessions.
F.23.1. Overview
The default TPC-B-like transaction test requires specific tables to be set up beforehand. pgbench should be invoked with the -i (initialize) option to create and populate these tables. (When you are testing a custom script, you don't need this step, but will instead need to do whatever setup your test needs.) Initialization looks like:
pgbench -i [ other-options ] dbname
where dbname is the name of the already-created database to test in. (You may also need -h, -p, and/or -U options to specify how to connect to the database server.)
| Caution |
pgbench -i creates four tables pgbench_accounts, pgbench_branches, pgbench_history, and pgbench_tellers, destroying any existing tables of these names. Be very careful to use another database if you have tables having these names! |
At the default "scale factor" of 1, the tables initially contain this many rows:
table # of rows --------------------------------- pgbench_branches 1 pgbench_tellers 10 pgbench_accounts 100000 pgbench_history 0
You can (and, for most purposes, probably should) increase the number of rows by using the -s (scale factor) option. The -F (fillfactor) option might also be used at this point.
Once you have done the necessary setup, you can run your benchmark with a command that doesn't include -i, that is
pgbench [ options ] dbname
In nearly all cases, you'll need some options to make a useful test. The most important options are -c (number of clients), -t (number of transactions), -T (time limit), and -f (specify a custom script file). See below for a full list.
Section F.23.2 shows options that are used during database initialization, while Section F.23.3 shows options that are used while running benchmarks, and Section F.23.4 shows options that are useful in both cases.
F.23.2. pgbench Initialization Options
pgbench accepts the following command-line initialization arguments:
- -F fillfactor
Create the pgbench_accounts, pgbench_tellers and pgbench_branches tables with the given fillfactor. Default is 100.
- -i
Required to invoke initialization mode.
- -s scale_factor
Multiply the number of rows generated by the scale factor. For example, -s 100 will create 10,000,000 rows in the pgbench_accounts table. Default is 1.
F.23.3. pgbench Benchmarking Options
pgbench accepts the following command-line benchmarking arguments:
- -c clients
Number of clients simulated, that is, number of concurrent database sessions. Default is 1.
- -C
Establish a new connection for each transaction, rather than doing it just once per client session. This is useful to measure the connection overhead.
- -d
Print debugging output.
- -D varname=value
Define a variable for use by a custom script (see below). Multiple -D options are allowed.
- -f filename
Read transaction script from filename. See below for details. -N, -S, and -f are mutually exclusive.
- -j threads
Number of worker threads within pgbench. Using more than one thread can be helpful on multi-CPU machines. The number of clients must be a multiple of the number of threads, since each thread is given the same number of client sessions to manage. Default is 1.
- -l
Write the time taken by each transaction to a log file. See below for details.
- -M querymode
-
Protocol to use for submitting queries to the server:
simple: use simple query protocol.
extended: use extended query protocol.
prepared: use extended query protocol with prepared statements.
The default is simple query protocol. (See Chapter 46 for more information.)
- -n
Perform no vacuuming before running the test. This option is necessary if you are running a custom test scenario that does not include the standard tables pgbench_accounts, pgbench_branches, pgbench_history, and pgbench_tellers.
- -N
Do not update pgbench_tellers and pgbench_branches. This will avoid update contention on these tables, but it makes the test case even less like TPC-B.
- -s scale_factor
Report the specified scale factor in pgbench's output. With the built-in tests, this is not necessary; the correct scale factor will be detected by counting the number of rows in the pgbench_branches table. However, when testing custom benchmarks (-f option), the scale factor will be reported as 1 unless this option is used.
- -S
Perform select-only transactions instead of TPC-B-like test.
- -t transactions
Number of transactions each client runs. Default is 10.
- -T seconds
Run the test for this many seconds, rather than a fixed number of transactions per client. -t and -T are mutually exclusive.
- -v
Vacuum all four standard tables before running the test. With neither -n nor -v, pgbench will vacuum the pgbench_tellers and pgbench_branches tables, and will truncate pgbench_history.
F.23.4. pgbench Common Options
pgbench accepts the following command-line common arguments:
- -h hostname
The database server's host name
- -p port
The database server's port number
- -U login
The user name to connect as
F.23.5. What is the "transaction" actually performed in pgbench?
The default transaction script issues seven commands per transaction:
BEGIN;
UPDATE pgbench_accounts SET abalance = abalance + :delta WHERE aid = :aid;
SELECT abalance FROM pgbench_accounts WHERE aid = :aid;
UPDATE pgbench_tellers SET tbalance = tbalance + :delta WHERE tid = :tid;
UPDATE pgbench_branches SET bbalance = bbalance + :delta WHERE bid = :bid;
INSERT INTO pgbench_history (tid, bid, aid, delta, mtime) VALUES (:tid, :bid, :aid, :delta, CURRENT_TIMESTAMP);
END;
If you specify -N, steps 4 and 5 aren't included in the transaction. If you specify -S, only the SELECT is issued.
F.23.6. Custom Scripts
pgbench has support for running custom benchmark scenarios by replacing the default transaction script (described above) with a transaction script read from a file (-f option). In this case a "transaction" counts as one execution of a script file. You can even specify multiple scripts (multiple -f options), in which case a random one of the scripts is chosen each time a client session starts a new transaction.
The format of a script file is one SQL command per line; multiline SQL commands are not supported. Empty lines and lines beginning with -- are ignored. Script file lines can also be "meta commands", which are interpreted by pgbench itself, as described below.
There is a simple variable-substitution facility for script files. Variables can be set by the command-line -D option, explained above, or by the meta commands explained below. In addition to any variables preset by -D command-line options, the variable scale is preset to the current scale factor. Once set, a variable's value can be inserted into a SQL command by writing :variablename. When running more than one client session, each session has its own set of variables.
Script file meta commands begin with a backslash (\). Arguments to a meta command are separated by white space. These meta commands are supported:
- \set varname operand1 [ operator operand2 ]
-
Sets variable varname to a calculated integer value. Each operand is either an integer constant or a :variablename reference to a variable having an integer value. The operator can be +, -, *, or /.
Example:
\set ntellers 10 * :scale
- \setrandom varname min max
-
Sets variable varname to a random integer value between the limits min and max inclusive. Each limit can be either an integer constant or a :variablename reference to a variable having an integer value.
Example:
\setrandom aid 1 :naccounts
- \sleep number [ us | ms | s ]
-
Causes script execution to sleep for the specified duration in microseconds (us), milliseconds (ms) or seconds (s). If the unit is omitted then seconds are the default. number can be either an integer constant or a :variablename reference to a variable having an integer value.
Example:
\sleep 10 ms
- \setshell varname command [ argument ... ]
-
Sets variable varname to the result of the shell command command. The command must return an integer value through its standard output.
argument can be either a text constant or a :variablename reference to a variable of any types. If you want to use argument starting with colons, you need to add an additional colon at the beginning of argument.
Example:
\setshell variable_to_be_assigned command literal_argument :variable ::literal_starting_with_colon
- \shell command [ argument ... ]
-
Same as \setshell, but the result is ignored.
Example:
\shell command literal_argument :variable ::literal_starting_with_colon
As an example, the full definition of the built-in TPC-B-like transaction is:
\set nbranches :scale \set ntellers 10 * :scale \set naccounts 100000 * :scale \setrandom aid 1 :naccounts \setrandom bid 1 :nbranches \setrandom tid 1 :ntellers \setrandom delta -5000 5000 BEGIN; UPDATE pgbench_accounts SET abalance = abalance + :delta WHERE aid = :aid; SELECT abalance FROM pgbench_accounts WHERE aid = :aid; UPDATE pgbench_tellers SET tbalance = tbalance + :delta WHERE tid = :tid; UPDATE pgbench_branches SET bbalance = bbalance + :delta WHERE bid = :bid; INSERT INTO pgbench_history (tid, bid, aid, delta, mtime) VALUES (:tid, :bid, :aid, :delta, CURRENT_TIMESTAMP); END;
This script allows each iteration of the transaction to reference different, randomly-chosen rows. (This example also shows why it's important for each client session to have its own variables — otherwise they'd not be independently touching different rows.)
F.23.7. Per-transaction logging
With the -l option, pgbench writes the time taken by each transaction to a log file. The log file will be named pgbench_log.nnn, where nnn is the PID of the pgbench process. If the -j option is 2 or higher, creating multiple worker threads, each will have its own log file. The first worker will use the same name for its log file as in the standard single worker case. The additional log files for the other workers will be named pgbench_log.nnn.mmm, where mmm is a sequential number for each worker starting with 1.
The format of the log is:
client_id transaction_no time file_no time_epoch time_us
where time is the elapsed transaction time in microseconds, file_no identifies which script file was used (useful when multiple scripts were specified with -f), and time_epoch/time_us are a UNIX epoch format timestamp and an offset in microseconds (suitable for creating a ISO 8601 timestamp with fractional seconds) showing when the transaction completed.
Here are example outputs:
0 199 2241 0 1175850568 995598 0 200 2465 0 1175850568 998079 0 201 2513 0 1175850569 608 0 202 2038 0 1175850569 2663
F.23.8. Good Practices
It is very easy to use pgbench to produce completely meaningless numbers. Here are some guidelines to help you get useful results.
In the first place, never believe any test that runs for only a few seconds. Use the -t or -T option to make the run last at least a few minutes, so as to average out noise. In some cases you could need hours to get numbers that are reproducible. It's a good idea to try the test run a few times, to find out if your numbers are reproducible or not.
For the default TPC-B-like test scenario, the initialization scale factor (-s) should be at least as large as the largest number of clients you intend to test (-c); else you'll mostly be measuring update contention. There are only -s rows in the pgbench_branches table, and every transaction wants to update one of them, so -c values in excess of -s will undoubtedly result in lots of transactions blocked waiting for other transactions.
The default test scenario is also quite sensitive to how long it's been since the tables were initialized: accumulation of dead rows and dead space in the tables changes the results. To understand the results you must keep track of the total number of updates and when vacuuming happens. If autovacuum is enabled it can result in unpredictable changes in measured performance.
A limitation of pgbench is that it can itself become the bottleneck when trying to test a large number of client sessions. This can be alleviated by running pgbench on a different machine from the database server, although low network latency will be essential. It might even be useful to run several pgbench instances concurrently, on several client machines, against the same database server.