pgbench Man page

PGBENCH(1) PostgreSQL 9.5.5 Documentation PGBENCH(1)

NAME

pgbench – run a benchmark test on PostgreSQL

SYNOPSIS

pgbench -i [option…] [dbname]

pgbench [option…] [dbname]

DESCRIPTION

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. (In -T mode, only the actual number of
transactions is printed.) The last two lines report the number of
transactions per second, figured with and without counting the time to
start database sessions.

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.

OPTIONS

The following is divided into three subsections: Different options are
used during database initialization and while running benchmarks, some
options are useful in both cases.

Initialization Options
pgbench accepts the following command-line initialization arguments:

-i
–initialize
Required to invoke initialization mode.

-F fillfactor
–fillfactor=fillfactor
Create the pgbench_accounts, pgbench_tellers and pgbench_branches
tables with the given fillfactor. Default is 100.

-n
–no-vacuum
Perform no vacuuming after initialization.

-q
–quiet
Switch logging to quiet mode, producing only one progress message
per 5 seconds. The default logging prints one message each 100000
rows, which often outputs many lines per second (especially on good
hardware).

-s scale_factor
–scale=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. When the scale is 20,000 or larger, the
columns used to hold account identifiers (aid columns) will switch
to using larger integers (bigint), in order to be big enough to
hold the range of account identifiers.

–foreign-keys
Create foreign key constraints between the standard tables.

–index-tablespace=index_tablespace
Create indexes in the specified tablespace, rather than the default
tablespace.

–tablespace=tablespace
Create tables in the specified tablespace, rather than the default
tablespace.

–unlogged-tables
Create all tables as unlogged tables, rather than permanent tables.

Benchmarking Options
pgbench accepts the following command-line benchmarking arguments:

-c clients
–client=clients
Number of clients simulated, that is, number of concurrent database
sessions. Default is 1.

-C
–connect
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
–debug
Print debugging output.

-D varname=value
–define=varname=value
Define a variable for use by a custom script (see below). Multiple
-D options are allowed.

-f filename
–file=filename
Read transaction script from filename. See below for details. -N,
-S, and -f are mutually exclusive.

-j threads
–jobs=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
–log
Write the time taken by each transaction to a log file. See below
for details.

-L limit
–latency-limit=limit
Transaction which last more than limit milliseconds are counted and
reported separately, as late.

When throttling is used (–rate=…), transactions that lag behind
schedule by more than limit ms, and thus have no hope of meeting
the latency limit, are not sent to the server at all. They are
counted and reported separately as skipped.

-M querymode
–protocol=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 50,
Frontend/Backend Protocol, in the documentation for more
information.)

-n
–no-vacuum
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
–skip-some-updates
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.

-P sec
–progress=sec
Show progress report every sec seconds. The report includes the
time since the beginning of the run, the tps since the last report,
and the transaction latency average and standard deviation since
the last report. Under throttling (-R), the latency is computed
with respect to the transaction scheduled start time, not the
actual transaction beginning time, thus it also includes the
average schedule lag time.

-r
–report-latencies
Report the average per-statement latency (execution time from the
perspective of the client) of each command after the benchmark
finishes. See below for details.

-R rate
–rate=rate
Execute transactions targeting the specified rate instead of
running as fast as possible (the default). The rate is given in
transactions per second. If the targeted rate is above the maximum
possible rate, the rate limit won’t impact the results.

The rate is targeted by starting transactions along a
Poisson-distributed schedule time line. The expected start time
schedule moves forward based on when the client first started, not
when the previous transaction ended. That approach means that when
transactions go past their original scheduled end time, it is
possible for later ones to catch up again.

When throttling is active, the transaction latency reported at the
end of the run is calculated from the scheduled start times, so it
includes the time each transaction had to wait for the previous
transaction to finish. The wait time is called the schedule lag
time, and its average and maximum are also reported separately. The
transaction latency with respect to the actual transaction start
time, i.e. the time spent executing the transaction in the
database, can be computed by subtracting the schedule lag time from
the reported latency.

If –latency-limit is used together with –rate, a transaction can
lag behind so much that it is already over the latency limit when
the previous transaction ends, because the latency is calculated
from the scheduled start time. Such transactions are not sent to
the server, but are skipped altogether and counted separately.

A high schedule lag time is an indication that the system cannot
process transactions at the specified rate, with the chosen number
of clients and threads. When the average transaction execution time
is longer than the scheduled interval between each transaction,
each successive transaction will fall further behind, and the
schedule lag time will keep increasing the longer the test run is.
When that happens, you will have to reduce the specified
transaction rate.

-s scale_factor
–scale=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
–select-only
Perform select-only transactions instead of TPC-B-like test.

-t transactions
–transactions=transactions
Number of transactions each client runs. Default is 10.

-T seconds
–time=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
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.

–aggregate-interval=seconds
Length of aggregation interval (in seconds). May be used only
together with -l – with this option, the log contains per-interval
summary (number of transactions, min/max latency and two additional
fields useful for variance estimation).

This option is not currently supported on Windows.

–sampling-rate=rate
Sampling rate, used when writing data into the log, to reduce the
amount of log generated. If this option is given, only the
specified fraction of transactions are logged. 1.0 means all
transactions will be logged, 0.05 means only 5% of the transactions
will be logged.

Remember to take the sampling rate into account when processing the
log file. For example, when computing tps values, you need to
multiply the numbers accordingly (e.g. with 0.01 sample rate,
you’ll only get 1/100 of the actual tps).

Common Options
pgbench accepts the following command-line common arguments:

-h hostname
–host=hostname
The database server’s host name

-p port
–port=port
The database server’s port number

-U login
–username=login
The user name to connect as

-V
–version
Print the pgbench version and exit.

-?
–help
Show help about pgbench command line arguments, and exit.

NOTES
What is the “Transaction” Actually Performed in pgbench?
The default transaction script issues seven commands per transaction:

1. BEGIN;

2. UPDATE pgbench_accounts SET abalance = abalance + :delta WHERE aid
= :aid;

3. SELECT abalance FROM pgbench_accounts WHERE aid = :aid;

4. UPDATE pgbench_tellers SET tbalance = tbalance + :delta WHERE tid =
:tid;

5. UPDATE pgbench_branches SET bbalance = bbalance + :delta WHERE bid
= :bid;

6. INSERT INTO pgbench_history (tid, bid, aid, delta, mtime) VALUES
(:tid, :bid, :aid, :delta, CURRENT_TIMESTAMP);

7. END;

If you specify -N, steps 4 and 5 aren’t included in the transaction. If
you specify -S, only the SELECT is issued.

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, there are a few variables that are
preset automatically, listed in Table 222, “Automatic variables”. A
value specified for these variables using -D takes precedence over the
automatic presets. 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.

Table 222. Automatic variables
┌──────────┬────────────────────────────┐
│Variable │ Description │
├──────────┼────────────────────────────┤
│scale │ current scale factor │
├──────────┼────────────────────────────┤
│client_id │ unique number identifying │
│ │ the client session (starts │
│ │ from zero) │
└──────────┴────────────────────────────┘

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 expression
Sets variable varname to an integer value calculated from
expression. The expression may contain integer constants such as
5432, references to variables :variablename, and expressions
composed of unary (-) or binary operators (+, -, *, /, %) with
their usual associativity, and parentheses.

Examples:

\set ntellers 10 * :scale
\set aid (1021 * :aid) % (100000 * :scale) + 1

\setrandom varname min max [ uniform | { gaussian | exponential }
parameter ] 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.

By default, or when uniform is specified, all values in the range
are drawn with equal probability. Specifying gaussian or
exponential options modifies this behavior; each requires a
mandatory parameter which determines the precise shape of the
distribution.

For a Gaussian distribution, the interval is mapped onto a standard
normal distribution (the classical bell-shaped Gaussian curve)
truncated at -parameter on the left and +parameter on the right.
Values in the middle of the interval are more likely to be drawn.
To be precise, if PHI(x) is the cumulative distribution function of
the standard normal distribution, with mean mu defined as (max +
min) / 2.0, with

f(x) = PHI(2.0 * parameter * (x – mu) / (max – min + 1)) /
(2.0 * PHI(parameter) – 1.0)

then value i between min and max inclusive is drawn with
probability: f(i + 0.5) – f(i – 0.5). Intuitively, the larger
parameter, the more frequently values close to the middle of the
interval are drawn, and the less frequently values close to the min
and max bounds. About 67% of values are drawn from the middle 1.0 /
parameter, that is a relative 0.5 / parameter around the mean, and
95% in the middle 2.0 / parameter, that is a relative 1.0 /
parameter around the mean; for instance, if parameter is 4.0, 67%
of values are drawn from the middle quarter (1.0 / 4.0) of the
interval (i.e. from 3.0 / 8.0 to 5.0 / 8.0) and 95% from the middle
half (2.0 / 4.0) of the interval (second and third quartiles). The
minimum parameter is 2.0 for performance of the Box-Muller
transform.

For an exponential distribution, parameter controls the
distribution by truncating a quickly-decreasing exponential
distribution at parameter, and then projecting onto integers
between the bounds. To be precise, with

f(x) = exp(-parameter * (x – min) / (max – min + 1)) / (1.0 – exp(-parameter))

Then value i between min and max inclusive is drawn with
probability: f(x) – f(x + 1). Intuitively, the larger parameter,
the more frequently values close to min are accessed, and the less
frequently values close to max are accessed. The closer to 0
parameter, the flatter (more uniform) the access distribution. A
crude approximation of the distribution is that the most frequent
1% values in the range, close to min, are drawn parameter% of the
time. parameter value must be strictly positive.

Example:

\setrandom aid 1 :naccounts gaussian 5.0

\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.)

Per-Transaction Logging
With the -l option but without the –aggregate-interval, 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 [schedule_lag]

where time is the total 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 time stamp and an offset in microseconds (suitable for
creating an ISO 8601 time stamp with fractional seconds) showing when
the transaction completed. Field schedule_lag is the difference between
the transaction’s scheduled start time, and the time it actually
started, in microseconds. It is only present when the –rate option is
used. When both –rate and –latency-limit are used, the time for a
skipped transaction will be reported as skipped.

Here is a snippet of the log file generated:

0 199 2241 0 1175850568 995598
0 200 2465 0 1175850568 998079
0 201 2513 0 1175850569 608
0 202 2038 0 1175850569 2663

Another example with –rate=100 and –latency-limit=5 (note the
additional schedule_lag column):

0 81 4621 0 1412881037 912698 3005
0 82 6173 0 1412881037 914578 4304
0 83 skipped 0 1412881037 914578 5217
0 83 skipped 0 1412881037 914578 5099
0 83 4722 0 1412881037 916203 3108
0 84 4142 0 1412881037 918023 2333
0 85 2465 0 1412881037 919759 740

In this example, transaction 82 was late, because its latency (6.173
ms) was over the 5 ms limit. The next two transactions were skipped,
because they were already late before they were even started.

When running a long test on hardware that can handle a lot of
transactions, the log files can become very large. The –sampling-rate
option can be used to log only a random sample of transactions.

Aggregated Logging
With the –aggregate-interval option, the logs use a bit different
format:

interval_start num_of_transactions latency_sum latency_2_sum min_latency max_latency [lag_sum lag_2_sum min_lag max_lag [skipped_transactions]]

where interval_start is the start of the interval (Unix epoch format
time stamp), num_of_transactions is the number of transactions within
the interval, latency_sum is a sum of latencies (so you can compute
average latency easily). The following two fields are useful for
variance estimation – latency_sum is a sum of latencies and
latency_2_sum is a sum of 2nd powers of latencies. The next two fields
are min_latency – a minimum latency within the interval, and
max_latency – maximum latency within the interval. A transaction is
counted into the interval when it was committed. The fields in the end,
lag_sum, lag_2_sum, min_lag, and max_lag, are only present if the
–rate option is used. The very last one, skipped_transactions, is only
present if the option –latency-limit is present, too. They are
calculated from the time each transaction had to wait for the previous
one to finish, i.e. the difference between each transaction’s scheduled
start time and the time it actually started.

Here is example output:

1345828501 5601 1542744 483552416 61 2573
1345828503 7884 1979812 565806736 60 1479
1345828505 7208 1979422 567277552 59 1391
1345828507 7685 1980268 569784714 60 1398
1345828509 7073 1979779 573489941 236 1411

Notice that while the plain (unaggregated) log file contains a
reference to the custom script files, the aggregated log does not.
Therefore if you need per script data, you need to aggregate the data
on your own.

Per-Statement Latencies
With the -r option, pgbench collects the elapsed transaction time of
each statement executed by every client. It then reports an average of
those values, referred to as the latency for each statement, after the
benchmark has finished.

For the default script, the output will look similar to this:

starting vacuum…end.
transaction type: TPC-B (sort of)
scaling factor: 1
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 = 618.764555 (including connections establishing)
tps = 622.977698 (excluding connections establishing)
statement latencies in milliseconds:
0.004386 \set nbranches 1 * :scale
0.001343 \set ntellers 10 * :scale
0.001212 \set naccounts 100000 * :scale
0.001310 \setrandom aid 1 :naccounts
0.001073 \setrandom bid 1 :nbranches
0.001005 \setrandom tid 1 :ntellers
0.001078 \setrandom delta -5000 5000
0.326152 BEGIN;
0.603376 UPDATE pgbench_accounts SET abalance = abalance + :delta WHERE aid = :aid;
0.454643 SELECT abalance FROM pgbench_accounts WHERE aid = :aid;
5.528491 UPDATE pgbench_tellers SET tbalance = tbalance + :delta WHERE tid = :tid;
7.335435 UPDATE pgbench_branches SET bbalance = bbalance + :delta WHERE bid = :bid;
0.371851 INSERT INTO pgbench_history (tid, bid, aid, delta, mtime) VALUES (:tid, :bid, :aid, :delta, CURRENT_TIMESTAMP);
1.212976 END;

If multiple script files are specified, the averages are reported
separately for each script file.

Note that collecting the additional timing information needed for
per-statement latency computation adds some overhead. This will slow
average execution speed and lower the computed TPS. The amount of
slowdown varies significantly depending on platform and hardware.
Comparing average TPS values with and without latency reporting enabled
is a good way to measure if the timing overhead is significant.

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.

PostgreSQL 9.5.5 2016 PGBENCH(1)

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