823 lines
27 KiB
Plaintext
823 lines
27 KiB
Plaintext
<!-- doc/src/sgml/pgbench.sgml -->
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<refentry id="pgbench">
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<refmeta>
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<refentrytitle><application>pgbench</application></refentrytitle>
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<manvolnum>1</manvolnum>
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<refmiscinfo>Application</refmiscinfo>
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</refmeta>
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<refnamediv>
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<refname>pgbench</refname>
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<refpurpose>run a benchmark test on <productname>PostgreSQL</productname></refpurpose>
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</refnamediv>
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<indexterm zone="pgbench">
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<primary>pgbench</primary>
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</indexterm>
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<refsynopsisdiv>
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<cmdsynopsis>
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<command>pgbench</command>
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<arg choice="plain"><option>-i</option></arg>
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<arg rep="repeat"><replaceable>option</replaceable></arg>
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<arg choice="opt"><replaceable>dbname</replaceable></arg>
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</cmdsynopsis>
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<cmdsynopsis>
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<command>pgbench</command>
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<arg rep="repeat"><replaceable>option</replaceable></arg>
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<arg choice="opt"><replaceable>dbname</replaceable></arg>
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</cmdsynopsis>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para>
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<application>pgbench</application> is a simple program for running benchmark
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tests on <productname>PostgreSQL</>. It runs the same sequence of SQL
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commands over and over, possibly in multiple concurrent database sessions,
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and then calculates the average transaction rate (transactions per second).
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By default, <application>pgbench</application> tests a scenario that is
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loosely based on TPC-B, involving five <command>SELECT</>,
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<command>UPDATE</>, and <command>INSERT</> commands per transaction.
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However, it is easy to test other cases by writing your own transaction
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script files.
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</para>
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<para>
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Typical output from pgbench looks like:
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<screen>
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transaction type: TPC-B (sort of)
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scaling factor: 10
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query mode: simple
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number of clients: 10
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number of threads: 1
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number of transactions per client: 1000
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number of transactions actually processed: 10000/10000
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tps = 85.184871 (including connections establishing)
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tps = 85.296346 (excluding connections establishing)
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</screen>
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The first six lines report some of the most important parameter
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settings. The next line reports the number of transactions completed
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and intended (the latter being just the product of number of clients
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and number of transactions per client); these will be equal unless the run
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failed before completion. (In <option>-T</> mode, only the actual
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number of transactions is printed.)
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The last two lines report the number of transactions per second,
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figured with and without counting the time to start database sessions.
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</para>
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<para>
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The default TPC-B-like transaction test requires specific tables to be
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set up beforehand. <application>pgbench</> should be invoked with
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the <option>-i</> (initialize) option to create and populate these
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tables. (When you are testing a custom script, you don't need this
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step, but will instead need to do whatever setup your test needs.)
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Initialization looks like:
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<programlisting>
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pgbench -i <optional> <replaceable>other-options</> </optional> <replaceable>dbname</>
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</programlisting>
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where <replaceable>dbname</> is the name of the already-created
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database to test in. (You may also need <option>-h</>,
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<option>-p</>, and/or <option>-U</> options to specify how to
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connect to the database server.)
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</para>
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<caution>
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<para>
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<literal>pgbench -i</> creates four tables <structname>pgbench_accounts</>,
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<structname>pgbench_branches</>, <structname>pgbench_history</>, and
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<structname>pgbench_tellers</>,
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destroying any existing tables of these names.
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Be very careful to use another database if you have tables having these
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names!
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</para>
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</caution>
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<para>
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At the default <quote>scale factor</> of 1, the tables initially
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contain this many rows:
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<screen>
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table # of rows
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---------------------------------
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pgbench_branches 1
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pgbench_tellers 10
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pgbench_accounts 100000
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pgbench_history 0
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</screen>
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You can (and, for most purposes, probably should) increase the number
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of rows by using the <option>-s</> (scale factor) option. The
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<option>-F</> (fillfactor) option might also be used at this point.
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</para>
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<para>
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Once you have done the necessary setup, you can run your benchmark
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with a command that doesn't include <option>-i</>, that is
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<programlisting>
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pgbench <optional> <replaceable>options</> </optional> <replaceable>dbname</>
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</programlisting>
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In nearly all cases, you'll need some options to make a useful test.
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The most important options are <option>-c</> (number of clients),
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<option>-t</> (number of transactions), <option>-T</> (time limit),
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and <option>-f</> (specify a custom script file).
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See below for a full list.
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</para>
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</refsect1>
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<refsect1>
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<title>Options</title>
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<para>
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The following is divided into three subsections: Different options are used
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during database initialization and while running benchmarks, some options
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are useful in both cases.
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</para>
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<refsect2 id="pgbench-init-options">
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<title>Initialization Options</title>
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<para>
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<application>pgbench</application> accepts the following command-line
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initialization arguments:
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<variablelist>
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<varlistentry>
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<term><option>-i</option></term>
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<listitem>
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<para>
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Required to invoke initialization mode.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><option>-F</option> <replaceable>fillfactor</></term>
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<listitem>
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<para>
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Create the <structname>pgbench_accounts</>,
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<structname>pgbench_tellers</> and
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<structname>pgbench_branches</> tables with the given fillfactor.
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Default is 100.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><option>-s</option> <replaceable>scale_factor</></term>
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<listitem>
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<para>
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Multiply the number of rows generated by the scale factor.
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For example, <literal>-s 100</> will create 10,000,000 rows
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in the <structname>pgbench_accounts</> table. Default is 1.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><option>--index-tablespace=<replaceable>index_tablespace</replaceable></option></term>
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<listitem>
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<para>
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Create indexes in the specified tablespace, rather than the default
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tablespace.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><option>--tablespace=<replaceable>tablespace</replaceable></option></term>
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<listitem>
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<para>
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Create tables in the specified tablespace, rather than the default
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tablespace.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><option>--unlogged-tables</option></term>
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<listitem>
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<para>
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Create all tables as unlogged tables, rather than permanent tables.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</para>
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</refsect2>
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<refsect2 id="pgbench-run-options">
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<title>Benchmarking Options</title>
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<para>
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<application>pgbench</application> accepts the following command-line
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benchmarking arguments:
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<variablelist>
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<varlistentry>
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<term><option>-c</option> <replaceable>clients</></term>
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<listitem>
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<para>
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Number of clients simulated, that is, number of concurrent database
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sessions. Default is 1.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><option>-C</option></term>
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<listitem>
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<para>
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Establish a new connection for each transaction, rather than
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doing it just once per client session.
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This is useful to measure the connection overhead.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><option>-d</option></term>
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<listitem>
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<para>
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Print debugging output.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><option>-D</option> <replaceable>varname</><literal>=</><replaceable>value</></term>
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<listitem>
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<para>
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Define a variable for use by a custom script (see below).
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Multiple <option>-D</> options are allowed.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><option>-f</option> <replaceable>filename</></term>
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<listitem>
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<para>
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Read transaction script from <replaceable>filename</>.
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See below for details.
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<option>-N</option>, <option>-S</option>, and <option>-f</option>
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are mutually exclusive.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><option>-j</option> <replaceable>threads</></term>
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<listitem>
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<para>
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Number of worker threads within <application>pgbench</application>.
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Using more than one thread can be helpful on multi-CPU machines.
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The number of clients must be a multiple of the number of threads,
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since each thread is given the same number of client sessions to manage.
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Default is 1.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><option>-l</option></term>
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<listitem>
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<para>
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Write the time taken by each transaction to a log file.
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See below for details.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><option>-M</option> <replaceable>querymode</></term>
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<listitem>
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<para>
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Protocol to use for submitting queries to the server:
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<itemizedlist>
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<listitem>
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<para><literal>simple</>: use simple query protocol.</para>
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</listitem>
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<listitem>
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<para><literal>extended</>: use extended query protocol.</para>
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</listitem>
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<listitem>
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<para><literal>prepared</>: use extended query protocol with prepared statements.</para>
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</listitem>
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</itemizedlist>
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The default is simple query protocol. (See <xref linkend="protocol">
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for more information.)
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><option>-n</option></term>
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<listitem>
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<para>
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Perform no vacuuming before running the test.
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This option is <emphasis>necessary</>
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if you are running a custom test scenario that does not include
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the standard tables <structname>pgbench_accounts</>,
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<structname>pgbench_branches</>, <structname>pgbench_history</>, and
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<structname>pgbench_tellers</>.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><option>-N</option></term>
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<listitem>
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<para>
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Do not update <structname>pgbench_tellers</> and
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<structname>pgbench_branches</>.
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This will avoid update contention on these tables, but
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it makes the test case even less like TPC-B.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><option>-r</option></term>
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<listitem>
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<para>
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Report the average per-statement latency (execution time from the
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perspective of the client) of each command after the benchmark
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finishes. See below for details.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><option>-s</option> <replaceable>scale_factor</></term>
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<listitem>
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<para>
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Report the specified scale factor in <application>pgbench</>'s
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output. With the built-in tests, this is not necessary; the
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correct scale factor will be detected by counting the number of
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rows in the <structname>pgbench_branches</> table. However, when testing
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custom benchmarks (<option>-f</> option), the scale factor
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will be reported as 1 unless this option is used.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><option>-S</option></term>
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<listitem>
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<para>
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Perform select-only transactions instead of TPC-B-like test.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><option>-t</option> <replaceable>transactions</></term>
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<listitem>
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<para>
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Number of transactions each client runs. Default is 10.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><option>-T</option> <replaceable>seconds</></term>
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<listitem>
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<para>
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Run the test for this many seconds, rather than a fixed number of
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transactions per client. <option>-t</option> and
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<option>-T</option> are mutually exclusive.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><option>-v</option></term>
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<listitem>
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<para>
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Vacuum all four standard tables before running the test.
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With neither <option>-n</> nor <option>-v</>, pgbench will vacuum the
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<structname>pgbench_tellers</> and <structname>pgbench_branches</>
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tables, and will truncate <structname>pgbench_history</>.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</para>
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</refsect2>
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<refsect2 id="pgbench-common-options">
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<title>Common Options</title>
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<para>
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<application>pgbench</application> accepts the following command-line
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common arguments:
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<variablelist>
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<varlistentry>
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<term><option>-h</option> <replaceable>hostname</></term>
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<listitem>
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<para>
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The database server's host name
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><option>-p</option> <replaceable>port</></term>
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<listitem>
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<para>
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The database server's port number
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><option>-U</option> <replaceable>login</></term>
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<listitem>
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<para>
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The user name to connect as
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</para>
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</refsect2>
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</refsect1>
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<refsect1>
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<title>Notes</title>
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<refsect2>
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<title>What is the <quote>Transaction</> Actually Performed in pgbench?</title>
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<para>
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The default transaction script issues seven commands per transaction:
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</para>
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<orderedlist>
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<listitem><para><literal>BEGIN;</literal></para></listitem>
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<listitem><para><literal>UPDATE pgbench_accounts SET abalance = abalance + :delta WHERE aid = :aid;</literal></para></listitem>
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<listitem><para><literal>SELECT abalance FROM pgbench_accounts WHERE aid = :aid;</literal></para></listitem>
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<listitem><para><literal>UPDATE pgbench_tellers SET tbalance = tbalance + :delta WHERE tid = :tid;</literal></para></listitem>
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<listitem><para><literal>UPDATE pgbench_branches SET bbalance = bbalance + :delta WHERE bid = :bid;</literal></para></listitem>
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<listitem><para><literal>INSERT INTO pgbench_history (tid, bid, aid, delta, mtime) VALUES (:tid, :bid, :aid, :delta, CURRENT_TIMESTAMP);</literal></para></listitem>
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<listitem><para><literal>END;</literal></para></listitem>
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</orderedlist>
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<para>
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If you specify <option>-N</>, steps 4 and 5 aren't included in the
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transaction. If you specify <option>-S</>, only the <command>SELECT</> is
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issued.
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</para>
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</refsect2>
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<refsect2>
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<title>Custom Scripts</title>
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<para>
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<application>pgbench</application> has support for running custom
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benchmark scenarios by replacing the default transaction script
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(described above) with a transaction script read from a file
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(<option>-f</option> option). In this case a <quote>transaction</>
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counts as one execution of a script file. You can even specify
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multiple scripts (multiple <option>-f</option> options), in which
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case a random one of the scripts is chosen each time a client session
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starts a new transaction.
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</para>
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<para>
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The format of a script file is one SQL command per line; multiline
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SQL commands are not supported. Empty lines and lines beginning with
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<literal>--</> are ignored. Script file lines can also be
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<quote>meta commands</>, which are interpreted by <application>pgbench</>
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itself, as described below.
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</para>
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<para>
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There is a simple variable-substitution facility for script files.
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Variables can be set by the command-line <option>-D</> option,
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explained above, or by the meta commands explained below.
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In addition to any variables preset by <option>-D</> command-line options,
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the variable <literal>scale</> is preset to the current scale factor.
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Once set, a variable's
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value can be inserted into a SQL command by writing
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<literal>:</><replaceable>variablename</>. When running more than
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one client session, each session has its own set of variables.
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</para>
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<para>
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Script file meta commands begin with a backslash (<literal>\</>).
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Arguments to a meta command are separated by white space.
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These meta commands are supported:
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</para>
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|
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<variablelist>
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<varlistentry>
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<term>
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|
<literal>\set <replaceable>varname</> <replaceable>operand1</> [ <replaceable>operator</> <replaceable>operand2</> ]</literal>
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</term>
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|
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<listitem>
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<para>
|
|
Sets variable <replaceable>varname</> to a calculated integer value.
|
|
Each <replaceable>operand</> is either an integer constant or a
|
|
<literal>:</><replaceable>variablename</> reference to a variable
|
|
having an integer value. The <replaceable>operator</> can be
|
|
<literal>+</>, <literal>-</>, <literal>*</>, or <literal>/</>.
|
|
</para>
|
|
|
|
<para>
|
|
Example:
|
|
<programlisting>
|
|
\set ntellers 10 * :scale
|
|
</programlisting></para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term>
|
|
<literal>\setrandom <replaceable>varname</> <replaceable>min</> <replaceable>max</></literal>
|
|
</term>
|
|
|
|
<listitem>
|
|
<para>
|
|
Sets variable <replaceable>varname</> to a random integer value
|
|
between the limits <replaceable>min</> and <replaceable>max</> inclusive.
|
|
Each limit can be either an integer constant or a
|
|
<literal>:</><replaceable>variablename</> reference to a variable
|
|
having an integer value.
|
|
</para>
|
|
|
|
<para>
|
|
Example:
|
|
<programlisting>
|
|
\setrandom aid 1 :naccounts
|
|
</programlisting></para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term>
|
|
<literal>\sleep <replaceable>number</> [ us | ms | s ]</literal>
|
|
</term>
|
|
|
|
<listitem>
|
|
<para>
|
|
Causes script execution to sleep for the specified duration in
|
|
microseconds (<literal>us</>), milliseconds (<literal>ms</>) or seconds
|
|
(<literal>s</>). If the unit is omitted then seconds are the default.
|
|
<replaceable>number</> can be either an integer constant or a
|
|
<literal>:</><replaceable>variablename</> reference to a variable
|
|
having an integer value.
|
|
</para>
|
|
|
|
<para>
|
|
Example:
|
|
<programlisting>
|
|
\sleep 10 ms
|
|
</programlisting></para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term>
|
|
<literal>\setshell <replaceable>varname</> <replaceable>command</> [ <replaceable>argument</> ... ]</literal>
|
|
</term>
|
|
|
|
<listitem>
|
|
<para>
|
|
Sets variable <replaceable>varname</> to the result of the shell command
|
|
<replaceable>command</>. The command must return an integer value
|
|
through its standard output.
|
|
</para>
|
|
|
|
<para>
|
|
<replaceable>argument</> can be either a text constant or a
|
|
<literal>:</><replaceable>variablename</> reference to a variable of
|
|
any types. If you want to use <replaceable>argument</> starting with
|
|
colons, you need to add an additional colon at the beginning of
|
|
<replaceable>argument</>.
|
|
</para>
|
|
|
|
<para>
|
|
Example:
|
|
<programlisting>
|
|
\setshell variable_to_be_assigned command literal_argument :variable ::literal_starting_with_colon
|
|
</programlisting></para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term>
|
|
<literal>\shell <replaceable>command</> [ <replaceable>argument</> ... ]</literal>
|
|
</term>
|
|
|
|
<listitem>
|
|
<para>
|
|
Same as <literal>\setshell</literal>, but the result is ignored.
|
|
</para>
|
|
|
|
<para>
|
|
Example:
|
|
<programlisting>
|
|
\shell command literal_argument :variable ::literal_starting_with_colon
|
|
</programlisting></para>
|
|
</listitem>
|
|
</varlistentry>
|
|
</variablelist>
|
|
|
|
<para>
|
|
As an example, the full definition of the built-in TPC-B-like
|
|
transaction is:
|
|
|
|
<programlisting>
|
|
\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;
|
|
</programlisting>
|
|
|
|
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.)
|
|
</para>
|
|
|
|
</refsect2>
|
|
|
|
<refsect2>
|
|
<title>Per-Transaction Logging</title>
|
|
|
|
<para>
|
|
With the <option>-l</> option, <application>pgbench</> writes the time
|
|
taken by each transaction to a log file. The log file will be named
|
|
<filename>pgbench_log.<replaceable>nnn</></filename>, where
|
|
<replaceable>nnn</> is the PID of the pgbench process.
|
|
If the <option>-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
|
|
<filename>pgbench_log.<replaceable>nnn</>.<replaceable>mmm</></filename>,
|
|
where <replaceable>mmm</> is a sequential number for each worker starting
|
|
with 1.
|
|
</para>
|
|
|
|
<para>
|
|
The format of the log is:
|
|
|
|
<synopsis>
|
|
<replaceable>client_id</> <replaceable>transaction_no</> <replaceable>time</> <replaceable>file_no</> <replaceable>time_epoch</> <replaceable>time_us</>
|
|
</synopsis>
|
|
|
|
where <replaceable>time</> is the total elapsed transaction time in microseconds,
|
|
<replaceable>file_no</> identifies which script file was used
|
|
(useful when multiple scripts were specified with <option>-f</>),
|
|
and <replaceable>time_epoch</>/<replaceable>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.
|
|
</para>
|
|
|
|
<para>
|
|
Here are example outputs:
|
|
<screen>
|
|
0 199 2241 0 1175850568 995598
|
|
0 200 2465 0 1175850568 998079
|
|
0 201 2513 0 1175850569 608
|
|
0 202 2038 0 1175850569 2663
|
|
</screen></para>
|
|
</refsect2>
|
|
|
|
<refsect2>
|
|
<title>Per-Statement Latencies</title>
|
|
|
|
<para>
|
|
With the <option>-r</> option, <application>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.
|
|
</para>
|
|
|
|
<para>
|
|
For the default script, the output will look similar to this:
|
|
<screen>
|
|
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;
|
|
</screen>
|
|
</para>
|
|
|
|
<para>
|
|
If multiple script files are specified, the averages are reported
|
|
separately for each script file.
|
|
</para>
|
|
|
|
<para>
|
|
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.
|
|
</para>
|
|
</refsect2>
|
|
|
|
<refsect2>
|
|
<title>Good Practices</title>
|
|
|
|
<para>
|
|
It is very easy to use <application>pgbench</> to produce completely
|
|
meaningless numbers. Here are some guidelines to help you get useful
|
|
results.
|
|
</para>
|
|
|
|
<para>
|
|
In the first place, <emphasis>never</> believe any test that runs
|
|
for only a few seconds. Use the <option>-t</> or <option>-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.
|
|
</para>
|
|
|
|
<para>
|
|
For the default TPC-B-like test scenario, the initialization scale factor
|
|
(<option>-s</>) should be at least as large as the largest number of
|
|
clients you intend to test (<option>-c</>); else you'll mostly be
|
|
measuring update contention. There are only <option>-s</> rows in
|
|
the <structname>pgbench_branches</> table, and every transaction wants to
|
|
update one of them, so <option>-c</> values in excess of <option>-s</>
|
|
will undoubtedly result in lots of transactions blocked waiting for
|
|
other transactions.
|
|
</para>
|
|
|
|
<para>
|
|
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.
|
|
</para>
|
|
|
|
<para>
|
|
A limitation of <application>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 <application>pgbench</> on a different
|
|
machine from the database server, although low network latency will be
|
|
essential. It might even be useful to run several <application>pgbench</>
|
|
instances concurrently, on several client machines, against the same
|
|
database server.
|
|
</para>
|
|
</refsect2>
|
|
</refsect1>
|
|
</refentry>
|