postgresql.conf optimization

Question

My postgresql server is extremely slow. Especially when I execute several queries at the same time in different threads, which somtimes causes the postgresql server to be unresponsive for 5 to 15 seconds. I wonder if I made a mistake in the postgresql.conf.

My dedicated server has 2 cores and 4 gb of ram, and a standard SATA-2 disk. There are 6gb of data in the postgres db (version 8.4) and a few hundred users simultaneously connected. I'm using JDBC to access postgress, running 1 to 10 simultaneous threads accessing the database. My server is a debian lenny. The Cpu is not used at 100%, neither is the memory.

Thanks for any sort of help.

    # -----------------------------
    # PostgreSQL configuration file
    # -----------------------------
    #
    # This file consists of lines of the form:
    #
    #   name = value
    #
    # (The "=" is optional.)  Whitespace may be used.  Comments are introduced with
    # "#" anywhere on a line.  The complete list of parameter names and allowed
    # values can be found in the PostgreSQL documentation.
    #
    # The commented-out settings shown in this file represent the default values.
    # Re-commenting a setting is NOT sufficient to revert it to the default value;
    # you need to reload the server.
    #
    # This file is read on server startup and when the server receives a SIGHUP
    # signal.  If you edit the file on a running system, you have to SIGHUP the
    # server for the changes to take effect, or use "pg_ctl reload".  Some
    # parameters, which are marked below, require a server shutdown and restart to
    # take effect.
    #
    # Any parameter can also be given as a command-line option to the server, e.g.,
    # "postgres -c log_connections=on".  Some parameters can be changed at run time
    # with the "SET" SQL command.
    #
    # Memory units:  kB = kilobytes        Time units:  ms  = milliseconds
    #                MB = megabytes                     s   = seconds
    #                GB = gigabytes                     min = minutes
    #                                                   h   = hours
    #                                                   d   = days


    #------------------------------------------------------------------------------
    # FILE LOCATIONS
    #------------------------------------------------------------------------------

    # The default values of these variables are driven from the -D command-line
    # option or PGDATA environment variable, represented here as ConfigDir.

    data_directory = '/home/keyja/dbdata'       # use data in another directory
                        # (change requires restart)
    hba_file = '/etc/postgresql/8.4/main/pg_hba.conf'   # host-based authentication file
                        # (change requires restart)
    ident_file = '/etc/postgresql/8.4/main/pg_ident.conf'   # ident configuration file
                        # (change requires restart)

    # If external_pid_file is not explicitly set, no extra PID file is written.
    external_pid_file = '/var/run/postgresql/8.4-main.pid'      # write an extra PID file
                        # (change requires restart)


    #------------------------------------------------------------------------------
    # CONNECTIONS AND AUTHENTICATION
    #------------------------------------------------------------------------------

    # - Connection Settings -

    listen_addresses = '*'      # what IP address(es) to listen on;
                        # comma-separated list of addresses;
                        # defaults to 'localhost', '*' = all
                        # (change requires restart)
    port = 5432             # (change requires restart)
    max_connections = 100           # (change requires restart)
    # Note:  Increasing max_connections costs ~400 bytes of shared memory per 
    # connection slot, plus lock space (see max_locks_per_transaction).
    #superuser_reserved_connections = 3 # (change requires restart)
    unix_socket_directory = '/var/run/postgresql'       # (change requires restart)
    #unix_socket_group = ''         # (change requires restart)
    #unix_socket_permissions = 0777     # begin with 0 to use octal notation
                        # (change requires restart)
    #bonjour_name = ''          # defaults to the computer name
                        # (change requires restart)

    # - Security and Authentication -

    #authentication_timeout = 1min      # 1s-600s
    ssl = true              # (change requires restart)
    #ssl_ciphers = 'ALL:!ADH:!LOW:!EXP:!MD5:@STRENGTH'  # allowed SSL ciphers
                        # (change requires restart)
    #ssl_renegotiation_limit = 512MB    # amount of data between renegotiations
    #password_encryption = on
    #db_user_namespace = off

    # Kerberos and GSSAPI
    #krb_server_keyfile = ''
    #krb_srvname = 'postgres'       # (Kerberos only)
    #krb_caseins_users = off

    # - TCP Keepalives -
    # see "man 7 tcp" for details

    #tcp_keepalives_idle = 0        # TCP_KEEPIDLE, in seconds;
                        # 0 selects the system default
    #tcp_keepalives_interval = 0        # TCP_KEEPINTVL, in seconds;
                        # 0 selects the system default
    #tcp_keepalives_count = 0       # TCP_KEEPCNT;
                        # 0 selects the system default


    #------------------------------------------------------------------------------
    # RESOURCE USAGE (except WAL)
    #------------------------------------------------------------------------------

    # - Memory -

    shared_buffers = 1024MB         # min 128kB
                        # (change requires restart)
    #temp_buffers = 8MB         # min 800kB
    #max_prepared_transactions = 0      # zero disables the feature
                        # (change requires restart)
    # Note:  Increasing max_prepared_transactions costs ~600 bytes of shared memory
    # per transaction slot, plus lock space (see max_locks_per_transaction).
    # It is not advisable to set max_prepared_transactions nonzero unless you
    # actively intend to use prepared transactions.
    work_mem = 100MB                # min 64kB
    maintenance_work_mem = 160MB        # min 1MB
    #max_stack_depth = 2MB          # min 100kB

    # - Kernel Resource Usage -

    #max_files_per_process = 1000       # min 25
                        # (change requires restart)
    #shared_preload_libraries = ''      # (change requires restart)

    # - Cost-Based Vacuum Delay -

    #vacuum_cost_delay = 0ms        # 0-100 milliseconds
    #vacuum_cost_page_hit = 1       # 0-10000 credits
    #vacuum_cost_page_miss = 10     # 0-10000 credits
    #vacuum_cost_page_dirty = 20        # 0-10000 credits
    #vacuum_cost_limit = 200        # 1-10000 credits

    # - Background Writer -

    #bgwriter_delay = 200ms         # 10-10000ms between rounds
    #bgwriter_lru_maxpages = 100        # 0-1000 max buffers written/round
    #bgwriter_lru_multiplier = 2.0      # 0-10.0 multipler on buffers scanned/round

    # - Asynchronous Behavior -

    #effective_io_concurrency = 1       # 1-1000. 0 disables prefetching


    #------------------------------------------------------------------------------
    # WRITE AHEAD LOG
    #------------------------------------------------------------------------------

    # - Settings -

    fsync = off             # turns forced synchronization on or off
    #synchronous_commit = on        # immediate fsync at commit
    #wal_sync_method = fsync        # the default is the first option 
                        # supported by the operating system:
                        #   open_datasync
                        #   fdatasync (default on Linux)
                        #   fsync
                        #   fsync_writethrough
                        #   open_sync
    full_page_writes = off          # recover from partial page writes
    #wal_buffers = 64kB         # min 32kB
                        # (change requires restart)
    #wal_writer_delay = 200ms       # 1-10000 milliseconds

    #commit_delay = 0           # range 0-100000, in microseconds
    #commit_siblings = 5            # range 1-1000

    # - Checkpoints -

    checkpoint_segments = 20        # in logfile segments, min 1, 16MB each
    #checkpoint_timeout = 5min      # range 30s-1h
    checkpoint_completion_target = 0.9  # checkpoint target duration, 0.0 - 1.0
    #checkpoint_warning = 30s       # 0 disables

    # - Archiving -

    #archive_mode = off     # allows archiving to be done
                    # (change requires restart)
    #archive_command = ''       # command to use to archive a logfile segment
    #archive_timeout = 0        # force a logfile segment switch after this
                    # number of seconds; 0 disables


    #------------------------------------------------------------------------------
    # QUERY TUNING
    #------------------------------------------------------------------------------

    # - Planner Method Configuration -

    #enable_bitmapscan = on
    #enable_hashagg = on
    #enable_hashjoin = on
    #enable_indexscan = on
    #enable_mergejoin = on
    #enable_nestloop = on
    #enable_seqscan = on
    #enable_sort = on
    #enable_tidscan = on

    # - Planner Cost Constants -

    #seq_page_cost = 1.0            # measured on an arbitrary scale
    #random_page_cost = 4.0         # same scale as above
    #cpu_tuple_cost = 0.01          # same scale as above
    #cpu_index_tuple_cost = 0.005       # same scale as above
    #cpu_operator_cost = 0.0025     # same scale as above
    effective_cache_size = 256MB

    # - Genetic Query Optimizer -

    #geqo = on
    #geqo_threshold = 12
    #geqo_effort = 5            # range 1-10
    #geqo_pool_size = 0         # selects default based on effort
    #geqo_generations = 0           # selects default based on effort
    #geqo_selection_bias = 2.0      # range 1.5-2.0

    # - Other Planner Options -

    #default_statistics_target = 100    # range 1-10000
    #constraint_exclusion = partition   # on, off, or partition
    #cursor_tuple_fraction = 0.1        # range 0.0-1.0
    #from_collapse_limit = 8
    #join_collapse_limit = 8        # 1 disables collapsing of explicit 
                        # JOIN clauses


    #------------------------------------------------------------------------------
    # ERROR REPORTING AND LOGGING
    #------------------------------------------------------------------------------

    # - Where to Log -

    log_destination = 'stderr'      # Valid values are combinations of
                        # stderr, csvlog, syslog and eventlog,
                        # depending on platform.  csvlog
                        # requires logging_collector to be on.

    # This is used when logging to stderr:
    logging_collector = ON      # Enable capturing of stderr and csvlog
                        # into log files. Required to be on for
                        # csvlogs.
                        # (change requires restart)

    # These are only used if logging_collector is on:
    log_directory = 'pg_log'        # directory where log files are written,
                        # can be absolute or relative to PGDATA
    log_filename = 'postgresql_%A.log'  # log file name pattern,
                        # can include strftime() escapes
    log_truncate_on_rotation = on       # If on, an existing log file of the
                        # same name as the new log file will be
                        # truncated rather than appended to.
                        # But such truncation only occurs on
                        # time-driven rotation, not on restarts
                        # or size-driven rotation.  Default is
                        # off, meaning append to existing files
                        # in all cases.
    log_rotation_age = 1d           # Automatic rotation of logfiles will
                        # happen after that time.  0 disables.
    #log_rotation_size = 10MB       # Automatic rotation of logfiles will 
                        # happen after that much log output.
                        # 0 disables.

    # These are relevant when logging to syslog:
    #syslog_facility = 'LOCAL0'
    #syslog_ident = 'postgres'

    silent_mode = on            # Run server silently.
                        # DO NOT USE without syslog or
                        # logging_collector
                        # (change requires restart)


    # - When to Log -

    #client_min_messages = notice       # values in order of decreasing detail:
                        #   debug5
                        #   debug4
                        #   debug3
                        #   debug2
                        #   debug1
                        #   log
                        #   notice
                        #   warning
                        #   error

    #log_min_messages = warning     # values in order of decreasing detail:
                        #   debug5
                        #   debug4
                        #   debug3
                        #   debug2
                        #   debug1
                        #   info
                        #   notice
                        #   warning
                        #   error
                        #   log
                        #   fatal
                        #   panic

    #log_error_verbosity = default      # terse, default, or verbose messages

    #log_min_error_statement = error    # values in order of decreasing detail:
                        #   debug5
                        #   debug4
                        #   debug3
                        #   debug2
                        #   debug1
                        #   info
                        #   notice
                        #   warning
                        #   error
                        #   log
                        #   fatal
                        #   panic (effectively off)

    log_min_duration_statement = 1000   # -1 is disabled, 0 logs all statements
                        # and their durations, > 0 logs only
                        # statements running at least this number
                        # of milliseconds


    # - What to Log -

    #debug_print_parse = off
    #debug_print_rewritten = off
    #debug_print_plan = off
    #debug_pretty_print = on
    #log_checkpoints = off
    #log_connections = off
    #log_disconnections = off
    log_duration = off
    #log_hostname = off
    log_line_prefix = '%t [%p]: [%l-1] '            # special values:
                        #   %u = user name
                        #   %d = database name
                        #   %r = remote host and port
                        #   %h = remote host
                        #   %p = process ID
                        #   %t = timestamp without milliseconds
                        #   %m = timestamp with milliseconds
                        #   %i = command tag
                        #   %c = session ID
                        #   %l = session line number
                        #   %s = session start timestamp
                        #   %v = virtual transaction ID
                        #   %x = transaction ID (0 if none)
                        #   %q = stop here in non-session
                        #        processes
                        #   %% = '%'
                        # e.g. '<%u%%%d> '
    #log_lock_waits = off           # log lock waits >= deadlock_timeout
    log_statement = 'all'           # none, ddl, mod, all
    #log_temp_files = -1            # log temporary files equal or larger
                        # than the specified size in kilobytes;
                        # -1 disables, 0 logs all temp files
    #log_timezone = unknown         # actually, defaults to TZ environment
                        # setting


    #------------------------------------------------------------------------------
    # RUNTIME STATISTICS
    #------------------------------------------------------------------------------

    # - Query/Index Statistics Collector -

    #track_activities = on
    track_counts = on
    #track_functions = none         # none, pl, all
    #track_activity_query_size = 1024
    #update_process_title = on
    #stats_temp_directory = 'pg_stat_tmp'


    # - Statistics Monitoring -

    #log_parser_stats = off
    #log_planner_stats = off
    #log_executor_stats = off
    #log_statement_stats = off


    #------------------------------------------------------------------------------
    # AUTOVACUUM PARAMETERS
    #------------------------------------------------------------------------------

    autovacuum = on         # Enable autovacuum subprocess?  'on' 
                        # requires track_counts to also be on.
    log_autovacuum_min_duration = -1    # -1 disables, 0 logs all actions and
                        # their durations, > 0 logs only
                        # actions running at least this number
                        # of milliseconds.
    autovacuum_max_workers = 3      # max number of autovacuum subprocesses
    autovacuum_naptime = 1min       # time between autovacuum runs
    autovacuum_vacuum_threshold = 50    # min number of row updates before
                        # vacuum
    autovacuum_analyze_threshold = 50   # min number of row updates before 
                        # analyze
    autovacuum_vacuum_scale_factor = 0.2    # fraction of table size before vacuum
    autovacuum_analyze_scale_factor = 0.1   # fraction of table size before analyze
    autovacuum_freeze_max_age = 200000000   # maximum XID age before forced vacuum
                        # (change requires restart)
    autovacuum_vacuum_cost_delay = 20ms # default vacuum cost delay for
                        # autovacuum, in milliseconds;
                        # -1 means use vacuum_cost_delay
    autovacuum_vacuum_cost_limit = -1   # default vacuum cost limit for
                        # autovacuum, -1 means use
                        # vacuum_cost_limit


    #------------------------------------------------------------------------------
    # CLIENT CONNECTION DEFAULTS
    #------------------------------------------------------------------------------

    # - Statement Behavior -

    #search_path = '"$user",public'     # schema names
    #default_tablespace = ''        # a tablespace name, '' uses the default
    #temp_tablespaces = ''          # a list of tablespace names, '' uses
                        # only default tablespace
    #check_function_bodies = on
    #default_transaction_isolation = 'read committed'
    #default_transaction_read_only = off
    #session_replication_role = 'origin'
    #statement_timeout = 0          # in milliseconds, 0 is disabled
    #vacuum_freeze_min_age = 50000000
    #vacuum_freeze_table_age = 150000000
    #xmlbinary = 'base64'
    #xmloption = 'content'

    # - Locale and Formatting -

    datestyle = 'iso, dmy'
    #intervalstyle = 'postgres'
    #timezone = unknown         # actually, defaults to TZ environment
                        # setting
    #timezone_abbreviations = 'Default'     # Select the set of available time zone
                        # abbreviations.  Currently, there are
                        #   Default
                        #   Australia
                        #   India
                        # You can create your own file in
                        # share/timezonesets/.
    #extra_float_digits = 0         # min -15, max 2
    #client_encoding = sql_ascii        # actually, defaults to database
                        # encoding

    # These settings are initialized by initdb, but they can be changed.
    lc_messages = 'en_GB'           # locale for system error message
                        # strings
    lc_monetary = 'en_GB'           # locale for monetary formatting
    lc_numeric = 'en_GB'            # locale for number formatting
    lc_time = 'en_GB'               # locale for time formatting

    # default configuration for text search
    default_text_search_config = 'pg_catalog.english'

    # - Other Defaults -

    #dynamic_library_path = '$libdir'
    #local_preload_libraries = ''


    #------------------------------------------------------------------------------
    # LOCK MANAGEMENT
    #------------------------------------------------------------------------------

    #deadlock_timeout = 1s
    #max_locks_per_transaction = 64     # min 10
                        # (change requires restart)
    # Note:  Each lock table slot uses ~270 bytes of shared memory, and there are
    # max_locks_per_transaction * (max_connections + max_prepared_transactions)
    # lock table slots.


    #------------------------------------------------------------------------------
    # VERSION/PLATFORM COMPATIBILITY
    #------------------------------------------------------------------------------

    # - Previous PostgreSQL Versions -

    #add_missing_from = off
    #array_nulls = on
    #backslash_quote = safe_encoding    # on, off, or safe_encoding
    #default_with_oids = off
    #escape_string_warning = on
    #regex_flavor = advanced        # advanced, extended, or basic
    #sql_inheritance = on
    #standard_conforming_strings = off
    #synchronize_seqscans = on

    # - Other Platforms and Clients -

    #transform_null_equals = off


    #------------------------------------------------------------------------------
    # CUSTOMIZED OPTIONS
    #------------------------------------------------------------------------------

    #custom_variable_classes = ''       # list of custom variable class names

Answer 1

a few hundred users simultaneously connected

max_connections = 100

Are you using a connection pooler? If so, which one, and how is it configured?

I'm using JDBC to access postgress, running 1 to 10 simultaneous threads accessing the database.

... or is connection pooling effectively handled by your Java application? If so, is 10 a hard limit for its database connections?

2 cores and 4 gb of ram, and a standard SATA-2 disk

How many concurrent queries tends to cause a problem? You don't have a lot of resources to share. You should probably be limiting active database transactions to about 5.

shared_buffers = 1024MB
work_mem = 100MB

Since each connection can allocate work_mem space once per plan node, it won't take many concurrent queries to totally tank performance.

fsync = off
full_page_writes = off

With these settings you are telling PostgreSQL that it is OK to lose all the data in the database if the OS, hardware, or VM crashes -- you either don't care about the data in the database or you're willing to go to your latest known good backup. If you turn these on and turn off synchronous_commit your performance will be almost the same, but you won't risk unrecoverable database corruption on an OS crash; it's just that the when a transaction is committed, the data might not actually be persisted for a short time, usually less than a second.

version 8.4

Each release since then has included improvements in checkpoint handling which would probably reduce the frequency and severity of the type of problem you describe. You should consider upgrading.