From 6e3e6cc0e884a6091e1094dff29db430af08fb93 Mon Sep 17 00:00:00 2001 From: Tomas Vondra Date: Mon, 4 Nov 2019 01:57:45 +0100 Subject: [PATCH] Allow sampling of statements depending on duration This allows logging a sample of statements, without incurring excessive log traffic (which may impact performance). This can be useful when analyzing workloads with lots of short queries. The sampling is configured using two new GUC parameters: * log_min_duration_sample - minimum required statement duration * log_statement_sample_rate - sample rate (0.0 - 1.0) Only statements with duration exceeding log_min_duration_sample are considered for sampling. To enable sampling, both those GUCs have to be set correctly. The existing log_min_duration_statement GUC has a higher priority, i.e. statements with duration exceeding log_min_duration_statement will be always logged, irrespectedly of how the sampling is configured. This means only configurations log_min_duration_sample < log_min_duration_statement do actually sample the statements, instead of logging everything. Author: Adrien Nayrat Reviewed-by: David Rowley, Vik Fearing, Tomas Vondra Discussion: https://postgr.es/m/bbe0a1a8-a8f7-3be2-155a-888e661cc06c@anayrat.info --- doc/src/sgml/config.sgml | 85 +++++++++++++++++++ src/backend/tcop/postgres.c | 40 ++++++--- src/backend/utils/misc/guc.c | 27 +++++- src/backend/utils/misc/postgresql.conf.sample | 11 +++ src/include/utils/guc.h | 2 + 5 files changed, 153 insertions(+), 12 deletions(-) diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml index 48d7939d2d..46bc31de4c 100644 --- a/doc/src/sgml/config.sgml +++ b/doc/src/sgml/config.sgml @@ -5950,6 +5950,12 @@ local0.* /var/log/postgresql Only superusers can change this setting. + + This overrides , + meaning that queries with duration exceeding this setting are not + subject to sampling and are always logged. + + For clients using extended query protocol, durations of the Parse, Bind, and Execute steps are logged independently. @@ -5972,6 +5978,85 @@ local0.* /var/log/postgresql + + log_min_duration_sample (integer) + + log_min_duration_sample configuration parameter + + + + + Allows to sample the logging of the duration of each completed + statement if the statement ran for at least the specified amount of + time. If this value is specified without units, it is taken as milliseconds. + Setting this to zero samples all statement durations. + Minus-one (the default) disables sampling statement durations. + For example, if you set it to 250ms + then all SQL statements that run 250ms or longer will be considered + for sampling, with sample rate is controlled by . + Enabling this parameter can be helpful when the traffic too high to + sample all queries. + Only superusers can change this setting. + + + + This option has lower priority than , + meaning that statements with durations exceeding + are not subject to sampling and are always logged. + + + + For clients using extended query protocol, durations of the Parse, + Bind, and Execute steps are logged independently. + + + + + When using this option together with + , + the text of statements that are logged because of + log_statement will not be repeated in the + duration log message. + If you are not using syslog, it is recommended + that you log the PID or session ID using + + so that you can link the statement message to the later + duration message using the process ID or session ID. + + + + + + + log_statement_sample_rate (real) + + log_statement_sample_rate configuration parameter + + + + + Determines the fraction of statements with duration exceeding + to be logged. + This is a statistical parameter, for example 0.5 + means there is statistically one in two chances to log the statement. + The default is 1.0, meaning log all such + statements. + Setting this to zero disables sampling logging, same as setting + log_min_duration_sample to + -1. + log_statement_sample_rate is helpful when the + traffic is too high to log all queries. + Only superusers can change this setting. + + + + Like all statement-logging options, this option can add significant + overhead. + + + + + log_transaction_sample_rate (real) diff --git a/src/backend/tcop/postgres.c b/src/backend/tcop/postgres.c index 4bec40aa28..82894eadc6 100644 --- a/src/backend/tcop/postgres.c +++ b/src/backend/tcop/postgres.c @@ -2235,12 +2235,15 @@ check_log_statement(List *stmt_list) int check_log_duration(char *msec_str, bool was_logged) { - if (log_duration || log_min_duration_statement >= 0 || xact_is_sampled) + if (log_duration || log_min_duration_sample >= 0 || + log_min_duration_statement >= 0 || xact_is_sampled) { long secs; int usecs; int msecs; - bool exceeded; + bool exceeded_duration; + bool exceeded_sample_duration; + bool in_sample = false; TimestampDifference(GetCurrentStatementStartTimestamp(), GetCurrentTimestamp(), @@ -2248,20 +2251,35 @@ check_log_duration(char *msec_str, bool was_logged) msecs = usecs / 1000; /* - * This odd-looking test for log_min_duration_statement being exceeded - * is designed to avoid integer overflow with very long durations: - * don't compute secs * 1000 until we've verified it will fit in int. + * This odd-looking test for log_min_duration_* being exceeded is + * designed to avoid integer overflow with very long durations: don't + * compute secs * 1000 until we've verified it will fit in int. */ - exceeded = (log_min_duration_statement == 0 || - (log_min_duration_statement > 0 && - (secs > log_min_duration_statement / 1000 || - secs * 1000 + msecs >= log_min_duration_statement))); + exceeded_duration = (log_min_duration_statement == 0 || + (log_min_duration_statement > 0 && + (secs > log_min_duration_statement / 1000 || + secs * 1000 + msecs >= log_min_duration_statement))); - if (exceeded || log_duration || xact_is_sampled) + exceeded_sample_duration = (log_min_duration_sample == 0 || + (log_min_duration_sample > 0 && + (secs > log_min_duration_sample / 1000 || + secs * 1000 + msecs >= log_min_duration_sample))); + + /* + * Do not log if log_statement_sample_rate = 0. Log a sample if + * log_statement_sample_rate <= 1 and avoid unecessary random() call + * if log_statement_sample_rate = 1. + */ + if (exceeded_sample_duration) + in_sample = log_statement_sample_rate != 0 && + (log_statement_sample_rate == 1 || + random() <= log_statement_sample_rate * MAX_RANDOM_VALUE); + + if (exceeded_duration || in_sample || log_duration || xact_is_sampled) { snprintf(msec_str, 32, "%ld.%03d", secs * 1000 + msecs, usecs % 1000); - if ((exceeded || xact_is_sampled) && !was_logged) + if ((exceeded_duration || in_sample || xact_is_sampled) && !was_logged) return 2; else return 1; diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c index 31a5ef0474..e84c8cc4cf 100644 --- a/src/backend/utils/misc/guc.c +++ b/src/backend/utils/misc/guc.c @@ -509,8 +509,10 @@ bool session_auth_is_superuser; int log_min_error_statement = ERROR; int log_min_messages = WARNING; int client_min_messages = NOTICE; +int log_min_duration_sample = -1; int log_min_duration_statement = -1; int log_temp_files = -1; +double log_statement_sample_rate = 1.0; double log_xact_sample_rate = 0; int trace_recovery_messages = LOG; @@ -2700,10 +2702,23 @@ static struct config_int ConfigureNamesInt[] = NULL, NULL, NULL }, + { + {"log_min_duration_sample", PGC_SUSET, LOGGING_WHEN, + gettext_noop("Sets the minimum execution time above which " + "a sample of statements will be logged." + " Sampling is determined by log_statement_sample_rate."), + gettext_noop("Zero log a sample of all queries. -1 turns this feature off."), + GUC_UNIT_MS + }, + &log_min_duration_sample, + -1, -1, INT_MAX, + NULL, NULL, NULL + }, + { {"log_min_duration_statement", PGC_SUSET, LOGGING_WHEN, gettext_noop("Sets the minimum execution time above which " - "statements will be logged."), + "all statements will be logged."), gettext_noop("Zero prints all queries. -1 turns this feature off."), GUC_UNIT_MS }, @@ -3430,6 +3445,16 @@ static struct config_real ConfigureNamesReal[] = NULL, NULL, NULL }, + { + {"log_statement_sample_rate", PGC_SUSET, LOGGING_WHEN, + gettext_noop("Fraction of statements exceeding log_min_duration_sample to be logged."), + gettext_noop("Use a value between 0.0 (never log) and 1.0 (always log).") + }, + &log_statement_sample_rate, + 1.0, 0.0, 1.0, + NULL, NULL, NULL + }, + { {"log_transaction_sample_rate", PGC_SUSET, LOGGING_WHEN, gettext_noop("Set the fraction of transactions to log for new transactions."), diff --git a/src/backend/utils/misc/postgresql.conf.sample b/src/backend/utils/misc/postgresql.conf.sample index 0fc23e3a61..be02a76d9d 100644 --- a/src/backend/utils/misc/postgresql.conf.sample +++ b/src/backend/utils/misc/postgresql.conf.sample @@ -493,6 +493,17 @@ # statements running at least this number # of milliseconds +#log_min_duration_sample = -1 # -1 is disabled, 0 logs a sample of statements + # and their durations, > 0 logs only a sample of + # statements running at least this number + # of milliseconds + # Sample fraction is determined by log_statement_sample_rate + +#log_statement_sample_rate = 1.0 # Fraction of logged statements exceeding + # log_min_duration_sample to be logged. + # 1.0 logs all such statements, 0.0 never logs. + + #log_transaction_sample_rate = 0.0 # Fraction of transactions whose statements # are logged regardless of their duration. 1.0 logs all # statements from all transactions, 0.0 never logs. diff --git a/src/include/utils/guc.h b/src/include/utils/guc.h index 6791e0cbc2..9aa3d02596 100644 --- a/src/include/utils/guc.h +++ b/src/include/utils/guc.h @@ -251,8 +251,10 @@ extern bool session_auth_is_superuser; extern int log_min_error_statement; extern PGDLLIMPORT int log_min_messages; extern PGDLLIMPORT int client_min_messages; +extern int log_min_duration_sample; extern int log_min_duration_statement; extern int log_temp_files; +extern double log_statement_sample_rate; extern double log_xact_sample_rate; extern int temp_file_limit;