Re: Log a sample of transactions

From: Adrien NAYRAT <adrien(dot)nayrat(at)anayrat(dot)info>
To: Andres Freund <andres(at)anarazel(dot)de>
Cc: "Kuroda, Hayato" <kuroda(dot)hayato(at)jp(dot)fujitsu(dot)com>, "'pgsql-hackers(at)lists(dot)postgresql(dot)org'" <pgsql-hackers(at)lists(dot)postgresql(dot)org>
Subject: Re: Log a sample of transactions
Date: 2019-02-15 14:24:39
Message-ID: 32b611f6-5b98-3687-df9c-16fc5f808c8a@anayrat.info
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On 2/14/19 9:14 PM, Andres Freund wrote:
> Hi,
>

Hello Andres,

> On 2019-01-26 11:44:58 +0100, Adrien NAYRAT wrote:
>> + <varlistentry id="guc-transaction-sample-rate" xreflabel="log_transaction_sample_rate">
>> + <term><varname>log_transaction_sample_rate</varname> (<type>real</type>)
>> + <indexterm>
>> + <primary><varname>log_transaction_sample_rate</varname> configuration parameter</primary>
>> + </indexterm>
>> + </term>
>> + <listitem>
>> + <para>
>> + Set the fraction of transactions whose statements are logged. It applies
>> + to each new transaction regardless of their duration. The default is
>> + <literal>0</literal>, meaning do not log statements from this transaction.
>> + Setting this to <literal>1</literal> logs all statements for all transactions.
>> + Logging can be disabled inside a transaction by setting this to 0.
>> + <varname>log_transaction_sample_rate</varname> is helpful to track a
>> + sample of transaction.
>> + </para>
>> + </listitem>
>> + </varlistentry>
>> +
>> </variablelist>
>
> I wonder if this doesn't need a warning, explaining that using this when
> there are large transactions could lead to slowdowns.

Yes, I will add some wording

>
>
>> <para>
>> diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c
>> index 7c3a9c1e89..1a52c10c39 100644
>> --- a/src/backend/access/transam/xact.c
>> +++ b/src/backend/access/transam/xact.c
>> @@ -1821,6 +1821,9 @@ StartTransaction(void)
>> s->state = TRANS_START;
>> s->transactionId = InvalidTransactionId; /* until assigned */
>>
>> + /* Determine if we log statements in this transaction */
>> + xact_is_sampled = random() <= log_xact_sample_rate * MAX_RANDOM_VALUE;
>> +
>> /*
>> * initialize current transaction state fields
>> *
>> diff --git a/src/backend/tcop/postgres.c b/src/backend/tcop/postgres.c
>> index e773f20d9f..a11667834e 100644
>> --- a/src/backend/tcop/postgres.c
>> +++ b/src/backend/tcop/postgres.c
>> @@ -100,7 +100,8 @@ int max_stack_depth = 100;
>> /* wait N seconds to allow attach from a debugger */
>> int PostAuthDelay = 0;
>>
>> -
>> +/* flag for logging statements in a transaction */
>> +bool xact_is_sampled = false;
>
> It seems pretty weird to have this in postgres.c, given you declared it
> in xact.h?

Yes, I have to revise my C. I will move it to
src/backend/access/transam/xact.c

>
>
>> @@ -2218,7 +2221,8 @@ check_log_statement(List *stmt_list)
>> int
>> check_log_duration(char *msec_str, bool was_logged)
>> {
>> - if (log_duration || log_min_duration_statement >= 0)
>> + if (log_duration || log_min_duration_statement >= 0 ||
>> + (xact_is_sampled && log_xact_sample_rate > 0))
>
> Why are both of these checked? ISTM once xact_is_sampled is set, we
> ought not to respect a different value of log_xact_sample_rate for the
> rest of that transaction.

I added theses checks to allow to disable logging during a sampled
transaction, per suggestion of Masahiko Sawada:
https://www.postgresql.org/message-id/CAD21AoD4t%2BhTV6XfK5Yz%3DEocB8oMyJSYFfjAryCDYtqfib2GrA%40mail.gmail.com

>
>
> As far as I can tell xact_is_sampled is not properly reset in case of
> errors?
>

Yes, I wonder where we should reset it? Is it in AbortTransaction() or
CleanupTransaction()?

Thanks!

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