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Thread: PG Logging is Slow

PG Logging is Slow

From
Theo Galanakis
Date:

Under postgres 7.3 logging is incredibly slow!

I have applied the following settings:

syslog = 2
syslog_facility = 'LOCAL0'
syslog_ident = 'postgres'
 
 log_connections =  true 
log_duration =  true 
log_pid =  true 
log_statement =  true 
log_timestamp =  true 
 
This severely impacted the performance of our production system, a search page which took 1-3 seconds now takes over 30, is this normal?
 
I need to get some performance indicators from our production db, however I cant turn on logging with such performance degradation.
 
Theo



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Re: PG Logging is Slow

From
Bruce Momjian
Date:
Theo Galanakis wrote:
> Under postgres 7.3 logging is incredibly slow!
>
> I have applied the following settings:
>
> syslog = 2
> syslog_facility = 'LOCAL0'
> syslog_ident = 'postgres'
>
>  log_connections =  true
> log_duration =  true
> log_pid =  true
> log_statement =  true
> log_timestamp =  true
>
> This severely impacted the performance of our production system, a search
> page which took 1-3 seconds now takes over 30, is this normal?
>
> I need to get some performance indicators from our production db, however I
> cant turn on logging with such performance degradation.

Linux syslog has this bad behavior of fsync'ing all log writes.  See the
syslog manual page for a way to turn off the fsync.

--
  Bruce Momjian                        |  http://candle.pha.pa.us
  pgman@candle.pha.pa.us               |  (610) 359-1001
  +  If your life is a hard drive,     |  13 Roberts Road
  +  Christ can be your backup.        |  Newtown Square, Pennsylvania 19073

Re: PG Logging is Slow

From
Grega Bremec
Date:
...and on Mon, Dec 20, 2004 at 03:17:11PM +1100, Theo Galanakis used the keyboard:
> Under postgres 7.3 logging is incredibly slow!
>
> I have applied the following settings:
>
> syslog = 2
> syslog_facility = 'LOCAL0'
> syslog_ident = 'postgres'
>
>  log_connections =  true
> log_duration =  true
> log_pid =  true
> log_statement =  true
> log_timestamp =  true
>
> This severely impacted the performance of our production system, a search
> page which took 1-3 seconds now takes over 30, is this normal?
>
> I need to get some performance indicators from our production db, however I
> cant turn on logging with such performance degradation.
>

Hi Theo,

One thing you should be sure about is that whichever logfile you have
configured for the local0 facility is being written to asynchronously.
Synchronous logging is REALLY expensive.

If you're using the standard syslogd, you can achieve that by prefixing
the filename in syslogd.conf with a dash. For example,

    local0.*        /var/log/postgresql.log

would become

    local0.*        -/var/log/postgresql.log

One other option would be to turn off syslog logging completely and let
postmaster take care of the log on its own, which may or may not be
possible for you, depending on the policy in effect (remote logging, etc.).

Hope this helped,
--
    Grega Bremec
    gregab at p0f dot net
Attachment

Re: PG Logging is Slow

From
Michael Adler
Date:
On Mon, Dec 20, 2004 at 03:17:11PM +1100, Theo Galanakis wrote:
> Under postgres 7.3 logging is incredibly slow!
>
> I have applied the following settings:
>
> syslog = 2
> syslog_facility = 'LOCAL0'
> syslog_ident = 'postgres'
>
>  log_connections =  true
> log_duration =  true
> log_pid =  true
> log_statement =  true
> log_timestamp =  true
>
> This severely impacted the performance of our production system, a search
> page which took 1-3 seconds now takes over 30, is this normal?
>
> I need to get some performance indicators from our production db, however I
> cant turn on logging with such performance degradation.


I've experienced this problem many times due to hanging dns
lookups. /etc/resolv.conf may point to a nonexistent
nameserver. Comment it out and restart syslogd. Or use a syslog
implementation that allows you to disable dns lookups. Or just give
the nameserver a kick.

 -Mike Adler

Re: PG Logging is Slow

From
Theo Galanakis
Date:

Thank-you Grega,

        I ended up using the pg_ctl -l parameter to write the output to a specified file. Much quicker to do so.

        I tried the -/var/log/postgresql.log option however I noticed no performance improvement. May be the fact that we use redhad linux and syslog, I'm no sys-admin, so I'm not sure if there is a difference between syslogd and syslog.

Theo

-----Original Message-----
From: Grega Bremec [mailto:gregab@p0f.net]
Sent: Monday, 20 December 2004 3:49 PM
To: Theo Galanakis
Cc: pgsql-performance@postgresql.org
Subject: Re: [PERFORM] PG Logging is Slow

...and on Mon, Dec 20, 2004 at 03:17:11PM +1100, Theo Galanakis used the keyboard:
> Under postgres 7.3 logging is incredibly slow!
>
> I have applied the following settings:
>
> syslog = 2
> syslog_facility = 'LOCAL0'
> syslog_ident = 'postgres'
> 
>  log_connections =  true
> log_duration =  true
> log_pid =  true
> log_statement =  true
> log_timestamp =  true
> 
> This severely impacted the performance of our production system, a
> search page which took 1-3 seconds now takes over 30, is this normal?
> 
> I need to get some performance indicators from our production db,
> however I cant turn on logging with such performance degradation.
> 

Hi Theo,

One thing you should be sure about is that whichever logfile you have configured for the local0 facility is being written to asynchronously. Synchronous logging is REALLY expensive.

If you're using the standard syslogd, you can achieve that by prefixing the filename in syslogd.conf with a dash. For example,

    local0.*            /var/log/postgresql.log

would become

    local0.*            -/var/log/postgresql.log

One other option would be to turn off syslog logging completely and let postmaster take care of the log on its own, which may or may not be possible for you, depending on the policy in effect (remote logging, etc.).

Hope this helped,
--
    Grega Bremec
    gregab at p0f dot net

______________________________________________________________________
This email, including attachments, is intended only for the addressee
and may be confidential, privileged and subject to copyright. If you
have received this email in error, please advise the sender and delete
it. If you are not the intended recipient of this email, you must not
use, copy or disclose its content to anyone. You must not copy or
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Re: PG Logging is Slow

From
Alexander Borkowski
Date:
Theo,

 >     I tried the -/var/log/postgresql.log option however I noticed no
 > performance improvement. May be the fact that we use redhad linux and
 > syslog, I'm no sys-admin, so I'm not sure if there is a difference
between
 > syslogd and syslog.

Did you restart syslogd (that's the server process implementing the
syslog (= system log) service) after you changed its configuration?

In order to do so, try running

/etc/init.d/syslog restart

as root from a commandline.

HTH

Alex

Re: PG Logging is Slow

From
Theo Galanakis
Date:

Thankyou Alexander,

        That has worked and appears to have fixed the issue with syslog.

Theo

-----Original Message-----
From: Alexander Borkowski [mailto:alexander.borkowski@abri.une.edu.au]
Sent: Tuesday, 21 December 2004 10:09 AM
To: Theo Galanakis
Cc: 'pgsql-performance@postgresql.org'
Subject: Re: [PERFORM] PG Logging is Slow

Theo,

 >      I tried the -/var/log/postgresql.log option however I noticed no
 > performance improvement. May be the fact that we use redhad linux and  > syslog, I'm no sys-admin, so I'm not sure if there is a difference

between
 > syslogd and syslog.

Did you restart syslogd (that's the server process implementing the
syslog (= system log) service) after you changed its configuration?

In order to do so, try running

/etc/init.d/syslog restart

as root from a commandline.

HTH

Alex

______________________________________________________________________
This email, including attachments, is intended only for the addressee
and may be confidential, privileged and subject to copyright. If you
have received this email in error, please advise the sender and delete
it. If you are not the intended recipient of this email, you must not
use, copy or disclose its content to anyone. You must not copy or
communicate to others content that is confidential or subject to
copyright, unless you have the consent of the content owner.