Subject: Re: bgwriter tunables vs pg_stat_bgwriter
From: Brad Nicholson <bnichols@ca.afilias.info>
To: Jeff <threshar@real.jefftrout.com>
Cc: pgsql-performance@postgresql.org
In-Reply-To: <777025E3-813C-4EA6-B654-E4C54F83AA1A@torgo.978.org>
References: <777025E3-813C-4EA6-B654-E4C54F83AA1A@torgo.978.org>
Content-Type: text/plain
Date: Wed, 17 Feb 2010 14:01:16 -0500
Message-Id: <1266433276.4412.39.camel@bnicholson-desktop>
Mime-Version: 1.0
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On Wed, 2010-02-17 at 08:30 -0500, Jeff wrote:
> Since getting on 8.4 I've been monitoring things fairly closely.
> I whipped up a quick script to monitor pg_stat_bgwriter and save  
> deltas every minute so I can ensure my bgwriter is beating out the  
> backends for writes (as it is supposed to do).
> 
> Now, the odd thing I'm running into is this:
> 
> bgwriter_delay is 100ms (ie 10 times a second, give or take)
> bgwriter_lru_maxpages is 500 (~5000 pages / second)
> bgwriter_lru_multiplier is 4
> 
> Now, assuming I understand these values right the following is what  
> should typically happen:
> 
> while(true)
> {
>      if buffers_written > bgwriter_lru_maxpages
>        or buffers_written > anticipated_pages_needed *  
> bgwriter_lru_multiplier
>     {
>           sleep(bgwriter_delay ms)
>            continue;
>     }
>     ...
> }

Correct.

> so I should not be able to have more than ~5000 bgwriter_clean pages  
> per minute. (this assumes writing takes 0ms, which of course is  
> inaccurate)

That works out to 5000/second - 300,000/minute.

> However, I see this in my stats (they are deltas), and I'm reasonably  
> sure it is not a bug in the code:
> 
> (timestamp, buffers clean, buffers_checkpoint, buffers backend)
>   2010-02-17 08:23:51.184018 |             1 |               1686  
> |               5
>   2010-02-17 08:22:51.170863 |         15289 |              12676  
> |             207
>   2010-02-17 08:21:51.155793 |         38467 |               8993  
> |            4277
>   2010-02-17 08:20:51.139199 |         35582 |                  0  
> |            9437
>   2010-02-17 08:19:51.125025 |             8 |                  0  
> |               3
>   2010-02-17 08:18:51.111184 |          1140 |               1464  
> |               6
>   2010-02-17 08:17:51.098422 |             0 |               1682  
> |             228
>   2010-02-17 08:16:51.082804 |            50 |                  0  
> |               6
>   2010-02-17 08:15:51.067886 |           789 |                  0  
> |               1
> 
> perhaps some stats buffering occurring or something or some general  
> misunderstanding of some of these tunables?
>
> --
> Jeff Trout <jeff@jefftrout.com>
> http://www.stuarthamm.net/
> http://www.dellsmartexitin.com/
> 
> 
> 
> 
-- 
Brad Nicholson  416-673-4106
Database Administrator, Afilias Canada Corp.