Re: proposal: Set effective_cache_size to greater of .conf value, shared_buffers

On Thu, Sep  5, 2013 at 06:14:33PM +0200, Magnus Hagander wrote:
> > I have developed the attached patch which implements an auto-tuned
> > effective_cache_size which is 4x the size of shared buffers.  I had to
> > set effective_cache_size to its old 128MB default so the EXPLAIN
> > regression tests would pass unchanged.
> 
> That's not really autotuning though. ISTM that making the *default* 4
> x shared_buffers might make perfect sense, but do we really need to
> hijack the value of "-1" for that? That might be useful for some time
> when we have actual autotuning, that somehow inspects the system and
> tunes it from there.
>
> I also don't think it should be called autotuning, when it's just a
> "smarter default value".
> 
> I like the feature, though, just not the packaging.

That "auto-tuning" text came from the wal_buffer documentation, which
does exactly this based on shared_buffers:
       The contents of the WAL buffers are written out to disk at every       transaction commit, so extremely large
valuesare unlikely to       provide a significant benefit.  However, setting this value to at       least a few
megabytescan improve write performance on a busy
 
-->     server where many clients are committing at once.  The auto-tuning
               -----------       selected by the default setting of -1 should give reasonable       results in most
cases.

I am fine with rewording and not using -1, but we should change the
wal_buffer default and documentation too then.  I am not sure what other
value than -1 to use?  0?  I figure if we ever get better auto-tuning,
we would just remove this functionality and make it better.

--  Bruce Momjian  <bruce@momjian.us>        http://momjian.us EnterpriseDB
http://enterprisedb.com
 + It's impossible for everything to be true. +