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

On Thu, Sep 5, 2013 at 05:14:37PM -0400, Bruce Momjian wrote:
> 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 values are unlikely to
> provide a significant benefit. However, setting this value to at
> least a few megabytes can 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.

Patch applied with a default of 4x shared buffers. I have added a 9.4
TODO that we might want to revisit this.

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ It's impossible for everything to be true. +