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

On Thu, Sep 5, 2013 at 5:11 PM, Josh Berkus <josh(at)agliodbs(dot)com> wrote:
> On 09/05/2013 02:16 PM, Bruce Momjian wrote:
>>> Well, the real problem with this patch is that it documents what the
>>> auto-tuning algorithm is; without that commitment, just saying "-1 means
>>> autotune" might be fine.
>>
>> OK, but I did this based on wal_buffers, which has a -1 default, calls
>> it auto-tuning, and explains how the default is computed.
>
> I don't see a real problem with this. For users who have set their
> shared_buffers correctly, effective_cache_size should also be correct.

Agreed. I think -1 is the right setting for autotune as things stand
today. If we want something else, then we should change other settings
as well (like wal_buffers) and that is not in the scope of this patch.

>> The problem there is that many users are told to tune shared_buffers,
>> but don't touch effective cache size. Having initdb set the
>> effective_cache_size value would not help there. Again, this is all
>> based on the auto-tuning of wal_buffers.
>
> Standard advice we've given in the past is 25% shared buffers, 75%
> effective_cache_size. Which would make EFS *3X* shared_buffers, not 4X.
> Maybe we're changing the conventional calculation, but I thought I'd
> point that out.

This was debated upthread.

merlin