Re: checkpointer continuous flushing

From: Andres Freund <andres(at)anarazel(dot)de>
To: Fabien COELHO <coelho(at)cri(dot)ensmp(dot)fr>
Cc: PostgreSQL Developers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: checkpointer continuous flushing
Date: 2016-01-19 17:04:59
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On 2016-01-19 13:34:14 +0100, Fabien COELHO wrote:
> >synchronous_commit = off does make a significant difference.
> Sure, but I had thought about that and kept this one...

But why are you then saying this is fundamentally limited to 160

> I think I found one possible culprit: I automatically wrote 300 seconds for
> checkpoint_timeout, instead of 30 seconds in your settings. I'll have to
> rerun the tests with this (unreasonnable) figure to check whether I really
> get a regression.

I've not seen meaningful changes in the size of the regression between 30/300s.

> Other tests I ran with "reasonnable" settings on a large (scale=800) db did
> not show any significant performance regression, up to know.

Try running it so that the data set nearly, but not entirely fit into
the OS page cache, while definitely not fitting into shared_buffers. The
scale=800 just worked for that on my hardware, no idea how it's for yours.

That seems to be the point where the effect is the worst.

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