Re: CLOG contention

On Thu, Dec 22, 2011 at 12:28 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:

> But on the flip side, I feel like your discussion of the problems is a
> bit hand-wavy.  I think we need some real test cases that we can look
> at and measure, not just an informal description of what we think is
> happening.

I understand why you say that and take no offence. All I can say is
last time I has access to a good test rig and well structured
reporting and analysis I was able to see evidence of what I described
to you here.

I no longer have that access, which is the main reason I've not done
anything in the last few years. We both know you do have good access
and that's the main reason I'm telling you about it rather than just
doing it myself.

>> * We allocate a new clog page every 32k xids. At the rates you have
>> now measured, we will do this every 1-2 seconds.
>
> And a new pg_subtrans page quite a bit more frequently than that.

It is less of a concern, all the same. In most cases we can simply
drop pg_subtrans pages (though we don't do that as often as we could),
no fsync is required on write, no WAL record required for extension
and no update required at commit.

--
 Simon Riggs                   http://www.2ndQuadrant.com/
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