Re: Speed up Clog Access by increasing CLOG buffers

On 09/23/2016 01:44 AM, Tomas Vondra wrote:
>...
> The 4.5 kernel clearly changed the results significantly:
>
...
>
> (c) Although it's not visible in the results, 4.5.5 almost perfectly
> eliminated the fluctuations in the results. For example when 3.2.80
> produced this results (10 runs with the same parameters):
>
> 12118 11610 27939 11771 18065
> 12152 14375 10983 13614 11077
>
> we get this on 4.5.5
>
> 37354 37650 37371 37190 37233
> 38498 37166 36862 37928 38509
>
> Notice how much more even the 4.5.5 results are, compared to 3.2.80.
>

The more I think about these random spikes in pgbench performance on
3.2.80, the more I find them intriguing. Let me show you another example
(from Dilip's workload and group-update patch on 64 clients).

This is on 3.2.80:

44175 34619 51944 38384 49066
37004 47242 36296 46353 36180

and on 4.5.5 it looks like this:

34400 35559 35436 34890 34626
35233 35756 34876 35347 35486

So the 4.5.5 results are much more even, but overall clearly below
3.2.80. How does 3.2.80 manage to do ~50k tps in some of the runs?
Clearly we randomly do something right, but what is it and why doesn't
it happen on the new kernel? And how could we do it every time?

regards

--
Tomas Vondra http://www.2ndQuadrant.com
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