Re: checkpointer continuous flushing

Hi,

On 03/22/2016 07:35 AM, Fabien COELHO wrote:
>
> Hello Tomas,
>
> Thanks again for these interesting benches.
>
>> Overall, this means ~300M transactions in total for the un-throttled
>> case, so sample with ~15M transactions available when computing the
>> following charts.
>
> Still a very sizable run!
>
>> There results (including scripts for generating the charts) are here:
>>
>>    https://github.com/tvondra/flushing-benchmark-2
>
> This repository seems empty.

Strange. Apparently I forgot to push, or maybe it did not complete 
before I closed the terminal. Anyway, pushing now (it'll take a bit more 
time to complete).

>
>> 1) regular-latency.png
>
> I'm wondering whether it would be clearer if the percentiles where
> relative to the largest sample, not to itself, so that the figures
> from the largest one would still be between 0 and 1, but the other
> (unpatched) one would go between 0 and 0.85, that is would be cut
> short proportionnaly to the actual performance.
>

I'm not sure what you mean by 'relative to largest sample'?

>> The two curves intersect at ~4ms, where both CDF reach ~85%. For
>> the shorter transactions, the old code is slightly faster (i.e.
>> apparently there's some per-transaction overhead).
>
> I'm not sure how meaningfull is the crossing, because both curves do
> not reflect the same performance. I think that they may not cross at
> all if the normalization is with the same reference, i.e. the better
> run.

Well, I think the curves illustrate exactly the performance difference, 
because with the old code the percentiles after p=0.85 get much higher. 
Which is the point of the crossing, although I agree the exact point 
does not have a particular meaning.

>> 2) throttled-latency.png
>>
>> In the throttled case (i.e. when the system is not 100% utilized,
>> so it's more representative of actual production use), the
>> difference is quite clearly in favor of the new code.
>
> Indeed, it is a no brainer.

Yep.

>
>> 3) throttled-schedule-lag.png
>>
>> Mostly just an alternative view on the previous chart, showing how
>> much later the transactions were scheduled. Again, the new code is
>> winning.
>
> No brainer again. I infer from this figure that with the initial
> version 60% of transactions have trouble being processed on time,
> while this is maybe about 35% with the new version.

Yep.

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services