Re: Abbreviated keys for Numeric

On 24.2.2015 05:09, Andrew Gierth wrote:
>>>>>> "Tomas" == Tomas Vondra <tomas.vondra@2ndquadrant.com> writes:
> 
>  Tomas> I believe the small regressions (1-10%) for small data sets,
>  Tomas> might be caused by this 'random padding' effect, because that's
>  Tomas> probably where L1/L2 cache is most important. For large datasets
>  Tomas> the caches are probably not as efficient anyway, so the random
>  Tomas> padding makes no difference,
> 
> Except that _your own results_ show that this is not the case.
> 
> In your first set of results you claimed a reduction in performance 
> to 91% for a query which is _in no way whatsoever_ affected by any
> of the code changes. How is this not noise?
> 
> I refer specifically to this case from your spreadsheet:
> 
> query select * from (select * from stuff_text order by randtxt
> offset 100000000000) foo
> type    text
> scale   5
> master  26.949
> datum   28.051
> numeric 29.734
> datum   96%
> numeric 91%    
> 
> This query does not invoke any code path touched by either the datum 
> or numeric patches! The observed slowdown is therefore just noise 
> (assuming here that your timings are correct).

I don't see how that contradicts what I said? Weren't you claiming that
changes / random padding in unrelated functions may impact other places
by introducing different patterns of cache misses etc?

But yes, I do agree the first set of results is rather noisy, at least
partially due to running on CPU (i7-4770R) with some power-management
features etc. That's why I did the follow-up tests on a different system
with CPUs that don't not do that - at least the i5 does not, and the
results are much better IMHO.


> Whether that case can be improved by tweaking the _text_
> abbreviation code is another question, one which is not relevant to
> either of the patches currently in play.

I don't think I suggested that.


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