Re: pgbench throttling latency limit

On 09/11/2014 03:36 PM, Fabien COELHO wrote:
>
> Hello Heikki,
>
>> Now that I've finished the detour and committed and backpatched the changes
>> to the way latency is calculated, we can get back to this patch. It needs to
>> be rebased.
>
> Before rebasing, I think that there are a few small problems with the
> modification applied to switch from an integer range to double.
>
> (1) ISTM that the + 0.5 which remains in the PoissonRand computation comes
> from the previous integer approach and is not needed here. If I'm not
> mistaken the formula should be plain:
>
>        -log(uniform) * center

No. The +0.5 is to round the result to the nearest integer, instead of 
truncating it down.

> (2) I'm not sure of the name "center", I think that "lambda" or
>       "mean" would be more appropriate.

(shrug), I guess. The comment says that it's the value the average of a 
series values is centered on, so "center" doesn't seem too bad. I guess 
the mathematically accurate term would be "expected value".

> (3) I wish that the maximum implied multiplier could be explicitely
>       documented in the source code. From pg_rand48 source code, I think
>       that it is 33.27106466687737

Oh, ok. That's an even smaller multiplier than I got just by feeding 
DBL_MIN to the formula. I don't think that's worth worrying about. That 
might change if the implementation of pg_erand48() is changed, so I'm a 
bit reluctant to state it explicitly.

- Heikki