Re: pgbench throttling latency limit
От | Fabien COELHO |
---|---|
Тема | Re: pgbench throttling latency limit |
Дата | |
Msg-id | alpine.DEB.2.10.1409121904010.28521@sto обсуждение исходный текст |
Ответ на | Re: pgbench throttling latency limit (Heikki Linnakangas <hlinnakangas@vmware.com>) |
Список | pgsql-hackers |
>> (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. Hmmm... probably ok. I'll have to think about it a bit. In that case, it seems much clearer to do: "round(-log(uniform) * xxx)" instead of relying on the truncation of the cast. >> (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". The word "center" does not appear once of the wikipedia page about "Poisson distribution". Its "mean" is called "lambda" (or rather λ:-) all over the place. I find "expected value" rather too generic, but it is better than "center". >> (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. I think that it is an important information, so it deserves to appear. If pg_erand48 implementation changes, it should not be called "48", because the max value and the above multiplier limit is completely linked to the "16*3=48" structure of the random construction. If the code change then the comments need be updated, that is life. -- Fabien.
В списке pgsql-hackers по дате отправления: