From: | Fabien COELHO <coelho(at)cri(dot)ensmp(dot)fr> |
---|---|
To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>, Adrien Nayrat <adrien(dot)nayrat(at)anayrat(dot)info>, Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com>, Dmitry Dolgov <9erthalion6(at)gmail(dot)com>, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>, vik(dot)fearing(at)2ndquadrant(dot)com, Robert Haas <robertmhaas(at)gmail(dot)com>, Michael Paquier <michael(at)paquier(dot)xyz>, David Rowley <david(dot)rowley(at)2ndquadrant(dot)com>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: random() (was Re: New GUC to sample log queries) |
Date: | 2018-12-28 14:27:30 |
Message-ID: | alpine.DEB.2.21.1812281458460.32444@lancre |
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Lists: | pgsql-hackers |
Hello again,
>>> - lrand48 (48 bits state as 3 uint16) is 29 ops
>>> (10 =, 8 *, 7 +, 4 >>)
>>
>> - xoshiro256** (256 bits states as 4 uint64) is 24 ops (18 if rot in hw)
>> 8 =, 2 *, 2 +, 5 <<, 5 ^, 2 |
>
> Small benchmark on my laptop with gcc-7.3 -O3:
>
> - pg_lrand48 takes 4.0 seconds to generate 1 billion 32-bit ints
> - xoshiro256** takes 1.6 seconds to generate 1 billion 64-bit ints
These too-good-to-be-true figures have raised doubt in my mind, so after
giving it some thought, I do not trust them: the function call is probably
inlined and other optimizations are performed which would not apply in a
more realistic case.
> With -O2 it is 4.8 and 3.4 seconds, respectively.
I trust this one, which is reasonably consistent with the operation count.
More tests with "clang -O2" yielded 3.2 and 1.6 respectively, that I do
not trust either.
I did separate compilation to prevent inlining and other undesirable
optimizations: clang -Ofast or -O2 gives 5.2 and 3.9, gcc -Ofast gives 5.4
and 3.5.
It seems that clang is better at compiling pg_erand48 but gcc is better at
xoroshi256**.
> So significantly better speed _and_ quality are quite achievable.
xoroshi256** is about 1/3 faster at producing twice as much pseudo-randoms
stuff, and it passed significant randomness tests that an LCG PRNG would
not.
--
Fabien.
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