Re: How to efficiently count Proth primes with GP parfor?

[hermann@stamm-wilbrandt.de]

On 2024-03-20 11:26, Bill Allombert wrote:
> The function isProth is too fast compared to the overhead of 
> parallelism.
> As you see, PARI only manage to schedule 3.36 job on average before the 
> first
> one complete, so most of the time is spent scheduling jobs.
Ok.

> You need to do work by batch.
>
> isProth(p)=my(v=valuation(p-1,2));p>>v<2^v;
> export(isProth);
> {
>   my(nbt=default(nbthreads));
>   my(c=0,B=323092481\nbt);
>   parforstep(i=1,323092481,B,
>     my(cc=0);
>     forprime(p=i,min(i+B-1,323092481),cc+=isProth(p));cc
>   ,C,c+=C);c
> }
> %3 = 3000
> ? ##
>   ***   last result: cpu time 17,843 ms, real time 1,070 ms.
>
> (with 20 cores).
Which GP version do you run?
I run 2.15.4 and get error for your code:

? isProth(p)=my(v=valuation(p-1,2));p>>v<2^v;
? export(isProth);
? {
  my(nbt=default(nbthreads));
  my(c=0,B=323092481\nbt);
  parforstep(i=1,323092481,B,
    my(cc=0);
    forprime(p=i,min(i+B-1,323092481),cc+=isProth(p));cc
  ,C,c+=C);c
}
  ***   unexpected ';': ...23092481),cc+=isProth(p));cc,C,c+=C);c
  ***                                               ^-------------
?



> The best value of nbt will depend on your system.
My sysmtem value waits to see what can be done ...

? print(default(nbthreads))
32
?


Regards,

Hermann.