Discussion related to cado-nfs

[Guy - <pleasechoose@hotmail.com>]

Hi,

I'm trying to run cado on RSA-640 (193 digits), I have a server with 32 cores and 32GB RAM.

I've noticed that when sieving the las process takes approx. 3.3GB, this limits the sieving to 8-9 cores.

When running cado-nfs.py the configuration is chosen automatically to the rounded-up matching number (params.c195), and the sieving region is defined to tasks.I = 15

I also noticed that params.c190 (closest rounded down matching number) defines tasks.I = 14

According to the parameters documentation (params.c90):

Increasing I by 1 multiplies the amount of required RAM for the siever by a factor of 4.

As expected, decreasing I to 14, reduces the amount of ram to approx. 1GB, which allows me to run sieving on about 30 cores.

My questions are:

1. Why was 15 chosen? what is the penalty for choosing 14 instead? I can see that each workunit sieves less (~7000 relations instead of ~17000),

2. What happens if you decrease task.I during a run? (after some of the relations were gathered with I=15) I've noticed that the polynomial MurphyE's score is affected by reducing to tasks. does it mean that this polynomial is not necessary the best with the new chosen I?

3. Decreasing to 14 improved sieving by 20% (~230 rels/min instead of ~190 rels/min)

Thanks for your help!

Guy