Discussion related to cado-nfs

[Emmanuel Thomé <Emmanuel.Thome@inria.fr>]

Actually commit c0b46653f might be a better fix.

The code required the hint file to be complete. The patch waives this
requirement, and completes the p60 hint file anyway.

E.

On Tue, Feb 28, 2017 at 08:57:41AM +0100, Emmanuel Thomé wrote:
> Hi,
> 
> On Mon, Feb 27, 2017 at 09:31:06PM +0300, Христофор Бобров wrote:
> > Hello! In some cases, the computation of an individual logarithm takes
> > a very long time. The descent In the file.you receive a stream of
> > output to stdout # to walk the path: 20@0 -> loop (#1) 0 980963
> > 334145... It's related in the fact that the found correlations not
> > found the logarithm of the target ? When reducing tasks.sieve.qmin this
> > problem does not occur. I would like to know what caused this problem
> > and how to solve it. To the letter attach 4 files with examples. Target
> > p60_1 is present this trouble.
> 
> Thanks for your report.
> 
> This is apparently stuck in an endless loop, and will never finish,
> unfortunately.
> 
> I've been able to reproduce this behaviour.
> 
> What happens is that when trying to descend some ideal q1, the program
> picks a relation involving an ideal q2, and when it tries to descend q2,
> all it gets is a relation involving q1. Since the program is not
> satisfied with this looping behaviour, it tries harder on q2, increasing
> the lpb, to see whether a better relation can be found.
> 
> This approach has several shortcomings.
> 
>  - First off, it's admittedly rather simple-minded. One may consider
>    backtracking to q1 instead, for example. It's not clear that this
>    backtracking approach would be a way to go, though: the inherently
>    hairy logic that would go with it would be error-prone, at best.
> 
>  - "Trying harder" by just increasing lpb is also very crude. It could
>    make sense to increase I as well (which governs the size of the
>    sieving area). This stuff happens around line 3144 in las.cpp. I would
>    give a try to adding the line:
>     current_config->logI += next_todo->iteration / 4;
>    for instance, but currently this does not work (see later).
> 
>  - The size of the sieving area, again, is controled by I (actually,
>    tasks.descent.init_I). One expects a sieving area of size roughly
>    2^(2*I-1). In some cases though, it can be much much less, depending
>    on the ideal basis depending on q. This is a misbehaviour of cado-nfs
>    which we should fix. To diagnose this, it would be interesting if you
>    could send the file /home/user/dlp/60_ok/p60.descent.4.middle.rels ; 
> 
> For more serious computations, one would counter the "I is too small"
> effect by crafting a "hint file", which decides on sieving parameters (I,
> factor base bounds, large prime bounds) which depend on the size of the
> special-q. It is not anticipated that going to such trouble is necessary
> for a p60, though. A stub p60.hint can be found in
> parameters/dlp/p60.hint. It seems to me that that file could be improved,
> although the mechanism to really make it useful has been inadvertently
> castrated (a failing assert in the code).
> 
> Short of any better suggestion for now before we look at how to fix this,
> you can adjust the parameters as in the attached patch. But it's really a
> quick and dirty fix.
> 
> E.
> 

> diff --git a/parameters/dlp/p60.hint b/parameters/dlp/p60.hint
> index 44d4f0274..6cda55bda 100644
> --- a/parameters/dlp/p60.hint
> +++ b/parameters/dlp/p60.hint
> @@ -1,5 +1,9 @@
> -22@0 0.0038 0.9940 I=11 100000,22,24 100000,22,24
> -22@1 0.0038 0.9995 I=11 100000,22,24 100000,22,24
> -23@0 0.0038 0.9945 I=11 100000,22,24 100000,23,25
> -23@1 0.0038 0.9990 I=11 100000,22,24 100000,22,24
> -24@0 0.0038 0.9945 I=11 100000,22,24 100000,24,45
> +20@0 0.1 1 I=12 200000,19,3 200000,20,3
> +20@1 0.1 1 I=12 200000,19,3 200000,19,3
> +21@0 0.1 1 I=12 200000,20,3 200000,21,3
> +21@1 0.1 1 I=12 200000,20,3 200000,20,3
> +22@0 0.1 1 I=12 200000,21,3 200000,22,3
> +22@1 0.1 1 I=12 200000,21,3 200000,21,3
> +23@0 0.1 1 I=12 200000,22,3 200000,23,3
> +23@1 0.1 1 I=12 200000,22,3 200000,22,3
> +24@0 0.1 1 I=12 200000,22,3 200000,24,3
> diff --git a/parameters/dlp/params.p60 b/parameters/dlp/params.p60
> index 895075b04..ffa9b2e32 100644
> --- a/parameters/dlp/params.p60
> +++ b/parameters/dlp/params.p60
> @@ -77,16 +77,16 @@ tasks.reconstructlog.partial=false
>  # Individual log
>  ###########################################################################
>  
> -tasks.descent.init_I = 11
> +tasks.descent.init_I = 12
>  tasks.descent.init_ncurves = 10
>  tasks.descent.init_lpb = 24
>  tasks.descent.init_lim = 100000
>  tasks.descent.init_mfb = 48
>  tasks.descent.init_tkewness = 200000
> -tasks.descent.I = 11
> -tasks.descent.lim0 = 100000
> -tasks.descent.lim1 = 100000
> +tasks.descent.I = 12
> +tasks.descent.lim0 = 200000
> +tasks.descent.lim1 = 200000
>  tasks.descent.lpb0 = 22
>  tasks.descent.lpb1 = 22
> -tasks.descent.mfb0 = 44
> -tasks.descent.mfb1 = 44
> +tasks.descent.mfb0 = 45
> +tasks.descent.mfb1 = 45

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