Re: Optimising generated rules for SAT solving (5/12 are duplicates)

On 24/11/2016 13:05, Hans Petter Selasky wrote:
> On 11/24/16 13:13, Vsevolod Stakhov wrote:
>> On 23/11/2016 16:27, Ed Schouten wrote:
>>> Hi Hans,
>>>
>>> 2016-11-23 15:27 GMT+01:00 Hans Petter Selasky <hps_at_selasky.org>:
>>>> I've made a patch to hopefully optimise SAT solving in our pkg utility.
>>>
>>> Nice! Do you by any chance have any numbers that show the performance
>>> improvements made by this change? Assuming that the SAT solver of
>>> pkg(1) uses an algorithm similar to DPLL[1], a change like this would
>>> affect performance linearly. My guess is therefore that the running
>>> time is reduced by approximately 5/12. Is this correct?
>>
>> There won't be any improvement if you just remove duplicates from SAT
>> formula. This situation is handled by picosat internally and even for
>> naive DPLL there is no significant influence of duplicate KNF clauses:
>> once you've assumed all vars in some clause, you automatically resolve
>> all duplicates.
>>
>> Is there any real improvement of SAT solver speed with this patch? From
>> my experiences, SAT solving is negligible in terms of CPU time comparing
>> to other tasks performed by pkg.
> 
> Hi,
> 
> I added some code to measure the time for SAT solving. During my test
> run I'm seeing values in the range 8-10ms for both versions, so I
> consider that neglible. However, the unpatched version wants to
> reinstall 185 packages while the non-patched version wants to reinstall
> 1 package. That has a huge time influential. I'm not that familar with
> PKG that I can draw any conclusions from this.
> 
> # Test1:
> echo "n" | /xxx/pkg/src/pkg-static upgrade --no-repo-update > b.txt
> 
> # Test2:
> echo "n" | env PKG_NO_SORT=YES /xxx/pkg/src/pkg-static upgrade
> --no-repo-update > a.txt
> 

Then I don't understand how your patch should affect the solving
procedure. If pkg tries to reinstall something without *reason* it is a
good sign of bug in pkg itself and/or your database/repo and not in SAT
solver.

I'll try to review your issue but I'll likely need your local packages
database for this test.

-- 
Vsevolod Stakhov