All posts by msoos

Memory layout of clauses in MiniSat

SAT,

I have been trying to debug why some MiniSat-based solvers perform better at unit propagation than CryptoMiniSat. It took me exactly 3 full days to find out and I’d like to share this tidbit of information with everyone, as I think it might be of interest and I hardly believe many understand it.

The mystery of the faster unit propagation was that I tried my best to make my solver behave exactly as MiniSat to debug the issue, but even though it was behaving almost identically, it was still slower. It made no sense and I had to dig deeper. You have to understand that both solvers use their own memory managers. In fact, CryptoMiniSat had a memory manager before MiniSat. Memory managers are used so that the clauses are put close to one another so there is a chance that they are in the same memory page, or even better, close enough for them not to waste memory in the cache. This means that a contiguous memory space is reserved where the clauses are placed.

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Benchmark randomisation for SAT Comp’16

Uncategorized

Things are heating up for the SAT competition 2016. I will of course compete. However, I would publicly like to ask the organisers to please for the love of whatever you believe in, please randomise the benchmarks. Just a tiny, little bit. It’s ridiculous that people are tuning their solvers so they can solve some randomly solveable instance like the vmpc* series. It’s laughable and it’s making the whole community look bad. Really, it’s time to stop this madness. I wrote that article with a bunch of ideas in 2013. It’s time. Not even the largest of organisations move this slowly, and this is a research group of about 50 people max.

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On Testing and Security Engineering

Personal,

I have been working in a large organization for quite a while now where I have seen a lot of testing going on. As an information security engineer, I naturally aligned more with testing and indeed, information security assurance does align well with testing: It’s done on a continuous basis, its results usually mean work for developers, operations people, system architects, etc. and not caring about it is equivalent to accepting unknown risks. Since I have been working in an environment where testing was paramount, I have been digging more and more into the testing literature.

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Testing CryptoMiniSat using GoogleTest

Development, SAT,

Lately, I have been working quite hard on writing module tests for CryptoMinisat using GoogleTest. I’d like to share what I’ve learnt and what surprised me most about this exercise.

An example

First of all, let me show how a typical test looks like:

TEST_F(intree, fail_1)
{
    s->add_clause_outer(str_to_cl(" 1,  2"));
    s->add_clause_outer(str_to_cl("-2,  3"));
    s->add_clause_outer(str_to_cl("-2, -3"));

    inp->intree_probe();
    check_zero_assigned_lits_contains(s, "-2");
}

Here we are checking that intree probing finds that the set of three binary clauses cause a failure and it enqueues “-2” at top level. If one looks at it, it’s a fairly trivial test. It turns out that most are in fact, fairly trivial if the system is set up well. This test’s setup is the following test fixture:

struct intree : public ::testing::Test {
    intree()
    {
        must_inter = false;
        s = new Solver(NULL, &must_inter);
        s->new_vars(30);
        inp = s->intree;
    }
    ~intree()
    {
        delete s;
    }

    Solver* s;
    InTree* inp;
    bool must_inter;
};

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CryptoMiniSat: 8000 commits later

Personal, SAT, , ,

The GitHub repository for CryptoMiniSat just hit 8000 commits. To celebrate this rather weird and crazy fact, let me put together a bit of a history.

The Beginnings

CryptoMiniSat began as a way of trying to prove that a probabilistic cryptographic scheme was not possible to break using SAT solvers. This was the year 2009, and I was working in Grenoble at INRIA. It was a fun time and I was working really hard to prove what I wanted to prove, to the point that I created a probabilistic SAT solver where one could add probability weights to clauses. The propagations and conflict engine would only work if the conflict or propagation was supported by multiple clauses. Thus began a long range of my unpublished work in SAT. This SAT solver, curiously, still works and solves problems quite well. It’s a lot of fun, actually — just add some random clause into the database that bans the only correct solution and the solver will find the correct solution. A bit of a hackery, but hey, it works. Also, it’s so much faster than doing that solving with the “right” systems, it’s not even worth comparing.
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