1,900 research outputs found
Bounds on series-parallel slowdown
We use activity networks (task graphs) to model parallel programs and
consider series-parallel extensions of these networks. Our motivation is
two-fold: the benefits of series-parallel activity networks and the modelling
of programming constructs, such as those imposed by current parallel computing
environments. Series-parallelisation adds precedence constraints to an activity
network, usually increasing its makespan (execution time). The slowdown ratio
describes how additional constraints affect the makespan. We disprove an
existing conjecture positing a bound of two on the slowdown when workload is
not considered. Where workload is known, we conjecture that 4/3 slowdown is
always achievable, and prove our conjecture for small networks using max-plus
algebra. We analyse a polynomial-time algorithm showing that achieving 4/3
slowdown is in exp-APX. Finally, we discuss the implications of our results.Comment: 12 pages, 4 figure
Effective guessing has unlikely consequences
Funding: EPSRC Grant number EP/P015638/1.A classic result of Paul, Pippenger, Szemeredi and Trotter states that DTIME(n) ⊊ NTIME(n). The natural question then arises: could the inclusion DTIME(t (n)) ⊆ NTIME(n) hold for some superlinear time-constructible function t (n)? If such a function t (n) does exist, then there also exist effective nondeterministic guessing strategies to speed up deterministic computations. In this work, we prove limitations on the effectiveness of nondeterministic guessing to speed up deterministic computations by showing that the existence of effective nondeterministic guessing strategies would have unlikely consequences. In particular, we show that if a subpolynomial amount of nondeterministic guessing could be used to speed up deterministic computation by a polynomial factor, then P ⊊ NTIME(n). Furthermore, even achieving a logarithmic speedup at the cost of making every step nondeterministic would show that SAT ∈ NTIME(n) under appropriate encodings. Of possibly independent interest, under such encodings we also show that SAT can be decided in O(n log n) steps on a nondeterministic multitape Turing machine, improving on the well-known O(n(log n)c) bound for some constant but undetermined exponent c ≥ 1.Publisher PDFPeer reviewe
Superlinear lower bounds based on ETH
Andras Z. Salamon acknowledges support from EPSRC grants EP/P015638/1 and EP/V027182/1.We introduce techniques for proving superlinear conditional lower bounds for polynomial time problems. In particular, we show that CircuitSAT for circuits with m gates and log(m) inputs (denoted by log-CircuitSAT) is not decidable in essentially-linear time unless the exponential time hypothesis (ETH) is false and k-Clique is decidable in essentially-linear time in terms of the graph's size for all fixed k. Such conditional lower bounds have previously only been demonstrated relative to the strong exponential time hypothesis (SETH). Our results therefore offer significant progress towards proving unconditional s uperlinear time complexity lower bounds for natural problems in polynomial time.Postprin
Macroscopes: models for collective decision making
We introduce a new model of collective decision making, when a global
decision needs to be made but the parties only possess partial information, and
are unwilling (or unable) to first create a globalcomposite of their local
views. Our macroscope model captures two key features of many real-world
problems: allotment structure (how access to local information is apportioned
between parties, including overlaps between the parties) and the possible
presence of meta-information (what each party knows about the allotment
structure of the overall problem). Using the framework of communication
complexity, we formalize the efficient solution of a macroscope. We present
general results about the macroscope model, and also results that abstract the
essential computational operations underpinning practical applications,
including in financial markets and decentralized sensor networks. We illustrate
the computational problem inherent in real-world collective decision making
processes using results for specific functions, involving detecting a change in
state (constant and step functions), and computing statistical properties (the
mean).Comment: Presented at Collective Intelligence conference, 2012
(arXiv:1204.2991), 8 page
Towards Exploratory Reformulation of Constraint Models
It is well established that formulating an effective constraint model of a
problem of interest is crucial to the efficiency with which it can subsequently
be solved. Following from the observation that it is difficult, if not
impossible, to know a priori which of a set of candidate models will perform
best in practice, we envisage a system that explores the space of models
through a process of reformulation from an initial model, guided by performance
on a set of training instances from the problem class under consideration. We
plan to situate this system in a refinement-based approach, where a user writes
a constraint specification describing a problem above the level of abstraction
at which many modelling decisions are made. In this position paper we set out
our plan for an exploratory reformulation system, and discuss progress made so
far.Comment: 13 pages, 6 figure
CHANGES IN FATTY ACID COMPOSITION OF DIFFERENT MILK PRODUCTS CAUSED BY DIFFERENT TECHNOLOGY
The fatty acid composition of cow’s milk with fat contents of 3.6%, Dalia cheese with fat contents of 44%, butter with fat contents of 80% and margarine with fat contents of 24% was determined after a heat treatment performed on cooking plate and microwave treatment, respectively of different durations. The biggest difference was obtained for oleic acid and elaidic acid since, with the exception of the margarine, in each case proportion of the cis-configurated oleic acid decreased while that of the trans-configurated elaidic acid increased. For all of the other fatty acids in the foodstuffs examined no such differences were obtained regarding change in fatty acid composition whose differences could influence healthy nutrition to considerable extent. Therefore we can take it as a fact that neither heat treatment performed on a traditional cooking plate nor microwave treatment affects considerably the composition of food fats
Classification of annotation semirings over containment of conjunctive queries
Funding: This work is supported under SOCIAM: The Theory and Practice of Social Machines, a project funded by the UK Engineering and Physical Sciences Research Council (EPSRC) under grant number EP/J017728/1. This work was also supported by FET-Open Project FoX, grant agreement 233599; EPSRC grants EP/F028288/1, G049165 and J015377; and the Laboratory for Foundations of Computer Science.We study the problem of query containment of conjunctive queries over annotated databases. Annotations are typically attached to tuples and represent metadata, such as probability, multiplicity, comments, or provenance. It is usually assumed that annotations are drawn from a commutative semiring. Such databases pose new challenges in query optimization, since many related fundamental tasks, such as query containment, have to be reconsidered in the presence of propagation of annotations. We axiomatize several classes of semirings for each of which containment of conjunctive queries is equivalent to existence of a particular type of homomorphism. For each of these types, we also specify all semirings for which existence of a corresponding homomorphism is a sufficient (or necessary) condition for the containment. We develop new decision procedures for containment for some semirings which are not in any of these classes. This generalizes and systematizes previous approaches.PostprintPeer reviewe
Towards exploratory reformulation of constraint models
Funding: Ian Miguel: EPSRC grant EP/V027182/1; Christopher Stone: EPSRC grant EP/V027182/1.It is well established that formulating an effective constraint model of a problem of interest is crucial to the efficiency with which it can subsequently be solved. Following from the observation that it is difficult, if not impossible, to know a priori which of a set of candidate models will perform best in practice, we envisage a system that explores the space of models through a process of reformulation from an initial model, guided by performance on a set of training instances from the problem class under consideration. We plan to situate this system in a refinement-based approach, where a user writes a constraint specification describing a problem above the level of abstraction at which many modelling decisions are made. In this position paper we set out our plan for an exploratory reformulation system, and discuss progress made so far.PostprintPeer reviewe
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