137 research outputs found
Anonymous Asynchronous Systems: The Case of Failure Detectors
Due the multiplicity of loci of control, a main issue distributed systems have to cope with lies in the uncertainty on the system state created by the adversaries that are asynchrony, failures, dynamicity, mobility, etc. Considering message-passing systems, this paper considers the uncertainty created by the net effect of three of these adversaries, namely, asynchrony, failures, and anonymity. This means that, in addition to be asynchronous and crash-prone, the processes have no identity. Trivially, agreement problems (e.g., consensus) that cannot be solved in presence of asynchrony and failures cannot be solved either when adding anonymity. The paper consequently proposes anonymous failure detectors to circumvent these impossibilities. It has several contributions. First it presents three classes of failure detectors (denoted AP, Aâ© and Aâ) and show that they are the anonymous counterparts of the classes of perfect failure detectors, eventual leader failure detectors and quorum failure detectors, respectively. The class Aâ is new and showing it is the anonymous counterpart of the class â is not trivial. Then, the paper presents and proves correct a genuinely anonymous consensus algorithm based on the pair of anonymous failure detector classes (Aâ©, Aâ) (âgenuinelyâ means that, not only processes have no identity, but no process is aware of the total number of processes). This new algorithm is not a âstraightforward extensionâ of an algorithm designed for non-anonymous systems. To benefit from Aâ, it uses a novel message exchange pattern where each phase of every round is made up of sub-rounds in which appropriate control information is exchanged. Finally, the paper discusses the notions of failure detector class hierarchy and weakest failure detector class for a given problem in the context of anonymous systems
Diversification in the Archean Biosphere: Insight from NanoSIMS of Microstructures in the Farrel Quartzite of Australia
The nature of early life on Earth is difficult to assess because potential Early Archean biosignatures are commonly poorly preserved. Interpretations of such materials have been contested, and abiotic or epigenetic derivations have been proposed (summarized in [1]). Yet, an understanding of Archean life is of astrobiological importance, as knowledge of early evolutionary processes on Earth could provide insight to development of life on other planets. A recently-discovered assemblage of organic microstructures in approx.3 Ga charts of the Farrel Quartzite (FQ) of Australia [2-4] includes unusual spindle-like forms and a variety of spheroids. If biogenicity and syngeneity of these forms could be substantiated, the FQ assemblage would provide a new view of Archean life. Our work uses NanoSIMS to further assess the biogenicity and syngeneity of FQ microstructures. In prior NanoSIMS studies [5-6], we gained an understanding of nano-scale elemental distributions in undisputed microfossils from the Neoproterozoic Bitter Springs Formation of Australia. Those results provide a new tool with which to evaluate poorly preserved materials that we might find in Archean sediments and possibly in extraterrestrial materials. We have applied this tool to the FQ forms
"Nano" Scale Biosignatures and the Search for Extraterrestrial Life
A critical step in the search for remnants of potential life forms on other planets lies in our ability to recognize indigenous fragments of ancient microbes preserved in some of Earth's oldest rocks. To this end, we are building a database of nano-scale chemical and morphological characteristics of some of Earth's oldest organic microfossils. We are primarily using the new technology of Nano-Secondary ion mass spectrometry (NanoSIMS) which provides in-situ, nano-scale elemental analysis of trace quantities of organic residues. The initial step was to characterize element composition of well-preserved, organic microfossils from the late Proterozoic (0.8 Ga) Bitter Springs Formation of Australia. Results from that work provide morphologic detail and nitrogen/carbon ratios that appear to reflect the well-established biological origin of these 0.8 Ga fossils
Coordinated Analyses of Presolar Grains in the Allan Hills 77307 and Queen Elizabeth Range 99177 Meteorites
We report the identification of presolar silicates (~177 ppm), presolar
oxides (~11 ppm), and one presolar SiO2 grain in the Allan Hills (ALHA) 77307
chondrite. Three grains having Si isotopic compositions similar to SiC X and Z
grains were also identified, though the mineral phases are unconfirmed. Similar
abundances of presolar silicates (~152 ppm) and oxides (~8 ppm) were also
uncovered in the primitive CR chondrite Queen Elizabeth Range (QUE) 99177,
along with 13 presolar SiC grains and one presolar silicon nitride. The O
isotopic compositions of the presolar silicates and oxides indicate that most
of the grains condensed in low-mass red giant and asymptotic giant branch
stars. Interestingly, unlike presolar oxides, few presolar silicate grains have
isotopic compositions pointing to low-metallicity, low-mass stars (Group 3).
The 18O-rich (Group 4) silicates, along with the few Group 3 silicates that
were identified, likely have origins in supernova outflows. This is supported
by their O and Si isotopic compositions. Elemental compositions for 74 presolar
silicate grains were determined by scanning Auger spectroscopy. Most of the
grains have non-stoichiometric elemental compositions inconsistent with
pyroxene or olivine, the phases commonly used to fit astronomical spectra, and
have comparable Mg and Fe contents. Non-equilibrium condensation and/or
secondary alteration could produce the high Fe contents. Transmission electron
microscopic analysis of three silicate grains also reveals non-stoichiometric
compositions, attributable to non-equilibrium or multistep condensation, and
very fine scale elemental heterogeneity, possibly due to subsequent annealing.
The mineralogies of presolar silicates identified in meteorites thus far seem
to differ from those in interplanetary dust particles.Comment: 23 pages, 16 figure
Conditions for Set Agreement with an Application to Synchronous Systems
The -set agreement problem is a generalization of the consensus problem: considering a system made up of processes where each process proposes a value, each non-faulty process has to decide a value such that a decided value is a proposed value, and no more than different values are decided. While this problem cannot be solved in an asynchronous system prone to process crashes when , it can always be solved in a synchronous system; is then a lower bound on the number of rounds (consecutive communication steps) for the non-faulty processes to decide. The {\it condition-based} approach has been introduced in the consensus context. Its aim was to both circumvent the consensus impossibility in asynchronous systems, and allow for more efficient consensus algorithms in synchronous systems. This paper addresses the condition-based approach in the context of the -set agreement problem. It has two main contributions. The first is the definition of a framework that allows defining conditions suited to the -set agreement problem. More precisely, a condition is defined as a set of input vectors such that each of its input vectors can be seen as ``encoding'' values, namely, the values that can be decided from that vector. A condition is characterized by the parameters , , and a parameter denoted such that the greater , the least constraining the condition (i.e., it includes more and more input vectors when increases, and there is a condition that includes all the input vectors when ). The conditions characterized by the triple of parameters , and define the class of conditions denoted , , . The properties of the sets are investigated, and it is shown that they have a lattice structure. The second contribution is a generic synchronous -set agreement algorithm based on a condition , i.e., a condition suitedto the -set agreement problem, for . This algorithm requires at most rounds when the input vector belongs to , and rounds otherwise. (Interestingly, this algorithm includes as particular cases the classical synchronous -set agreement algorithm that requires rounds (case and ), and the synchronous consensus condition-based algorithm that terminates in rounds when the input vector belongs to the condition, and in rounds otherwise (case ).
On the asymptotic giant branch star origin of peculiar spinel grain OC2
Microscopic presolar grains extracted from primitive meteorites have
extremely anomalous isotopic compositions revealing the stellar origin of these
grains. The composition of presolar spinel grain OC2 is different from that of
all other presolar spinel grains. Large excesses of the heavy Mg isotopes are
present and thus an origin from an intermediate-mass (IM) asymptotic giant
branch (AGB) star was previously proposed for this grain. We discuss the
isotopic compositions of presolar spinel grain OC2 and compare them to
theoretical predictions. We show that the isotopic composition of O, Mg and Al
in OC2 could be the signature of an AGB star of IM and metallicity close to
solar experiencing hot bottom burning, or of an AGB star of low mass (LM) and
low metallicity suffering very efficient cool bottom processing. Large
measurement uncertainty in the Fe isotopic composition prevents us from
discriminating which model better represents the parent star of OC2. However,
the Cr isotopic composition of the grain favors an origin in an IM-AGB star of
metallicity close to solar. Our IM-AGB models produce a self-consistent
solution to match the composition of OC2 within the uncertainties related to
reaction rates. Within this solution we predict that the 16O(p,g)17F and the
17O(p,a)14N reaction rates should be close to their lower and upper limits,
respectively. By finding more grains like OC2 and by precisely measuring their
Fe and Cr isotopic compositions, it may be possible in the future to derive
constraints on massive AGB models from the study of presolar grains.Comment: 10 pages, 8 figures, accepted for publication on Astronomy &
Astrophysic
Short-lived Nuclei in the Early Solar System: Possible AGB Sources
(Abridged) We review abundances of short-lived nuclides in the early solar
system (ESS) and the methods used to determine them. We compare them to the
inventory for a uniform galactic production model. Within a factor of two,
observed abundances of several isotopes are compatible with this model. I-129
is an exception, with an ESS inventory much lower than expected. The isotopes
Pd-107, Fe-60, Ca-41, Cl-36, Al-26, and Be-10 require late addition to the
solar nebula. Be-10 is the product of particle irradiation of the solar system
as probably is Cl-36. Late injection by a supernova (SN) cannot be responsible
for most short-lived nuclei without excessively producing Mn-53; it can be the
source of Mn-53 and maybe Fe-60. If a late SN is responsible for these two
nuclei, it still cannot make Pd-107 and other isotopes. We emphasize an AGB
star as a source of nuclei, including Fe-60 and explore this possibility with
new stellar models. A dilution factor of about 4e-3 gives reasonable amounts of
many nuclei. We discuss the role of irradiation for Al-26, Cl-36 and Ca-41.
Conflict between scenarios is emphasized as well as the absence of a global
interpretation for the existing data. Abundances of actinides indicate a
quiescent interval of about 1e8 years for actinide group production in order to
explain the data on Pu-244 and new bounds on Cm-247. This interval is not
compatible with Hf-182 data, so a separate type of r-process is needed for at
least the actinides, distinct from the two types previously identified. The
apparent coincidence of the I-129 and trans-actinide time scales suggests that
the last actinide contribution was from an r-process that produced actinides
without fission recycling so that the yields at Ba and below were governed by
fission.Comment: 92 pages, 14 figure files, in press at Nuclear Physics
A Nomadic Testbed for Teaching Computer Architecture
A nomadic laboratory or testbed, based on Raspberry Pi 3 computers and Arduino microcontrollers, has been developed in order to teach subjects related to computer architecture. The testbed can be transported to the classroom. Students can access it through the available network, which can be a wireless LAN, wired LAN o a custom network. The student can access without constraints to the platforms, therefore there are a wide range of possible experiments.
This laboratory was used during 2017 for practical works in the course Introduction to Technology, and during 2018 in the course Computers Architecture at Universidad Nacional of Cuyo. Some of the experiments that are been carried out by students are: to explore and analyse the architecture of the computers through Linux commands, write and run programs on different programing languages, input and output operations through memory mapped addressing and isolated addressing, write interrupt service routines in order to service interrupts, multithreading programing, explore memory maps, CPU features, etc.
This paper describes the testbed architecture, experiments performed by students in the mentioned subjects, present the students feedback, and describes the possible methods in order to integrate it to a remote laboratory.XVII Workshop TecnologĂa InformĂĄtica Aplicada en EducaciĂłn (WTIAE)Red de Universidades con Carreras en InformĂĄtica (RedUNCI
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Overview of the results of the organics PET Study of the cometary samples returned from comet Wild 2 by the Stardust mission
This presenation will provide an overview of the efforts and results produced by the Organics Preliminary Examination Team during their studies of the samples returned from comet Wild 2 by the Stardust spacecraft
The Kuiper Belt and Other Debris Disks
We discuss the current knowledge of the Solar system, focusing on bodies in
the outer regions, on the information they provide concerning Solar system
formation, and on the possible relationships that may exist between our system
and the debris disks of other stars. Beyond the domains of the Terrestrial and
giant planets, the comets in the Kuiper belt and the Oort cloud preserve some
of our most pristine materials. The Kuiper belt, in particular, is a
collisional dust source and a scientific bridge to the dusty "debris disks"
observed around many nearby main-sequence stars. Study of the Solar system
provides a level of detail that we cannot discern in the distant disks while
observations of the disks may help to set the Solar system in proper context.Comment: 50 pages, 25 Figures. To appear in conference proceedings book
"Astrophysics in the Next Decade
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