3,576 research outputs found
Revealing the unseen: how to expose cloud usage while protecting user privacy
Cloud users have little visibility into the performance characteristics and utilization of the physical machines underpinning the virtualized cloud resources they use. This uncertainty forces users and researchers to reverse engineer the inner workings of cloud systems in order to understand and optimize the conditions their applications operate. At Massachusetts Open Cloud (MOC), as a public cloud operator, we'd like to expose the utilization of our physical infrastructure to stop this wasteful effort. Mindful that such exposure can be used maliciously for gaining insight into other user's workloads, in this position paper we argue for the need for an approach that balances openness of the cloud overall with privacy for each tenant inside of it. We believe that this approach can be instantiated via a novel combination of several security and privacy technologies. We discuss the potential benefits, implications of transparency for cloud systems and users, and technical challenges/possibilities.Accepted manuscrip
Drift rate control of a Brownian processing system
A system manager dynamically controls a diffusion process Z that lives in a
finite interval [0,b]. Control takes the form of a negative drift rate \theta
that is chosen from a fixed set A of available values. The controlled process
evolves according to the differential relationship dZ=dX-\theta(Z) dt+dL-dU,
where X is a (0,\sigma) Brownian motion, and L and U are increasing processes
that enforce a lower reflecting barrier at Z=0 and an upper reflecting barrier
at Z=b, respectively. The cumulative cost process increases according to the
differential relationship d\xi =c(\theta(Z)) dt+p dU, where c(\cdot) is a
nondecreasing cost of control and p>0 is a penalty rate associated with
displacement at the upper boundary. The objective is to minimize long-run
average cost. This problem is solved explicitly, which allows one to also solve
the following, essentially equivalent formulation: minimize the long-run
average cost of control subject to an upper bound constraint on the average
rate at which U increases. The two special problem features that allow an
explicit solution are the use of a long-run average cost criterion, as opposed
to a discounted cost criterion, and the lack of state-related costs other than
boundary displacement penalties. The application of this theory to power
control in wireless communication is discussed.Comment: Published at http://dx.doi.org/10.1214/105051604000000855 in the
Annals of Applied Probability (http://www.imstat.org/aap/) by the Institute
of Mathematical Statistics (http://www.imstat.org
Electronic mean free path in as-produced and purified single-wall carbon nanotubes
The effect of purification on room temperature electronic transport
properties of single-wall carbon nanotubes (SWNT) was studied by submerging
samples into liquid mercury. The conductance plots of purified SWNTs showed
plateaus, indicating weak dependence of the electrical resistance on the length
of the tube connecting the electrodes, providing evidence of quasi-ballistic
conduction in SWNTs. The electronic mean free path of the purified SWNTs
reached a few microns, which is longer than that of the as-produced SWNTs, and
which is consistent with the calculation based on the scattering by acoustic
phonons
RR Lyrae Variables in M33. I. Evidence For a Field Halo Population
We present observations of RR Lyrae variables in the Local Group late-type
spiral galaxy M33. Using the Advanced Camera for Surveys on the Hubble Space
Telescope, we have identified 64 ab-type RR Lyraes in M33. We have estimated
reddenings for these stars based on their minimum light V-I colors and
metallicities based on their periods. From the distributions of these
properties, we conclude that the RR Lyraes belong to two populations - one
associated with the halo of M33 and the other with its disk. Given that RR
Lyraes are produced by populations older than ~10 Gyr, this suggests that not
only does the field halo of M33 contain an old component, but so does its disk.
This is one of the best pieces of evidence for the existence of a halo field
component in M33. Using a relation between RR Lyrae absolute magnitude and
metallicity (Mv(RR) = 0.23[Fe/H] + 0.93), we estimate a mean distance modulus
of = 24.67 +/- 0.08 for M33. This places M33 approximately 70 kpc
beyond M31 in line-of-sight distance.Comment: 20 pages, 17 figures, accepted for publication in The Astronomical
Journa
The Metallicity Distribution Function of Field Stars in M31's Bulge
We have used Hubble Space Telescope Wide Field Planetary Camera 2
observations to construct a color-magnitude diagram (CMD) for the bulge of M31
at a location ~1.6 kpc from the galaxy's center. Using scaled-solar abundance
theoretical red giant branches with a range of metallicities, we have
translated the observed colors of the stars in the CMD to abundances and
constructed a metallicity distribution function (MDF) for this region. The MDF
shows a peak at [M/H]~0 with a steep decline at higher metallicities and a more
gradual tail to lower metallicities. This is similar in shape to the MDF of the
Milky Way bulge but shifted to higher metallicities by ~0.1 dex. As is the case
with the Milky Way bulge MDF, a pure closed box model of chemical evolution,
even with significant pre-enrichment, appears to be inconsistent with the M31
bulge MDF. However, a scenario in which an initial infall of gas enriched the
bulge to an abundance of [M/H] ~ -1.6 with subsequent evolution proceeding as a
closed box provides a better fit to the observed MDF. The similarity between
the MDF of the M31 bulge and that of the Milky Way stands in stark contrast to
the significant differences in the MDFs of their halo populations. This
suggests that the bulk of the stars in the bulges of both galaxies were in
place before the accretion events that occurred in the halos could influence
them.Comment: 12 pages, 9 figures, accepted for publication in The Astronomical
Journal, October 200
Large Magellanic Cloud Near-Infrared Synoptic Survey. II. The Wesenheit relations and their application to the Distance scale
We present new near-infrared Cepheid Period-Wesenheit relations in the LMC
using time-series observations from the Large Magellanic Cloud Near-Infrared
Synoptic Survey. We also derive opticalnear-infrared P-W relations using
and ~magnitudes from OGLE-III. We employ our new data to determine
an independent distance to the LMC of ~mag, using an absolute calibration of the Galactic
relations based on several distance determination methods and accounting for
the intrinsic scatter of each technique. We also derive new near-infrared
Period-Luminosity and Wesenheit relations for Cepheids in M31 using
observations from the PHAT survey. We use the absolute calibrations of the
Galactic and LMC relations to determine the distance modulus of M31,
~mag. We apply a simultaneous fit to Cepheids in
several Local Group galaxies covering a range of metallicities
(~dex) to determine a global slope of
-~mag/dex for the relation and obtain robust
distance estimates. Our distances are in good agreement with recent TRGB based
distance estimates and we do not find any evidence for a metallicity dependence
in the near-infrared P-W relations.Comment: 16 pages, 8 figures, Accepted for publication in The Astronomical
Journa
Thrombolytic therapy in acute myocardial infarction in Pakistan.
To characterize features of acute myocardial infarction (AMI) in a Pakistani population, and assess the role of thrombolysis in our country, we studied 194 consecutive admitted patients with enzyme positive AMI. Males were affected three times more frequently; women, although affected less, had a higher incidence of complications than men. Premature coronary artery disease (CAD) was present in 50% patients presenting with AMI. An unusually high incidence of anterior wall myocardial infarction (39%) was seen. Complications were frequent with a predominance of LV failure symptoms. Cardiogenic shock was associated with a very high mortality, in excess of 93%. Streptokinase (SK) was administered in 60% patients with suspected transmural AMI. A statistically significant reduction in mortality was seen in the group that received SK (15.2%) compared to those who did not receive SK (24.7%), (p = \u3c 0.05)
Thermal profiles within the channel of planar gunn diodes using micro-particle sensors
The paper describes the use of a novel microparticle sensor (~3 μm diameter) and infra-red (IR) microscopy to measure the temperature profile within the active channel (typically 3 μm length and 120 μm width) of planar Gunn diodes. The method has enabled detailed temperature measurements showing an asymmetrical temperature profile along the active width of these devices. The asymmetrical temperature profile suggests a similar behaviour in the channel current density, which may contribute to the lower than expected RF output power
Vibration of circumferentially stepped-thickness piezoelectric cylindrical shells
In this work, thickness variations are introduced around the circumference of a piezoelectric cylindrical shell. The aim is to investigate the vibration characteristics of the shells and the effect of these step-thickness variations on the mode shape of vibration. These thickness variations require stress distribution analysis as well to avoid failure of the cylindrical shell. To this aim, two configurations of stepped-thickness shells with two and three circumferential thickness variations are investigated using FEA software, ANSYS. The results show that these steps assist in localizing vibration in the thin sections and excite mode shapes having the same circumferential wave number as the number of thickness variations. This can be a suitable approach to control and forcibly excite certain vibration mode shapes, which might be required for some applications
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