89 research outputs found
Coupling Memory and Computation for Locality Management
We articulate the need for managing (data) locality automatically rather than leaving it to the programmer, especially in parallel programming systems. To this end, we propose techniques for coupling tightly the computation (including the thread scheduler) and the memory manager so that data and computation can be positioned closely in hardware. Such tight coupling of computation and memory management is in sharp contrast with the prevailing practice of considering each in isolation. For example, memory-management techniques usually abstract the computation as an unknown "mutator", which is treated as a "black box". As an example of the approach, in this paper we consider a specific class of parallel computations, nested-parallel computations. Such computations dynamically create a nesting of parallel tasks. We propose a method for organizing memory as a tree of heaps reflecting the structure of the nesting. More specifically, our approach creates a heap for a task if it is separately scheduled on a processor. This allows us to couple garbage collection with the structure of the computation and the way in which it is dynamically scheduled on the processors. This coupling enables taking advantage of locality in the program by mapping it to the locality of the hardware. For example for improved locality a heap can be garbage collected immediately after its task finishes when the heap contents is likely in cache
Spatially Coupled Turbo Codes
In this paper, we introduce the concept of spatially coupled turbo codes
(SC-TCs), as the turbo codes counterpart of spatially coupled low-density
parity-check codes. We describe spatial coupling for both Berrou et al. and
Benedetto et al. parallel and serially concatenated codes. For the binary
erasure channel, we derive the exact density evolution (DE) equations of SC-TCs
by using the method proposed by Kurkoski et al. to compute the decoding erasure
probability of convolutional encoders. Using DE, we then analyze the asymptotic
behavior of SC-TCs. We observe that the belief propagation (BP) threshold of
SC-TCs improves with respect to that of the uncoupled ensemble and approaches
its maximum a posteriori threshold. This phenomenon is especially significant
for serially concatenated codes, whose uncoupled ensemble suffers from a poor
BP threshold.Comment: in Proc. 8th International Symposium on Turbo Codes & Iterative
Information Processing 2014, Bremen, Germany, August 2014. To appear. (The
PCC ensemble is changed with respect to the one in the previous version of
the paper. However, it gives identical thresholds
Threshold Saturation for Spatially Coupled Turbo-like Codes over the Binary Erasure Channel
In this paper we prove threshold saturation for spatially coupled turbo codes
(SC-TCs) and braided convolutional codes (BCCs) over the binary erasure
channel. We introduce a compact graph representation for the ensembles of SC-TC
and BCC codes which simplifies their description and the analysis of the
message passing decoding. We demonstrate that by few assumptions in the
ensembles of these codes, it is possible to rewrite their vector recursions in
a form which places these ensembles under the category of scalar admissible
systems. This allows us to define potential functions and prove threshold
saturation using the proof technique introduced by Yedla et al..Comment: 5 pages, 3figure
Analysis and Design of Partially Information- and Partially Parity-Coupled Turbo Codes
In this paper, we study a class of spatially coupled turbo codes, namely
partially information- and partially parity-coupled turbo codes. This class of
codes enjoy several advantages such as flexible code rate adjustment by varying
the coupling ratio and the encoding and decoding architectures of the
underlying component codes can remain unchanged. For this work, we first
provide the construction methods for partially coupled turbo codes with
coupling memory and study the corresponding graph models. We then derive
the density evolution equations for the corresponding ensembles on the binary
erasure channel to precisely compute their iterative decoding thresholds.
Rate-compatible designs and their decoding thresholds are also provided, where
the coupling and puncturing ratios are jointly optimized to achieve the largest
decoding threshold for a given target code rate. Our results show that for a
wide range of code rates, the proposed codes attain close-to-capacity
performance and the decoding performance improves with increasing the coupling
memory. In particular, the proposed partially parity-coupled turbo codes have
thresholds within 0.0002 of the BEC capacity for rates ranging from to
, yielding an attractive way for constructing rate-compatible
capacity-approaching channel codes.Comment: 15 pages, 13 figures. Accepted for publication in IEEE Transactions
on Communication
Thresholds of Braided Convolutional Codes on the AWGN Channel
In this paper, we perform a threshold analysis of braided convolutional codes
(BCCs) on the additive white Gaussian noise (AWGN) channel. The decoding
thresholds are estimated by Monte-Carlo density evolution (MC-DE) techniques
and compared with approximate thresholds from an erasure channel prediction.
The results show that, with spatial coupling, the predicted thresholds are very
accurate and quickly approach capacity if the coupling memory is increased. For
uncoupled ensembles with random puncturing, the prediction can be improved with
help of the AWGN threshold of the unpunctured ensemble
Binary Message Passing Decoding of Product-like Codes
We propose a novel binary message passing decoding algorithm for product-like
codes based on bounded distance decoding (BDD) of the component codes. The
algorithm, dubbed iterative BDD with scaled reliability (iBDD-SR), exploits the
channel reliabilities and is therefore soft in nature. However, the messages
exchanged by the component decoders are binary (hard) messages, which
significantly reduces the decoder data flow. The exchanged binary messages are
obtained by combining the channel reliability with the BDD decoder output
reliabilities, properly conveyed by a scaling factor applied to the BDD
decisions. We perform a density evolution analysis for generalized low-density
parity-check (GLDPC) code ensembles and spatially coupled GLDPC code ensembles,
from which the scaling factors of the iBDD-SR for product and staircase codes,
respectively, can be obtained. For the white additive Gaussian noise channel,
we show performance gains up to dB and dB for product and
staircase codes compared to conventional iterative BDD (iBDD) with the same
decoder data flow. Furthermore, we show that iBDD-SR approaches the performance
of ideal iBDD that prevents miscorrections.Comment: Accepted for publication in the IEEE Transactions on Communication
Pressure dependence of diffusion coefficient and orientational relaxation time for acetonitrile and methanol in water: DRISM/mode-coupling study
We present results of theoretical description and numerical calculation of
the dynamics of molecular liquids based on the Reference Interaction Site Model
/ Mode-Coupling Theory. They include the temperature-pressure(density)
dependence of the translational diffusion coefficients and orientational
relaxation times for acetonitrile and methanol in water at infinite dilution.
Anomalous behavior, i.e. the increase in mobility with density, is observed for
the orientational relaxation time of methanol, while acetonitrile does not show
any deviations from the usual. This effect is in qualitative agreement with the
recent data of MD simulation and with experimental measurements, which tells us
that presented theory is a good candidate to explain such kind of anomalies
from the microscopical point of view and with the connection to the structure
of the molecules.Comment: 10 pages, 2 eps-figures, 3 table
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