4,123 research outputs found
The Blacklisting Memory Scheduler: Balancing Performance, Fairness and Complexity
In a multicore system, applications running on different cores interfere at
main memory. This inter-application interference degrades overall system
performance and unfairly slows down applications. Prior works have developed
application-aware memory schedulers to tackle this problem. State-of-the-art
application-aware memory schedulers prioritize requests of applications that
are vulnerable to interference, by ranking individual applications based on
their memory access characteristics and enforcing a total rank order.
In this paper, we observe that state-of-the-art application-aware memory
schedulers have two major shortcomings. First, such schedulers trade off
hardware complexity in order to achieve high performance or fairness, since
ranking applications with a total order leads to high hardware complexity.
Second, ranking can unfairly slow down applications that are at the bottom of
the ranking stack. To overcome these shortcomings, we propose the Blacklisting
Memory Scheduler (BLISS), which achieves high system performance and fairness
while incurring low hardware complexity, based on two observations. First, we
find that, to mitigate interference, it is sufficient to separate applications
into only two groups. Second, we show that this grouping can be efficiently
performed by simply counting the number of consecutive requests served from
each application.
We evaluate BLISS across a wide variety of workloads/system configurations
and compare its performance and hardware complexity, with five state-of-the-art
memory schedulers. Our evaluations show that BLISS achieves 5% better system
performance and 25% better fairness than the best-performing previous scheduler
while greatly reducing critical path latency and hardware area cost of the
memory scheduler (by 79% and 43%, respectively), thereby achieving a good
trade-off between performance, fairness and hardware complexity
Quantum Valence Criticality as Origin of Unconventional Critical Phenomena
It is shown that unconventional critical phenomena commonly observed in
paramagnetic metals YbRh2Si2, YbRh2(Si0.95Ge0.05)2, and beta-YbAlB4 is
naturally explained by the quantum criticality of Yb-valence fluctuations. We
construct the mode coupling theory taking account of local correlation effects
of f electrons and find that unconventional criticality is caused by the
locality of the valence fluctuation mode. We show that measured low-temperature
anomalies such as divergence of uniform spin susceptibility \chi T^{-\zeta)
with giving rise to a huge enhancement of the Wilson ratio and the
emergence of T-linear resistivity are explained in a unified way.Comment: 5 pages, 3 figures, to be published in Physical Review Letter
Crossover component in non critical dissipative sandpile models
The effect of bulk dissipation on non critical sandpile models is studied
using both multifractal and finite size scaling analyses. We show numerically
that the local limited (LL) model exhibits a crossover from multifractal to
self-similar behavior as the control parameters and turn
towards their critical values, i.e. and . The critical exponents are not universal and exhibit a continuous
variation with . On the other hand, the finite size effects for the
local unlimited (LU), non local limited (NLL), and non local unlimited (NLU)
models are well described by the multifractal analysis for all values of
dissipation rate . The space-time avalanche structure is studied in
order to give a deeper understanding of the finite size effects and the origin
of the crossover behavior. This result is confirmed by the calculation of the
susceptibility.Comment: 13 pages, 10 figures, Published in European Physical Journal
Optimized Surface Code Communication in Superconducting Quantum Computers
Quantum computing (QC) is at the cusp of a revolution. Machines with 100
quantum bits (qubits) are anticipated to be operational by 2020
[googlemachine,gambetta2015building], and several-hundred-qubit machines are
around the corner. Machines of this scale have the capacity to demonstrate
quantum supremacy, the tipping point where QC is faster than the fastest
classical alternative for a particular problem. Because error correction
techniques will be central to QC and will be the most expensive component of
quantum computation, choosing the lowest-overhead error correction scheme is
critical to overall QC success. This paper evaluates two established quantum
error correction codes---planar and double-defect surface codes---using a set
of compilation, scheduling and network simulation tools. In considering
scalable methods for optimizing both codes, we do so in the context of a full
microarchitectural and compiler analysis. Contrary to previous predictions, we
find that the simpler planar codes are sometimes more favorable for
implementation on superconducting quantum computers, especially under
conditions of high communication congestion.Comment: 14 pages, 9 figures, The 50th Annual IEEE/ACM International Symposium
on Microarchitectur
Finite size scaling for quantum criticality using the finite-element method
Finite size scaling for the Schr\"{o}dinger equation is a systematic approach
to calculate the quantum critical parameters for a given Hamiltonian. This
approach has been shown to give very accurate results for critical parameters
by using a systematic expansion with global basis-type functions. Recently, the
finite element method was shown to be a powerful numerical method for ab initio
electronic structure calculations with a variable real-space resolution. In
this work, we demonstrate how to obtain quantum critical parameters by
combining the finite element method (FEM) with finite size scaling (FSS) using
different ab initio approximations and exact formulations. The critical
parameters could be atomic nuclear charges, internuclear distances, electron
density, disorder, lattice structure, and external fields for stability of
atomic, molecular systems and quantum phase transitions of extended systems. To
illustrate the effectiveness of this approach we provide detailed calculations
of applying FEM to approximate solutions for the two-electron atom with varying
nuclear charge; these include Hartree-Fock, density functional theory under the
local density approximation, and an "exact"' formulation using FEM. We then use
the FSS approach to determine its critical nuclear charge for stability; here,
the size of the system is related to the number of elements used in the
calculations. Results prove to be in good agreement with previous Slater-basis
set calculations and demonstrate that it is possible to combine finite size
scaling with the finite-element method by using ab initio calculations to
obtain quantum critical parameters. The combined approach provides a promising
first-principles approach to describe quantum phase transitions for materials
and extended systems.Comment: 15 pages, 19 figures, revision based on suggestions by referee,
accepted in Phys. Rev.
Effects of Crystalline Electronic Field and Onsite Interorbital Interaction in Yb-based Quasicrystal and Approximant Crystal
To get an insight into a new type of quantum critical phenomena recently
discovered in the quasicrystal YbAlAu and approximant
crystal (AC) YbAlAu under pressure, we discuss the
property of the crystalline electronic field (CEF) at Yb in the AC and show
that uneven CEF levels at each Yb site can appear because of the Al/Au mixed
sites. Then we construct the minimal model for the electronic state on the AC
by introducing the onsite Coulomb repulsion between the 4f and 5d orbitals at
Yb. Numerical calculation for the ground state shows that the lattice constant
dependence of the Yb valence well explains the recent measurement done by
systematic substitution of elements of Al and Au in the quasicrystal and AC,
where the quasicrystal YbAlAu is just located at the point
from where the Yb-valence starts to change drastically. Our calculation
convincingly demonstrates that this is indeed the evidence that this material
is just located at the quantum critical point of the Yb-valence transition.Comment: 12 pages, 8 figures, Invited Paper in the 26th International
Conference on High Pressure Science & Technology (AIRAPT 26
Entanglement renormalization
In the context of real-space renormalization group methods, we propose a
novel scheme for quantum systems defined on a D-dimensional lattice. It is
based on a coarse-graining transformation that attempts to reduce the amount of
entanglement of a block of lattice sites before truncating its Hilbert space.
Numerical simulations involving the ground state of a 1D system at criticality
show that the resulting coarse-grained site requires a Hilbert space dimension
that does not grow with successive rescaling transformations. As a result we
can address, in a quasi-exact way, tens of thousands of quantum spins with a
computational effort that scales logarithmically in the system's size. The
calculations unveil that ground state entanglement in extended quantum systems
is organized in layers corresponding to different length scales. At a quantum
critical point, each rellevant length scale makes an equivalent contribution to
the entanglement of a block with the rest of the system.Comment: 4 pages, 4 figures, updated versio
An order parameter for impurity systems at quantum criticality
A quantum phase transition may occur in the ground state of a system at zero
temperature when a controlling field or interaction is varied. The resulting
quantum fluctuations which trigger the transition produce scaling behavior of
various observables, governed by universal critical exponents. A particularly
interesting class of such transitions appear in systems with quantum impurities
where a non-extensive term in the free energy becomes singular at the critical
point. Curiously, the notion of a conventional order parameter which exhibits
scaling at the critical point is generically missing in these systems. We here
explore the possibility to use the Schmidt gap, which is an observable obtained
from the entanglement spectrum, as an order parameter. A case study of the
two-impurity Kondo model confirms that the Schmidt gap faithfully captures the
scaling behavior by correctly predicting the critical exponent of the
dynamically generated length scale at the critical point.Comment: 6 pages, 5 figure
- …