25 research outputs found
Doctor of Philosophy
dissertationA modern software system is a composition of parts that are themselves highly complex: operating systems, middleware, libraries, servers, and so on. In principle, compositionality of interfaces means that we can understand any given module independently of the internal workings of other parts. In practice, however, abstractions are leaky, and with every generation, modern software systems grow in complexity. Traditional ways of understanding failures, explaining anomalous executions, and analyzing performance are reaching their limits in the face of emergent behavior, unrepeatability, cross-component execution, software aging, and adversarial changes to the system at run time. Deterministic systems analysis has a potential to change the way we analyze and debug software systems. Recorded once, the execution of the system becomes an independent artifact, which can be analyzed offline. The availability of the complete system state, the guaranteed behavior of re-execution, and the absence of limitations on the run-time complexity of analysis collectively enable the deep, iterative, and automatic exploration of the dynamic properties of the system. This work creates a foundation for making deterministic replay a ubiquitous system analysis tool. It defines design and engineering principles for building fast and practical replay machines capable of capturing complete execution of the entire operating system with an overhead of several percents, on a realistic workload, and with minimal installation costs. To enable an intuitive interface of constructing replay analysis tools, this work implements a powerful virtual machine introspection layer that enables an analysis algorithm to be programmed against the state of the recorded system through familiar terms of source-level variable and type names. To support performance analysis, the replay engine provides a faithful performance model of the original execution during replay
Linear modal instabilities around post-stall swept finite-aspect ratio wings at low Reynolds numbers
Linear modal instabilities of flow over finite-span untapered wings have been
investigated numerically at Reynolds number 400, at a range of angles of attack
and sweep on two wings having aspect ratios 4 and 8. Base flows have been
generated by direct numerical simulation, marching the unsteady incompressible
three-dimensional Navier-Stokes equations to a steady state, or using selective
frequency damping to obtain stationary linearly unstable flows. Unstable
three-dimensional linear global modes of swept wings have been identified for
the first time using spectral-element time-stepping solvers. The effect of the
wing geometry and flow parameters on these modes has been examined in detail.
An increase of the angle of attack was found to destabilize the flow, while an
increase of the sweep angle had the opposite effect. On unswept wings,
TriGlobal analysis revealed that the most unstable global mode peaks in the
midspan region of the wake; the peak of the mode structure moves towards the
tip as sweep is increased. Data-driven analysis was then employed to study the
effects of wing geometry and flow conditions on the nonlinear wake. On unswept
wings, the dominant mode at low angles of attack is a Kelvin-Helmholtz-like
instability, qualitatively analogous with global modes of infinite-span wings
under same conditions. At higher angles of attack and moderate sweep angles,
the dominant mode is a structure denominated the interaction mode. At high
sweep angles, this mode evolves into elongated streamwise vortices on higher
aspect ratio wings, while on shorter wings it becomes indistinguishable from
tip-vortex instability.Comment: 41 pages, 27 figure
Laminar post-stall wakes of tapered swept wings
While tapered swept wings are widely used, the influence of taper on their
post-stall wake characteristics remains largely unexplored. To address this
issue, we conduct an extensive study using direct numerical simulations to
characterize the wing taper and sweep effects on laminar separated wakes. We
analyze flows behind NACA 0015 cross-sectional profile wings at post-stall
angles of attack -- with taper ratios
--, leading edge sweep angles --, and semi
aspect ratios and at a mean-chord-based Reynolds number of .
Tapered wings have smaller tip chord length, which generates a weaker tip
vortex, and attenuates inboard downwash. This results in the development of
unsteadiness over a large portion of the wingspan at high angles of attack. For
tapered wings with backward-swept leading edges unsteadiness emerges near the
wing tip. On the other hand, wings with forward-swept trailing edges are shown
to concentrate wake shedding structures near the wing root. For highly swept
untapered wings, the wake is steady, while unsteady shedding vortices appear
near the tip for tapered wings with high leading edge sweep angles. For such
wings, larger wake oscillations emerge near the root as the taper ratio
decreases. While the combination of taper and sweep increases flow
unsteadiness, we find that tapered swept wings have more enhanced aerodynamic
performance than untapered and unswept wings, exhibiting higher time-averaged
lift and lift-to-drag ratio. The current findings shed light on the fundamental
aspects of flow separation over tapered wings in the absence of turbulent flow
effects
Remote attestation of SEV-SNP confidential VMs using e-vTPMs
Departing from "your data is safe with us" model where the cloud
infrastructure is trusted, cloud tenants are shifting towards a model in which
the cloud provider is not part of the trust domain. Both silicon and cloud
vendors are trying to address this shift by introducing confidential computing
- an umbrella term that provides mechanisms for protecting the data in-use
through encryption below the hardware boundary of the CPU, e.g., Intel Software
Guard Extensions (SGX), AMD secure encrypted virtualization (SEV), Intel trust
domain extensions (TDX), etc.
In this work, we design and implement a virtual trusted platform module
(vTPM) that virtualizes the hardware root-of-trust without requiring to trust
the cloud provider. To ensure the security of a vTPM in a provider-controlled
environment, we leverage unique isolation properties of the SEV-SNP hardware
and a novel approach to ephemeral TPM state management. Specifically, we
develop a stateless ephemeral vTPM that supports remote attestation without
persistent state. This allows us to pair each confidential VM with a private
instance of a vTPM that is completely isolated from the provider-controlled
environment and other VMs. We built our prototype entirely on open-source
components - Qemu, Linux, and Keylime. Though our work is AMD-specific, a
similar approach could be used to build remote attestation protocol on other
trusted execution environments (TEE).Comment: 12 pages, 4 figure
Learning to Solve Voxel Building Embodied Tasks from Pixels and Natural Language Instructions
The adoption of pre-trained language models to generate action plans for
embodied agents is a promising research strategy. However, execution of
instructions in real or simulated environments requires verification of the
feasibility of actions as well as their relevance to the completion of a goal.
We propose a new method that combines a language model and reinforcement
learning for the task of building objects in a Minecraft-like environment
according to the natural language instructions. Our method first generates a
set of consistently achievable sub-goals from the instructions and then
completes associated sub-tasks with a pre-trained RL policy. The proposed
method formed the RL baseline at the IGLU 2022 competition.Comment: 6 pages, 3 figure
Linear Instability Mechanisms over Three-Dimensional Wings at Low Reynolds Numbers
Identification of flow instabilities that cause a laminar flow to transition to turbulence is of great importance. Most instability studies of flows over lifting surfaces have focused on sim- plified models of laminar separation, which fail to address the essential three-dimensionality of the flow. The limited knowledge of linear instability mechanisms associated with three- dimensional separation on the wing surface and a lack of a deep understanding of the complex vortex dynamics arising from the wake instabilities behind a finite wing have motivated this work.
The present thesis documents the instability mechanisms and vortex dynamics of a range of finite wing planforms at low Reynolds numbers (100 ≤ Re ≤ 400) and high angles of attack. Global linear stability analysis is conducted on steady base flows to identify the leading unstable modes of the separated flow. A parametric study revealed the existence of three families of unstable global modes, denominated A, B and C, that manifest themselves in a range of geometrical configurations. The origin of wake unsteadiness is associated with unstable global mode A, which originates at the peak recirculation zone of the three- dimensional laminar separation bubble (LSB) formed on the wing. There is a mostly linear relationship between reversed flow and the leading mode amplification rate when the three-dimensionality of the LSB is low. Adjoint global modes and structural sensitivity were computed for informing future flow control. The wavemaker of the leading global eigenmode lies inside the LSB at the spanwise location of peak recirculation. Increasing Re leads to the structure of the wavemaker becoming more compact in the spanwise direction and more dependent on the top and bottom shear layers of the LSB.
The wake dynamics of the unsteady flow are analysed once the growth of the leading three-dimensional global mode has led the flow to nonlinear saturation. Several mechanisms (namely, some aspects of the LSB’s wing surface topology and the tip vortex meandering phenomenon) typically observed in turbulent flows at higher Re have been identified. This suggests that the undelaying physical mechanisms observed here may extend beyond the laminar regime and play a role in turbulent flows at higher Reynolds numbers. Analysis of the flow using data-driven techniques revealed the structure of the Wake mode associated with the higher frequency shedding of predominantly spanwise vortices near midspan and the Interaction mode responsible for the shedding of spanwise and braid vortices on the outboard section of the wing. Differences in the frequency between these modes manifest in the appearance of vortex dislocations. Present results establish a basis for understanding flow dynamics and instabilities on
finite three-dimensional wings at low Reynolds numbers as a first step towards understand- ing turbulent flow at higher Reynolds numbers