We meet weekly monthly to discuss papers from A* system conferences: OSDI, SOSP, Eurosys, FAST, ATC, ASPLOS, POPL, PLDI, NSDI, SIGCOMM, MobiSys, and MobiCom.

Logistics

  • When: Fridays 1-2:30pm IST TBD
  • Where: SIT 113 or virtually in MS Teams.

If you are having trouble joining the Team from the link above, please fill this form, and we will add you. Joining Teams will also let you access any previous talk recordings, and you will get notified about the upcoming talks. Otherwise, you may directly join the meeting by clicking on the “Talk link” in the “Schedule” below.

How can I give a talk?

  • Let Abhilash know when you’d like to give a talk.

Honor code

If you will be unable to give the talk in your committed slot due to unforeseen emergency, let Abhilash or Sorav know ASAP.

Schedule

  • [NSDI 2014] MICA: a holistic approach to fast in-memory key-value storage
    When: 1-2:30 pm, 22 Jul 2022 Lead: Saurav Bharti Talk recording
    Abstract MICA is a scalable in-memory key-value store that handles 65.6 to 76.9 million key-value operations per second using a single general-purpose multi-core system. MICA is over 4-13.5x faster than current state-of-the-art systems, while providing consistently high throughput over a variety of mixed read and write workloads. MICA takes a holistic approach that encompasses all aspects of request handling, including parallel data access, network request handling, and data structure design, but makes unconventional choices in each of the three domains. First, MICA optimizes for multi-core architectures by enabling parallel access to partitioned data. Second, for efficient parallel data access, MICA maps client requests directly to specific CPU cores at the server NIC level by using client-supplied information and adopts a light-weight networking stack that bypasses the kernel. Finally, MICA's new data structures--circular logs, lossy concurrent hash indexes, and bulk chaining--handle both read-and write-intensive workloads at low overhead.

  • [PLDI 2022] Progressive polynomial approximations for fast correctly rounded math libraries
    When: 1-2:30 pm, 15 Jul 2022 Lead: Sorav Bansal Talk slides Talk recording
    Abstract This paper presents a novel method for generating a single polynomial approximation that produces correctly rounded results for all inputs of an elementary function for multiple representations. The generated polynomial approximation has the nice property that the first few lower degree terms produce correctly rounded results for specific representations of smaller bitwidths, which we call progressive performance. To generate such progressive polynomial approximations, we approximate the correctly rounded result and formulate the computation of correctly rounded polynomial approximations as a linear program similar to our prior work on the RLIBM project. To enable the use of resulting polynomial approximations in mainstream libraries, we want to avoid piecewise polynomials with large lookup tables. We observe that the problem of computing polynomial approximations for elementary functions is a linear programming problem in low dimensions, i.e., with a small number of unknowns. We design a fast randomized algorithm for computing polynomial approximations with progressive performance. Our method produces correct and fast polynomials that require a small amount of storage. A few polynomial approximations from our prototype have already been incorporated into LLVM’s math library.

  • [SOSP 2021] Syrup: User-Defined Scheduling Across the Stack
    When: 1-2:30 pm, 8 Jul 2022 Lead: Rijurekha Sen Talk slides Talk recording
    Abstract Suboptimal scheduling decisions in operating systems, networking stacks, and application runtimes are often responsible for poor application performance, including higher latency and lower throughput. These poor decisions stem from a lack of insight into the applications and requests the scheduler is handling and a lack of coherence and coordination between the various layers of the stack, including NICs, kernels, and applications. We propose Syrup, a framework for user-defined scheduling. Syrup enables untrusted application developers to express application-specific scheduling policies across these system layers without being burdened with the low-level system mechanisms that implement them. Application developers write a scheduling policy with Syrup as a set of matching functions between inputs (threads, network packets, network connections) and executors (cores, network sockets, NIC queues) and then deploy it across system layers without modifying their code. Syrup supports multi-tenancy as multiple co-located applications can each safely and securely specify a custom policy. We present several examples of uses of Syrup to define application and workload-specific scheduling policies in a few lines of code, deploy them across the stack, and improve performance up to 8x compared with default policies.

  • [SOSP 2021] Snowboard: Finding Kernel Concurrency Bugs through Systematic Inter-thread Communication Analysis
    When: 1-2:30 pm, 1 Jul 2022 Lead: Subodh Vishnu Sharma Talk slides Talk recording
    Abstract Kernel concurrency bugs are challenging to find because they depend on very specific thread interleavings and test inputs. While separately exploring kernel thread interleavings or test inputs has been closely examined, jointly exploring interleavings and test inputs has received little attention, in part due to the resulting vast search space. Using precious, limited testing resources to explore this search space and execute just the right concurrent tests in the proper order is critical. This paper proposes Snowboard a testing framework that generates and executes concurrent tests by intelligently exploring thread interleavings and test inputs jointly. The design of Snowboard is based on a concept called potential memory communication (PMC), a guess about pairs of tests that, when executed concurrently, are likely to perform memory accesses to shared addresses, which in turn may trigger concurrency bugs. To identify PMCs, Snowboard runs tests sequentially from a fixed initial kernel state, collecting their memory accesses. It then pairs up tests that write and read the same region into candidate concurrent tests. It executes those tests using the associated PMC as a scheduling hint to focus interleaving search only on those schedules that directly affect the relevant memory accesses. By clustering candidate tests on various features of their PMCs, Snowboard avoids testing similar behaviors, which would be inefficient. Finally, by executing tests from small clusters first, it prioritizes uncommon suspicious behaviors that may have received less scrutiny. Snowboard discovered 14 new concurrency bugs in Linux kernels 5.3.10 and 5.12-rc3, of which 12 have been confirmed by developers. Six of these bugs cause kernel panics and filesystem errors, and at least two have existed in the kernel for many years, showing that this approach can uncover hard-to-find, critical bugs. Furthermore, we show that covering as many distinct pairs of uncommon read/write instructions as possible is the test-prioritization strategy with the highest bug yield for a given test-time budget.

  • [PLDI 2022] Lasagne: a static binary translator for weak memory model architectures
    When: 1-2:30 pm, 24 Jun 2022 Lead: Divyanjali Talk slides Talk recording
    Abstract The emergence of new architectures create a recurring challenge to ensure that existing programs still work on them. Manually porting legacy code is often impractical. Static binary translation (SBT) is a process where a program's binary is automatically translated from one architecture to another, while preserving their original semantics. However, these SBT tools have limited support to various advanced architectural features. Importantly, they are currently unable to translate concurrent binaries. The main challenge arises from the mismatches of the memory consistency model specified by the different architectures, especially when porting existing binaries to a weak memory model architecture. In this paper, we propose Lasagne, an end-to-end static binary translator with precise translation rules between x86 and Arm concurrency semantics. First, we propose a concurrency model for Lasagne's intermediate representation (IR) and formally proved mappings between the IR and the two architectures. The memory ordering is preserved by introducing fences in the translated code. Finally, we propose optimizations focused on raising the level of abstraction of memory address calculations and reducing the number of fences. Our evaluation shows that Lasagne reduces the number of fences by up to about 65%, with an average reduction of 45.5%, significantly reducing their runtime overhead.

  • [SOSP 2021] Log-structured Protocols in Delos
    When: 1-2:30 pm, 10 Jun 2022 Lead: Ramita Sardana Talk slides Talk recording
    Abstract Developers have access to a wide range of storage APIs and functionality in large-scale systems, such as relational databases, key-value stores, and namespaces. However, this diversity comes at a cost: each API is implemented by a complex distributed system that is difficult to develop and operate. Delos amortizes this cost by enabling different APIs on a shared codebase and operational platform. The primary innovation in Delos is a log-structured protocol: a fine-grained replicated state machine executing above a shared log that can be layered into reusable protocol stacks under different databases. We built and deployed two production databases using Delos at Facebook, creating nine different log-structured protocols in the process. We show via experiments and production data that log-structured protocols impose low overhead, while allowing optimizations that can improve latency by up to 100X (e.g., via leasing) and throughput by up to 2X (e.g., via batching).

  • [ASPLOS 2019] MVEDSUA: Higher Availability Dynamic Software Updates via Multi-Version Execution
    When: 1-2:30 pm, 3 Jun 2022 Lead: Vishal Singh Talk slides Talk recording
    Abstract Dynamic Software Updating (DSU) is a technique for patching stateful software without shutting it down, which enables both timely updates and non-stop service. Unfortunately, bugs in the update itself—whether in the changed code or in the way the change is introduced dynamically—may cause the updated software to crash or misbehave. Furthermore, the time taken to dynamically apply the update may be unacceptable if it introduces a long delay in service. This paper makes the key observation that both problems can be addressed by employing Multi-Version Execution (MVE). To avoid delay in service, the update is applied to a forked copy while the original system continues to operate. Once the update completes, the MVE system monitors that the responses of both versions agree for the same inputs. Expected divergences are specified by the programmer using an MVE-specific DSL. Unexpected divergences signal possible errors and roll back the update, which simply means terminating the updated version and reverting to the original version. This is safe because the MVE system keeps the state of both versions in sync. If the new version shows no problems after a warmup period, operators can make it permanent and discard the original version. We have implemented this approach, which we call Mvedsua,1 by extending the Kitsune DSU framework with Varan, a state-of-the-art MVE system. We have used Mvedsua to update several high-performance servers: Redis, Memcached, and Vsftpd. Our results show that Mvedsua significantly reduces the update-time delay, imposes little overhead in steady state, and easily recovers from a variety of update related errors.

  • [OSDI 2020] Virtual Consensus in Delos
    When: 1-2:30 pm, 27 May 2022 Lead: Prashant Agrawal Talk slides Talk recording
    Abstract Consensus-based replicated systems are complex, monolithic, and difficult to upgrade once deployed. As a result, deployed systems do not benefit from innovative research, and new consensus protocols rarely reach production. We propose virtualizing consensus by virtualizing the shared log API, allowing services to change consensus protocols without downtime. Virtualization splits the logic of consensus into the VirtualLog, a generic and reusable reconfiguration layer; and pluggable ordering protocols called Loglets. Loglets are simple, since they do not need to support reconfiguration or leader election; diverse, consisting of different protocols, codebases, and even deployment modes; and composable, via RAID-like stacking and striping. We describe a production database called Delos1 which leverages virtual consensus for rapid, incremental development and deployment. Delos reached production within 8 months, and 4 months later upgraded its consensus protocol without downtime for a 10X latency improvement. Delos can dynamically change its performance properties by changing consensus protocols: we can scale throughput by up to 10X by switching to a disaggregated Loglet, and double the failure threshold of an instance without sacrificing throughput via a striped Loglet.

  • [PLDI 2020] Predictable Accelerator Design with Time-Sensitive Affine Types
    When: 1-2:30 pm, 20 May 2022 Lead: Madhukar Yerraguntla Talk slides Talk recording
    Abstract Field-programmable gate arrays (FPGAs) provide an opportunity to co-design applications with hardware accelerators, yet they remain difficult to program. High-level synthesis (HLS) tools promise to raise the level of abstraction by compiling C or C++ to accelerator designs. Repurposing legacy software languages, however, requires complex heuristics to map imperative code onto hardware structures. We find that the black-box heuristics in HLS can be unpredictable: changing parameters in the program that should improve performance can counterintuitively yield slower and larger designs. This paper proposes a type system that restricts HLS to programs that can predictably compile to hardware accelerators. The key idea is to model consumable hardware resources with a time-sensitive affine type system that prevents simultaneous uses of the same hardware structure. We implement the type system in Dahlia, a language that compiles to HLS C++, and show that it can reduce the size of HLS parameter spaces while accepting Pareto-optimal designs.

  • [ASPLOS 2020] Learning-based Memory Allocation for C++ Server Workloads
    When: 1-2:30 pm, 13 May 2022 Lead: Ashish Panwar Talk slides Talk recording
    Abstract Modern C++ servers have memory footprints that vary widely over time, causing persistent heap fragmentation of up to 2x from long-lived objects allocated during peak memory usage. This fragmentation is exacerbated by the use of huge (2MB) pages, a requirement for high performance on large heap sizes. Reducing fragmentation automatically is challenging because C++ memory managers cannot move objects. This paper presents a new approach to huge page fragmentation. It combines modern machine learning techniques with a novel memory manager (LLAMA) that manages the heap based on object lifetimes and huge pages (divided into blocks and lines). A neural network-based language model predicts lifetime classes using symbolized calling contexts. The model learns context-sensitive per-allocation site lifetimes from previous runs, generalizes over different binary versions, and extrapolates from samples to unobserved calling contexts. Instead of size classes, LLAMA's heap is organized by lifetime classes that are dynamically adjusted based on observed behavior at a block granularity. LLAMA reduces memory fragmentation by up to 78% while only using huge pages on several production servers. We address ML-specific questions such as tolerating mispredictions and amortizing expensive predictions across application execution. Although our results focus on memory allocation, the questions we identify apply to other system-level problems with strict latency and resource requirements where machine learning could be applied.

  • [OSDI 2020] Theseus: an Experiment in Operating System Structure and State Management
    When: 1-2:30 pm, 06 May 2022 Lead: Indrajit Banerjee Talk slides Talk recording
    Abstract This paper describes an operating system (OS) called Theseus. Theseus is the result of multi-year experimentation to redesign and improve OS modularity by reducing the states one component holds for another, and to leverage a safe programming language, namely Rust, to shift as many OS responsibilities as possible to the compiler. Theseus embodies two primary contributions. First, an OS structure in which many tiny components with clearly-defined, runtime-persistent bounds interact without holding states for each other. Second, an intralingual approach that realizes the OS itself using language-level mechanisms such that the compiler can enforce invariants about OS semantics. Theseus's structure, intralingual design, and state management realize live evolution and fault recovery for core OS components in ways beyond that of existing works.

  • [PLDI 2018] iReplayer: In-situ and Identical Record-and-Replay for Multithreaded Applications
    When: 1-2:30 pm, 29 Apr 2022 Lead: Abhilash Jindal Talk slides Talk recording
    Abstract Reproducing executions of multithreaded programs is very challenging due to many intrinsic and external non-deterministic factors. Existing RnR systems achieve significant progress in terms of performance overhead, but none targets the in-situ setting, in which replay occurs within the same process as the recording process. Also, most existing work cannot achieve identical replay, which may prevent the reproduction of some errors. This paper presents iReplayer, which aims to identically replay multithreaded programs in the original process (under the "in-situ" setting). The novel in-situ and identical replay of iReplayer makes it more likely to reproduce errors, and allows it to directly employ debugging mechanisms (e.g. watchpoints) to aid failure diagnosis. Currently, iReplayer only incurs 3% performance overhead on average, which allows it to be always enabled in the production environment. iReplayer enables a range of possibilities, and this paper presents three examples: two automatic tools for detecting buffer overflows and use-after-free bugs, and one interactive debugging tool that is integrated with GDB.

  • [SOSP 2017] Optimizing Big-Data Queries Using Program Synthesis
    When: 1-2:30 pm, 22 Apr 2022 Lead: Aditya Senthilnathan Talk slides Talk recording
    Abstract Classical query optimization relies on a predefined set of rewrite rules to re-order and substitute SQL operators at a logical level. This paper proposes Blitz, a system that can synthesize efficient query-specific operators using automated program reasoning. Blitz uses static analysis to identify sub-queries as potential targets for optimization. For each sub-query, it constructs a template that defines a large space of possible operator implementations, all restricted to have linear time and space complexity. Blitz then employs program synthesis to instantiate the template and obtain a data-parallel operator implementation that is functionally equivalent to the original sub-query up to a bound on the input size. Program synthesis is an undecidable problem in general and often difficult to scale, even for bounded inputs. Blitz therefore uses a series of analyses to judiciously use program synthesis and incrementally construct complex operators. We integrated Blitz with existing big-data query languages by embedding the synthesized operators back into the query as User Defined Operators. We evaluated Blitz on several production queries from Microsoft running on two state-of-the-art query engines: SparkSQL as well as Scope, the big-data engine of Microsoft. Blitz produces correct optimizations despite the synthesis being bounded. The resulting queries have much more succinct query plans and demonstrate significant performance improvements on both big-data systems (1.3x --- 4.7x).

  • [ASPLOS 2022] Understanding and Exploiting Optimal Function Inlining
    When: 1-2:30 pm, 15 Apr 2022 Lead: Sorav Bansal Talk slides Talk recording
    Abstract Inlining is a core transformation in optimizing compilers. It replaces a function call (call site) with the body of the called function (callee). It helps reduce function call overhead and binary size, and more importantly, enables other optimizations. The problem of inlining has been extensively studied, but it is far from being solved; predicting which inlining decisions are beneficial is nontrivial due to interactions with the rest of the compiler pipeline. Previous work has mainly focused on designing heuristics for better inlining decisions and has not investigated optimal inlining, i.e., exhaustively finding the optimal inlining decisions. Optimal inlining is necessary for identifying and exploiting missed opportunities and evaluating the state of the art. This paper fills this gap through an extensive empirical analysis of optimal inlining using the SPEC2017 benchmark suite. Our novel formulation drastically reduces the inlining search space size (from 2^349 down to 2^25) and allows us to exhaustively evaluate all inlining choices on 1,135 SPEC2017 files. We show a significant gap between the state-of-the-art strategy in LLVM and optimal inlining when optimizing for binary size, an important, deterministic metric independent of workload (in contrast to performance, another important metric). Inspired by our analysis, we introduce a simple, effective autotuning strategy for inlining that outperforms the state of the art by 7% on average (and up to 28%) on SPEC2017, 15% on the source code of LLVM itself, and 10% on the source code of SQLite. This work highlights the importance of exploring optimal inlining by providing new, actionable insight and an effective autotuning strategy that is of practical utility.