XPRESS

XPRESS
Team Members	SNL, IU, LBNL, LSU, ORNL, UNC/RENCI, UH, UO
PI	Ron Brightwell (SNL)
Chief Scientist	Thomas Sterling (IU)
Co-PIs	Andrew Lumsdaine (IU), Hartmut Kaiser (LSU), Barbara Chapman (UH), Allen Maloney (UO), Chris Baker (ORNL), Allan Porterfield (UNC/RENCI), Alice Koniges (LBNL)
Website	http://xstack.sandia.gov/xpress

eXascale Programming Environment and System Software or XPRESS

Team Members

• Sandia National Laboratories (SNL): LXK operating system, OpenX software architecture, Applications
• Indiana University (IU): Parallex Execution Model, OpenX software architecture, HPX‐4 runtime system, XPI
• Lawrence Berkeley National Laboratory (LBNL): Applications
• Louisiana State University (LSU): HPX‐4 runtime system
• Oak Ridge National Laboratory (ORNL): Applications
• University of Houston (UH): Application migration
• University of North Carolina at Chapel Hill/RENCI (UNC/RENCI): XPI, HPX‐4, APEX
• University of Oregon (UO): Performance instrumentation (APEX)

Products

This document lists all of the products for the XPRESS project.

• Publications
  • A complete list of publications is available here.

• Software
  • A list of software for this project is available here.

The OpenX Software Architecture

Goals, Objectives, and Approach

• Goals:
  • Enable exascale performance capability for current and future DOE applications
  • Develop and deliver a practical computing system software X‐stack, “OpenX”, for future practical DOE computing systems
  • Provide programming methods, environments, languages, and tools for effective means of expressing application and system software for portable exascale system execution

• Objectives:
  • Derive a dynamic adaptive introspective strategy for exploiting opportunities and addressing critical exascale technology challenges in the form of an abstract execution model
  • Devise a software architecture as a framework for future exascale system design and implementation
  • Implement core interrelated and interoperable components of the software architecture to realize a fully working and usable system
  • Test, evaluate, validate, and demonstrate correctness, performance, resiliency, and energy efficiency
  • Provide technology transfer through cooperative engagement of industry hardware and software vendors and national labs via documentation and training

• Approach:
  • Research, develop, and deploy a software stack to exploit the ParalleX execution model

Software Stack

ParalleX Execution Model

• An execution model to provide the governing principles of computation to guide the system co-design and interoperability of software component layers and portability across system classes

• Goal is to provide conceptual foundation to dramatically increase efficiency and scalability through transition from static to dynamic resource management and task scheduling and exploitation of new sources of parallelism

• Key semantic constructs
  • Active Global Address Space (“AGAS”) for single system image
  • First class lightweight user threads for medium­‐gain parallelism
  • “Parcels” message­‐driven computing for latency mitigation
  • Local Control Objects (“LCO”) for powerful system synchronization

• Performance strategy
  • Scalability through lightweight thread level parallelism, overlapping successive phases of computation with powerful synchronization and elimination of global barriers, automatic exposure/exploitation of intrinsic meta‐data parallelism, effective use of finer grain parallelism through reduction of overhead that bounds granularity
  • Latency mitigation through parcels by reducing numbers of long distance actions (split­‐phase transactions), localizing remote data and work, migrating continuations to change locus of continued execution with data structure, lightweight thread context switching, and direct in­‐memory and parcel generation without thread instantiation, and locality semantics
  • Overhead reduction is derived by powerful semantics of synchronization for minimum work, optimized thread control
  • Contention amelioration through dynamic resource management

LXK: Lightweight eXascale Kernel

• Based on Sandia’s Kitten lightweight kernel
• Boots idenDcally to Linux
  • Repurposes basic Linux funcDonality (PCI, NUMA, ACPI, etc.)
  • Supports POSIX threads (NPTL) and OpenMP
  • Allows innovaDon in key areas
    • Memory management
    • Multicore messaging
    • Network stack optimizations
    • Fully tick‐less operation
  • 20K LOC

• Allows for re‐thinking OS structure and implementation of
  • Lightweight asynchronous system services
  • Dynamic composability and modularity
  • Adaptive resource policy enforcement mechanisms
  • Interface to runtime system(s)
  • Integrated instrumentation and monitoring

Kitten Implementation

• Monolithic, C code, GNU toolchain, Kbuild configuration
• Supports x86­‐64 architecture only, considering port to ARM
  • Boots on standard PC architecture, Cray XT, and in virtual machines
  • Boots identically to Linux (Kitten bzImage and init_task)

• Repurposes basic functionality from Linux
  • Hardware bootstrap
  • Basic OS kernel primitives (lists, locks, wait queues, etc.)
  • PCI, NUMA, ACPI, IOMMU, …
  • Directory structure similar to Linux, arch dependent/independent directories

• Custom address space management and task management
  • User‐level API for managing physical memory, building virtual address spaces
  • User‐level API for creating tasks, which run in virtual address spaces
  • User‐level API for migrating tasks between cores

Kitten Thread Support

• Kitten user‐applications link with standard GNU C library (Glibc) and other system libraries installed on the Linux build host

• Functionality added to Kitten to support Glibc NPTL POSIX threads implementation
  • Futex() system call (fast user‐level locking)
  • Basic support for signals
  • Match Linux implementation of thread local storage
  • Support for multiple threads per CPU core, preemptively scheduled

• Kitten supports runtimes that work on top of POSIX threads
  • Glibc GOMP OpenMP implementation
  • Sandia Qthreads
  • Probably others with a little effort

HPX Runtime System

• A next‐generation runtime system software layer that supports the semantics of the ParalleX execution model for significant increase in efficiency and scalability
• HPX‐3 provides
  • Existing early proof‐of‐concept software dynamic adaptive resource management, task scheduling, global name space, efficient powerful synchronization, and Parcel message­‐driven computation. Interfaces with conventional Unix­‐like OS.
  • HPX­‐5
  • Modular system software architecture with specified functionality, interfaces and protocols for intra­‐operability and interfaces to OS and programming environment
  • Introspection closed­‐loop system for
    • Resiliency through microcheckpointing
    • Dynamic load balancing
    • Power monitoring and control

XPI: Low‐Level Intermediate Form

• User programming syntax and source‐to‐source compiler target for high‐level programming languages
• Provides stable application plakorm based on HPX, which is expected to change underneath throughout project
• A library of C­‐bindings to represent lowest­‐level semantics, policies, and mechanisms for ParalleX execution model
• XPI construct classes
  • Process
  • Thread
  • Locality
  • Parcel
  • Future
  • Dataflow
  • Housekeeping

XPRESS Information Flow

• Passing information between layers will be critical
  • In current systems information flows one direction

• For Exascale static scheduling decisions will not work
  • Dynamic environment
    • Billion‐way parallelism
    • Resilience
    • Reliability
    • Energy
    • Shared resource contention

• Feedback will be required

Performance Information as Glue

• Performance informaDon
  • Current: post‐execution performance tools
  • Exascale: dynamic application introspection

• For performance and reliability thread/core/node/system knowledge will be critical throughout the software stack
  • Interfaces designed to enable information flow
    • Utilities need to know current system performance
    • Utilities need to know how other utilities are reacting

Exascale Performance Observability

• Exascale requires a fundamentally new observability paradigm
  • Reflects translation of application and mapping of computation model to execution model
  • Designed specifically to support introspection of runtime performance for adaptation and control
  • Aware of multiple objectives
    • System‐level resource utilization data and analysis, energy consumption, and health information

• Exascale observability abstraction
  • Inherent state of exascale execution is dynamic
  • Embodies non‐stationarity of performance, energy, resilience during application execution
  • Constantly shaped by the adaptation of resources to meet computational needs and optimize execution objectives

APEX

• XPRESS performance measurement/analysis/introspection
  • Observation and runtime analysis of performance, energy, and reliability
  • Online introspection resource utilization, energy consumption, and health information
  • Coupling of introspection with OpenX software stack for self­‐adaptive control

• APEX: Autonomic Performance Environment for eXascale
  • Support performance awareness and performance reactivity
  • Couple with application and execution knowledge
  • Serve top­‐down AND bottom‐up requirements of OpenX
  • Performance overlay on OpenX

APEX’s Role for Top­‐Down OpenX Requirements

• Top-­‐down requirements are driven by:
  • Mapping of applications to the ParalleX model
  • Translation through the programming models and the language compilers into runtime operations and execution
  • Performance abstractions (PA) at each level define:
    • Set of parameters to be observed by the next levels down
    • Performance model to be evaluated and provide basis for control
  • Performance abstractions are coupled with observations through APEX’s hierarchical performance framework
  • Realization of control mechanisms (reactive to PA actualization)

• Top­‐down view sees APEX functionality as part of the application’s execution, specialized with observability and introspection support built into each OpenX layer:
  • LXK OS – system resource, utilization, job contention, overhead
  • HPX – threads, queues, concurrency, remote, parcels, memory
  • XPI/Legacy – language‐level performance semantics

APEX’s Role for Bottom­‐Up OpenX Requirements

• Bottom­‐up requirements are driven by:
  • Performance introspection across the OpenX layers
  • Enable dynamic, adaptive operation, and decision control
  • Online access and analysis of observations at different levels
  • Working model is multi‐parameter system optimization

• APEX creates the performance feedback mechanisms and builds an efficient hierarchical infrastructure for connecting subscribers to runtime performance state
  • Intra‐level performance awareness for HPX and LXK
  • Interplay with the overall application dynamics (top­‐down)
    • Top­‐down requirement implement constraints
  • Performance information is the result of runtime analysis

Top­‐Down Development Approach

• Define performance abstraction (focusing on higher level)
  • Specify observability requirements, and results semantics
  • Provide application context for association
  • What are the performance models, attributes, factors?

• Build into legacy languages and XPI
  • APEX programming of PA infrastructure
  • Invokes HPX performance measurement API

• Integrate TAU capabilities for APEX realization
  • Instrumentation
    • legacy programming (MPI, OpenMP, OpenACC)
    • imperative API (XPI) for ParalleX programming
  • TAU mapping for measurement contextualization
  • Wrapper interposition to intercept HPX runtime layer

• Create introspection API for feedback

Legacy Application Migration and Interoperability

• Seamless migration, no modification needed for apps
  • Retargeting OpenMP compiler to OpenX
  • Adapting MPI to OpenX, need MPI system‐level adjustment, and change of configuration logic

• Two approaches for retargeting OpenMP via XPI:
  • Through a POSIX compliant interface using XPI, e.g. pthreads on top of XPI
    • +: No (or minor) modification needed to OpenMP compiler
    • -­: May only use a limited subset of OpenX features
  • Rewrite OpenMP compiler and runtime to XPI
    • +: Explore OpenX advanced features

• OpenACC integration with OpenX
  • Adapt the OpenACC data movement mechanism to OpenX communication parcels in the AGAS
  • Integrate accelerator kernel execution of OpenACC with the OpenX execution model

• Evaluation:
  • Applications: Mini­‐apps with MPI+OpenMP/OpenACC
  • Performance and productivity feedback to the OpenX implementation team