Team, Visitors, External Collaborators
Overall Objectives
Research Program
Application Domains
Highlights of the Year
New Software and Platforms
New Results
Bilateral Contracts and Grants with Industry
Partnerships and Cooperations
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Section: New Results

Support for multicore machines

The complexity of computer architectures has risen since the early years of the Linux kernel: Simultaneous Multi-Threading (SMT), multicore processing, and frequency scaling with complex algorithms such as Intel Turbo Boost have all become omnipresent. In order to keep up with hardware innovations, the Linux scheduler has been rewritten several times, and many hardware-related heuristics have been added. Despite this, we have shown in a PLOS paper [16] that a fundamental problem was never identified: the POSIX process creation model, i.e., fork/wait, can behave inefficiently on current multicore architectures due to frequency scaling. We investigate this issue through a simple case study: the compilation of the Linux kernel source tree. To do this, we have developed SchedLog, a low-overhead scheduler tracing tool, and SchedDisplay, a scriptable tool to graphically analyze SchedLog's traces efficiently. We implement two solutions to the problem at the sched-uler level which improve the speed of compiling part of the Linux kernel by up to 26%, and the whole kernel by up to 10%.

In an Eurosys paperĀ [15], we address the problem of efficiently virtualizing NUMA architectures. The major challenge comes from the fact that the hypervisor regularly reconfigures the placement of a virtual machine (VM) over the NUMA topology. However, neither guest operating systems (OSes) nor system runtime libraries (e.g., Hotspot) are designed to consider NUMA topology changes at runtime, leading end user applications to unpredictable performance. We present eXtended Para-Virtualization (XPV), a new principle to efficiently virtualize a NUMA architecture. XPV consists in revisiting the interface between the hypervisor and the guest OS, and between the guest OS and system runtime libraries (SRL) so that they can dynamically take into account NUMA topology changes. We introduce a methodology for systematically adapting legacy hypervisors, OSes, and SRLs. We have applied our approach with less than 2k line of codes in two legacy hypervisors (Xen and KVM), two legacy guest OSes (Linux and FreeBSD), and three legacy SRLs (Hotspot, TCMalloc, and jemalloc). The evaluation results showed that XPV outperforms all existing solutions by up to 304%.

Memory interferences may introduce important slowdowns in applications running on COTS multi-core processors. They are caused by concurrent accesses to shared hardware resources of the memory system. The induced delays are difficult to predict, making memory interferences a major obstacle to the adoption of COTS multi-core processors in real-time systems. In an RTSS paper[18], we propose an experimental characterization of applications' memory consumption to determine their sensitivity to memory interferences. Thanks to a new set of microbenchmarks, we show the lack of precision of a purely quantitative characterization. To improve accuracy, we define new metrics quantifying qualitative aspects of memory consumption and implement a profiling tool using the VALGRIND framework. In addition, our profiling tool produces high resolution profiles allowing us to clearly distinguish the various phases in applications' behavior. Using our microbenchmarks and our new characterization, we train a state-of-the-art regressor. The validation on applications from the MIBENCH and the PARSEC suites indicates significant gain in prediction accuracy compared to a purely quantitative characterization.