Virtual memory | EPFL Graph Search

In computing, virtual memory, or virtual storage, is a memory management technique that provides an "idealized abstraction of the storage resources that are actually available on a given machine" which "creates the illusion to users of a very large (main) memory". The computer's operating system, using a combination of hardware and software, maps memory addresses used by a program, called virtual addresses, into physical addresses in computer memory. Main storage, as seen by a process or task, appears as a contiguous address space or collection of contiguous segments. The operating system manages virtual address spaces and the assignment of real memory to virtual memory. Address translation hardware in the CPU, often referred to as a memory management unit (MMU), automatically translates virtual addresses to physical addresses. Software within the operating system may extend these capabilities, utilizing, e.g., disk storage, to provide a virtual address space that can exceed the cap

Dynamic Resource Allocation for Database Servers Running on Virtual Storage

Jin Chen, Daniel Lupei, Adrian Daniel Popescu

We introduce a novel multi-resource allocator to dynamically allocate resources for database servers running on virtual storage. Multi-resource allocation involves proportioning the database and storage server caches, and the storage bandwidth between applications according to overall performance goals. The problem is challenging due to the interplay between different resources, e.g., changing any cache quota affects the access pattern at the cache/disk levels below it in the storage hierarchy. We use a combination of on-line modeling and sampling to arrive at near-optimal configurations within minutes. The key idea is to incorporate access tracking and known resource dependencies e.g., due to cache replacement policies, into our performance model. In our experimental evaluation, we use both microbenchmarks and the industry standard benchmarks TPC-W and TPC-C. We show that our multi-resource allocation approach improves application performance by up to factors of 2.9 and 2.4 compared to state-of-the-art single-resource controllers, and their ad-hoc combination, respectively.

2009

Rethinking Software Runtimes for Disaggregated Memory

Sanidhya Kashyap, Ivan Puddu

Disaggregated memory can address resource provisioning inefficiencies in current datacenters. Multiple software runtimes for disaggregated memory have been proposed in an attempt to make disaggregated memory practical. These systems rely on the virtual memory subsystem to transparently offer disaggregated memory to applications using a local memory abstraction. Unfortunately, using virtual memory for disaggregation has multiple limitations, including high overhead that comes from the use of page faults to identify what data to fetch and cache locally, and high dirty data amplification that comes from the use of page-granularity for tracking changes to the cached data (4KB or higher). In this paper, we propose a fundamentally new approach to designing software runtimes for disaggregated memory that addresses these limitations. Our main observation is that we can use cache coherence instead of virtual memory for tracking applications' memory accesses transparently, at cache-line granularity. This simple idea (1) eliminates page faults from the application critical path when accessing remote data, and (2) decouples the application memory access tracking from the virtual memory page size, enabling cache-line granularity dirty data tracking and eviction. Using this observation, we implemented a new software runtime for disaggregated memory that improves average memory access time by 1.7-5X and reduces dirty data amplification by 2-10X, compared to state-of-the-art systems.

ASSOC COMPUTING MACHINERY2021

Unifying software and hardware of multithreaded reconfigurable applications within operating system processes

Miljan Vuletic

Novel reconfigurable System-on-Chip (SoC) devices offer combining software with application-specific hardware accelerators to speed up applications. However, by mixing user software and user hardware, principal programming abstractions and system-software commodities are usually lost, since hardware accelerators (1) do not have execution context —it is typically the programmer who is supposed to provide it, for each accelerator, (2) do not have virtual memory abstraction —it is again programmer who shall communicate data from user software space to user hardware, even if it is usually burdensome (or sometimes impossible!), (3) cannot invoke system services (e.g., to allocate memory, open files, communicate), and (4) are not easily portable —they depend mostly on system-level interfacing, although they logically belong to the application level. We introduce a unified Operating System (OS) process for codesigned reconfigurable applications that provides (1) unified memory abstraction for software and hardware application parts, (2) execution transfers from software to hardware and vice versa, thus enabling hardware accelerators to use systems services and callback other software and hardware functions, and (3) multithreaded execution of multiple software and hardware threads. The unified OS process ensures portability of codesigned applications, by providing standardised means of interfacing. Having just-another abstraction layer usually affects performance: we show that the runtime optimisations in the system layer supporting the unified OS process can minimise the performance loss and even outperform typical approaches. The unified OS process also fosters unrestricted automated synthesis of software to hardware, thus allowing unlimited migration of application components. We demonstrate the advantages of the unified OS process in practice, for Linux systems running on Xilinx Virtex-II Pro and Altera Excalibur reconfigurable devices.

EPFL2006

Unifying software and hardware of multithreaded reconfigurable applications within operating system processes

Miljan Vuletic

EPFL2006