Ottawa Linux Symposium (OLS) Papers for 2014:

## Btr-Diff: An Innovative Approach to Differentiate BtrFs Snapshots - N. Mandliwala, S. Pimpale, N. P. Singh, G. Phatangare

Efficient storage and fast retrieval of data has always been of utmost importance. The BtrFs file system is a copy-on-write (COW) based B-tree file system that has an built-in support for snapshots and is considered a potential replacement for the EXT4 file system. It is designed specifically to address the need to scale, manage and administer large storage configurations of Linux systems. Snapshots are useful to have local online copies'' of the file system that can be referred back to, or to implement a form of deduplication, or for taking a full backup of the file system. The ability to compare these snapshots becomes crucial for system administrators as well as end users.

The existing snapshot management tools perform directory based comparison on block level in user space. This approach is generic and is not suitable for B-tree based file systems that are designed to cater to large storage. Simply traversing the directory structure is slow and only gets slower as the file system grows. With the BtrFs send/receive mechanism, the filesystem can be instructed to calculate the set of changes made between the two snapshots and serialize them to a file.

Our objective is to leverage the send part in the kernel to implement a new mechanism to list all the files that have been added, removed, changed or had their metadata changed in some way. The proposed solution takes advantage of the BtrFs B-tree structure and its powerful snapshot capabilities to speed up the tree traversal and detect changes in snapshots based on inode values. In addition, our tool can also detect changes between a snapshot and an explicitly mentioned parent. This lends itself for daily incremental backups of the file system, and can very easily be integrated with existing snapshot management tools.

## Leveraging MPST in Linux with Application Guidance to Achieve Power and Performance Goals - M.R. Jantz, K.A. Doshi, P.A. Kulkarni, H. Yun

In this work, we describe an approach that improves collaboration between applications, the Linux kernel, and hardware memory subsystem (controllers and the DIMMs) in order to balance power and performance objectives, and we present details of its implementation using the Linux 2.6.32 kernel (x64) as base. The implementation employs ACPI memory power state table (MPST) to organize system memory into power domains according to rank information. An application programming interface (API) in our implementation allows applications to efficiently communicate various provisioning goals concerning groups of virtual ranges to the kernel. The kernel biases allocation and reclamation algorithms in line with the provisioning goals. The goals may vary over time; thus at one time, the applications may request high power efficiency; and at another time, they may ask for bandwidth or capacity reservations, and so on. This paper describes the framework, the changes for incorporating MPST information, policy modifications, and examples and use cases for invoking the new capabilities.

## CPU Time Jitter Based Non-Physical True Random Number Generator - S. Müller

Today's operating systems provide non-physical true random number generators which are based on hardware events. With the advent of virtualization and the ever growing need of more high-quality entropy, these random number generators reach their limits. Additional sources of entropy must be opened up. This document introduces an entropy source based on CPU execution time jitter. The design and implementation of a non-physical true random number generator, the CPU Jitter random number generator, its statistical properties and the maintenance and behavior of entropy is discussed in this document.

The complete version of the analysis together with large amounts of test results is provided at http://www.chronox.de.

## Policy-extendable LMK filter framework for embedded system - K. Baik, J. Kim, D. Kim

Background application management by low memory killer (LMK) is one of the outstanding features of Linux-based platforms such as Android or Tizen. However, LMK has been debated in the Linux community because victim selection mechanism with a specific policy is not suitable for the Linux kernel and a flexible way to apply new policies has been required. Thus, several developers have tried implementing a userspace LMK like the ulmkd (userspace low memory killer daemon). However, not much work has been done regarding applying new polices.

In this paper, we present a policy-extendable LMK filter framework similar to the out-of-memory killer filter discussed at the 2013 LSF/MM Summit. The framework is integrated into the native LMK. When the LMK is triggered, each LMK filter module manages processes in the background like packet filters in the network stack. While the native LMK keeps background applications based on a specific policy, the framework can enhance background application management policy. We describe several benefits of the enhanced policies, including managing undesirable power-consuming background applications and memory-leaking background applications. We also present a new LMK filter module to improve the accuracy of victim selection. The module keeps the applications which could be used in the near future by predicting which applications are likely to be used next from the latest used application based on a Markov model.

We implemented the framework and the module on Galaxy S4 and Odroid-XU device using the Linux 3.4.5 kernel and acquired a preliminary result. The result shows that the number of application terminations was reduced by 14%. Although we implemented it in the kernel, it can be implemented as a userspace daemon by using ulmkd. We expect that the policy-extendable LMK filter framework and LMK filter will improve user experience.

## Scalable Tools for Non-Intrusive Performance Debugging of Parallel Linux Workloads - R. Schöne, J. Schuchart, T. Ilsche, D. Hackenberg

There are a variety of tools to measure the performance of Linux systems and the applications running on them. However, the resulting performance data is often presented in plain text format or only with a very basic user interface. For large systems with many cores and concurrent threads, it is increasingly difficult to present the data in a clear way for analysis. Moreover, certain performance analysis and debugging tasks require the use of a high-resolution time-line based approach, again entailing data visualization challenges. Tools in the area of High Performance Computing (HPC) have long been able to scale to hundreds or thousands of parallel threads and are able to help find performance anomalies. We therefore present a solution to gather performance data using Linux performance monitoring interfaces. A combination of sampling and careful instrumentation allows us to obtain detailed performance traces with manageable overhead. We then convert the resulting output to the Open Trace Format (OTF) to bridge the gap between the recording infrastructure and HPC analysis tools. We explore ways to visualize the data by using the graphical tool Vampir. The combination of established Linux and HPC tools allows us to create an interface for easy navigation through time-ordered performance data grouped by thread or CPU and to help users find opportunities for performance optimizations.

## Veloces: An Efficient I/O Scheduler for Solid State Devices - V.R. Damle, A.N. Palnitkar, S.D. Rangnekar, O.D. Pawar, S.A. Pimpale, N.O. Mandliwala

Solid State Devices (SSD) use NAND-based Flash memory for storage of data. They have the potential to alleviate the ever-existing I/O bottleneck problem in data-intensive computing environments, due to their advantages over conventional Hard Disk Drives (HDD). SSDs differ from traditional mechanical HDDs in various respects. The SSDs have no moving parts and are thus free from rotational latency which dominates the disk access time of HDDs. However, on the other hand, due to the long existence of HDDs as persistent storage devices, conventional I/O schedulers are largely designed for HDDs. They mitigate the high seek and rotational costs in mechanical disks through elevator-style I/O request ordering and anticipatory I/O. As a consequence, just by replacing conventional HDDs with SSDs in the storage systems without taking into consideration other properties like low latency, minimal access time and absence of rotary head, we may not be able to make the best use of SSDs. We propose Veloces, an I/O scheduler which will leverage the inherent properties of SSDs. Since SSDs perform read I/O operations faster than write operations, Veloces is designed to provide preference to reads over writes. Secondly, Veloces implements optional front-merging of contiguous I/O requests. Lastly, writing in the same block of the SSD is faster than writing to different blocks. Therefore, Veloces bundles write requests belonging to the same block. Above implementation has shown to enhance the overall performance of SSDs for various workloads like File-server, Web-server and Mail-server.

## Dmdedup: Device Mapper Target for Data Deduplication - V. Tarasov, D. Jain, G. Kuenning, S. Mandal, K. Palanisami, P. Shilane, S. Trehan, E. Zadok

We present Dmdedup, a versatile and practical primary-storage deduplication platform suitable for both regular users and researchers. Dmdedup operates at the block layer, so it is usable with existing file systems and applications. Since most deduplication research focuses on metadata management, we designed and implemented a flexible backend API that lets developers easily build and evaluate various metadata management policies. We implemented and evaluated three backends: an in-RAM table, an on-disk table, and an on-disk COW B-tree. We have evaluated Dmdedup under a variety of workloads and report the evaluation results here. Although it was initially designed for research flexibility, Dmdedup is fully functional and can be used in production. Under many real-world workloads, Dmdedup's throughput exceeds that of a raw block device by 1.5--6\times.

## SkyPat: C++ Performance Analysis and Testing Framework - P.H. Chang, K.H. Kuo, D.Y. Tsai, K. Chen, Luba W.L. Tang

This paper introduces SkyPat, a C++ performance analysis toolkit on Linux. SkyPat combines unit tests and perf_event to give programmers the power of white-box performance analysis.

Unlike traditional tools that manipulate entire program as a black-box, SkyPat works on a region of code like a white-box. It is used as a normal unit test library. It provides macros and assertions, into which perf_events are embedded, to ensure correctness and to evaluate performance of a region of code. With perf_events, SkyPat can evaluate running time precisely without interference to scheduler. Moreover, perf_event also gives SkyPat accurate cycle counts that are useful for tools that are sensitive to variance of timing, such as compilers.

We develop SkyPat under the new BSD license, and it is also the unit-test library of the bold'' project.

## Computationally Efficient Multiplexing of Events on Hardware Counters - R.V. Lim, D. Carrillo-Cisneros, W. Alkowaileet, I.D. Scherson

This paper proposes a novel approach for scheduling n performance monitoring events onto m hardware performance counters, where n > m. Whereas existing scheduling approaches overlook monitored task information, the proposed algorithm utilizes the monitored task's behavior and schedules the combination of the most costly events. The proposed algorithm was implemented in Linux Perf Event subsystem in kernel space (build 3.11.3), which provides finer granularity and less system perturbation in event monitoring when compared to existing user space approaches. Benchmark experiments in PARSEC and SPLASH.2x suites compared the existing round-robin scheme with the proposed rate-of-change approach. Results demonstrate that the rate-of-change approach reduces the mean-squared error on average by 22%, confirming that the proposed methodology not only improves the accuracy of performance measurements read, but also makes scheduling multiple event measurements feasible with a limited number of hardware performance counters.

## The maxwell(8) random number generator - S. Harris

I propose a daemon process for use on Linux. It gathers entropy from timer calls, distills into a concentrated form, and sends it to the kernel random(4) device. The program is small and does not require large resources. The entropy output is of high quality. The output rate varies with the parameters chosen; with the defaults it is about six kilobits per second, which is enough for many applications.

## Popcorn: a replicated-kernel OS based on Linux - A. Barbalace, B. Ravindran, D. Katz

In recent years, the number of CPUs per platform has continuously increased, affecting almost all segments of the computer market. Because of this trend, many researchers have investigated the problem of how to scale operating systems better on high core-count machines. While many projects have used Linux as a vehicle for this investigation, others have proposed new OS designs. Among them, the replicated-kernel OS model, specifically the multikernel, has gained traction. In this paper, we present Popcorn: a replicated-kernel OS based on Linux. Popcorn boots multiple Linux kernel instances on multicore hardware, one per core or group of cores. Kernels communicate to give to applications the illusion that they are running on top of a single OS. Applications can freely migrate between kernels, exploiting all the hardware resources available on the platform, as in SMP Linux.