Omitting resource-release operations in systems error handling code can lead to memory leaks, crashes, and deadlocks. Finding omission faults is challenging due to the difficulty of reproducing system errors, the diversity of system resources, and the lack of appropriate abstractions in the C language. To address these issues, numerous approaches have been proposed that globally scan a code base for common resource-release operations. Such macroscopic approaches are notorious for their many false positives, while also leaving many faults undetected. We propose a novel microscopic approach to finding resourcerelease omission faults in systems software. Rather than generalizing from the entire source code, our approach focuses on the error-handling code of each function. Using our tool, Hector, we have found over 370 faults in six systems software projects, including Linux, with a 23% false positive rate. Some of these faults allow an unprivileged malicious user to crash the entire system.
The scalability of multithreaded applications on current multicore systems is hampered by the performance of lock algorithms, due to the costs of access contention and cache misses. The main contribution presented in this article is a new locking technique, Remote Core Locking (RCL), that aims to accelerate the execution of critical sections in legacy applications on multicore architectures. The idea of RCL is to replace lock acquisitions by optimized remote procedure calls to a dedicated server hardware thread. RCL limits the performance collapse observed with other lock algorithms when many threads try to acquire a lock concurrently and removes the need to transfer lock-protected shared data to the hardware thread acquiring the lock, because such data can typically remain in the server’s cache. Other contributions presented in this article include a profiler that identifies the locks that are the bottlenecks in multithreaded applications and that can thus benefit from RCL, and a reengineering tool that transforms POSIX lock acquisitions into RCL locks. Eighteen applications were used to evaluate RCL: the nine applications of the SPLASH-2 benchmark suite, the seven applications of the Phoenix 2 benchmark suite, Memcached, and Berkeley DB with a TPC-C client. Eight of these applications are unable to scale because of locks and benefit from RCL on an ×86 machine with four AMD Opteron processors and 48 hardware threads. By using RCL instead of Linux POSIX locks, performance is improved by up to 2.5 times on Memcached, and up to 11.6 times on Berkeley DB with the TPC-C client. On a SPARC machine with two Sun Ultrasparc T2+ processors and 128 hardware threads, three applications benefit from RCL. In particular, performance is improved by up to 1.3 times with respect to Solaris POSIX locks on Memcached, and up to 7.9 times on Berkeley DB with the TPC-C client.
As a central part of resource management, the OS thread scheduler must maintain the following, simple, invariant: make sure that ready threads are scheduled on available cores. As simple as it may seem, we found that this invariant is often broken in Linux. Cores may stay idle for seconds while ready threads are waiting in runqueues. In our experiments, these performance bugs caused many-fold performance degradation for synchronization-heavy scientific applications, 13% higher latency for kernel make, and a 14-23% decrease in TPC-H throughput for a widely used commercial database. The main contribution of this work is the discovery and analysis of these bugs and providing the fixes. Conventional testing techniques and debugging tools are ineffective at confirming or understanding this kind of bugs, because their symptoms are often evasive. To drive our investigation, we built new tools that check for violation of the invariant online and visualize scheduling activity. They are simple, easily portable across kernel versions, and run with a negligible overhead. We believe that making these tools part of the kernel developers' tool belt can help keep this type of bug at bay.
Recent research and bug reports have shown that work conservation, the property that a core is idle only if no other core is overloaded, is not guaranteed by Linux's CFS or FreeBSD's ULE multicore schedulers. Indeed, multicore schedulers are challenging to specify and verify: they must operate under stringent performance requirements, while handling very large numbers of concurrent operations on threads. As a consequence, the verification of correctness properties of schedulers has not yet been considered. In this paper, we propose an approach, based on a domainspecific language and theorem provers, for developing schedulers with provable properties. We introduce the notion of concurrent work conservation (CWC), a relaxed definition of work conservation that can be achieved in a concurrent system where threads can be created, unblocked and blocked concurrently with other scheduling events. We implement several scheduling policies, inspired by CFS and ULE. We show that our schedulers obtain the same level of performance as production schedulers, while concurrent work conservation is satisfied.
To cite this version:Jean-Pierre Lozi, René Lozi, Oleg Garasym. How useful randomness for cryptography can emerge from multicore-implemented complex networks of chaotic maps.
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