ABSTRACT Concurrent garbage collectors require write barriers to preserveconsistency, but these barriers impose significant direct and indirect costs. While there has been a lot of work on optimizing write barri-ers, we present the first study of their elision in a concurrent collector. We show conditions under which write barriers are redundant,and describe how these conditions can be applied to both incremental update or snapshot-at-the-beginning barriers. We then evaluatethe potential for write barrier elimination with a trace-based limit study, which shows that a significant percentage of write barriersare redundant. On average, 54 % of incremental barriers and 83 % of snapshot barriers are unnecessary.

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Multithreaded applications with multi-gigabyte heaps running on modern servers provide new challenges for garbage collection (GC). The challenges for “server-oriented ” GC include: ensuring short pause times on a multi-gigabyte heap while minimizing throughput penalty, good scaling on multiprocessor hardware, and keeping the number of expensive multi-cycle fence instructions required by weak ordering to a minimum. We designed and implemented a collector facing these demands building on the mostly concurrent garbage collector proposed by Boehm et al. Our collector incorporates new ideas into the original collector. We make it parallel and incremental; we employ concurrent low-priority background GC threads to take advantage of processor idle time; we propose novel algorithmic improvements to the basic mostly concurrent algorithm improving its efficiency and shortening its pause times; and finally, we use advanced techniques, such as a low-overhead work packet mechanism to enable full parallelism among the incremental and concurrent collecting threads and ensure load balancing. We compared the new collector to the mature, well-optimized, parallel, stop-the-world marksweep collector already in the IBM JVM. When allowed to run aggressively, using 72 % of the CPU utilization during a short concurrent phase, our collector prototype reduces the maximum pause time from 161ms to 46ms while only losing 11.5 % throughput when running the SPECjbb2000 benchmark on a 600 MB heap on an 8-way PowerPC 1.1 GHz processors. When the collector is limited to a non-intrusive operation using only 29 % of the CPU utilization, the maximum pause time obtained is 79ms and the loss in throughput is 15.4%.

This paper describes an incremental conservative garbage collector that significantly reduces pause time of an existing collector by Boehm et al. Like their collector, it is a true conservative collector that does not require compiler cooperation but uses virtual memory primitives (page protection) of operating systems for write barriers. While much successful work has been done on incremental collectors in general, achieving small pause time of the order of a few milliseconds in such uncooperative settings has been challenging. Our collector combines several ideas that bound pause times without introducing significant overheads. They in- clude: (1) bounding the number of dirty (writable) pages during concurrent marking, (2) adaptive repetition of concurrent marking phases, and (3) allocating objects in black (marked) in later stages of a collection. With these tech- niques, we have achieved the maximum pause time of 2.6 4.5ms for five application benchmarks on 400MHz UltraSPARC processors. They are up to forty times shorter than the basic incremental collector similar to Boehm et al.'s. The overhead (total work time) of our collector is 1.2 53% to the stop-the-world collector and 9% or less to the basic incremental collector.

Parallel and concurrent garbage collectors are increasingly employed by managed runtime environments (MREs) to maintain scalability, as multi-core architectures and multi-threaded applications become pervasive. Moreover, state-of-the-art MREs commonly implement compaction to eliminate heap fragmentation and enable fast linear object allocation. Our empirical analysis of object demographics reveals that unreachable objects in the heap tend to form clusters large enough to be effectively managed at the granularity of virtual memory pages. Even though processes can manipulate the mapping of the virtual address space through the standard operating system (OS) interface on most platforms, extant parallel/concurrent compactors do not do so to exploit this clustering behavior and instead achieve compaction by performing, relatively expensive, object moving and pointer adjustment. We introduce the Mapping Collector (MC), which leverages virtual memory operations to reclaim and consolidate free space without moving objects and updating pointers. MC is a nearly-singlephase compactor that is simpler and more efficient than previously reported compactors that comprise two to four phases. Through effective MRE-OS coordination, MC maintains the simplicity of a non-moving collector while providing efficient parallel and concurrent compaction. We implement both stop-the-world and concurrent MC in a generational garbage collection framework within the open-source HotSpot Java Virtual Machine. Our experimental evaluation using a multiprocessor indicates that MC significantly increases throughput and scalability as well as reduces pause times, relative to stateof-the-art, parallel and concurrent compactors.

Abstract. We describe the effect of a particular form of “noise ” in benchmarking. We investigate the source of anomalous measurement data in a series of optimization strategies that attempt to improve data cache performance in the garbage collector of a Java virtual machine. The results of our experiments can be explained in terms of the difference in code positioning, and hence instruction and data cache behaviour. We show that unintended changes in code positioning due to code modifications as trivial as symbol renaming can contribute up to 2.7 % of measured machine cycle cost, 20 % in data cache misses, and 37 % in instruction cache misses.

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A reference counting garbage collector cannot reclaim unreachable cyclic structures of objects. Therefore, reference counting collectors either use a backup tracing collector seldom, or employ a cycle collectors to reclaim cyclic structures. Recently, the first on-the-fly cycle collector, that may run concurrently with program threads, was presented by Bacon and Rajan [3]. This demonstrated the ability to run on-the-fly reference counting without resorting to an auxiliary tracing collector.

Generational garbage collection divides a heap up into two or more generations, and usually collects a youngest generation most frequently. Collection of the youngest generation requires identification of pointers into that generation from older generations; a data structure that supports such identification is called a remembered set. Various remembered set mechanisms have been proposed; these generally require mutator code to execute a write barrier when modifying pointer fields. Remembered set data structures can vary in their precision: an imprecise structure requires the garbage collector to do more work to find old-to-young pointers. Generally there is a tradeo# between remembered set precision and barrier cost: a more precise remembered set requires a more elaborate barrier. Many current systems tend to favor more e#cient barriers in this tradeo#, as shown by the widespread popularity of relatively imprecise card marking techniques. This imprecision becomes increasingly costly as the ratio between old- and young-generation sizes grows. We propose a technique that maintains more precise remembered sets that scale with old-generation size, using a barrier whose cost is not significantly greater than card marking.

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