The Collie

Abstract

We describe the Collie collector, a fully concurrent compacting collector that uses transactional memory techniques to achieve waitfree compaction. The collector uses compaction as the primary means of reclaiming unused memory, and performs "individual object transplantations" as transactions. We introduce new terms and requirements useful for analyzing concurrent relocating collectors, including definitions of referrer sets, object transplantation and the notion of individually transplantable objects. The Collie collector builds on these terms and on a detailed analysis of an object's legal states during compaction.Collie uses a combination of read barriers, write barriers and transactional memory operations. Its read-barrier supports fast, direct object referencing while using a bound, constant time, waitfree triggering path. Collie thereby avoids the constant indirection cost of Brooks [9] style barriers or handle-based heaps [25]. Collie is demonstrated using speculative multi-address atomicity [11], a form of hardware transactional memory supported by the Azul Vega architecture [2].We evaluate the Collie collector on the Azul platform, on which previous concurrent collectors such as the Pauseless Collector [12] and its generational variant [30] have been commercially available for several years. We discuss Collie's performance while running sustained workloads, and compare it to the Pauseless collector on the same platform. The Collie collector provides significant MMU [5] improvements even in the 1-msec time windows compared to the Pauseless collector. At the same time, it matches Pauseless in throughput and in the ability to scale to large heap sizes.We believe that the Collie collector is the first garbage collector to leverage hardware-assisted transactional memory. While Collie directly leverages Vega's speculative multi-address atomicity feature (SMA) [11], its design can be easily adapted to other hardwareassisted transactional memory systems. Specifically, the upcoming Intel TSX instruction set extensions [21] include capabilities similar to SMA. We expect Collie to be easily implementable on future Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. To copy otherwise, to republish, to post on servers or to redistribute to lists, requires prior specific permission and/or a fee.

Transactional Sapphire

“…The Collie on-the-ly collector [Iyengar et al 2012] uses a hardware-supported read barrier on Azul Systems Vega processors to ensure that mutators always access tospace replicas, unlike Sapphire which accesses fromspace until its RootFlip phase. To provide wait-freedom, an object referenced from a root or accessed by a mutator while Collie is trying to move it is not physically but virtually copied, by mapping its fromspace page to the same physical memory as its mirrored tospace page.…”

Concurrent GCs and Modern Java Workloads: A Cache Perspective

“…Historically, GCs harmed application performance due to unacceptably long stop-theworld (STW) pauses. To reduce the performance overhead and responsiveness issues created by STW events, significant effort was put into re-designing and implementing more efficient GCs [7,12,16,22,26,28,29]. Therefore, most modern GCs [10,32] have increasingly shorter pauses, while performing most of the work concurrently with the application threads (e.g., ZGC [32]).…”