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Using Page Residency to Balance Tradeoffs in Tracing Garbage Collection Daniel Spoonhower spoons cs cmu edu Guy Blelloch blelloch cs cmu edu Robert Harper rwh cs cmu edu School of Computer Science Carnegie Mellon University Pittsburgh PA 15213 ABSTRACT 1 We introduce an extension of mostly copying collection that uses page residency to determine when to relocate objects Our collector promotes in place pages with high residency avoiding unnecessary work and wasted space It predicts the residency of each page but when its predictions prove to be inaccurate our collector reclaims unoccupied space by using it to satisfy allocation requests Using residency allows our collector to dynamically balance the tradeoffs of copying and non copying collection Our technique requires less space than a pure copying collector and supports object pinning without otherwise sacrificing the ability to relocate objects Unlike other hybrids our collector does not depend on application specific configuration and can quickly respond to changing application behavior Our measurements show that our hybrid performs well under a variety of conditions it prefers copying collection when there is ample heap space but falls back on non copying collection when space becomes limited Garbage collectors in modern virtual machines must satisfy the demands of both a high level language and a native runtime implementation While the former takes an abstract view of memory the latter requires a transparent interface where the position and layout of heap objects are made explicit To interoperate with existing libraries many languages also provide foreign function interfaces e g Haskell 10 Java 27 Each of these imposes constraints on how garbage can be collected for example by limiting when objects can be relocated In this work we explore the tradeoffs of tracing garbage collection We show how the two simplest forms of tracing collection mark sweep and semi space collection complement each other not only in

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