Technical Report

Supporting Per-processor Local-allocation Buffers Using Multi-processor Restartable Critical Sections
March 2004

One challenge for runtime systems like the Java™ platform that depend on garbage collection is the ability to scale performance with the number of allocating threads. As the number of such threads grows, allocation of memory in the heap becomes a point of contention. To relieve this contention, many collectors allow threads to preallocate blocks of memory from the shared heap. These per-thread local-allocation buffers (LABs) allow threads to allocate most objects without any need for further synchronization. As the number of threads exceeds the number of processors, however, the cost of committing memory to local-allocation buffers becomes a challenge and sophisticated LAB-sizing policies must be employed.

To reduce this complexity, we implement support for local-allocation buffers associated with processors instead of threads using multiprocess restartable critical sections (MP-RCSs). MP-RCSs allow threads to manipulate processor-local data safely. To support processor-spe-cific transactions in dynamically generated code, we have developed a novel mechanism for implementing these critical sections that is efficient, allows preemption-notification at known points in a given critical section, and does not require explicit registration of the critical sec-tions. Finally, we analyze the performance of per-processor LABs and show that, for highly threaded applications, this approach performs better than per-thread LABs, and allows for simpler LAB-sizing policies.

Authors: Derek White, David Dice, Alex Garthwaite


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