[PATCH] Exploit Empty Regions in Young Gen to Enhance PS Full GC Performance

[Haoyu Li]

Hi all，
I have developed a patch to enhance the full GC performance of Parallel
Scavenge on OpenJDK 11, may I have some reviews? The patch is described as
follows and attached in this mail.

*Problem*
Parallel Scavenge(PS) implements a compacting algorithm to do the full GC,
and we find that this algorithm leads in terrible GC thread utilization
(like only 8% on Derby benchmark in SPECjvm2008 suite) since there are
serious dependencies between heap regions, i.e., a region is available to
receive live objects from its source regions only after it has been
collected. The work stealing does not solve this problem, idle GC threads
cannot steal anything because most regions are unavailable to collect.

*Optimization*
We propose *shadow region* to solve the above problem. The basic idea is to
let GC threads collect unavailable regions in advance by copying their live
data into newly allocated empty regions, i.e., shadow regions, to resolve
the region dependencies. The contents of shadow regions will be copied back
to the corresponding regions later. With our approach, GC threads can keep
working at most of the time without suffering from any work stealing
failure (except the work stealing failure happened in the end of a full
GC). And we notice that the to space in young gen is always empty, so we
exploit the empty regions in to space to play the role of shadow regoins
(if the ScavengeBeforeFullGC option is on, regions in eden space may be
used, too) and avoid allocating shadow regions from off heap memory.

*Evaluation*
We evaluate the full GC performance with our patch on DaCapo, SPECjvm2008,
JOlden benchmark suits, and the results shows that shadow region
optimization could improve full GC throughput by 2.1X on average, up to
3.2X.

The patch and evaluation result are attached.

Best Regrads,
Haoyu Li,
Institute of Parallel and Distributed Systems(IPADS),
School of Software,
Shanghai Jiao Tong University
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