Concurrent GCs and Modern JavaWorkloads: A Cache Perspective

The garbage collector (GC) is a crucial component of language runtimes, offering correctness guarantees and high productivity in exchange for a run-time overhead. Concurrent collectors run alongside application threads (mutators) and share CPU resources. A likely point of contention between mutators and GC threads and, consequently, a potential overhead source is the shared last-level cache (LLC). This work builds on the hypothesis that the cache pollution caused by concurrent GCs hurts application performance. We validate this hypothesis with a cache-sensitive Java micro-benchmark. We find that concurrent GC activity may slow down the application by up to 3 x and increase the LLC misses by 3 orders of magnitude. However, when we extend our analysis to a suite of benchmarks representative for today's server workloads (Renaissance), we find that only 5 out of 23 benchmarks show a statistically significant correlation between GC-induced cache pollution and performance. Even for these, the performance overhead of GC does not exceed 10 %. Based on further analysis, we conclude that the lower impact of the GC on the performance of Renaissance benchmarks is due to their lack of sensitivity to LLC capacity.