Chapter 4. Basic configuration options of Shenandoah garbage collector
Shenandoah garbage collector (GC) has the following basic configuration options:
- -Xlog:gc
- Print the individual GC timing.
- -Xlog:gc+ergo
- Print the heuristics decisions, which might shed light on outliers, if any.
- -Xlog:gc+stats
Print the summary table on Shenandoah internal timings at the end of the run.
It is best to run this with logging enabled. This summary table conveys important information about GC performance. Heuristics logs are useful to figure out GC outliers.
- -XX:+AlwaysPreTouch
- Commit heap pages into memory and helps to reduce latency hiccups.
- -Xms and -Xmx
-
Making the heap non-resizeable with
-Xms = -Xmxreduces difficulties with heap management. Along withAlwaysPreTouch, the-Xms = -Xmxcommit all memory on startup, which avoids difficulties when memory is finally used.-Xmsalso defines the low boundary for memory uncommit, so with-Xms = -Xmxall memory stays committed. If you want to configure Shenandoah for a lower footprint, then setting lower-Xmsis recommended. You need to decide how low to set it to balance the commit/uncommit overhead versus memory footprint. In many cases, you can set-Xmsarbitrarily low. - -XX:+UseLargePages
-
Enables
hugetlbfsLinux support. - -XX:+UseTransparentHugePages
-
Enables huge pages transparently. With transparent huge pages, it is recommended to set
/sys/kernel/mm/transparent_hugepage/enabledand/sys/kernel/mm/transparent_hugepage/defragtomadvise. When running withAlwaysPreTouch, it will also pay thedefragtool costs upfront at startup. - -XX:+UseNUMA
-
While Shenandoah does not support NUMA explicitly yet, it is a good idea to enable NUMA interleaving on multi-socket hosts. Coupled with
AlwaysPreTouch, it provides better performance than the default out-of-the-box configuration. - -XX:-UseBiasedLocking
- There is a tradeoff between uncontended (biased) locking throughput, and the safepoints JVM does to enable and disable them. For latency-oriented workloads, turn biased locking off.
- -XX:+DisableExplicitGC
- Invoking System.gc() from user code forces Shenandoah to perform additional GC cycle. It usually does not harm, as -XX:+ExplicitGCInvokesConcurrent gets enabled by default, which means the concurrent GC cycle would be invoked, not the STW Full GC.
Revised on 2024-05-09 14:54:37 UTC
