This presentation from the November 17, 2011 Boundary Meetup takes us through the architecture of Boundary's stream processing infrastructure and how the architecture is pushing the bounds of JVM throughput.
Scott Andreas - Garbage, Garbage Everywhere: GC Strategies for Event Processing Systems on the JVM, Boundary Tech Talks 11/17/11
Garbage, Garbage EverywhereGC Strategies for Event Processing Systems on the JVMC. Scott AndreasPizza, Beer, and Tech TalksNovember 17, 2011
What’s ESP / CEP?• Event Stream Processing Selecting events on dimensions among a stream of moving data, maintaining them for a brief period, emitting aggregations.• Complex Event Processing Identifying correlations between events, predicting trends, and programmatically reacting to emergent trends.
ESP and Network Analytics• Packet ﬂows are event streams with many dimensions.• Blast them into the engine, select over the stream, emit aggregations based on queries.• Ipﬁx data ﬂows in, JSON comes out.
Back of the Envelope• 500 Mbps / sec data comes into the JVM xxx Mbps / sec data goes out of the JVM• This memory must be allocated, retained for processing, freed, and collected.• Actual allocation rates far higher than data in / out (Memory also used for deserializing, aggregations, etc).
Opening the Grimoire• GC Tuning to the rescue• Oracle guide with 84 -XX: options• Stas’s guide: 830 options (OpenJDK debug builds) http://www.oracle.com/technetwork/java/javase/tech/vmoptions-jsp-140102.html http://stas-blogspot.blogspot.com/2011/07/most-complete-list-of-xx-options-for.html
Moment of Pause• Don’t touch the knobs unless you need to• Server defaults are a decent place to start for local development• Defaults shipped with Cassandra decent for bimodal GC proﬁles• Basic rule of thumb: if you’re aggressively tuning garbage collection, you can trade hours of frustration for ~10% gain
Generational Garbage Collection• Modern JVMs divide heap space up into multiple “generations.”• Most applications have a lot of objects which live for a very short time, and a lot which live (nearly) forever.• Generational collection enables the JVM to collect unused memory more efﬁciently by avoiding unnecessarily scanning heap / object graphs for references or free regions.
Low-Pause Collector: The G1 v% => threshold violations. lower is better.
G1 Collector• Hundreds of tiny 1ms collections / second rather than ParNew’s ~100 - 200ms larger collections.• Capable of meeting ambitious pause targets.• Powered by a gang of threads working in parallel • ...cooperating to chew through CPU like it’s free.
Unsafe• A OpenJDK/HotSpot class exposing direct access to the underlying VM, OS, and memory.• This includes the ability to allocate, manage, and free memory.• Perhaps we can outsmart the JVM and do a better job than it!
Learned While Astray• Finalization occurs in a single thread.• Jumping from native ﬁnalization back into Java is expensive.• Attempting to outsmart the garbage collector by creating hundreds of thousands of tiny ByteBuffers is...a thing.• Java’s collectors are very good at collecting garbage. Your home-grown in-app GC go-kart is probably not.
Lessons from Science• Your rate of “freeing” must be equal to or exceed your rate of object allocation on the heap.• High rates of allocation speed up heap fragmentation, which compounds the problem.• Creating less garbage reduces your rate of allocation (and freeing).• This means less work for the garbage collector.
attempt 3[b]:responsible tuning of the old hat
Optimizing for Infant Mortality default newgen ratios in java 6• Java 6 AMD64 (server) defaults to allocating 1/3 of heap to the new gen, 2/3 to the old gen.• ESP/CEP workloads place tremendous pressure on the newgen. The vast majority of objects survive less than ﬁve seconds.• Experiment: Allocate 80% of heap to the new gen, set a higher tenuring threshold, and lean hard on the ParNew collector.
CMS Collector• Guardian of the tenured generation, favorite workhorse for years.• Primarily parallel, easier on the CPU than the G1.• ...But contains a signiﬁcant pause phase, is less suited to meeting low pause targets.
ParNew Collector• Designed for the small, but works great in the large. Excellent throughput, parallel collection.• Can collect ~5GB in ~200ms on a quad-core Xeon w/HT.• 200ms pause every several seconds favorable compared to less frequent multi-second pauses and promotion failures.
Real Time and the JVM• Real Time Ability to meet speciﬁc targets with low variance is critical to the bare minimum functionality of the product (e.g., air bags).• “Soft” Real Time Ability to meet targets important but not critical. Value of system’s functionality is diminished but not eliminated by delay.
Real Time and “The Pause”• To what extent can a system which can endure pauses of unpredictable duration be considered “real-time”?• Is it sufﬁcient to mitigate the frequency and duration of pauses for a system to still deliver value as “soft real-time”?• Is the alternative worth the cost?