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Comment by runeks | Hacker News Reader

> 10 milliseconds is far too long for STP for anything in the financial industry.

Does Java give any guarantees about garbage collection latency?

The LMAX Disruptor inter-thread communication library[1], that is the heart of the high performance[2][3] limit order book used by the LMAX Exchange is written in Java.

Just because a language can't guarantee that the duration of GC stops are below a certain threshold, does not mean that you cannot write programs in this language that offer better latency 99.99% of the time.

[1] https://lmax-exchange.github.io/disruptor/

[2] http://www.lmax.com/execution-performance

[3] http://www.infoq.com/presentations/LMAX

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> Does Java give any guarantees about garbage collection latency?

Java has multiple JVMs and garbage collectors available.

The concurrent GC (CMS) in Oracle's JVM doesn't give any guarantee. The new G1 GC in Oracle's Java 7 does provide guarantees while trying to minimize STW pauses. Azul Systems' Pauseless GC is advertised as being completely pauseless, though it is commercial.

Quite painful is that for small heaps, the more you try to do concurrent garbage collection with small latencies, the more throughput suffers. On the other hand, the more you add memory, the bigger the latency. So depending on the app, the memory layout and its access patterns and the hardware used for deployment, one has to pick a GC strategy, as there's no one size fits all.

FWIW, the LMAX Exchange currently uses Oracle's HotSpot JVM, but they've announced they want to switch to Azul's Zing.

I guess my overall point is that the GC guarantee in question is an upper bound. It's perfectly acceptable for a financial application to only deliver x ms latency 99.9% of the time.

Also, the JVM is only one component in the stack. If you aren't using a real-time OS kernel, you're not guaranteed any maximum latency anyway.

or, you know, aim to never GC during market open...

I've heard of people who just "GC on market close". In the applications I've worked on you don't have the luxury of a "market close". They're 24/7 applications and literally 90B transactions per day at peaks of 1.8M qps...

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