Saturday, August 20, 2011

tips on how to incur just 1 major GC/day -- low-latency java

Big eq trading desks in Citi and other banks claim they incur a single full-GC "penalty" a day in most jvm instances (where an instance is typically a node in a scalable grid). Here are some techniques.

0) "pre-allocate" long-living objects. Probably allocate a large array of primitive objects. Each "primitive object" could be a byteArray of fixed size. No POJO no java beans.
)) don't let GC do the clean-up; reuse the memory pre-allocated. Reuse by dropping in new data. Circular array of fixed-size byteArray is one of my ideas.

2) avoid promoting objects beyond the nursery/eden -- see other posts in this blog.

3) run 64bit and allocate enough heap (typically 10G) to last a day.
)) Each jvm instance presumably serves only US, or only Hongkong..., so it could shutdown nightly.
)) If an instance must run 24x7, then keep it simple and small.
)) If we still need more heap, split to 2 jvm instances.

)) avoid distributed cache and latency. Barc DMA guys also avoide distributed caching.
)) How do you size heap? See post on heap sizing

5) During a scheduled quiet period, trigger a full GC throughout the entire (huge) heap, which could actually take 30 minutes. Just call Runtime.gc(). If this doesn't trigger GC, then tweak -XdisableExplicitGC

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