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RFC: Patch Ice-3.5.1 to add class lookup caching to BasicStream (java)

I've been experiencing severe performance issues decoding complex message in a java-hosted application.

Consider a slice definition such as:
class Example(5) {

sequence<Example> ExampleSeq;

Now consider a message that passes an ExampleSeq with thousands of entries.

I found in such a case, BasicStream#getConcreteClass would be called several times for each of these thousands of entries:

"IceCompactId.TypeId_5" (lookup fails)
"" (lookup succeeds)
"" (lookup succeeds)

The accumulated total run time of all these lookups would result in 60+ second decode times for one message in real world test case I have. Profiling showed 137,350 calls to java.lang.ClassLoader.loadClass. After adopting a caching strategy for these lookups, the run time was reduced to under one second. Calls to loadClass dropped to about 379.

In my patch, I create a ConcurrentHashMap per instance of BasicStream, not knowing if BasicStream instances are ever shared between threads. I chose to wrap the Class instances in WeakReference so the cache doesn't prevent these classes from being unloaded. To get the desired performance benefit, I had to also cache lookup misses. This part worries me because if the class is later loaded, the cache would be returning false negatives.

I thought that ideally the cache would only be around for the decoding of a single message, but I found that BasicStream instances are created on a per-thread basis and reused.

In summary, I believe this patch works correctly for my purposes but is not an ideal general case solution. I would appreciate any feedback. Thanks!


  • mes
    mes California
    Hi Robert,

    Just to follow up on this, we've added a fix for this issue in our 3.6 branch. It will be included in the upcoming 3.6.2 release.

    Note that the performance issue only affected Java programs that unmarshal objects-by-value whose definitions use compact type IDs.