We've encountered GC spikes at Etsy after adding new ExternalFileFields a decent number of times. I was always a little confused by this behavior -- isn't it just one big float[]? why does that cause problems for the GC? -- but looking at the FileFloatSource code a little more carefully, I wonder if this is due to using a WeakHashMap that is only cleaned by GC or manual invocation of a request handler.
FileFloatSource stores a WeakHashMap containing <IndexReader,<float[] or CreationPlaceholder>>. In the code[1], it mentions that the implementation is modeled after the FieldCache implementation. However, the FieldCacheImpl adds listeners for IndexReader close events and uses those to purge its caches. [2] Should we be doing the same in FileFloatSource? Here's a mostly untested patch[3] with a possible implementation. There are probably better ways to do it (e.g. I don't love using another WeakHashMap), but I found it tough to hook into the IndexReader lifecycle without a) relying on classes other than FileFloatSource b) changing the public API of FIleFloatSource or c) changing the implementation too much. There is a RequestHandler inside of FileFloatSource (ReloadCacheRequestHandler) that can be used to clear the cache entirely[4], but this is sub-optimal for us for a few reasons: --It clears the entire cache. ExternalFileFields often take some non-trivial time to load and we prefer to do so during SolrCore warmups. Clearing the entire cache while serving traffic would likely cause user-facing requests to timeout. --It forces an extra commit with its consequent cache cycling, etc.. I'm thinking of ways to monitor the size of FileFloatSource's cache to track its size against GC pause times, but it seems tricky because even calling WeakHashMap#size() has side-effects. Any ideas? Overall, what do you think? Does relying on GC to clean this cache make sense as a possible cause of GC spikiness? If so, does the patch [3] look like a decent approach? Thanks! --Gregg [1] https://github.com/apache/lucene-solr/blob/a3914cb5c0243913b827762db2d616ad7cc6801d/solr/core/src/java/org/apache/solr/search/function/FileFloatSource.java#L135 [2] https://github.com/apache/lucene-solr/blob/1c0eee5c5cdfddcc715369dad9d35c81027bddca/lucene/core/src/java/org/apache/lucene/search/FieldCacheImpl.java#L166 [3] https://gist.github.com/2876371 [4] https://github.com/apache/lucene-solr/blob/a3914cb5c0243913b827762db2d616ad7cc6801d/solr/core/src/java/org/apache/solr/search/function/FileFloatSource.java#L310
