Shawn Heisey

Java 8 or Java 11 recommendation for Solr

Java 8 is *required* for Solr 6.x. For 4.x and 5.x releases, I strongly recommend it. Earlier releases only required Java 5 and have not been extensively tested with 8.

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Solr 7 is tested with Java 9 and should work. Earlier versions may encounter problems.

Java 11, like Java 8, has long term support and will have many new releases.  In most cases, it is just as recommended as 8.  When Solr 9 is finally released, it will require Java 11.

GC Tuning for Solr

Starting in version 4.10, and greatly improved in 5.0, startup scripts were added that do GC tuning for you, using CMS (ConcurrentMarkSweep) settings very similar to the CMS settings that you can find further down on this page. If you are using the bin/solr or bin\solr script to start Solr, you already have GC tuning and probably won't need to worry about the recommendations here.  Starting with Solr 8.1, the default collector for Solr is G1, so manual tuning should be even less of a requirement for Solr going forward.

The secret to good Solr GC tuning: Eliminating full garbage collections. A full garbage collection is slow, no matter what collector you're using. If you can set up the options for GC such that both young and old generations are always kept below max size by their generation-specific collection algorithms, performance is almost guaranteed to be good.

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It's important to point out a warning from the Lucene project about G1. They say "Do not, under any circumstances, run Lucene with the G1 garbage collector." Solr is a Lucene program. I personally have never had any problems at all with the G1 collector, but if you're going to use it, it's important that you know you're going against advice from smart people. If this worries you at all, use the CMS settings instead.

With the newest Oracle Java versions, G1GC is looking extremely good. Do not try these options unless you're willing to run the latest Java 7 or Java 8, preferably from Oracle. Experiments with G1 on early Java 7 releases were very disappointing. My most recent testing has been with Oracle 7u72, and I have been informed on multiple occasions that Oracle 8u40 and later has much better G1 performance.

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Note to self: The -XX:ParGCCardsPerStrideChunk option was mentioned to me on the #solr IRC channel. With values of 4096 and 32768, the IRC user was able to achieve 15% and 19% reductions in average pause time, respectively, with the maximum pause cut nearly in half. That option was added to the options you can find in the Solr 5.x start script, on a 12GB heap.

The UseLargePages Option

A note about the UseLargePages option: Currently I do not have huge pages allocated in my operating system. This option will not actually do anything unless you allocate memory to huge pages. If you do so, memory usage reporting with the "top" command will probably only show a few hundred MB of resident memory used by your Solr process, even if it is in fact using several gigabytes of heap. If you do enable huge pages in Linux, be aware that you might wish to turn off an operating system feature called transparent huge pages.

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