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Solr Cache Sizing - filterCache queryResultCache documentCache

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Every Solr cache trades heap for speed. Sized against your own document count, they help; copied from an example, they can take the heap.

Solr caches are worth having, and they are also the easiest way to lose a heap. Four of them are configurable in solrconfig.xml, and each entry they hold is a live object on the heap for as long as the searcher lives.

THE FOUR CONFIGURABLE CACHESfilterCacheDocument ids matching one filter query.A wide filter is one bit per document.queryResultCacheOrdered id lists per query, sort and page.Every deep page adds another entry.documentCacheStored fields of the documents returned.Large stored fields, large entries.fieldValueCacheUn-inverted multi valued fields.docValues make it unnecessary.In Solr 9 all of them are solr.CaffeineCache. FastLRUCache and LRUCache were removed.

What each cache holds. The sizes are entry counts, not bytes — the bytes depend on your index.

The filterCache is the one that surprises people, because its entries are sized by the index and not by the filter. A filter matching a large share of the documents is kept as a bitset of one bit per document, so on a 10 million document index a single entry can be around 1.25 MB. Configure size="512" and you have authorised roughly 640 MB of heap for that cache alone. On a small index the same setting costs almost nothing. The number to think about is not the cache size, it is the cache size multiplied by your document count.

<query>
  <!-- entry counts, not megabytes. autowarmCount=0 means a new searcher starts cold
       instead of replaying old queries while the old searcher is still holding its heap. -->
  <filterCache      size="512" initialSize="512" autowarmCount="0"/>
  <queryResultCache size="512" initialSize="512" autowarmCount="0"/>
  <documentCache    size="512" initialSize="512" autowarmCount="0"/>

  <!-- how many documents of a result page are worth caching at all -->
  <queryResultWindowSize>20</queryResultWindowSize>
  <queryResultMaxDocsCached>200</queryResultMaxDocsCached>
</query>

Start small and measure, do not start large and hope

Cache hit ratios are visible per index in the Solr admin interface. A cache with a low hit ratio is pure heap cost; a cache with a high hit ratio and evictions is the one worth growing.

Autowarming is not free

While a new searcher warms, the old one is still open and both hold their caches. Aggressive autowarmCount values on an index that commits often are a classic way to run two full cache sets at once.

Cache classes changed in Solr 9

Every cache is now solr.CaffeineCache. A config carried over from Solr 8 that names solr.FastLRUCache or solr.LRUCache will not load — see ClassNotFoundException: solr.FastLRUCache.

Solr best practices

Cache sizing (this page)