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https://issues.apache.org/jira/browse/LUCENE-9038?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16969675#comment-16969675
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Adrien Grand commented on LUCENE-9038:
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This is not due to invalidation but to how Lucene groups data into segments,
that get regularly merged together into fewer bigger segments. When segments
get merged away, they are closed, which triggers a callback on the cache that
tells it that it may remove all entries that are about these segments, since
they will never be used again. Before Lucene introduced a query cache,
Elasticsearch used to have its own query cache that was based on Guava and used
(Query,CacheKey.Helper) pairs as keys, and used to evict all entries for a
segment by iterating over all cached entries and removing those that were about
this segment. It triggered some interesting behaviors when closing top-level
readers, which in-turn closes all their segments in sequence, which in-turn
iterates over all remaining cached entries. So if you want to cache Q queries
and have S segments, then you may have up to QxS entries in your cache, and
thus closing the reader runs in O(QxS^2), and we were seeing users whose
clusters would take ages to close indices because of this. One could make the
argument that is is not required to evict those entries and that we could wait
for them to get evicted naturally, but I don't like the idea of spending some
of the JVM memory on unused data.
> Evaluate Caffeine for LruQueryCache
> -----------------------------------
>
> Key: LUCENE-9038
> URL: https://issues.apache.org/jira/browse/LUCENE-9038
> Project: Lucene - Core
> Issue Type: Improvement
> Reporter: Ben Manes
> Priority: Major
>
> [LRUQueryCache|https://github.com/apache/lucene-solr/blob/master/lucene/core/src/java/org/apache/lucene/search/LRUQueryCache.java]
> appears to play a central role in Lucene's performance. There are many
> issues discussing its performance, such as LUCENE-7235, LUCENE-7237,
> LUCENE-8027, LUCENE-8213, and LUCENE-9002. It appears that the cache's
> overhead can be just as much of a benefit as a liability, causing various
> workarounds and complexity.
> When reviewing the discussions and code, the following issues are concerning:
> # The cache is guarded by a single lock for all reads and writes.
> # All computations are performed outside of the any locking to avoid
> penalizing other callers. This doesn't handle the cache stampedes meaning
> that multiple threads may cache miss, compute the value, and try to store it.
> That redundant work becomes expensive under load and can be mitigated with ~
> per-key locks.
> # The cache queries the entry to see if it's even worth caching. At first
> glance one assumes that is so that inexpensive entries don't bang on the lock
> or thrash the LRU. However, this is also used to indicate data dependencies
> for uncachable items (per JIRA), which perhaps shouldn't be invoking the
> cache.
> # The cache lookup is skipped if the global lock is held and the value is
> computed, but not stored. This means a busy lock reduces performance across
> all usages and the cache's effectiveness degrades. This is not counted in the
> miss rate, giving a false impression.
> # An attempt was made to perform computations asynchronously, due to their
> heavy cost on tail latencies. That work was reverted due to test failures and
> is being worked on.
> # An [in-progress change|https://github.com/apache/lucene-solr/pull/940]
> tries to avoid LRU thrashing due to large, infrequently used items being
> cached.
> # The cache is tightly intertwined with business logic, making it hard to
> tease apart core algorithms and data structures from the usage scenarios.
> It seems that more and more items skip being cached because of concurrency
> and hit rate performance, causing special case fixes based on knowledge of
> the external code flows. Since the developers are experts on search, not
> caching, it seems justified to evaluate if an off-the-shelf library would be
> more helpful in terms of developer time, code complexity, and performance.
> Solr has already introduced [Caffeine|https://github.com/ben-manes/caffeine]
> in SOLR-8241 and SOLR-13817.
> The proposal is to replace the internals {{LruQueryCache}} so that external
> usages are not affected in terms of the API. However, like in {{SolrCache}},
> a difference is that Caffeine only bounds by either the number of entries or
> an accumulated size (e.g. bytes), but not both constraints. This likely is an
> acceptable divergence in how the configuration is honored.
> cc [~ab], [~dsmiley]
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