jgq2008303393 commented on a change in pull request #940: LUCENE-9002: Query
caching leads to absurdly slow queries
URL: https://github.com/apache/lucene-solr/pull/940#discussion_r333896831
##########
File path: lucene/core/src/java/org/apache/lucene/search/LRUQueryCache.java
##########
@@ -732,8 +741,39 @@ public ScorerSupplier scorerSupplier(LeafReaderContext
context) throws IOExcepti
if (docIdSet == null) {
if (policy.shouldCache(in.getQuery())) {
- docIdSet = cache(context);
- putIfAbsent(in.getQuery(), docIdSet, cacheHelper);
+ final ScorerSupplier supplier = in.scorerSupplier(context);
+ if (supplier == null) {
+ putIfAbsent(in.getQuery(), DocIdSet.EMPTY, cacheHelper);
+ return null;
+ }
+
+ final long cost = supplier.cost();
+ return new ScorerSupplier() {
+ @Override
+ public Scorer get(long leadCost) throws IOException {
+ // skip cache operation which would slow query down too much
+ if ((cost > skipCacheCost || cost > leadCost * skipCacheFactor)
Review comment:
We have tested different scenarios to observe the query latency with/without
cacheing in an online ES cluster. Here is the result:
| queryPattern | latencyWithoutCaching | latencyWithCaching | leadCost
| rangeQueryCost | skipCacheFactor |
| ---------- | :-----------: | :-----------: | :-----------: |
:-----------: | :-----------: |
| ip:xxx AND time:[t-1h, t] | 10ms | 36ms(+260%) | 20528 | 878979 | 42 |
| ip:xxx AND time:[t-4h, t] | 10ms | 100ms(+900%) | 20528 | 4365870 | 212 |
| ip:xxx AND time:[t-8h, t] | 11ms | 200ms(+1700%) | 20528 | 8724483 | 425 |
| ip:xxx AND time:[t-12h, t] | 12ms | 300ms(+2400%) | 20528 | 13083096 | 637
|
| ip:xxx AND time:[t-24h, t] | 16ms | 500ms(+3000%) | 20528 | 26158936 |
1274 |
| ip:xxx AND time:[t-48h, t] | 30ms | 1200ms(3900%) | 20528 | 52310616 |
2548 |
As the table shows, query latency without caching is low and it's related
with the final result set. Query latency with caching is much high and it's
mainly related with _rangeQueryCost_. According to the above test, we set the
default value of _skipCacheFactor_ to 250, which make the query slower by no
more than 10 times.
In addition to _skipCacheFactor_ which is similar to _maxCostFactor_ in
LUCENE-8027, we add a new parameter _skipCacheCost_. The mainly reasons are:
- control the time used for caching as the caching time is related to the
cost of range query.
- skip caching too large range queries which will consume too much memory
and evict cache entries frequently.
How do you think? Looking forward to your ideas. @jpountz
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