If hit_rate is below 1% drop match phase limiting. It has too high fixed cost and will liklely make things worse.

2 files changed, 12 insertions, 2 deletions

diff --git a/searchcore/src/tests/proton/matching/match_phase_limiter/match_phase_limiter_test.cpp b/searchcore/src/tests/proton/matching/match_phase_limiter/match_phase_limiter_test.cpp
index 21c572995d3..b26ed1d4765 100644
--- a/searchcore/src/tests/proton/matching/match_phase_limiter/match_phase_limiter_test.cpp
+++ b/searchcore/src/tests/proton/matching/match_phase_limiter/match_phase_limiter_test.cpp
@@ -352,6 +352,7 @@ TEST("require that the match phase limiter is able to pre-limit the query") {
         "            hit_rate: 0.1,"
         "            num_docs: 100000,"
         "            max_filter_docs: 100000,"
+        "            upper_limited_corpus_size: 100000,"
         "            wanted_docs: 5000,"
         "            action: 'Will limit with prefix filter',"
         "            max_group_size: 5000,"
diff --git a/searchcore/src/vespa/searchcore/proton/matching/match_phase_limiter.cpp b/searchcore/src/vespa/searchcore/proton/matching/match_phase_limiter.cpp
index b64d5ba4c05..784ce649c5f 100644
--- a/searchcore/src/vespa/searchcore/proton/matching/match_phase_limiter.cpp
+++ b/searchcore/src/vespa/searchcore/proton/matching/match_phase_limiter.cpp
@@ -102,6 +102,10 @@ do_limit(AttributeLimiter &limiter_factory, SearchIterator::UP search, double ma
     return search;
 }
 
+// When hitrate is below 1% limiting the query is often far more expensive than not.
+// TODO This limit should probably be a lot higher.
+constexpr double MIN_HIT_RATE_LIMIT = 0.01;
+
 } // namespace proton::matching::<unnamed>
 
 SearchIterator::UP
@@ -114,11 +118,16 @@ MatchPhaseLimiter::maybe_limit(SearchIterator::UP search, double match_freq, siz
         trace->setDouble("hit_rate", match_freq); 
         trace->setLong("num_docs", num_docs);
         trace->setLong("max_filter_docs", max_filter_docs);
+        trace->setLong("upper_limited_corpus_size", upper_limited_corpus_size);
         trace->setLong("wanted_docs", wanted_num_docs);
     }
-    if (upper_limited_corpus_size <= wanted_num_docs) {
+    if ((upper_limited_corpus_size <= wanted_num_docs) || (match_freq < MIN_HIT_RATE_LIMIT)) {
         if (trace) {
-            trace->setString("action", "Will not limit !");
+            if (upper_limited_corpus_size <= wanted_num_docs) {
+                trace->setString("action", "Will not limit due to upper_limited_corpus_size <= wanted_num_docs");
+            } else if (match_freq < MIN_HIT_RATE_LIMIT) {
+                trace->setString("action", "Will not limit due to match_freq < MIN_HIT_RATE_LIMIT(1%)");
+            }
         }
         LOG(debug, "Will not limit ! maybe_limit(hit_rate=%g, num_docs=%ld, max_filter_docs=%ld) = wanted_num_docs=%ld",
             match_freq, num_docs, max_filter_docs, wanted_num_docs);