Don't compact LOH just because a heap_hard_limit exists

This is based on a perf test with 100% survival in a container, before
and after dotnet/coreclr#22180. GC pause times were greater after that commit.
Debugging showed that the reason was that after, we were always doing
compacting GC, and objects were staying in generation 1 and not making it
to generation 2. The reason was that in the "after" build,
`should_compact_loh()` was always returning true if heap_hard_limit was
set; currently if we do an LOH compaction, we compact all other
generations too. As the comment indicates, we should decide that
automatically, not just set it to true all the time.

Commit migrated from https://github.com/dotnet/coreclr/commit/cc14e6cecf6984c991fe906c9fe4b327b4f93f96

diff --git a/src/coreclr/src/gc/gc.cpp b/src/coreclr/src/gc/gc.cpp
index 01f51c2..669aab2 100644 (file)
--- a/src/coreclr/src/gc/gc.cpp
+++ b/src/coreclr/src/gc/gc.cpp
@@ -5854,7 +5854,7 @@ void gc_mechanisms::init_mechanisms()
     promotion = FALSE;//TRUE;
     compaction = TRUE;
 #ifdef FEATURE_LOH_COMPACTION
-    loh_compaction = gc_heap::should_compact_loh();
+    loh_compaction = gc_heap::loh_compaction_requested();
 #else
     loh_compaction = FALSE;
 #endif //FEATURE_LOH_COMPACTION
@@ -21365,10 +21365,10 @@ retry:
     }
 }
 
-BOOL gc_heap::should_compact_loh()
+BOOL gc_heap::loh_compaction_requested()
 {
     // If hard limit is specified GC will automatically decide if LOH needs to be compacted.
-    return (heap_hard_limit || loh_compaction_always_p || (loh_compaction_mode != loh_compaction_default));
+    return (loh_compaction_always_p || (loh_compaction_mode != loh_compaction_default));
 }
 
 inline
@@ -21538,7 +21538,7 @@ BOOL gc_heap::plan_loh()
 
 void gc_heap::compact_loh()
 {
-    assert (should_compact_loh());
+    assert (loh_compaction_requested() || heap_hard_limit);
 
     generation* gen        = large_object_generation;
     heap_segment* start_seg = heap_segment_rw (generation_start_segment (gen));

diff --git a/src/coreclr/src/gc/gcpriv.h b/src/coreclr/src/gc/gcpriv.h
index 18785d5..10b5896 100644 (file)
--- a/src/coreclr/src/gc/gcpriv.h
+++ b/src/coreclr/src/gc/gcpriv.h
@@ -2223,7 +2223,7 @@ protected:
     BOOL loh_object_p (uint8_t* o);
 
     PER_HEAP_ISOLATED
-    BOOL should_compact_loh();
+    BOOL loh_compaction_requested();
 
     // If the LOH compaction mode is just to compact once,
     // we need to see if we should reset it back to not compact.