RFR: Compact heuristics adjustments

[Aleksey Shipilev]

I want to do a few "compact" heuristics adjustments after my yesterday's experiments with it: more
aggressive uncommit, denser compaction, reacting on minimum free threshold it case allocation
threshold is too high.

diff -r 509403d31498 src/hotspot/share/gc/shenandoah/heuristics/shenandoahCompactHeuristics.cpp
--- a/src/hotspot/share/gc/shenandoah/heuristics/shenandoahCompactHeuristics.cpp        Thu Nov 15
11:12:27 2018 +0100
+++ b/src/hotspot/share/gc/shenandoah/heuristics/shenandoahCompactHeuristics.cpp        Thu Nov 15
17:10:24 2018 +0100
@@ -35,20 +35,27 @@
   SHENANDOAH_ERGO_ENABLE_FLAG(ShenandoahImplicitGCInvokesConcurrent);
   SHENANDOAH_ERGO_ENABLE_FLAG(ShenandoahUncommit);
   SHENANDOAH_ERGO_ENABLE_FLAG(ShenandoahAlwaysClearSoftRefs);
   SHENANDOAH_ERGO_OVERRIDE_DEFAULT(ShenandoahAllocationThreshold,  10);
   SHENANDOAH_ERGO_OVERRIDE_DEFAULT(ShenandoahImmediateThreshold,   100);
-  SHENANDOAH_ERGO_OVERRIDE_DEFAULT(ShenandoahUncommitDelay,        5000);
+  SHENANDOAH_ERGO_OVERRIDE_DEFAULT(ShenandoahUncommitDelay,        1000);
   SHENANDOAH_ERGO_OVERRIDE_DEFAULT(ShenandoahGuaranteedGCInterval, 30000);
-  SHENANDOAH_ERGO_OVERRIDE_DEFAULT(ShenandoahGarbageThreshold,     20);
+  SHENANDOAH_ERGO_OVERRIDE_DEFAULT(ShenandoahGarbageThreshold,     10);
 }

 bool ShenandoahCompactHeuristics::should_start_normal_gc() const {
   ShenandoahHeap* heap = ShenandoahHeap::heap();

   size_t available = heap->free_set()->available();
   size_t threshold_bytes_allocated = heap->capacity() * ShenandoahAllocationThreshold / 100;
+  size_t min_threshold = ShenandoahMinFreeThreshold * heap->capacity() / 100;
+
+  if (available < min_threshold) {
+    log_info(gc)("Trigger: Free (" SIZE_FORMAT "M) is below minimum threshold (" SIZE_FORMAT "M)",
+                 available / M, min_threshold / M);
+    return true;
+  }

   if (available < threshold_bytes_allocated) {
     log_info(gc)("Trigger: Free (" SIZE_FORMAT "M) is lower than allocated recently (" SIZE_FORMAT
"M)",
                  available / M, threshold_bytes_allocated / M);
     return true;

Testing: tier3_gc_shenandoah

Thanks,
-Aleksey