From c6185c8034444aa6b08941280607f4eadb52d571 Mon Sep 17 00:00:00 2001
From: Joel Dice <joel.dice@gmail.com>
Date: Mon, 4 Mar 2013 16:02:59 -0700
Subject: [PATCH] remove "low memory" heuristic from heap code

The original goal was to minimize memory usage by garbage collecting
more frequently and more comprehensively as we got closer to the heap
limit.  In practice, though, this just slowed the VM to a crawl as
memory pressure increased.  If an app really wants to use a lot of
memory, the VM shouldn't penalize it aside from throwing an
OutOfMemoryError if it exceeds the limit.
---
 src/heap/heap.cpp | 51 ++++++-----------------------------------------
 1 file changed, 6 insertions(+), 45 deletions(-)

diff --git a/src/heap/heap.cpp b/src/heap/heap.cpp
index 01b6cffec8..d70286dd15 100644
--- a/src/heap/heap.cpp
+++ b/src/heap/heap.cpp
@@ -27,8 +27,6 @@ const unsigned Top = ~static_cast<unsigned>(0);
 const unsigned InitialGen2CapacityInBytes = 4 * 1024 * 1024;
 const unsigned InitialTenuredFixieCeilingInBytes = 4 * 1024 * 1024;
 
-const unsigned LowMemoryPaddingInBytes = 1024 * 1024;
-
 const bool Verbose = false;
 const bool Verbose2 = false;
 const bool Debug = false;
@@ -62,6 +60,7 @@ Aborter* getAborter(Context* c);
 
 void* tryAllocate(Context* c, unsigned size);
 void* allocate(Context* c, unsigned size);
+void* allocate(Context* c, unsigned size, bool limit);
 void free(Context* c, const void* p, unsigned size);
 
 #ifdef USE_ATOMIC_OPERATIONS
@@ -348,7 +347,8 @@ class Segment {
       capacity_ = desired;
       while (data == 0) {
         data = static_cast<uintptr_t*>
-          (tryAllocate(context, (footprint(capacity_)) * BytesPerWord));
+          (local::allocate
+           (context, (footprint(capacity_)) * BytesPerWord, false));
 
         if (data == 0) {
           if (capacity_ > minimum) {
@@ -609,7 +609,6 @@ class Context {
     client(0),
     count(0),
     limit(limit),
-    lowMemoryThreshold(limit / 2),
     lock(0),
     
     immortalHeapStart(0),
@@ -680,7 +679,6 @@ class Context {
 
   unsigned count;
   unsigned limit;
-  unsigned lowMemoryThreshold;
 
   System::Mutex* lock;
 
@@ -770,21 +768,6 @@ oversizedGen2(Context* c)
     and c->gen2.position() < (c->gen2.capacity() / 4);
 }
 
-inline unsigned
-memoryNeeded(Context* c)
-{
-  return c->count
-    + ((c->gen1.footprint(minimumNextGen1Capacity(c))
-        + c->gen2.footprint(minimumNextGen2Capacity(c))) * BytesPerWord)
-    + LowMemoryPaddingInBytes;
-}
-
-inline bool
-lowMemory(Context* c)
-{
-  return memoryNeeded(c) > c->lowMemoryThreshold;
-}
-
 inline void
 initNextGen1(Context* c)
 {
@@ -818,7 +801,7 @@ initNextGen2(Context* c)
   unsigned minimum = minimumNextGen2Capacity(c);
   unsigned desired = minimum;
 
-  if (not (lowMemory(c) or oversizedGen2(c))) {
+  if (not oversizedGen2(c)) {
     desired *= 2;
   }
 
@@ -1666,16 +1649,13 @@ collect2(Context* c)
 void
 collect(Context* c)
 {
-  if (lowMemory(c)
-      or oversizedGen2(c)
+  if (oversizedGen2(c)
       or c->tenureFootprint + c->tenurePadding > c->gen2.remaining()
       or c->fixieTenureFootprint + c->tenuredFixieFootprint
       > c->tenuredFixieCeiling)
   {
     if (Verbose) {
-      if (lowMemory(c)) {
-        fprintf(stderr, "low memory causes ");        
-      } else if (oversizedGen2(c)) {
+      if (oversizedGen2(c)) {
         fprintf(stderr, "oversized gen2 causes ");
       } else if (c->tenureFootprint + c->tenurePadding > c->gen2.remaining())
       {
@@ -1699,25 +1679,6 @@ collect(Context* c)
     then = c->system->now();
   }
 
-  unsigned count = memoryNeeded(c);
-  if (count > c->lowMemoryThreshold) {
-    if (Verbose) {
-      fprintf(stderr, "increase low memory threshold from %d to %d\n",
-              c->lowMemoryThreshold,
-              avg(c->limit, c->lowMemoryThreshold));
-    }
-
-    c->lowMemoryThreshold = avg(c->limit, c->lowMemoryThreshold);
-  } else if (count + (count / 16) < c->lowMemoryThreshold) {
-    if (Verbose) {
-      fprintf(stderr, "decrease low memory threshold from %d to %d\n",
-              c->lowMemoryThreshold,
-              avg(count, c->lowMemoryThreshold));
-    }
-
-    c->lowMemoryThreshold = avg(count, c->lowMemoryThreshold);
-  }
-
   initNextGen1(c);
 
   if (c->mode == Heap::MajorCollection) {