commit 6e6078ab06e16bb4032b363316360f3b76685e30
author Mathieu Chartier <mathieuc@google.com> Mon Oct 24 15:45:41 2016 -0700
committer Mathieu Chartier <mathieuc@google.com> Mon Oct 24 19:23:06 2016 -0700
tree 539b5b28159f2b8e6a3893b03915c65c91474761
parent 3209925b7c0e173741885e63f747af16dac78469

Clear (madvise) card table for CC

Since the cards are not really used, we can madvise them to reduce
RAM. Image and zygote cards are cleared in pause to prevent races
between with mutators.

Pause time goes up by only 1.5us on N6P maps:
(Paused)ClearCards: Sum: 755us 99% C.I. 0.250us-28us Avg: 1.540us Max: 28us

AOSP N6P before (60s after boot system wide CC):
4,194K: .GC
4,172K: .GC
4,110K: .GC

After:
3,253K: .GC
3,205K: .GC
3,245K: .GC

Bug: 12687968

Test: test-art-host

Change-Id: I3194b5b8044c2dca427302c32d9974920fecb289

runtime/gc/accounting/card_table-inl.h

diff --git a/runtime/gc/accounting/card_table-inl.h b/runtime/gc/accounting/card_table-inl.h
index f72f219..6ff5359 100644
--- a/runtime/gc/accounting/card_table-inl.h
+++ b/runtime/gc/accounting/card_table-inl.h
@@ -50,13 +50,17 @@
 }
 
 template <bool kClearCard, typename Visitor>
-inline size_t CardTable::Scan(ContinuousSpaceBitmap* bitmap, uint8_t* scan_begin, uint8_t* scan_end,
-                              const Visitor& visitor, const uint8_t minimum_age) const {
+inline size_t CardTable::Scan(ContinuousSpaceBitmap* bitmap,
+                              uint8_t* const scan_begin,
+                              uint8_t* const scan_end,
+                              const Visitor& visitor,
+                              const uint8_t minimum_age) {
   DCHECK_GE(scan_begin, reinterpret_cast<uint8_t*>(bitmap->HeapBegin()));
   // scan_end is the byte after the last byte we scan.
   DCHECK_LE(scan_end, reinterpret_cast<uint8_t*>(bitmap->HeapLimit()));
-  uint8_t* card_cur = CardFromAddr(scan_begin);
-  uint8_t* card_end = CardFromAddr(AlignUp(scan_end, kCardSize));
+  uint8_t* const card_begin = CardFromAddr(scan_begin);
+  uint8_t* const card_end = CardFromAddr(AlignUp(scan_end, kCardSize));
+  uint8_t* card_cur = card_begin;
   CheckCardValid(card_cur);
   CheckCardValid(card_end);
   size_t cards_scanned = 0;
@@ -67,9 +71,6 @@
       uintptr_t start = reinterpret_cast<uintptr_t>(AddrFromCard(card_cur));
       bitmap->VisitMarkedRange(start, start + kCardSize, visitor);
       ++cards_scanned;
-      if (kClearCard) {
-        *card_cur = 0;
-      }
     }
     ++card_cur;
   }
@@ -99,9 +100,6 @@
             << "card " << static_cast<size_t>(*card) << " intptr_t " << (start_word & 0xFF);
         bitmap->VisitMarkedRange(start, start + kCardSize, visitor);
         ++cards_scanned;
-        if (kClearCard) {
-          *card = 0;
-        }
       }
       start_word >>= 8;
       start += kCardSize;
@@ -116,13 +114,14 @@
       uintptr_t start = reinterpret_cast<uintptr_t>(AddrFromCard(card_cur));
       bitmap->VisitMarkedRange(start, start + kCardSize, visitor);
       ++cards_scanned;
-      if (kClearCard) {
-        *card_cur = 0;
-      }
     }
     ++card_cur;
   }
 
+  if (kClearCard) {
+    ClearCardRange(scan_begin, scan_end);
+  }
+
   return cards_scanned;
 }
 
@@ -135,7 +134,9 @@
  * us to know which cards got cleared.
  */
 template <typename Visitor, typename ModifiedVisitor>
-inline void CardTable::ModifyCardsAtomic(uint8_t* scan_begin, uint8_t* scan_end, const Visitor& visitor,
+inline void CardTable::ModifyCardsAtomic(uint8_t* scan_begin,
+                                         uint8_t* scan_end,
+                                         const Visitor& visitor,
                                          const ModifiedVisitor& modified) {
   uint8_t* card_cur = CardFromAddr(scan_begin);
   uint8_t* card_end = CardFromAddr(AlignUp(scan_end, kCardSize));