Oilpan: Discard unused system pages when sweeping NormalPageHeaps

third_party/WebKit/Source/platform/heap/HeapPage.cpp

Index: third_party/WebKit/Source/platform/heap/HeapPage.cpp
diff --git a/third_party/WebKit/Source/platform/heap/HeapPage.cpp b/third_party/WebKit/Source/platform/heap/HeapPage.cpp
index 637100d5f8eb42d30327a9ce160010aaeeb1702a..ce37af2f280fec49c7798ba9e254ad7c0df0a29d 100644
--- a/third_party/WebKit/Source/platform/heap/HeapPage.cpp
+++ b/third_party/WebKit/Source/platform/heap/HeapPage.cpp
@@ -1104,6 +1104,14 @@ void NormalPage::removeFromHeap()
     heapForNormalPage()->freePage(this);
 }
 
+static void discardPages(Address begin, Address end)
+{
+    uintptr_t beginAddress = WTF::roundUpToSystemPage(reinterpret_cast<uintptr_t>(begin));
+    uintptr_t endAddress = WTF::roundDownToSystemPage(reinterpret_cast<uintptr_t>(end));
+    if (beginAddress < endAddress)

[sof, 2016/02/04 10:20:04]

I understand the concern; for other live objects on that page, their next access
will be potentially costly.

[Yuta Kitamura, 2016/02/04 10:50:11]

We could make the condition a little bit tighter, i.e. discard memory
only when the unused interval is more than e.g. 32KB.

According to my experiment, the major contributor is large-sized
buckets, so I suspect there's enough memory that can be collected
by just discarding longer contiguous unused regions.

But that's kind of trade-offs; on low mem devices we probably should
return every unused 4K page, while on desktops we have some more
freedom on delaying the disposal. Just large contiguous memory should
be more prioritized than shorter regions.

+        WTF::discardSystemPages(reinterpret_cast<void*>(beginAddress), endAddress - beginAddress);
+}
+
 void NormalPage::sweep()
 {
     size_t markedObjectSize = 0;
@@ -1146,15 +1154,23 @@ void NormalPage::sweep()
             headerAddress += size;
             continue;
         }
-        if (startOfGap != headerAddress)
+        if (startOfGap != headerAddress) {
             heapForNormalPage()->addToFreeList(startOfGap, headerAddress - startOfGap);
+#if !ENABLE(ASSERT) && !defined(LEAK_SANITIZER) && !defined(ADDRESS_SANITIZER)
+            discardPages(startOfGap, headerAddress);
+#endif
+        }
         header->unmark();
         headerAddress += header->size();
         markedObjectSize += header->size();
         startOfGap = headerAddress;
     }
-    if (startOfGap != payloadEnd())
+    if (startOfGap != payloadEnd()) {
         heapForNormalPage()->addToFreeList(startOfGap, payloadEnd() - startOfGap);
+#if !ENABLE(ASSERT) && !defined(LEAK_SANITIZER) && !defined(ADDRESS_SANITIZER)
+        discardPages(startOfGap, payloadEnd());
+#endif
+    }
 
     if (markedObjectSize)
         Heap::increaseMarkedObjectSize(markedObjectSize);