Use Fibonacci instead of 2^n for block growth.

Chrome on android showed an increase of 5% memory use when 2^n block
growth was introduced. Use Fibonacci instead.

BUG=chromium:699130

Change-Id: I228d66385c63d487e72db46356f44e9efb5fa0f3
Reviewed-on: https://skia-review.googlesource.com/9447
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Herb Derby <herb@google.com>

2 files changed, 9 insertions, 7 deletions

diff --git a/src/core/SkArenaAlloc.cpp b/src/core/SkArenaAlloc.cpp
index 779eaff0f1..eca3aa97d7 100644
--- a/src/core/SkArenaAlloc.cpp
+++ b/src/core/SkArenaAlloc.cpp
@@ -100,8 +100,9 @@ void SkArenaAlloc::ensureSpace(uint32_t size, uint32_t alignment) {
         objSizeAndOverhead += alignment - 1;
     }
 
-    uint32_t allocationSize = std::max(objSizeAndOverhead, fExtraSize << fLogGrowth);
-    fLogGrowth++;
+    uint32_t allocationSize = std::max(objSizeAndOverhead, fExtraSize * fFib0);
+    fFib0 += fFib1;
+    std::swap(fFib0, fFib1);
 
     // Round up to a nice size. If > 32K align to 4K boundary else up to max_align_t. The > 32K
     // heuristic is from the JEMalloc behavior.
diff --git a/src/core/SkArenaAlloc.h b/src/core/SkArenaAlloc.h
index 04f929181c..494696ce76 100644
--- a/src/core/SkArenaAlloc.h
+++ b/src/core/SkArenaAlloc.h
@@ -54,8 +54,9 @@
 // addition overhead when switching from POD data to non-POD data of typically 8 bytes.
 //
 // If additional blocks are needed they are increased exponentially. This strategy bounds the
-// recursion of the RunDtorsOnBlock to be limited to O(ln size-of-memory). In practical terms, this
-// is a maximum recursion depth of 33 for an 8GB machine but usually much less.
+// recursion of the RunDtorsOnBlock to be limited to O(log size-of-memory). Block size grow using
+// the Fibonacci sequence which means that for 2^32 memory there are 48 allocations, and for 2^48
+// there are 71 allocations.
 class SkArenaAlloc {
 public:
     SkArenaAlloc(char* block, size_t size, size_t extraSize = 0);
@@ -196,9 +197,9 @@ private:
     char* const    fFirstBlock;
     const uint32_t fFirstSize;
     const uint32_t fExtraSize;
-    // The extra size allocations grow exponentially:
-    //    size-allocated = extraSize * 2 ^ fLogGrowth.
-    uint8_t        fLogGrowth {0};
+    // Use the Fibonacci sequence as the growth factor for block size. The size of the block
+    // allocated is fFib0 * fExtraSize. Using 2 ^ n * fExtraSize had too much slop for Android.
+    uint32_t       fFib0 {1}, fFib1 {1};
 };
 
 #endif//SkFixedAlloc_DEFINED