Drop de-duplication of heap sample (aka heapz) entries.

pprof can aggregate them, and it can do it way better than we can. With
proper unsampling etc.

src/stack_trace_table.cc

@@ -42,27 +42,15 @@
 
 namespace tcmalloc {
 
-bool StackTraceTable::Bucket::KeyEqual(uintptr_t h,
-                                       const StackTrace& t) const {
-  const bool eq = (this->hash == h && this->trace.depth == t.depth);
-  for (int i = 0; eq && i < t.depth; ++i) {
-    if (this->trace.stack[i] != t.stack[i]) {
-      return false;
-    }
-  }
-  return eq;
-}
-
 StackTraceTable::StackTraceTable()
     : error_(false),
       depth_total_(0),
       bucket_total_(0),
-      table_(new Bucket*[kHashTableSize]()) {
-  memset(table_, 0, kHashTableSize * sizeof(Bucket*));
+      head_(nullptr) {
 }
 
 StackTraceTable::~StackTraceTable() {
-  delete[] table_;
+  ASSERT(head_ == nullptr);
 }
 
 void StackTraceTable::AddTrace(const StackTrace& t) {
@@ -70,89 +58,64 @@ void StackTraceTable::AddTrace(const StackTrace& t) {
     return;
   }
 
-  // Hash function borrowed from base/heap-profile-table.cc
-  uintptr_t h = 0;
-  for (int i = 0; i < t.depth; ++i) {
-    h += reinterpret_cast<uintptr_t>(t.stack[i]);
-    h += h << 10;
-    h ^= h >> 6;
-  }
-  h += h << 3;
-  h ^= h >> 11;
-
-  const int idx = h % kHashTableSize;
-
-  Bucket* b = table_[idx];
-  while (b != NULL && !b->KeyEqual(h, t)) {
-    b = b->next;
-  }
-  if (b != NULL) {
-    b->count++;
-    b->trace.size += t.size;  // keep cumulative size
-  } else {
   depth_total_ += t.depth;
   bucket_total_++;
-    b = Static::bucket_allocator()->New();
-    if (b == NULL) {
+  Entry* entry = allocator_.allocate(1);
+  if (entry == nullptr) {
     Log(kLog, __FILE__, __LINE__,
-          "tcmalloc: could not allocate bucket", sizeof(*b));
+        "tcmalloc: could not allocate bucket", sizeof(*entry));
     error_ = true;
   } else {
-      b->hash = h;
-      b->trace = t;
-      b->count = 1;
-      b->next = table_[idx];
-      table_[idx] = b;
-    }
+    entry->trace = t;
+    entry->next = head_;
+    head_ = entry;
   }
 }
 
 void** StackTraceTable::ReadStackTracesAndClear() {
-  if (error_) {
-    return NULL;
-  }
+  void** out = nullptr;
 
-  // Allocate output array
   const int out_len = bucket_total_ * 3 + depth_total_ + 1;
-  void** out = new void*[out_len];
-  if (out == NULL) {
+  if (!error_) {
+    // Allocate output array
+    out = new (std::nothrow_t{}) void*[out_len];
+    if (out == nullptr) {
       Log(kLog, __FILE__, __LINE__,
           "tcmalloc: allocation failed for stack traces",
           out_len * sizeof(*out));
-    return NULL;
+    }
   }
 
+  if (out) {
     // Fill output array
     int idx = 0;
-  for (int i = 0; i < kHashTableSize; ++i) {
-    Bucket* b = table_[i];
-    while (b != NULL) {
-      out[idx++] = reinterpret_cast<void*>(static_cast<uintptr_t>(b->count));
-      out[idx++] = reinterpret_cast<void*>(b->trace.size);  // cumulative size
-      out[idx++] = reinterpret_cast<void*>(b->trace.depth);
-      for (int d = 0; d < b->trace.depth; ++d) {
-        out[idx++] = b->trace.stack[d];
-      }
-      b = b->next;
+    Entry* entry = head_;
+    while (entry != NULL) {
+      out[idx++] = reinterpret_cast<void*>(uintptr_t{1});   // count
+      out[idx++] = reinterpret_cast<void*>(entry->trace.size);  // cumulative size
+      out[idx++] = reinterpret_cast<void*>(entry->trace.depth);
+      for (int d = 0; d < entry->trace.depth; ++d) {
+        out[idx++] = entry->trace.stack[d];
       }
+      entry = entry->next;
     }
     out[idx++] = NULL;
     ASSERT(idx == out_len);
+  }
 
   // Clear state
   error_ = false;
   depth_total_ = 0;
   bucket_total_ = 0;
+
   SpinLockHolder h(Static::pageheap_lock());
-  for (int i = 0; i < kHashTableSize; ++i) {
-    Bucket* b = table_[i];
-    while (b != NULL) {
-      Bucket* next = b->next;
-      Static::bucket_allocator()->Delete(b);
-      b = next;
-    }
-    table_[i] = NULL;
+  Entry* entry = head_;
+  while (entry != nullptr) {
+    Entry* next = entry->next;
+    allocator_.deallocate(entry, 1);
+    entry = next;
   }
+  head_ = nullptr;
 
   return out;
 }

src/stack_trace_table.h

@@ -41,6 +41,7 @@
 #include <stdint.h>                     // for uintptr_t
 #endif
 #include "common.h"
+#include "page_heap_allocator.h"
 
 namespace tcmalloc {
 
@@ -62,29 +63,21 @@ class PERFTOOLS_DLL_DECL StackTraceTable {
   void** ReadStackTracesAndClear();
 
   // Exposed for PageHeapAllocator
-  struct Bucket {
-    // Key
-    uintptr_t hash;
-    StackTrace trace;
-
-    // Payload
-    int count;
-    Bucket* next;
-
-    bool KeyEqual(uintptr_t h, const StackTrace& t) const;
-  };
-
   // For testing
   int depth_total() const { return depth_total_; }
   int bucket_total() const { return bucket_total_; }
 
  private:
-  static const int kHashTableSize = 1 << 14; // => table_ is 128k
+  struct Entry {
+    Entry* next;
+    StackTrace trace;
+  };
 
   bool error_;
   int depth_total_;
   int bucket_total_;
-  Bucket** table_;
+  Entry* head_;
+  STLPageHeapAllocator<Entry, void> allocator_;
 };
 
 }  // namespace tcmalloc

src/static_vars.cc

@@ -74,7 +74,6 @@ CentralFreeListPadded Static::central_cache_[kClassSizesMax];
 PageHeapAllocator<Span> Static::span_allocator_;
 PageHeapAllocator<StackTrace> Static::stacktrace_allocator_;
 Span Static::sampled_objects_;
-PageHeapAllocator<StackTraceTable::Bucket> Static::bucket_allocator_;
 StackTrace* Static::growth_stacks_ = NULL;
 Static::PageHeapStorage Static::pageheap_;
 
@@ -84,7 +83,6 @@ void Static::InitStaticVars() {
   span_allocator_.New(); // Reduce cache conflicts
   span_allocator_.New(); // Reduce cache conflicts
   stacktrace_allocator_.Init();
-  bucket_allocator_.Init();
   // Do a bit of sanitizing: make sure central_cache is aligned properly
   CHECK_CONDITION((sizeof(central_cache_[0]) % 64) == 0);
   for (int i = 0; i < num_size_classes(); ++i) {

src/static_vars.h

@@ -83,9 +83,6 @@ class Static {
 
   // State kept for sampled allocations (/pprof/heap support)
   static Span* sampled_objects() { return &sampled_objects_; }
-  static PageHeapAllocator<StackTraceTable::Bucket>* bucket_allocator() {
-    return &bucket_allocator_;
-  }
 
   // Check if InitStaticVars() has been run.
   static bool IsInited() { return inited_; }
@@ -107,7 +104,6 @@ class Static {
   ATTRIBUTE_HIDDEN static PageHeapAllocator<Span> span_allocator_;
   ATTRIBUTE_HIDDEN static PageHeapAllocator<StackTrace> stacktrace_allocator_;
   ATTRIBUTE_HIDDEN static Span sampled_objects_;
-  ATTRIBUTE_HIDDEN static PageHeapAllocator<StackTraceTable::Bucket> bucket_allocator_;
 
   // Linked list of stack traces recorded every time we allocated memory
   // from the system.  Useful for finding allocation sites that cause

src/tests/stack_trace_table_test.cc

@@ -70,18 +70,10 @@ int main(int argc, char **argv) {
   AddTrace(&table, t2);
   CHECK_EQ(table.depth_total(), 4);
   CHECK_EQ(table.bucket_total(), 2);
-  static const uintptr_t k3[] = {1, 1024, 2, 1, 2, 1,  512, 2, 2, 1, 0};
+  static const uintptr_t k3[] = {1, 512, 2, 2, 1, 1, 1024, 2, 1, 2, 0};
   CheckTracesAndReset(&table, k3, ARRAYSIZE(k3));
 
-  // Table w/ 2 x t1, 1 x t2
-  AddTrace(&table, t1);
-  AddTrace(&table, t2);
-  AddTrace(&table, t1);
-  CHECK_EQ(table.depth_total(), 4);
-  CHECK_EQ(table.bucket_total(), 2);
-  static const uintptr_t k4[] = {2, 2048, 2, 1, 2, 1,  512, 2, 2, 1, 0};
-  CheckTracesAndReset(&table, k4, ARRAYSIZE(k4));
-
+  // Table w/ t1, t3
   // Same stack as t1, but w/ different size
   tcmalloc::StackTrace t3;
   t3.size = static_cast<uintptr_t>(2);
@@ -89,12 +81,11 @@ int main(int argc, char **argv) {
   t3.stack[0] = reinterpret_cast<void*>(1);
   t3.stack[1] = reinterpret_cast<void*>(2);
 
-  // Table w/ t1, t3
   AddTrace(&table, t1);
   AddTrace(&table, t3);
-  CHECK_EQ(table.depth_total(), 2);
-  CHECK_EQ(table.bucket_total(), 1);
-  static const uintptr_t k5[] = {2, 1026, 2, 1, 2, 0};
+  CHECK_EQ(table.depth_total(), 4);
+  CHECK_EQ(table.bucket_total(), 2);
+  static const uintptr_t k5[] = {1, 2, 2, 1, 2, 1, 1024, 2, 1, 2, 0};
   CheckTracesAndReset(&table, k5, ARRAYSIZE(k5));
 
   puts("PASS");