Thread-specific ticker statistics

Summary:
The global atomics we previously used for tickers had poor cache performance
since they were typically updated from different threads, causing frequent
invalidations. In this diff,

- recordTick() updates a local ticker value specific to the thread in which it was called
- When a thread exits, its local ticker value is added into merged_sum
- getTickerCount() returns the sum of all threads' local ticker values and the merged_sum
- setTickerCount() resets all threads' local ticker values and sets merged_sum to the value provided by the caller.

In a next diff I will make a similar change for histogram stats.

Test Plan:
before:

  $ TEST_TMPDIR=/dev/shm/ perf record -g ./db_bench --benchmarks=readwhilewriting --statistics --num=1000000 --use_existing_db --threads=64 --cache_size=250000000 --compression_type=lz4
  $ perf report -g --stdio | grep recordTick
  7.59%  db_bench     db_bench             [.] rocksdb::StatisticsImpl::recordTick
  ...

after:

  $ TEST_TMPDIR=/dev/shm/ perf record -g ./db_bench --benchmarks=readwhilewriting --statistics --num=1000000 --use_existing_db --threads=64 --cache_size=250000000 --compression_type=lz4
  $ perf report -g --stdio | grep recordTick
  1.46%  db_bench     db_bench             [.] rocksdb::StatisticsImpl::recordTick
  ...

Reviewers: kradhakrishnan, MarkCallaghan, IslamAbdelRahman, sdong

Reviewed By: sdong

Subscribers: yiwu, andrewkr, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D62337

util/statistics.cc

@@ -32,12 +32,19 @@ StatisticsImpl::StatisticsImpl(
 StatisticsImpl::~StatisticsImpl() {}
 
 uint64_t StatisticsImpl::getTickerCount(uint32_t tickerType) const {
+  MutexLock lock(&aggregate_lock_);
   assert(
     enable_internal_stats_ ?
       tickerType < INTERNAL_TICKER_ENUM_MAX :
       tickerType < TICKER_ENUM_MAX);
-  // Return its own ticker version
-  return tickers_[tickerType].value;
+  uint64_t thread_local_sum = 0;
+  tickers_[tickerType].thread_value->Fold(
+      [](void* curr_ptr, void* res) {
+        auto* sum_ptr = static_cast<uint64_t*>(res);
+        *sum_ptr += static_cast<std::atomic_uint_fast64_t*>(curr_ptr)->load();
+      },
+      &thread_local_sum);
+  return thread_local_sum + tickers_[tickerType].merged_sum.load();
 }
 
 void StatisticsImpl::histogramData(uint32_t histogramType,
@@ -56,13 +63,31 @@ std::string StatisticsImpl::getHistogramString(uint32_t histogramType) const {
   return histograms_[histogramType].ToString();
 }
 
+StatisticsImpl::ThreadTickerInfo* StatisticsImpl::getThreadTickerInfo(
+    uint32_t tickerType) {
+  auto info_ptr =
+      static_cast<ThreadTickerInfo*>(tickers_[tickerType].thread_value->Get());
+  if (info_ptr == nullptr) {
+    info_ptr =
+        new ThreadTickerInfo(0 /* value */, &tickers_[tickerType].merged_sum);
+    tickers_[tickerType].thread_value->Reset(info_ptr);
+  }
+  return info_ptr;
+}
+
 void StatisticsImpl::setTickerCount(uint32_t tickerType, uint64_t count) {
-  assert(
-    enable_internal_stats_ ?
-      tickerType < INTERNAL_TICKER_ENUM_MAX :
-      tickerType < TICKER_ENUM_MAX);
-  if (tickerType < TICKER_ENUM_MAX || enable_internal_stats_) {
-    tickers_[tickerType].value.store(count, std::memory_order_relaxed);
+  {
+    MutexLock lock(&aggregate_lock_);
+    assert(enable_internal_stats_ ? tickerType < INTERNAL_TICKER_ENUM_MAX
+                                  : tickerType < TICKER_ENUM_MAX);
+    if (tickerType < TICKER_ENUM_MAX || enable_internal_stats_) {
+      tickers_[tickerType].thread_value->Fold(
+          [](void* curr_ptr, void* res) {
+            static_cast<std::atomic<uint64_t>*>(curr_ptr)->store(0);
+          },
+          nullptr /* res */);
+      tickers_[tickerType].merged_sum.store(count);
+    }
   }
   if (stats_ && tickerType < TICKER_ENUM_MAX) {
     stats_->setTickerCount(tickerType, count);
@@ -75,7 +100,8 @@ void StatisticsImpl::recordTick(uint32_t tickerType, uint64_t count) {
       tickerType < INTERNAL_TICKER_ENUM_MAX :
       tickerType < TICKER_ENUM_MAX);
   if (tickerType < TICKER_ENUM_MAX || enable_internal_stats_) {
-    tickers_[tickerType].value.fetch_add(count, std::memory_order_relaxed);
+    auto info_ptr = getThreadTickerInfo(tickerType);
+    info_ptr->value.fetch_add(count);
   }
   if (stats_ && tickerType < TICKER_ENUM_MAX) {
     stats_->recordTick(tickerType, count);

util/statistics.h

@@ -10,10 +10,11 @@
 #include <atomic>
 #include <string>
 
+#include "port/likely.h"
+#include "port/port.h"
 #include "util/histogram.h"
 #include "util/mutexlock.h"
-#include "port/likely.h"
-
+#include "util/thread_local.h"
 
 namespace rocksdb {
 
@@ -50,24 +51,46 @@ class StatisticsImpl : public Statistics {
   std::shared_ptr<Statistics> stats_shared_;
   Statistics* stats_;
   bool enable_internal_stats_;
+  // Synchronizes setTickerCount()/getTickerCount() operations so partially
+  // completed setTickerCount() won't be visible.
+  mutable port::Mutex aggregate_lock_;
 
-  struct Ticker {
-    Ticker() : value(uint_fast64_t()) {}
-
+  // Holds data maintained by each thread for implementing tickers.
+  struct ThreadTickerInfo {
     std::atomic_uint_fast64_t value;
-    // Pad the structure to make it size of 64 bytes. A plain array of
-    // std::atomic_uint_fast64_t results in huge performance degradataion
-    // due to false sharing.
-    char padding[64 - sizeof(std::atomic_uint_fast64_t)];
+    // During teardown, value will be summed into *merged_sum.
+    std::atomic_uint_fast64_t* merged_sum;
+
+    ThreadTickerInfo(uint_fast64_t _value,
+                     std::atomic_uint_fast64_t* _merged_sum)
+        : value(_value), merged_sum(_merged_sum) {}
   };
 
-  static_assert(sizeof(Ticker) == 64, "Expecting to fit into 64 bytes");
+  struct Ticker {
+    Ticker()
+        : thread_value(new ThreadLocalPtr(&mergeThreadValue)), merged_sum(0) {}
+    // Holds thread-specific pointer to ThreadTickerInfo
+    std::unique_ptr<ThreadLocalPtr> thread_value;
+    // Sum of thread-specific values for tickers that have been reset due to
+    // thread termination or ThreadLocalPtr destruction. Also, this is used by
+    // setTickerCount() to conveniently change the global value by setting this
+    // while simultaneously zeroing all thread-local values.
+    std::atomic_uint_fast64_t merged_sum;
+
+    static void mergeThreadValue(void* ptr) {
+      auto info_ptr = static_cast<ThreadTickerInfo*>(ptr);
+      *info_ptr->merged_sum += info_ptr->value;
+      delete info_ptr;
+    }
+  };
 
+  // Returns the info for this tickerType/thread. It sets a new info with zeroed
+  // counter if none exists.
+  ThreadTickerInfo* getThreadTickerInfo(uint32_t tickerType);
+
+  Ticker tickers_[INTERNAL_TICKER_ENUM_MAX];
   // Attributes expand to nothing depending on the platform
   __declspec(align(64))
-  Ticker tickers_[INTERNAL_TICKER_ENUM_MAX]
-     __attribute__((aligned(64)));
-  __declspec(align(64))
   HistogramImpl histograms_[INTERNAL_HISTOGRAM_ENUM_MAX]
       __attribute__((aligned(64)));
 };