[RocksDB] Added nano second stopwatch and new perf counters to track block read cost

Summary: The pupose of this diff is to expose per user-call level precise timing of block read, so that we can answer questions like: a Get() costs me 100ms, is that somehow related to loading blocks from file system, or sth else? We will answer that with EXACTLY how many blocks have been read, how much time was spent on transfering the bytes from os, how much time was spent on checksum verification and how much time was spent on block decompression, just for that one Get. A nano second stopwatch was introduced to track time with higher precision. The cost/precision of the stopwatch is also measured in unit-test. On my dev box, retrieving one time instance costs about 30ns, on average. The deviation of timing results is good enough to track 100ns-1us level events. And the overhead could be safely ignored for 100us level events (10000 instances/s), for example, a viewstate thrift call.

Test Plan: perf_context_test, also testing with viewstate shadow traffic.

Reviewers: dhruba

Reviewed By: dhruba

CC: leveldb, xjin

Differential Revision: https://reviews.facebook.net/D12351

build_tools/build_detect_platform

@@ -82,7 +82,7 @@ case "$TARGET_OS" in
         if [ -z "$USE_CLANG" ]; then
             COMMON_FLAGS="$COMMON_FLAGS -fno-builtin-memcmp"
         fi
-        PLATFORM_LDFLAGS="$PLATFORM_LDFLAGS -lpthread"
+        PLATFORM_LDFLAGS="$PLATFORM_LDFLAGS -lpthread -lrt"
         # PORT_FILES=port/linux/linux_specific.cc
         ;;
     SunOS)

db/dbformat.cc

@@ -6,7 +6,7 @@
 #include "db/dbformat.h"
 #include "port/port.h"
 #include "util/coding.h"
-#include "rocksdb/perf_context.h"
+#include "util/perf_context_imp.h"
 
 namespace leveldb {
 
@@ -54,7 +54,7 @@ int InternalKeyComparator::Compare(const Slice& akey, const Slice& bkey) const {
   //    decreasing sequence number
   //    decreasing type (though sequence# should be enough to disambiguate)
   int r = user_comparator_->Compare(ExtractUserKey(akey), ExtractUserKey(bkey));
-  perf_context.user_key_comparison_count++;
+  BumpPerfCount(&perf_context.user_key_comparison_count);
   if (r == 0) {
     const uint64_t anum = DecodeFixed64(akey.data() + akey.size() - 8);
     const uint64_t bnum = DecodeFixed64(bkey.data() + bkey.size() - 8);

db/perf_context.cc

-#include "rocksdb/perf_context.h"
-
-
-namespace leveldb {
-
-void PerfContext::Reset() {
-  user_key_comparison_count = 0;
-}
-
-__thread PerfContext perf_context;
-
-}

db/perf_context_test.cc

 #include <iostream>
+#include <vector>
 
 #include "rocksdb/db.h"
 #include "rocksdb/perf_context.h"
+#include "util/histogram.h"
+#include "util/stop_watch.h"
 #include "util/testharness.h"
 
 
@@ -10,11 +13,11 @@ namespace leveldb {
 // Path to the database on file system
 const std::string kDbName = test::TmpDir() + "/perf_context_test";
 
-std::shared_ptr<DB> OpenDb() {
+std::shared_ptr<DB> OpenDb(size_t write_buffer_size) {
     DB* db;
     Options options;
     options.create_if_missing = true;
-    options.write_buffer_size = 1000000000;     // give it a big memtable
+    options.write_buffer_size = write_buffer_size;
     Status s = DB::Open(options, kDbName,  &db);
     ASSERT_OK(s);
     return std::shared_ptr<DB>(db);
@@ -24,11 +27,48 @@ class PerfContextTest { };
 
 int kTotalKeys = 100;
 
-TEST(PerfContextTest, KeyComparisonCount) {
+TEST(PerfContextTest, StopWatchNanoOverhead) {
+  // profile the timer cost by itself!
+  const int kTotalIterations = 1000000;
+  std::vector<uint64_t> timings(kTotalIterations);
+
+  StopWatchNano timer(Env::Default(), true);
+  for (auto& timing : timings) {
+    timing = timer.ElapsedNanos(true /* reset */);
+  }
+
+  HistogramImpl histogram;
+  for (const auto timing : timings) {
+    histogram.Add(timing);
+  }
+
+  std::cout << histogram.ToString();
+}
+
+TEST(PerfContextTest, StopWatchOverhead) {
+  // profile the timer cost by itself!
+  const int kTotalIterations = 1000000;
+  std::vector<uint64_t> timings(kTotalIterations);
+
+  StopWatch timer(Env::Default());
+  for (auto& timing : timings) {
+    timing = timer.ElapsedMicros();
+  }
+
+  HistogramImpl histogram;
+  uint64_t prev_timing = 0;
+  for (const auto timing : timings) {
+    histogram.Add(timing - prev_timing);
+    prev_timing = timing;
+  }
+
+  std::cout << histogram.ToString();
+}
 
+void ProfileKeyComparison() {
   DestroyDB(kDbName, Options());    // Start this test with a fresh DB
 
-  auto db = OpenDb();
+  auto db = OpenDb(1000000000);
 
   WriteOptions write_options;
   ReadOptions read_options;
@@ -63,12 +103,20 @@ TEST(PerfContextTest, KeyComparisonCount) {
             << max_user_key_comparison_get << "\n"
             << "avg user key comparison get:"
             << total_user_key_comparison_get/kTotalKeys << "\n";
-
 }
 
+TEST(PerfContextTest, KeyComparisonCount) {
+  SetPerfLevel(kDisable);
+  ProfileKeyComparison();
+
+  SetPerfLevel(kEnableCount);
+  ProfileKeyComparison();
 
+  SetPerfLevel(kEnableTime);
+  ProfileKeyComparison();
 }
 
+}
 
 int main(int argc, char** argv) {
 

include/rocksdb/env.h

@@ -190,6 +190,13 @@ class Env {
   // useful for computing deltas of time.
   virtual uint64_t NowMicros() = 0;
 
+  // Returns the number of nano-seconds since some fixed point in time. Only
+  // useful for computing deltas of time in one run.
+  // Default implementation simply relies on NowMicros
+  virtual uint64_t NowNanos() {
+    return NowMicros() * 1000;
+  }
+
   // Sleep/delay the thread for the perscribed number of micro-seconds.
   virtual void SleepForMicroseconds(int micros) = 0;
 

include/rocksdb/perf_context.h

@@ -5,6 +5,15 @@
 
 namespace leveldb {
 
+enum PerfLevel {
+  kDisable        = 0,  // disable perf stats
+  kEnableCount    = 1,  // enable only count stats
+  kEnableTime     = 2   // enable time stats too
+};
+
+// set the perf stats level
+void SetPerfLevel(PerfLevel level);
+
 // A thread local context for gathering performance counter efficiently
 // and transparently.
 
@@ -12,8 +21,13 @@ struct PerfContext {
 
   void Reset(); // reset all performance counters to zero
 
-
   uint64_t user_key_comparison_count; // total number of user key comparisons
+  uint64_t block_cache_hit_count;
+  uint64_t block_read_count;
+  uint64_t block_read_byte;
+  uint64_t block_read_time;
+  uint64_t block_checksum_time;
+  uint64_t block_decompress_time;
 };
 
 extern __thread PerfContext perf_context;

table/format.cc

@@ -4,11 +4,12 @@
 
 #include "table/format.h"
 
-#include "rocksdb/env.h"
 #include "port/port.h"
+#include "rocksdb/env.h"
 #include "table/block.h"
 #include "util/coding.h"
 #include "util/crc32c.h"
+#include "util/perf_context_imp.h"
 
 namespace leveldb {
 
@@ -69,7 +70,8 @@ Status Footer::DecodeFrom(Slice* input) {
 Status ReadBlockContents(RandomAccessFile* file,
                          const ReadOptions& options,
                          const BlockHandle& handle,
-                         BlockContents* result) {
+                         BlockContents* result,
+                         Env* env) {
   result->data = Slice();
   result->cachable = false;
   result->heap_allocated = false;
@@ -79,7 +81,14 @@ Status ReadBlockContents(RandomAccessFile* file,
   size_t n = static_cast<size_t>(handle.size());
   char* buf = new char[n + kBlockTrailerSize];
   Slice contents;
+
+  StopWatchNano timer(env);
+  StartPerfTimer(&timer);
   Status s = file->Read(handle.offset(), n + kBlockTrailerSize, &contents, buf);
+  BumpPerfCount(&perf_context.block_read_count);
+  BumpPerfCount(&perf_context.block_read_byte, n + kBlockTrailerSize);
+  BumpPerfTime(&perf_context.block_read_time, &timer);
+
   if (!s.ok()) {
     delete[] buf;
     return s;
@@ -99,6 +108,7 @@ Status ReadBlockContents(RandomAccessFile* file,
       s = Status::Corruption("block checksum mismatch");
       return s;
     }
+    BumpPerfTime(&perf_context.block_checksum_time, &timer);
   }
 
   char* ubuf = nullptr;
@@ -172,6 +182,8 @@ Status ReadBlockContents(RandomAccessFile* file,
       return Status::Corruption("bad block type");
   }
 
+  BumpPerfTime(&perf_context.block_decompress_time, &timer);
+
   return Status::OK();
 }
 

table/format.h

@@ -94,7 +94,8 @@ struct BlockContents {
 extern Status ReadBlockContents(RandomAccessFile* file,
                                 const ReadOptions& options,
                                 const BlockHandle& handle,
-                                BlockContents* result);
+                                BlockContents* result,
+                                Env* env);
 
 // Implementation details follow.  Clients should ignore,
 

table/table.cc

@@ -19,6 +19,7 @@
 #include "table/two_level_iterator.h"
 
 #include "util/coding.h"
+#include "util/perf_context_imp.h"
 #include "util/stop_watch.h"
 
 namespace leveldb {
@@ -81,9 +82,10 @@ Status ReadBlock(RandomAccessFile* file,
                  const ReadOptions& options,
                  const BlockHandle& handle,
                  Block** result,
+                 Env* env,
                  bool* didIO = nullptr) {
   BlockContents contents;
-  Status s = ReadBlockContents(file, options, handle, &contents);
+  Status s = ReadBlockContents(file, options, handle, &contents, env);
   if (s.ok()) {
     *result = new Block(contents);
   }
@@ -118,14 +120,14 @@ Status Table::Open(const Options& options,
     return Status::InvalidArgument("file is too short to be an sstable");
   }
 
-
   Footer footer;
   s = footer.DecodeFrom(&footer_input);
   if (!s.ok()) return s;
 
   Block* index_block = nullptr;
   // TODO: we never really verify check sum for index block
-  s = ReadBlock(file.get(), ReadOptions(), footer.index_handle(), &index_block);
+  s = ReadBlock(file.get(), ReadOptions(), footer.index_handle(), &index_block,
+                options.env);
 
   if (s.ok()) {
     // We've successfully read the footer and the index block: we're
@@ -176,7 +178,7 @@ void Table::ReadMeta(const Footer& footer) {
   //  TODO: we never really verify check sum for meta index block
   Block* meta = nullptr;
   if (!ReadBlock(rep_->file.get(), ReadOptions(), footer.metaindex_handle(),
-                 &meta).ok()) {
+                 &meta, rep_->options.env).ok()) {
     // Do not propagate errors since meta info is not needed for operation
     return;
   }
@@ -203,7 +205,8 @@ void Table::ReadFilter(const Slice& filter_handle_value) {
   // requiring checksum verification in Table::Open.
   ReadOptions opt;
   BlockContents block;
-  if (!ReadBlockContents(rep_->file.get(), opt, filter_handle, &block).ok()) {
+  if (!ReadBlockContents(rep_->file.get(), opt, filter_handle, &block,
+                        rep_->options.env).ok()) {
     return;
   }
   if (block.heap_allocated) {
@@ -266,6 +269,7 @@ Iterator* Table::BlockReader(void* arg,
       if (cache_handle != nullptr) {
         block = reinterpret_cast<Block*>(block_cache->Value(cache_handle));
 
+        BumpPerfCount(&perf_context.block_cache_hit_count);
         RecordTick(statistics, BLOCK_CACHE_HIT);
       } else if (no_io) {
         // Did not find in block_cache and can't do IO
@@ -280,6 +284,7 @@ Iterator* Table::BlockReader(void* arg,
                 options,
                 handle,
                 &block,
+                table->rep_->options.env,
                 didIO
               );
         }
@@ -295,8 +300,9 @@ Iterator* Table::BlockReader(void* arg,
     } else if (no_io) {
       // Could not read from block_cache and can't do IO
       return NewErrorIterator(Status::Incomplete("no blocking io"));
-    }else {
-      s = ReadBlock(table->rep_->file.get(), options, handle, &block, didIO);
+    } else {
+      s = ReadBlock(table->rep_->file.get(), options, handle, &block,
+                    table->rep_->options.env, didIO);
     }
   }
 

util/env_posix.cc

@@ -960,6 +960,12 @@ class PosixEnv : public Env {
     return static_cast<uint64_t>(tv.tv_sec) * 1000000 + tv.tv_usec;
   }
 
+  virtual uint64_t NowNanos() {
+    struct timespec ts;
+    clock_gettime(CLOCK_MONOTONIC, &ts);
+    return static_cast<uint64_t>(ts.tv_sec) * 1000000000 + ts.tv_nsec;
+  }
+
   virtual void SleepForMicroseconds(int micros) {
     usleep(micros);
   }

util/perf_context.cc

+#include "util/perf_context_imp.h"
+
+namespace leveldb {
+
+// by default, enable counts only
+PerfLevel perf_level = kEnableCount;
+
+void SetPerfLevel(PerfLevel level) { perf_level = level; }
+
+void PerfContext::Reset() {
+  user_key_comparison_count = 0;
+  block_cache_hit_count = 0;
+  block_read_count = 0;
+  block_read_byte = 0;
+  block_read_time = 0;
+  block_checksum_time = 0;
+  block_decompress_time = 0;
+}
+
+__thread PerfContext perf_context;
+
+}

util/perf_context_imp.h

+#ifndef PERF_CONTEXT_IMP_H
+#define PERF_CONTEXT_IMP_H
+
+#include "rocksdb/perf_context.h"
+#include "util/stop_watch.h"
+
+namespace leveldb {
+
+extern enum PerfLevel perf_level;
+
+inline void StartPerfTimer(StopWatchNano* timer) {
+  if (perf_level >= PerfLevel::kEnableTime) {
+    timer->Start();
+  }
+}
+
+inline void BumpPerfCount(uint64_t* count, uint64_t delta = 1) {
+  if (perf_level >= PerfLevel::kEnableCount) {
+    *count += delta;
+  }
+}
+
+inline void BumpPerfTime(uint64_t* time,
+                         StopWatchNano* timer,
+                         bool reset = true) {
+  if (perf_level >= PerfLevel::kEnableTime) {
+    *time += timer->ElapsedNanos(reset);
+  }
+}
+
+}
+
+#endif

util/stop_watch.h

@@ -11,8 +11,8 @@ class StopWatch {
  public:
   StopWatch(
     Env * const env,
-    std::shared_ptr<Statistics> statistics,
-    const Histograms histogram_name) :
+    std::shared_ptr<Statistics> statistics = nullptr,
+    const Histograms histogram_name = DB_GET) :
       env_(env),
       start_time_(env->NowMicros()),
       statistics_(statistics),
@@ -37,5 +37,32 @@ class StopWatch {
   const Histograms histogram_name_;
 
 };
+
+// a nano second precision stopwatch
+class StopWatchNano {
+ public:
+  StopWatchNano(Env* const env, bool auto_start = false)
+      : env_(env), start_(0) {
+    if (auto_start) {
+      Start();
+    }
+  }
+
+  void Start() { start_ = env_->NowNanos(); }
+
+  uint64_t ElapsedNanos(bool reset = false) {
+    auto now = env_->NowNanos();
+    auto elapsed = now - start_;
+    if (reset) {
+      start_ = now;
+    }
+    return elapsed;
+  }
+
+ private:
+  Env* const env_;
+  uint64_t start_;
+};
+
 } // namespace leveldb
 #endif // STORAGE_LEVELDB_UTIL_STOP_WATCH_H_