Make DBTest.ReadLatencyHistogramByLevel more robust

Summary:
Two fixes:
1. Wait compaction after generating each L0 file so that we are sure there are one L0 file left.
2. https://reviews.facebook.net/D48423 increased from 500 keys to 700 keys but in verification phase we are still querying the first 500 keys. It is a bug to fix.

Test Plan: Run the test in the same environment that fails by chance of one in tens of times. It doesn't fail after 1000 times.

Reviewers: yhchiang, IslamAbdelRahman, igor, rven, kradhakrishnan

Reviewed By: rven, kradhakrishnan

Subscribers: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D48759

db/db_test.cc

@@ -657,9 +657,10 @@ TEST_F(DBTest, ReadLatencyHistogramByLevel) {
   DestroyAndReopen(options);
   int key_index = 0;
   Random rnd(301);
-  for (int num = 0; num < 7; num++) {
+  for (int num = 0; num < 8; num++) {
     Put("foo", "bar");
     GenerateNewFile(&rnd, &key_index);
+    dbfull()->TEST_WaitForCompact();
   }
   dbfull()->TEST_WaitForCompact();
 
@@ -667,7 +668,7 @@ TEST_F(DBTest, ReadLatencyHistogramByLevel) {
   ASSERT_TRUE(dbfull()->GetProperty("rocksdb.dbstats", &prop));
 
   // Get() after flushes, See latency histogram tracked.
-  for (int key = 0; key < 500; key++) {
+  for (int key = 0; key < key_index; key++) {
     Get(Key(key));
   }
   ASSERT_TRUE(dbfull()->GetProperty("rocksdb.dbstats", &prop));
@@ -678,7 +679,7 @@ TEST_F(DBTest, ReadLatencyHistogramByLevel) {
   // Reopen and issue Get(). See thee latency tracked
   Reopen(options);
   dbfull()->TEST_WaitForCompact();
-  for (int key = 0; key < 500; key++) {
+  for (int key = 0; key < key_index; key++) {
     Get(Key(key));
   }
   ASSERT_TRUE(dbfull()->GetProperty("rocksdb.dbstats", &prop));
@@ -709,7 +710,7 @@ TEST_F(DBTest, ReadLatencyHistogramByLevel) {
   ASSERT_NE(std::string::npos, prop.find("** Level 0 read latency histogram"));
   ASSERT_NE(std::string::npos, prop.find("** Level 1 read latency histogram"));
   ASSERT_EQ(std::string::npos, prop.find("** Level 2 read latency histogram"));
-  for (int key = 0; key < 500; key++) {
+  for (int key = 0; key < key_index; key++) {
     Get(Key(key));
   }
   ASSERT_TRUE(dbfull()->GetProperty("rocksdb.dbstats", &prop));

util/histogram.cc

@@ -82,7 +82,7 @@ void HistogramImpl::Clear() {
   memset(buckets_, 0, sizeof buckets_);
 }
 
-bool HistogramImpl::Empty() { return sum_squares_ == 0; }
+bool HistogramImpl::Empty() { return num_ == 0; }
 
 void HistogramImpl::Add(uint64_t value) {
   const size_t index = bucketMapper.IndexForValue(value);