comments for DynamicCompactionOptions test

Summary: as title

Test Plan: n/a

Reviewers: yhchiang, sdong, igor

Reviewed By: igor

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D24963

db/db_test.cc

@@ -8702,8 +8702,9 @@ TEST(DBTest, DynamicCompactionOptions) {
     dbfull()->TEST_WaitForFlushMemTable();
   };
 
-  // Write 3 files that have the same key range, trigger compaction and
-  // result in one L1 file
+  // Write 3 files that have the same key range.
+  // Since level0_file_num_compaction_trigger is 3, compaction should be
+  // triggered. The compaction should result in one L1 file
   gen_l0_kb(0, 64, 1);
   ASSERT_EQ(NumTableFilesAtLevel(0), 1);
   gen_l0_kb(0, 64, 1);
@@ -8718,6 +8719,10 @@ TEST(DBTest, DynamicCompactionOptions) {
   ASSERT_GE(metadata[0].size, k64KB - k4KB);
 
   // Test compaction trigger and target_file_size_base
+  // Reduce compaction trigger to 2, and reduce L1 file size to 32KB.
+  // Writing to 64KB L0 files should trigger a compaction. Since these
+  // 2 L0 files have the same key range, compaction merge them and should
+  // result in 2 32KB L1 files.
   ASSERT_TRUE(dbfull()->SetOptions({
     {"level0_file_num_compaction_trigger", "2"},
     {"target_file_size_base", std::to_string(k32KB) }
@@ -8733,8 +8738,13 @@ TEST(DBTest, DynamicCompactionOptions) {
   ASSERT_EQ(2U, metadata.size());
   ASSERT_LE(metadata[0].size, k32KB + k4KB);
   ASSERT_GE(metadata[0].size, k32KB - k4KB);
+  ASSERT_LE(metadata[1].size, k32KB + k4KB);
+  ASSERT_GE(metadata[1].size, k32KB - k4KB);
 
   // Test max_bytes_for_level_base
+  // Increase level base size to 256KB and write enough data that will
+  // fill L1 and L2. L1 size should be around 256KB while L2 size should be
+  // around 256KB x 4.
   ASSERT_TRUE(dbfull()->SetOptions({
     {"max_bytes_for_level_base", std::to_string(k256KB) }
   }));
@@ -8751,7 +8761,9 @@ TEST(DBTest, DynamicCompactionOptions) {
               SizeAtLevel(2) < 4 * k256KB * 1.2);
 
   // Test max_bytes_for_level_multiplier and
-  // max_bytes_for_level_base (reduce)
+  // max_bytes_for_level_base. Now, reduce both mulitplier and level base,
+  // After filling enough data that can fit in L1 - L3, we should see L1 size
+  // reduces to 128KB from 256KB which was asserted previously. Same for L2.
   ASSERT_TRUE(dbfull()->SetOptions({
     {"max_bytes_for_level_multiplier", "2"},
     {"max_bytes_for_level_base", std::to_string(k128KB) }
@@ -8767,7 +8779,10 @@ TEST(DBTest, DynamicCompactionOptions) {
   ASSERT_TRUE(SizeAtLevel(2) < 2 * k128KB * 1.2);
   ASSERT_TRUE(SizeAtLevel(3) < 4 * k128KB * 1.2);
 
-  // Clean up memtable and L0
+  // Test level0_stop_writes_trigger.
+  // Clean up memtable and L0. Block compaction threads. If continue to write
+  // and flush memtables. We should see put timeout after 8 memtable flushes
+  // since level0_stop_writes_trigger = 8
   dbfull()->CompactRange(nullptr, nullptr);
   // Block compaction
   SleepingBackgroundTask sleeping_task_low1;
@@ -8788,7 +8803,9 @@ TEST(DBTest, DynamicCompactionOptions) {
   sleeping_task_low1.WakeUp();
   sleeping_task_low1.WaitUntilDone();
 
-  // Test: stop trigger (reduce)
+  // Now reduce level0_stop_writes_trigger to 6. Clear up memtables and L0.
+  // Block compaction thread again. Perform the put and memtable flushes
+  // until we see timeout after 6 memtable flushes.
   ASSERT_TRUE(dbfull()->SetOptions({
     {"level0_stop_writes_trigger", "6"}
   }));
@@ -8810,6 +8827,10 @@ TEST(DBTest, DynamicCompactionOptions) {
   sleeping_task_low2.WaitUntilDone();
 
   // Test disable_auto_compactions
+  // Compaction thread is unblocked but auto compaction is disabled. Write
+  // 4 L0 files and compaction should be triggered. If auto compaction is
+  // disabled, then TEST_WaitForCompact will be waiting for nothing. Number of
+  // L0 files do not change after the call.
   ASSERT_TRUE(dbfull()->SetOptions({
     {"disable_auto_compactions", "true"}
   }));
@@ -8824,6 +8845,8 @@ TEST(DBTest, DynamicCompactionOptions) {
   dbfull()->TEST_WaitForCompact();
   ASSERT_EQ(NumTableFilesAtLevel(0), 4);
 
+  // Enable auto compaction and perform the same test, # of L0 files should be
+  // reduced after compaction.
   ASSERT_TRUE(dbfull()->SetOptions({
     {"disable_auto_compactions", "false"}
   }));
@@ -8838,8 +8861,10 @@ TEST(DBTest, DynamicCompactionOptions) {
   dbfull()->TEST_WaitForCompact();
   ASSERT_LT(NumTableFilesAtLevel(0), 4);
 
-  // Test for hard_rate_limit, change max_bytes_for_level_base to make level
-  // size big
+  // Test for hard_rate_limit.
+  // First change max_bytes_for_level_base to a big value and populate
+  // L1 - L3. Then thrink max_bytes_for_level_base and disable auto compaction
+  // at the same time, we should see some level with score greater than 2.
   ASSERT_TRUE(dbfull()->SetOptions({
     {"max_bytes_for_level_base", std::to_string(k256KB) }
   }));
@@ -8869,7 +8894,9 @@ TEST(DBTest, DynamicCompactionOptions) {
               SizeAtLevel(2) / k64KB > 4 ||
               SizeAtLevel(3) / k64KB > 8);
 
-  // Enfoce hard rate limit, L0 score is not regulated by this limit
+  // Enfoce hard rate limit. Now set hard_rate_limit to 2,
+  // we should start to see put delay (1000 us) and timeout as a result
+  // (L0 score is not regulated by this limit).
   ASSERT_TRUE(dbfull()->SetOptions({
     {"hard_rate_limit", "2"}
   }));
@@ -8881,7 +8908,7 @@ TEST(DBTest, DynamicCompactionOptions) {
   wo.timeout_hint_us = 500;
   ASSERT_TRUE(Put(Key(count), RandomString(&rnd, 1024), wo).IsTimedOut());
 
-  // Bump up limit
+  // Lift the limit and no timeout
   ASSERT_TRUE(dbfull()->SetOptions({
     {"hard_rate_limit", "100"}
   }));