WriteBufferManager's dummy entry size to block cache 1MB -> 256KB (#5175)

Summary:
Dummy cache size of 1MB is too large for small block sizes. Our GetDefaultCacheShardBits() use min_shard_size = 512L * 1024L to determine number of shards, so 1MB will excceeds the size of the whole shard and make the cache excceeds the budget.
Change it to 256KB accordingly.
There shouldn't be obvious performance impact, since inserting a cache entry every 256KB of memtable inserts is still infrequently enough.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5175

Differential Revision: D14954289

Pulled By: siying

fbshipit-source-id: 2c275255c1ac3992174e06529e44c55538325c94

2 files changed, 40 insertions, 40 deletions

diff --git a/memtable/write_buffer_manager.cc b/memtable/write_buffer_manager.cc
index 7f2e664ab..cf7f53764 100644
--- a/memtable/write_buffer_manager.cc
+++ b/memtable/write_buffer_manager.cc
@@ -14,7 +14,7 @@
 namespace rocksdb {
 #ifndef ROCKSDB_LITE
 namespace {
-const size_t kSizeDummyEntry = 1024 * 1024;
+const size_t kSizeDummyEntry = 256 * 1024;
 // The key will be longer than keys for blocks in SST files so they won't
 // conflict.
 const size_t kCacheKeyPrefix = kMaxVarint64Length * 4 + 1;
@@ -86,7 +86,7 @@ void WriteBufferManager::ReserveMemWithCache(size_t mem) {
   size_t new_mem_used = memory_used_.load(std::memory_order_relaxed) + mem;
   memory_used_.store(new_mem_used, std::memory_order_relaxed);
   while (new_mem_used > cache_rep_->cache_allocated_size_) {
-    // Expand size by at least 1MB.
+    // Expand size by at least 256KB.
     // Add a dummy record to the cache
     Cache::Handle* handle;
     cache_rep_->cache_->Insert(cache_rep_->GetNextCacheKey(), nullptr,
diff --git a/memtable/write_buffer_manager_test.cc b/memtable/write_buffer_manager_test.cc
index 0fc9fd06c..3c89c8095 100644
--- a/memtable/write_buffer_manager_test.cc
+++ b/memtable/write_buffer_manager_test.cc
@@ -57,67 +57,67 @@ TEST_F(WriteBufferManagerTest, CacheCost) {
   std::unique_ptr<WriteBufferManager> wbf(
       new WriteBufferManager(50 * 1024 * 1024, cache));
 
-  // Allocate 1.5MB will allocate 2MB
-  wbf->ReserveMem(1536 * 1024);
-  ASSERT_GE(cache->GetPinnedUsage(), 2 * 1024 * 1024);
-  ASSERT_LT(cache->GetPinnedUsage(), 2 * 1024 * 1024 + 10000);
+  // Allocate 333KB will allocate 512KB
+  wbf->ReserveMem(333 * 1024);
+  ASSERT_GE(cache->GetPinnedUsage(), 2 * 256 * 1024);
+  ASSERT_LT(cache->GetPinnedUsage(), 2 * 256 * 1024 + 10000);
 
-  // Allocate another 2MB
-  wbf->ReserveMem(2 * 1024 * 1024);
-  ASSERT_GE(cache->GetPinnedUsage(), 4 * 1024 * 1024);
-  ASSERT_LT(cache->GetPinnedUsage(), 4 * 1024 * 1024 + 10000);
+  // Allocate another 512KB
+  wbf->ReserveMem(512 * 1024);
+  ASSERT_GE(cache->GetPinnedUsage(), 4 * 256 * 1024);
+  ASSERT_LT(cache->GetPinnedUsage(), 4 * 256 * 1024 + 10000);
 
-  // Allocate another 20MB
-  wbf->ReserveMem(20 * 1024 * 1024);
-  ASSERT_GE(cache->GetPinnedUsage(), 24 * 1024 * 1024);
-  ASSERT_LT(cache->GetPinnedUsage(), 24 * 1024 * 1024 + 10000);
+  // Allocate another 10MB
+  wbf->ReserveMem(10 * 1024 * 1024);
+  ASSERT_GE(cache->GetPinnedUsage(), 11 * 1024 * 1024);
+  ASSERT_LT(cache->GetPinnedUsage(), 11 * 1024 * 1024 + 10000);
 
-  // Free 2MB will not cause any change in cache cost
-  wbf->FreeMem(2 * 1024 * 1024);
-  ASSERT_GE(cache->GetPinnedUsage(), 24 * 1024 * 1024);
-  ASSERT_LT(cache->GetPinnedUsage(), 24 * 1024 * 1024 + 10000);
+  // Free 1MB will not cause any change in cache cost
+  wbf->FreeMem(1024 * 1024);
+  ASSERT_GE(cache->GetPinnedUsage(), 11 * 1024 * 1024);
+  ASSERT_LT(cache->GetPinnedUsage(), 11 * 1024 * 1024 + 10000);
 
   ASSERT_FALSE(wbf->ShouldFlush());
 
-  // Allocate another 30MB
-  wbf->ReserveMem(30 * 1024 * 1024);
-  ASSERT_GE(cache->GetPinnedUsage(), 52 * 1024 * 1024);
-  ASSERT_LT(cache->GetPinnedUsage(), 52 * 1024 * 1024 + 10000);
+  // Allocate another 41MB
+  wbf->ReserveMem(41 * 1024 * 1024);
+  ASSERT_GE(cache->GetPinnedUsage(), 51 * 1024 * 1024);
+  ASSERT_LT(cache->GetPinnedUsage(), 51 * 1024 * 1024 + 10000);
   ASSERT_TRUE(wbf->ShouldFlush());
 
   ASSERT_TRUE(wbf->ShouldFlush());
 
   wbf->ScheduleFreeMem(20 * 1024 * 1024);
-  ASSERT_GE(cache->GetPinnedUsage(), 52 * 1024 * 1024);
-  ASSERT_LT(cache->GetPinnedUsage(), 52 * 1024 * 1024 + 10000);
+  ASSERT_GE(cache->GetPinnedUsage(), 51 * 1024 * 1024);
+  ASSERT_LT(cache->GetPinnedUsage(), 51 * 1024 * 1024 + 10000);
 
   // Still need flush as the hard limit hits
   ASSERT_TRUE(wbf->ShouldFlush());
 
-  // Free 20MB will releae 1MB from cache
+  // Free 20MB will releae 256KB from cache
   wbf->FreeMem(20 * 1024 * 1024);
-  ASSERT_GE(cache->GetPinnedUsage(), 51 * 1024 * 1024);
-  ASSERT_LT(cache->GetPinnedUsage(), 51 * 1024 * 1024 + 10000);
+  ASSERT_GE(cache->GetPinnedUsage(), 51 * 1024 * 1024 - 256 * 1024);
+  ASSERT_LT(cache->GetPinnedUsage(), 51 * 1024 * 1024 - 256 * 1024 + 10000);
 
   ASSERT_FALSE(wbf->ShouldFlush());
 
-  // Every free will release 1MB if still not hit 3/4
+  // Every free will release 256KB if still not hit 3/4
   wbf->FreeMem(16 * 1024);
-  ASSERT_GE(cache->GetPinnedUsage(), 50 * 1024 * 1024);
-  ASSERT_LT(cache->GetPinnedUsage(), 50 * 1024 * 1024 + 10000);
+  ASSERT_GE(cache->GetPinnedUsage(), 51 * 1024 * 1024 - 2 * 256 * 1024);
+  ASSERT_LT(cache->GetPinnedUsage(), 51 * 1024 * 1024 - 2 * 256 * 1024 + 10000);
 
   wbf->FreeMem(16 * 1024);
-  ASSERT_GE(cache->GetPinnedUsage(), 49 * 1024 * 1024);
-  ASSERT_LT(cache->GetPinnedUsage(), 49 * 1024 * 1024 + 10000);
+  ASSERT_GE(cache->GetPinnedUsage(), 51 * 1024 * 1024 - 3 * 256 * 1024);
+  ASSERT_LT(cache->GetPinnedUsage(), 51 * 1024 * 1024 - 3 * 256 * 1024 + 10000);
 
-  // Free 2MB will not cause any change in cache cost
-  wbf->ReserveMem(2 * 1024 * 1024);
-  ASSERT_GE(cache->GetPinnedUsage(), 49 * 1024 * 1024);
-  ASSERT_LT(cache->GetPinnedUsage(), 49 * 1024 * 1024 + 10000);
+  // Reserve 512KB will not cause any change in cache cost
+  wbf->ReserveMem(512 * 1024);
+  ASSERT_GE(cache->GetPinnedUsage(), 51 * 1024 * 1024 - 3 * 256 * 1024);
+  ASSERT_LT(cache->GetPinnedUsage(), 51 * 1024 * 1024 - 3 * 256 * 1024 + 10000);
 
   wbf->FreeMem(16 * 1024);
-  ASSERT_GE(cache->GetPinnedUsage(), 48 * 1024 * 1024);
-  ASSERT_LT(cache->GetPinnedUsage(), 48 * 1024 * 1024 + 10000);
+  ASSERT_GE(cache->GetPinnedUsage(), 51 * 1024 * 1024 - 4 * 256 * 1024);
+  ASSERT_LT(cache->GetPinnedUsage(), 51 * 1024 * 1024 - 4 * 256 * 1024 + 10000);
 
   // Destory write buffer manger should free everything
   wbf.reset();
@@ -136,8 +136,8 @@ TEST_F(WriteBufferManagerTest, NoCapCacheCost) {
   ASSERT_FALSE(wbf->ShouldFlush());
 
   wbf->FreeMem(9 * 1024 * 1024);
-  for (int i = 0; i < 10; i++) {
-    wbf->FreeMem(16 * 1024);
+  for (int i = 0; i < 40; i++) {
+    wbf->FreeMem(4 * 1024);
   }
   ASSERT_GE(cache->GetPinnedUsage(), 1024 * 1024);
   ASSERT_LT(cache->GetPinnedUsage(), 1024 * 1024 + 10000);