Do not schedule memtable trimming if there is no history (facebook#6177)

Summary:
We have observed an increase in CPU load caused by frequent calls to
`ColumnFamilyData::InstallSuperVersion` from `DBImpl::TrimMemtableHistory`
when using `max_write_buffer_size_to_maintain` to limit the amount of
memtable history maintained for transaction conflict checking. Part of the issue
is that trimming can potentially be scheduled even if there is no memtable
history. The patch adds a check that fixes this.

See also facebook#6169.
Pull Request resolved: facebook#6177

Test Plan:
Compared `perf` output for

```
./db_bench -benchmarks=randomtransaction -optimistic_transaction_db=1 -statistics -stats_interval_seconds=1 -duration=90 -num=500000 --max_write_buffer_size_to_maintain=16000000 --transaction_set_snapshot=1 --threads=32
```

before and after the change. There is a significant reduction for the call chain
`rocksdb::DBImpl::TrimMemtableHistory` -> `rocksdb::ColumnFamilyData::InstallSuperVersion` ->
`rocksdb::ThreadLocalPtr::StaticMeta::Scrape` even without facebook#6169.

Differential Revision: D19057445

Pulled By: ltamasi

fbshipit-source-id: dff81882d7b280e17eda7d9b072a2d4882c50f79

db/memtable.h

@@ -138,7 +138,7 @@ class MemTable {
 
   // As a cheap version of `ApproximateMemoryUsage()`, this function doens't
   // require external synchronization. The value may be less accurate though
-  size_t ApproximateMemoryUsageFast() {
+  size_t ApproximateMemoryUsageFast() const {
     return approximate_memory_usage_.load(std::memory_order_relaxed);
   }
 

db/memtable_list.cc

@@ -280,7 +280,7 @@ void MemTableListVersion::Remove(MemTable* m,
 }
 
 // return the total memory usage assuming the oldest flushed memtable is dropped
-size_t MemTableListVersion::ApproximateMemoryUsageExcludingLast() {
+size_t MemTableListVersion::ApproximateMemoryUsageExcludingLast() const {
   size_t total_memtable_size = 0;
   for (auto& memtable : memlist_) {
     total_memtable_size += memtable->ApproximateMemoryUsage();
@@ -566,7 +566,7 @@ size_t MemTableList::ApproximateUnflushedMemTablesMemoryUsage() {
 
 size_t MemTableList::ApproximateMemoryUsage() { return current_memory_usage_; }
 
-size_t MemTableList::ApproximateMemoryUsageExcludingLast() {
+size_t MemTableList::ApproximateMemoryUsageExcludingLast() const {
   size_t usage =
       current_memory_usage_excluding_last_.load(std::memory_order_relaxed);
   return usage;

db/memtable_list.h

@@ -162,7 +162,7 @@ class MemTableListVersion {
   // excluding the last MemTable in memlist_history_. The reason for excluding
   // the last MemTable is to see if dropping the last MemTable will keep total
   // memory usage above or equal to max_write_buffer_size_to_maintain_
-  size_t ApproximateMemoryUsageExcludingLast();
+  size_t ApproximateMemoryUsageExcludingLast() const;
 
   bool MemtableLimitExceeded(size_t usage);
 
@@ -267,7 +267,7 @@ class MemTableList {
   size_t ApproximateMemoryUsage();
 
   // Returns the cached current_memory_usage_excluding_last_ value
-  size_t ApproximateMemoryUsageExcludingLast();
+  size_t ApproximateMemoryUsageExcludingLast() const;
 
   // Update current_memory_usage_excluding_last_ from MemtableListVersion
   void UpdateMemoryUsageExcludingLast();

db/write_batch.cc

@@ -1784,14 +1784,28 @@ class MemTableInserter : public WriteBatch::Handler {
     // check if memtable_list size exceeds max_write_buffer_size_to_maintain
     if (trim_history_scheduler_ != nullptr) {
       auto* cfd = cf_mems_->current();
-      assert(cfd != nullptr);
-      if (cfd->ioptions()->max_write_buffer_size_to_maintain > 0 &&
-          cfd->mem()->ApproximateMemoryUsageFast() +
-                  cfd->imm()->ApproximateMemoryUsageExcludingLast() >=
-              static_cast<size_t>(
-                  cfd->ioptions()->max_write_buffer_size_to_maintain) &&
-          cfd->imm()->MarkTrimHistoryNeeded()) {
-        trim_history_scheduler_->ScheduleWork(cfd);
+
+      assert(cfd);
+      assert(cfd->ioptions());
+
+      const size_t size_to_maintain = static_cast<size_t>(
+          cfd->ioptions()->max_write_buffer_size_to_maintain);
+
+      if (size_to_maintain > 0) {
+        MemTableList* const imm = cfd->imm();
+        assert(imm);
+
+        if (imm->NumFlushed() > 0) {
+          const MemTable* const mem = cfd->mem();
+          assert(mem);
+
+          if (mem->ApproximateMemoryUsageFast() +
+                      imm->ApproximateMemoryUsageExcludingLast() >=
+                  size_to_maintain &&
+              imm->MarkTrimHistoryNeeded()) {
+            trim_history_scheduler_->ScheduleWork(cfd);
+          }
+        }
       }
     }
   }