Cap concurrent arena's shard block size to 128KB (#4147)

Summary:
Users sometime see their memtable size far smaller than expected. They probably have hit a fragementation of shard blocks. Cap their size anyway to reduce the impact of problem. 128KB is conservative so I don't imagine it can cause any performance problem.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4147

Differential Revision: D8886706

Pulled By: siying

fbshipit-source-id: 8528a2a4196aa4457274522e2565fd3ff28f621e

util/concurrent_arena.cc

@@ -18,9 +18,17 @@ namespace rocksdb {
 __thread size_t ConcurrentArena::tls_cpuid = 0;
 #endif
 
+namespace {
+// If the shard block size is too large, in the worst case, every core
+// allocates a block without populate it. If the shared block size is
+// 1MB, 64 cores will quickly allocate 64MB, and may quickly trigger a
+// flush. Cap the size instead.
+const size_t kMaxShardBlockSize = size_t{128 * 1024};
+}  // namespace
+
 ConcurrentArena::ConcurrentArena(size_t block_size, AllocTracker* tracker,
                                  size_t huge_page_size)
-    : shard_block_size_(block_size / 8),
+    : shard_block_size_(std::min(kMaxShardBlockSize, block_size / 8)),
       shards_(),
       arena_(block_size, tracker, huge_page_size) {
   Fixup();