Add support to specify the number of shards for the Block cache. By default,...

Add support to specify the number of shards for the Block cache. By default, the block cache is sharded into 16 parts.

Summary:
Task ID: #

Blame Rev:

Test Plan: Revert Plan:

Differential Revision: https://reviews.facebook.net/D3273

db/db_bench.cc

@@ -102,6 +102,11 @@ static bool FLAGS_use_existing_db = false;
 // Use the db with the following name.
 static const char* FLAGS_db = "/tmp/dbbench";
 
+// Number of shards for the block cache is 2 ** FLAGS_cache_numshardbits.
+// Negative means use default settings. This is applied only
+// if FLAGS_cache_size is non-negative.
+static int FLAGS_cache_numshardbits = -1;
+
 namespace leveldb {
 
 namespace {
@@ -390,7 +395,10 @@ class Benchmark {
 
  public:
   Benchmark()
-  : cache_(FLAGS_cache_size >= 0 ? NewLRUCache(FLAGS_cache_size) : NULL),
+  : cache_(FLAGS_cache_size >= 0 ? 
+           (FLAGS_cache_numshardbits >= 1 ? 
+            NewLRUCache(FLAGS_cache_size, FLAGS_cache_numshardbits) : 
+            NewLRUCache(FLAGS_cache_size)) : NULL),
     filter_policy_(FLAGS_bloom_bits >= 0
                    ? NewBloomFilterPolicy(FLAGS_bloom_bits)
                    : NULL),
@@ -955,6 +963,8 @@ int main(int argc, char** argv) {
       FLAGS_write_buffer_size = n;
     } else if (sscanf(argv[i], "--cache_size=%ld%c", &n, &junk) == 1) {
       FLAGS_cache_size = n;
+    } else if (sscanf(argv[i], "--cache_numshardbits=%d%c", &n, &junk) == 1) {
+      FLAGS_cache_numshardbits = n;
     } else if (sscanf(argv[i], "--bloom_bits=%d%c", &n, &junk) == 1) {
       FLAGS_bloom_bits = n;
     } else if (sscanf(argv[i], "--open_files=%d%c", &n, &junk) == 1) {

include/leveldb/cache.h

@@ -28,6 +28,7 @@ class Cache;
 // Create a new cache with a fixed size capacity.  This implementation
 // of Cache uses a least-recently-used eviction policy.
 extern Cache* NewLRUCache(size_t capacity);
+extern Cache* NewLRUCache(size_t capacity, int numShardBits);
 
 class Cache {
  public:

util/cache.cc

@@ -267,12 +267,12 @@ void LRUCache::Erase(const Slice& key, uint32_t hash) {
   }
 }
 
-static const int kNumShardBits = 4;
-static const int kNumShards = 1 << kNumShardBits;
+static int kNumShardBits = 4;         // default values, can be overridden
+static int kNumShards = 1 << kNumShardBits;
 
 class ShardedLRUCache : public Cache {
  private:
-  LRUCache shard_[kNumShards];
+  LRUCache* shard_;
   port::Mutex id_mutex_;
   uint64_t last_id_;
 
@@ -284,14 +284,25 @@ class ShardedLRUCache : public Cache {
     return hash >> (32 - kNumShardBits);
   }
 
- public:
-  explicit ShardedLRUCache(size_t capacity)
-      : last_id_(0) {
+  void init(size_t capacity, int numShardBits) {
+    kNumShardBits = numShardBits;
+    kNumShards = 1 << kNumShardBits;
+    shard_ = new LRUCache[kNumShards];
     const size_t per_shard = (capacity + (kNumShards - 1)) / kNumShards;
     for (int s = 0; s < kNumShards; s++) {
       shard_[s].SetCapacity(per_shard);
     }
   }
+
+ public:
+  explicit ShardedLRUCache(size_t capacity)
+      : last_id_(0) {
+    init(capacity, kNumShardBits);
+  }
+  ShardedLRUCache(size_t capacity, int numShardBits)
+     : last_id_(0) {
+    init(capacity, numShardBits);
+  }
   virtual ~ShardedLRUCache() { }
   virtual Handle* Insert(const Slice& key, void* value, size_t charge,
                          void (*deleter)(const Slice& key, void* value)) {
@@ -325,4 +336,8 @@ Cache* NewLRUCache(size_t capacity) {
   return new ShardedLRUCache(capacity);
 }
 
+Cache* NewLRUCache(size_t capacity, int numShardBits) {
+  return new ShardedLRUCache(capacity, numShardBits);
+}
+
 }  // namespace leveldb