From a2a0e358cb2057634d350d113087e6eb01effa93 Mon Sep 17 00:00:00 2001
From: Dhruba Borthakur <dhruba@gmail.com>
Date: Wed, 16 May 2012 17:22:33 -0700
Subject: [PATCH] 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          | 12 +++++++++++-
 include/leveldb/cache.h |  1 +
 util/cache.cc           | 27 +++++++++++++++++++++------
 3 files changed, 33 insertions(+), 7 deletions(-)

diff --git a/db/db_bench.cc b/db/db_bench.cc
index 346ff9d5b..8912fad1d 100644
--- a/db/db_bench.cc
+++ b/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) {
diff --git a/include/leveldb/cache.h b/include/leveldb/cache.h
index 5e3b47637..a7bd2afa7 100644
--- a/include/leveldb/cache.h
+++ b/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:
diff --git a/util/cache.cc b/util/cache.cc
index 24f1f63f4..d76dc15c4 100644
--- a/util/cache.cc
+++ b/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
-- 
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