Add hash_seed to Caches (#11391)

Summary:
See motivation and description in new ShardedCacheOptions::hash_seed option.

Updated db_bench so that its seed param is used for the cache hash seed.
Made its code more safe to ensure seed is set before use.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11391

Test Plan:
unit tests added / updated

**Performance** - no discernible difference seen running cache_bench repeatedly before & after. With lru_cache and hyper_clock_cache.

Reviewed By: hx235

Differential Revision: D45557797

Pulled By: pdillinger

fbshipit-source-id: 40bf4da6d66f9d41a8a0eb8e5cf4246a4aa07934

HISTORY.md

@@ -3,6 +3,7 @@
 ### New Features
 * Introduced a new option `block_protection_bytes_per_key`, which can be used to enable per key-value integrity protection for in-memory blocks in block cache (#11287).
 * Added `JemallocAllocatorOptions::num_arenas`. Setting `num_arenas > 1` may mitigate mutex contention in the allocator, particularly in scenarios where block allocations commonly bypass jemalloc tcache.
+* Improve the operational safety of publishing a DB or SST files to many hosts by using different block cache hash seeds on different hosts. The exact behavior is controlled by new option `ShardedCacheOptions::hash_seed`, which also documents the solved problem in more detail.
 
 ### Public API Changes
 * Add `MakeSharedCache()` construction functions to various cache Options objects, and deprecated the `NewWhateverCache()` functions with long parameter lists.

cache/cache_test.cc

@@ -21,6 +21,7 @@
 #include "test_util/secondary_cache_test_util.h"
 #include "test_util/testharness.h"
 #include "util/coding.h"
+#include "util/hash_containers.h"
 #include "util/string_util.h"
 
 // HyperClockCache only supports 16-byte keys, so some of the tests
@@ -955,6 +956,97 @@ TEST_P(CacheTest, GetChargeAndDeleter) {
   cache_->Release(h1);
 }
 
+namespace {
+bool AreTwoCacheKeysOrdered(Cache* cache) {
+  std::vector<std::string> keys;
+  const auto callback = [&](const Slice& key, Cache::ObjectPtr /*value*/,
+                            size_t /*charge*/,
+                            const Cache::CacheItemHelper* /*helper*/) {
+    keys.push_back(key.ToString());
+  };
+  cache->ApplyToAllEntries(callback, /*opts*/ {});
+  EXPECT_EQ(keys.size(), 2U);
+  EXPECT_NE(keys[0], keys[1]);
+  return keys[0] < keys[1];
+}
+}  // namespace
+
+TEST_P(CacheTest, CacheUniqueSeeds) {
+  // kQuasiRandomHashSeed should generate unique seeds (up to 2 billion before
+  // repeating)
+  UnorderedSet<uint32_t> seeds_seen;
+  // Roughly sqrt(number of possible values) for a decent chance at detecting
+  // a random collision if it's possible (shouldn't be)
+  uint16_t kSamples = 20000;
+  seeds_seen.reserve(kSamples);
+
+  // Hash seed should affect ordering of entries in the table, so we should
+  // have extremely high chance of seeing two entries ordered both ways.
+  bool seen_forward_order = false;
+  bool seen_reverse_order = false;
+
+  for (int i = 0; i < kSamples; ++i) {
+    auto cache = NewCache(2, [=](ShardedCacheOptions& opts) {
+      opts.hash_seed = LRUCacheOptions::kQuasiRandomHashSeed;
+      opts.num_shard_bits = 0;
+      opts.metadata_charge_policy = kDontChargeCacheMetadata;
+    });
+    auto val = cache->GetHashSeed();
+    ASSERT_TRUE(seeds_seen.insert(val).second);
+
+    ASSERT_OK(cache->Insert(EncodeKey(1), nullptr, &kHelper, /*charge*/ 1));
+    ASSERT_OK(cache->Insert(EncodeKey(2), nullptr, &kHelper, /*charge*/ 1));
+
+    if (AreTwoCacheKeysOrdered(cache.get())) {
+      seen_forward_order = true;
+    } else {
+      seen_reverse_order = true;
+    }
+  }
+
+  ASSERT_TRUE(seen_forward_order);
+  ASSERT_TRUE(seen_reverse_order);
+}
+
+TEST_P(CacheTest, CacheHostSeed) {
+  // kHostHashSeed should generate a consistent seed within this process
+  // (and other processes on the same host, but not unit testing that).
+  // And we should be able to use that chosen seed as an explicit option
+  // (for debugging).
+  // And we should verify consistent ordering of entries.
+  uint32_t expected_seed = 0;
+  bool expected_order = false;
+  // 10 iterations -> chance of a random seed falsely appearing consistent
+  // should be low, just 1 in 2^9.
+  for (int i = 0; i < 10; ++i) {
+    auto cache = NewCache(2, [=](ShardedCacheOptions& opts) {
+      if (i != 5) {
+        opts.hash_seed = LRUCacheOptions::kHostHashSeed;
+      } else {
+        // Can be used as explicit seed
+        opts.hash_seed = static_cast<int32_t>(expected_seed);
+        ASSERT_GE(opts.hash_seed, 0);
+      }
+      opts.num_shard_bits = 0;
+      opts.metadata_charge_policy = kDontChargeCacheMetadata;
+    });
+    ASSERT_OK(cache->Insert(EncodeKey(1), nullptr, &kHelper, /*charge*/ 1));
+    ASSERT_OK(cache->Insert(EncodeKey(2), nullptr, &kHelper, /*charge*/ 1));
+    uint32_t val = cache->GetHashSeed();
+    bool order = AreTwoCacheKeysOrdered(cache.get());
+    if (i != 0) {
+      ASSERT_EQ(val, expected_seed);
+      ASSERT_EQ(order, expected_order);
+    } else {
+      expected_seed = val;
+      expected_order = order;
+    }
+  }
+  // Printed for reference in case it's needed to reproduce other unit test
+  // failures on another host
+  fprintf(stderr, "kHostHashSeed -> %u\n", (unsigned)expected_seed);
+}
+
 INSTANTIATE_TEST_CASE_P(CacheTestInstance, CacheTest,
                         secondary_cache_test_util::GetTestingCacheTypes());
 INSTANTIATE_TEST_CASE_P(CacheTestInstance, LRUCacheTest,

cache/clock_cache.cc

@@ -130,7 +130,8 @@ HyperClockTable::HyperClockTable(
     size_t capacity, bool /*strict_capacity_limit*/,
     CacheMetadataChargePolicy metadata_charge_policy,
     MemoryAllocator* allocator,
-    const Cache::EvictionCallback* eviction_callback, const Opts& opts)
+    const Cache::EvictionCallback* eviction_callback, const uint32_t* hash_seed,
+    const Opts& opts)
     : length_bits_(CalcHashBits(capacity, opts.estimated_value_size,
                                 metadata_charge_policy)),
       length_bits_mask_((size_t{1} << length_bits_) - 1),
@@ -138,7 +139,8 @@ HyperClockTable::HyperClockTable(
                                            kStrictLoadFactor)),
       array_(new HandleImpl[size_t{1} << length_bits_]),
       allocator_(allocator),
-      eviction_callback_(*eviction_callback) {
+      eviction_callback_(*eviction_callback),
+      hash_seed_(*hash_seed) {
   if (metadata_charge_policy ==
       CacheMetadataChargePolicy::kFullChargeCacheMetadata) {
     usage_ += size_t{GetTableSize()} * sizeof(HandleImpl);
@@ -1010,7 +1012,7 @@ inline void HyperClockTable::Evict(size_t requested_charge,
         if (eviction_callback_) {
           took_ownership =
               eviction_callback_(ClockCacheShard<HyperClockTable>::ReverseHash(
-                                     h.GetHash(), &unhashed),
+                                     h.GetHash(), &unhashed, hash_seed_),
                                  reinterpret_cast<Cache::Handle*>(&h));
         }
         if (!took_ownership) {
@@ -1039,11 +1041,11 @@ ClockCacheShard<Table>::ClockCacheShard(
     size_t capacity, bool strict_capacity_limit,
     CacheMetadataChargePolicy metadata_charge_policy,
     MemoryAllocator* allocator,
-    const Cache::EvictionCallback* eviction_callback,
+    const Cache::EvictionCallback* eviction_callback, const uint32_t* hash_seed,
     const typename Table::Opts& opts)
     : CacheShardBase(metadata_charge_policy),
       table_(capacity, strict_capacity_limit, metadata_charge_policy, allocator,
-             eviction_callback, opts),
+             eviction_callback, hash_seed, opts),
       capacity_(capacity),
       strict_capacity_limit_(strict_capacity_limit) {
   // Initial charge metadata should not exceed capacity
@@ -1082,10 +1084,11 @@ void ClockCacheShard<Table>::ApplyToSomeEntries(
     *state = index_end << (sizeof(size_t) * 8u - length_bits);
   }
 
+  auto hash_seed = table_.GetHashSeed();
   table_.ConstApplyToEntriesRange(
-      [callback](const HandleImpl& h) {
+      [callback, hash_seed](const HandleImpl& h) {
         UniqueId64x2 unhashed;
-        callback(ReverseHash(h.hashed_key, &unhashed), h.value,
+        callback(ReverseHash(h.hashed_key, &unhashed, hash_seed), h.value,
                  h.GetTotalCharge(), h.helper);
       },
       index_begin, index_end, false);
@@ -1295,7 +1298,7 @@ HyperClockCache::HyperClockCache(const HyperClockCacheOptions& opts)
     table_opts.estimated_value_size = opts.estimated_entry_charge;
     new (cs) Shard(per_shard, opts.strict_capacity_limit,
                    opts.metadata_charge_policy, alloc, &eviction_callback_,
-                   table_opts);
+                   &hash_seed_, table_opts);
   });
 }
 

cache/clock_cache.h

@@ -407,7 +407,7 @@ class HyperClockTable {
                   CacheMetadataChargePolicy metadata_charge_policy,
                   MemoryAllocator* allocator,
                   const Cache::EvictionCallback* eviction_callback,
-                  const Opts& opts);
+                  const uint32_t* hash_seed, const Opts& opts);
   ~HyperClockTable();
 
   Status Insert(const ClockHandleBasicData& proto, HandleImpl** handle,
@@ -448,6 +448,8 @@ class HyperClockTable {
     return standalone_usage_.load(std::memory_order_relaxed);
   }
 
+  uint32_t GetHashSeed() const { return hash_seed_; }
+
   // Acquire/release N references
   void TEST_RefN(HandleImpl& handle, size_t n);
   void TEST_ReleaseN(HandleImpl* handle, size_t n);
@@ -546,6 +548,9 @@ class HyperClockTable {
   // A reference to Cache::eviction_callback_
   const Cache::EvictionCallback& eviction_callback_;
 
+  // A reference to ShardedCacheBase::hash_seed_
+  const uint32_t& hash_seed_;
+
   // We partition the following members into different cache lines
   // to avoid false sharing among Lookup, Release, Erase and Insert
   // operations in ClockCacheShard.
@@ -573,7 +578,7 @@ class ALIGN_AS(CACHE_LINE_SIZE) ClockCacheShard final : public CacheShardBase {
                   CacheMetadataChargePolicy metadata_charge_policy,
                   MemoryAllocator* allocator,
                   const Cache::EvictionCallback* eviction_callback,
-                  const typename Table::Opts& opts);
+                  const uint32_t* hash_seed, const typename Table::Opts& opts);
 
   // For CacheShard concept
   using HandleImpl = typename Table::HandleImpl;
@@ -583,22 +588,23 @@ class ALIGN_AS(CACHE_LINE_SIZE) ClockCacheShard final : public CacheShardBase {
   static inline uint32_t HashPieceForSharding(HashCref hash) {
     return Upper32of64(hash[0]);
   }
-  static inline HashVal ComputeHash(const Slice& key) {
+  static inline HashVal ComputeHash(const Slice& key, uint32_t seed) {
     assert(key.size() == kCacheKeySize);
     HashVal in;
     HashVal out;
     // NOTE: endian dependence
     // TODO: use GetUnaligned?
     std::memcpy(&in, key.data(), kCacheKeySize);
-    BijectiveHash2x64(in[1], in[0], &out[1], &out[0]);
+    BijectiveHash2x64(in[1], in[0] ^ seed, &out[1], &out[0]);
     return out;
   }
 
   // For reconstructing key from hashed_key. Requires the caller to provide
   // backing storage for the Slice in `unhashed`
   static inline Slice ReverseHash(const UniqueId64x2& hashed,
-                                  UniqueId64x2* unhashed) {
+                                  UniqueId64x2* unhashed, uint32_t seed) {
     BijectiveUnhash2x64(hashed[1], hashed[0], &(*unhashed)[1], &(*unhashed)[0]);
+    (*unhashed)[0] ^= seed;
     // NOTE: endian dependence
     return Slice(reinterpret_cast<const char*>(unhashed), kCacheKeySize);
   }

cache/lru_cache.h

@@ -279,8 +279,8 @@ class ALIGN_AS(CACHE_LINE_SIZE) LRUCacheShard final : public CacheShardBase {
   using HashCref = uint32_t;
 
  public:  // Function definitions expected as parameter to ShardedCache
-  static inline HashVal ComputeHash(const Slice& key) {
-    return Lower32of64(GetSliceNPHash64(key));
+  static inline HashVal ComputeHash(const Slice& key, uint32_t seed) {
+    return Lower32of64(GetSliceNPHash64(key, seed));
   }
 
   // Separate from constructor so caller can easily make an array of LRUCache

cache/lru_cache_test.cc

@@ -397,7 +397,7 @@ class ClockCacheTest : public testing::Test {
     opts.estimated_value_size = 1;
     new (shard_)
         Shard(capacity, strict_capacity_limit, kDontChargeCacheMetadata,
-              /*allocator*/ nullptr, &eviction_callback_, opts);
+              /*allocator*/ nullptr, &eviction_callback_, &hash_seed_, opts);
   }
 
   Status Insert(const UniqueId64x2& hashed_key,
@@ -455,6 +455,7 @@ class ClockCacheTest : public testing::Test {
 
  private:
   Cache::EvictionCallback eviction_callback_;
+  uint32_t hash_seed_ = 0;
 };
 
 TEST_F(ClockCacheTest, Misc) {

cache/sharded_cache.cc

@@ -13,16 +13,55 @@
 #include <cstdint>
 #include <memory>
 
+#include "env/unique_id_gen.h"
+#include "rocksdb/env.h"
 #include "util/hash.h"
 #include "util/math.h"
 #include "util/mutexlock.h"
 
 namespace ROCKSDB_NAMESPACE {
+namespace {
+// The generated seeds must fit in 31 bits so that
+// ShardedCacheOptions::hash_seed can be set to it explicitly, for
+// diagnostic/debugging purposes.
+constexpr uint32_t kSeedMask = 0x7fffffff;
+uint32_t DetermineSeed(int32_t hash_seed_option) {
+  if (hash_seed_option >= 0) {
+    // User-specified exact seed
+    return static_cast<uint32_t>(hash_seed_option);
+  }
+  static SemiStructuredUniqueIdGen gen;
+  if (hash_seed_option == ShardedCacheOptions::kHostHashSeed) {
+    std::string hostname;
+    Status s = Env::Default()->GetHostNameString(&hostname);
+    if (s.ok()) {
+      return GetSliceHash(hostname) & kSeedMask;
+    } else {
+      // Fall back on something stable within the process.
+      return static_cast<uint32_t>(gen.GetBaseUpper()) & kSeedMask;
+    }
+  } else {
+    // for kQuasiRandomHashSeed and fallback
+    uint32_t val = gen.GenerateNext<uint32_t>() & kSeedMask;
+    // Perform some 31-bit bijective transformations so that we get
+    // quasirandom, not just incrementing. (An incrementing seed from a
+    // random starting point would be fine, but hard to describe in a name.)
+    // See https://en.wikipedia.org/wiki/Quasirandom and using a murmur-like
+    // transformation here for our bijection in the lower 31 bits.
+    // See https://en.wikipedia.org/wiki/MurmurHash
+    val *= /*31-bit prime*/ 1150630961;
+    val ^= (val & kSeedMask) >> 17;
+    val *= /*31-bit prime*/ 1320603883;
+    return val & kSeedMask;
+  }
+}
+}  // namespace
 
 ShardedCacheBase::ShardedCacheBase(const ShardedCacheOptions& opts)
     : Cache(opts.memory_allocator),
       last_id_(1),
       shard_mask_((uint32_t{1} << opts.num_shard_bits) - 1),
+      hash_seed_(DetermineSeed(opts.hash_seed)),
       strict_capacity_limit_(opts.strict_capacity_limit),
       capacity_(opts.capacity) {}
 

cache/sharded_cache.h

@@ -34,8 +34,8 @@ class CacheShardBase {
   std::string GetPrintableOptions() const { return ""; }
   using HashVal = uint64_t;
   using HashCref = uint64_t;
-  static inline HashVal ComputeHash(const Slice& key) {
-    return GetSliceNPHash64(key);
+  static inline HashVal ComputeHash(const Slice& key, uint32_t seed) {
+    return GetSliceNPHash64(key, seed);
   }
   static inline uint32_t HashPieceForSharding(HashCref hash) {
     return Lower32of64(hash);
@@ -104,6 +104,8 @@ class ShardedCacheBase : public Cache {
   size_t GetUsage(Handle* handle) const override;
   std::string GetPrintableOptions() const override;
 
+  uint32_t GetHashSeed() const override { return hash_seed_; }
+
  protected:  // fns
   virtual void AppendPrintableOptions(std::string& str) const = 0;
   size_t GetPerShardCapacity() const;
@@ -112,6 +114,7 @@ class ShardedCacheBase : public Cache {
  protected:                        // data
   std::atomic<uint64_t> last_id_;  // For NewId
   const uint32_t shard_mask_;
+  const uint32_t hash_seed_;
 
   // Dynamic configuration parameters, guarded by config_mutex_
   bool strict_capacity_limit_;
@@ -171,7 +174,7 @@ class ShardedCache : public ShardedCacheBase {
                 size_t charge, Handle** handle = nullptr,
                 Priority priority = Priority::LOW) override {
     assert(helper);
-    HashVal hash = CacheShard::ComputeHash(key);
+    HashVal hash = CacheShard::ComputeHash(key, hash_seed_);
     auto h_out = reinterpret_cast<HandleImpl**>(handle);
     return GetShard(hash).Insert(key, hash, obj, helper, charge, h_out,
                                  priority);
@@ -181,7 +184,7 @@ class ShardedCache : public ShardedCacheBase {
                            const CacheItemHelper* helper, size_t charge,
                            bool allow_uncharged) override {
     assert(helper);
-    HashVal hash = CacheShard::ComputeHash(key);
+    HashVal hash = CacheShard::ComputeHash(key, hash_seed_);
     HandleImpl* result = GetShard(hash).CreateStandalone(
         key, hash, obj, helper, charge, allow_uncharged);
     return reinterpret_cast<Handle*>(result);
@@ -191,14 +194,14 @@ class ShardedCache : public ShardedCacheBase {
                  CreateContext* create_context = nullptr,
                  Priority priority = Priority::LOW,
                  Statistics* stats = nullptr) override {
-    HashVal hash = CacheShard::ComputeHash(key);
+    HashVal hash = CacheShard::ComputeHash(key, hash_seed_);
     HandleImpl* result = GetShard(hash).Lookup(key, hash, helper,
                                                create_context, priority, stats);
     return reinterpret_cast<Handle*>(result);
   }
 
   void Erase(const Slice& key) override {
-    HashVal hash = CacheShard::ComputeHash(key);
+    HashVal hash = CacheShard::ComputeHash(key, hash_seed_);
     GetShard(hash).Erase(key, hash);
   }
 

db/db_block_cache_test.cc

@@ -979,12 +979,14 @@ TEST_F(DBBlockCacheTest, CacheEntryRoleStats) {
   const size_t capacity = size_t{1} << 25;
   int iterations_tested = 0;
   for (bool partition : {false, true}) {
+    SCOPED_TRACE("Partition? " + std::to_string(partition));
     for (std::shared_ptr<Cache> cache :
          {NewLRUCache(capacity),
           HyperClockCacheOptions(
               capacity,
               BlockBasedTableOptions().block_size /*estimated_value_size*/)
               .MakeSharedCache()}) {
+      SCOPED_TRACE(std::string("Cache: ") + cache->Name());
       ++iterations_tested;
 
       Options options = CurrentOptions();
@@ -1278,6 +1280,7 @@ TEST_F(DBBlockCacheTest, HyperClockCacheReportProblems) {
   HyperClockCacheOptions hcc_opts{capacity, value_size_est};
   hcc_opts.num_shard_bits = 2;  // 4 shards
   hcc_opts.metadata_charge_policy = kDontChargeCacheMetadata;
+  hcc_opts.hash_seed = 0;  // deterministic hashing
   std::shared_ptr<Cache> cache = hcc_opts.MakeSharedCache();
   std::shared_ptr<CountingLogger> logger = std::make_shared<CountingLogger>();
 

db/db_impl/db_impl.cc

@@ -258,6 +258,9 @@ DBImpl::DBImpl(const DBOptions& options, const std::string& dbname,
   co.capacity = table_cache_size;
   co.num_shard_bits = immutable_db_options_.table_cache_numshardbits;
   co.metadata_charge_policy = kDontChargeCacheMetadata;
+  // TODO: Consider a non-fixed seed once test fallout (prefetch_test) is
+  // dealt with
+  co.hash_seed = 0;
   table_cache_ = NewLRUCache(co);
   SetDbSessionId();
   assert(!db_session_id_.empty());

db/internal_stats.cc

@@ -700,6 +700,7 @@ void InternalStats::CacheEntryRoleStats::BeginCollection(
   cache_usage = cache->GetUsage();
   table_size = cache->GetTableAddressCount();
   occupancy = cache->GetOccupancyCount();
+  hash_seed = cache->GetHashSeed();
 }
 
 void InternalStats::CacheEntryRoleStats::EndCollection(
@@ -724,7 +725,7 @@ std::string InternalStats::CacheEntryRoleStats::ToString(
   std::ostringstream str;
   str << "Block cache " << cache_id
       << " capacity: " << BytesToHumanString(cache_capacity)
-      << " usage: " << BytesToHumanString(cache_usage)
+      << " seed: " << hash_seed << " usage: " << BytesToHumanString(cache_usage)
       << " table_size: " << table_size << " occupancy: " << occupancy
       << " collections: " << collection_count
       << " last_copies: " << copies_of_last_collection

db/internal_stats.h

@@ -478,6 +478,7 @@ class InternalStats {
     uint32_t copies_of_last_collection = 0;
     uint64_t last_start_time_micros_ = 0;
     uint64_t last_end_time_micros_ = 0;
+    uint32_t hash_seed = 0;
 
     void Clear() {
       // Wipe everything except collection_count

env/env_test.cc

@@ -3134,6 +3134,20 @@ TEST_F(EnvTest, SemiStructuredUniqueIdGenTest) {
   t.Run();
 }
 
+TEST_F(EnvTest, SemiStructuredUniqueIdGenTestSmaller) {
+  // For small generated types, will cycle through all the possible values.
+  SemiStructuredUniqueIdGen gen;
+  std::vector<bool> hit(256);
+  for (int i = 0; i < 256; ++i) {
+    auto val = gen.GenerateNext<uint8_t>();
+    ASSERT_FALSE(hit[val]);
+    hit[val] = true;
+  }
+  for (int i = 0; i < 256; ++i) {
+    ASSERT_TRUE(hit[i]);
+  }
+}
+
 TEST_F(EnvTest, FailureToCreateLockFile) {
   auto env = Env::Default();
   auto fs = env->GetFileSystem();

env/unique_id_gen.h

@@ -14,6 +14,7 @@
 
 #include <atomic>
 #include <cstdint>
+#include <type_traits>
 
 #include "rocksdb/rocksdb_namespace.h"
 
@@ -61,6 +62,19 @@ class SemiStructuredUniqueIdGen {
   // to the next (thread safe).
   void GenerateNext(uint64_t* upper, uint64_t* lower);
 
+  // For generating smaller values. Will cycle through all the possibilities
+  // before repeating.
+  template <typename T>
+  T GenerateNext() {
+    static_assert(sizeof(T) <= sizeof(uint64_t));
+    static_assert(std::is_integral_v<T>);
+    uint64_t ignore, val;
+    GenerateNext(&ignore, &val);
+    return static_cast<T>(val);
+  }
+
+  uint64_t GetBaseUpper() const { return base_upper_; }
+
  private:
   uint64_t base_upper_;
   uint64_t base_lower_;

include/rocksdb/advanced_cache.h

@@ -404,6 +404,9 @@ class Cache {
 
   MemoryAllocator* memory_allocator() const { return memory_allocator_.get(); }
 
+  // See ShardedCacheOptions::hash_seed
+  virtual uint32_t GetHashSeed() const { return 0; }
+
   // EXPERIMENTAL
   // The following APIs are experimental and might change in the future.
 

include/rocksdb/cache.h

@@ -138,6 +138,35 @@ struct ShardedCacheOptions {
   // A SecondaryCache instance to use the non-volatile tier.
   std::shared_ptr<SecondaryCache> secondary_cache;
 
+  // See hash_seed comments below
+  static constexpr int32_t kQuasiRandomHashSeed = -1;
+  static constexpr int32_t kHostHashSeed = -2;
+
+  // EXPERT OPTION: Specifies how a hash seed should be determined for the
+  // cache, or specifies a specific seed (only recommended for diagnostics or
+  // testing).
+  //
+  // Background: it could be dangerous to have different cache instances
+  // access the same SST files with the same hash seed, as correlated unlucky
+  // hashing across hosts or restarts could cause a widespread issue, rather
+  // than an isolated one. For example, with smaller block caches, it is
+  // possible for large full Bloom filters in a set of SST files to be randomly
+  // clustered into one cache shard, causing mutex contention or a thrashing
+  // condition as there's little or no space left for other entries assigned to
+  // the shard. If a set of SST files is broadcast and used on many hosts, we
+  // should ensure all have an independent chance of balanced shards.
+  //
+  // Values >= 0 will be treated as fixed hash seeds. Values < 0 are reserved
+  // for methods of dynamically choosing a seed, currently:
+  // * kQuasiRandomHashSeed - Each cache created chooses a seed mostly randomly,
+  //   except that within a process, no seed is repeated until all have been
+  //   issued.
+  // * kHostHashSeed - The seed is determined based on hashing the host name.
+  //   Although this is arguably slightly worse for production reliability, it
+  //   solves the essential problem of cross-host correlation while ensuring
+  //   repeatable behavior on a host, for diagnostic purposes.
+  int32_t hash_seed = kHostHashSeed;
+
   ShardedCacheOptions() {}
   ShardedCacheOptions(
       size_t _capacity, int _num_shard_bits, bool _strict_capacity_limit,

test_util/secondary_cache_test_util.h

@@ -56,13 +56,15 @@ class WithCacheType : public TestCreateContext {
     if (type == kLRU) {
       LRUCacheOptions lru_opts;
       lru_opts.capacity = capacity;
+      lru_opts.hash_seed = 0;  // deterministic tests
       if (modify_opts_fn) {
         modify_opts_fn(lru_opts);
       }
-      return NewLRUCache(lru_opts);
+      return lru_opts.MakeSharedCache();
     }
     if (type == kHyperClock) {
       HyperClockCacheOptions hc_opts{capacity, estimated_value_size_};
+      hc_opts.hash_seed = 0;  // deterministic tests
       if (modify_opts_fn) {
         modify_opts_fn(hc_opts);
       }

tools/db_bench_tool.cc

@@ -33,6 +33,7 @@
 #include <iostream>
 #include <memory>
 #include <mutex>
+#include <optional>
 #include <queue>
 #include <thread>
 #include <unordered_map>
@@ -287,7 +288,7 @@ DEFINE_int32(bloom_locality, 0, "Control bloom filter probes locality");
 DEFINE_int64(seed, 0,
              "Seed base for random number generators. "
              "When 0 it is derived from the current time.");
-static int64_t seed_base;
+static std::optional<int64_t> seed_base;
 
 DEFINE_int32(threads, 1, "Number of concurrent threads to run.");
 
@@ -2606,7 +2607,7 @@ struct ThreadState {
   SharedState* shared;
 
   explicit ThreadState(int index, int my_seed)
-      : tid(index), rand(seed_base + my_seed) {}
+      : tid(index), rand(*seed_base + my_seed) {}
 };
 
 class Duration {
@@ -3002,6 +3003,11 @@ class Benchmark {
     return allocator;
   }
 
+  static int32_t GetCacheHashSeed() {
+    // For a fixed Cache seed, need a non-negative int32
+    return static_cast<int32_t>(*seed_base) & 0x7fffffff;
+  }
+
   static std::shared_ptr<Cache> NewCache(int64_t capacity) {
     if (capacity <= 0) {
       return nullptr;
@@ -3010,17 +3016,19 @@ class Benchmark {
       fprintf(stderr, "Old clock cache implementation has been removed.\n");
       exit(1);
     } else if (FLAGS_cache_type == "hyper_clock_cache") {
-      return HyperClockCacheOptions(static_cast<size_t>(capacity),
-                                    FLAGS_block_size /*estimated_entry_charge*/,
-                                    FLAGS_cache_numshardbits)
-          .MakeSharedCache();
+      HyperClockCacheOptions hcco{
+          static_cast<size_t>(capacity),
+          static_cast<size_t>(FLAGS_block_size) /*estimated_entry_charge*/,
+          FLAGS_cache_numshardbits};
+      hcco.hash_seed = GetCacheHashSeed();
+      return hcco.MakeSharedCache();
     } else if (FLAGS_cache_type == "lru_cache") {
       LRUCacheOptions opts(
           static_cast<size_t>(capacity), FLAGS_cache_numshardbits,
           false /*strict_capacity_limit*/, FLAGS_cache_high_pri_pool_ratio,
           GetCacheAllocator(), kDefaultToAdaptiveMutex,
           kDefaultCacheMetadataChargePolicy, FLAGS_cache_low_pri_pool_ratio);
-
+      opts.hash_seed = GetCacheHashSeed();
       if (!FLAGS_secondary_cache_uri.empty()) {
         Status s = SecondaryCache::CreateFromString(
             ConfigOptions(), FLAGS_secondary_cache_uri, &secondary_cache);
@@ -3051,7 +3059,7 @@ class Benchmark {
             NewCompressedSecondaryCache(secondary_cache_opts);
       }
 
-      return NewLRUCache(opts);
+      return opts.MakeSharedCache();
     } else {
       fprintf(stderr, "Cache type not supported.");
       exit(1);
@@ -4891,7 +4899,7 @@ class Benchmark {
           values_[i] = i;
         }
         RandomShuffle(values_.begin(), values_.end(),
-                      static_cast<uint32_t>(seed_base));
+                      static_cast<uint32_t>(*seed_base));
       }
     }
 
@@ -5003,7 +5011,7 @@ class Benchmark {
     // Default_random_engine provides slightly
     // improved throughput over mt19937.
     std::default_random_engine overwrite_gen{
-        static_cast<unsigned int>(seed_base)};
+        static_cast<unsigned int>(*seed_base)};
     std::bernoulli_distribution overwrite_decider(p);
 
     // Inserted key window is filled with the last N
@@ -5013,7 +5021,7 @@ class Benchmark {
     // - random access is O(1)
     // - insertion/removal at beginning/end is also O(1).
     std::deque<int64_t> inserted_key_window;
-    Random64 reservoir_id_gen(seed_base);
+    Random64 reservoir_id_gen(*seed_base);
 
     // --- Variables used in disposable/persistent keys simulation:
     // The following variables are used when
@@ -5050,7 +5058,7 @@ class Benchmark {
         ErrorExit();
       }
     }
-    Random rnd_disposable_entry(static_cast<uint32_t>(seed_base));
+    Random rnd_disposable_entry(static_cast<uint32_t>(*seed_base));
     std::string random_value;
     // Queue that stores scheduled timestamp of disposable entries deletes,
     // along with starting index of disposable entry keys to delete.
@@ -8516,7 +8524,7 @@ int db_bench_tool(int argc, char** argv) {
     uint64_t now = FLAGS_env->GetSystemClock()->NowMicros();
     seed_base = static_cast<int64_t>(now);
     fprintf(stdout, "Set seed to %" PRIu64 " because --seed was 0\n",
-            seed_base);
+            *seed_base);
   } else {
     seed_base = FLAGS_seed;
   }