chore(tiering): add protection against overruning memory budget (#3327)

chore(tiering): Introduce second chance replacement strategy

Introduce hot/cold replacement strategy https://www.geeksforgeeks.org/second-chance-or-clock-page-replacement-policy/

Also, add protection against overruning memory budget
Finally, cancel in-flight offloading requests for entries that were looked up.

Signed-off-by: Roman Gershman <roman@dragonflydb.io>

src/core/compact_object.h

@@ -238,13 +238,13 @@ class CompactObj {
     }
   }
 
-  bool HasIoPending() const {
+  bool DefragIfNeeded(float ratio);
+
+  bool HasStashPending() const {
     return mask_ & IO_PENDING;
   }
 
-  bool DefragIfNeeded(float ratio);
-
-  void SetIoPending(bool b) {
+  void SetStashPending(bool b) {
     if (b) {
       mask_ |= IO_PENDING;
     } else {
@@ -451,7 +451,7 @@ class CompactObj {
   //
   static_assert(sizeof(u_) == 16, "");
 
-  mutable uint8_t mask_ = 0;
+  uint8_t mask_ = 0;
 
   // We currently reserve 5 bits for tags and 3 bits for extending the mask. currently reserved.
   uint8_t taglen_ = 0;

src/server/acl/acl_family_test.cc

@@ -4,10 +4,11 @@
 
 #include "server/acl/acl_family.h"
 
-#include "absl/container/flat_hash_map.h"
-#include "absl/flags/internal/flag.h"
-#include "absl/strings/ascii.h"
-#include "absl/strings/str_cat.h"
+#include <absl/container/flat_hash_map.h>
+#include <absl/strings/ascii.h>
+#include <absl/strings/str_cat.h>
+
+#include "base/flags.h"
 #include "base/gtest.h"
 #include "base/logging.h"
 #include "facade/facade_test.h"

src/server/acl/acl_log.cc

@@ -10,6 +10,7 @@
 #include "base/flags.h"
 #include "base/logging.h"
 #include "facade/dragonfly_connection.h"
+#include "server/conn_context.h"
 
 ABSL_FLAG(size_t, acllog_max_len, 32,
           "Specify the number of log entries. Logs are kept locally for each thread "

src/server/acl/acl_log.h

@@ -8,12 +8,11 @@
 #include <deque>
 #include <string>
 
-#include "base/flags.h"
-#include "server/conn_context.h"
+namespace dfly {
 
-ABSL_DECLARE_FLAG(size_t, acllog_max_len);
+class ConnectionContext;
 
-namespace dfly::acl {
+namespace acl {
 
 class AclLog {
  public:
@@ -49,4 +48,5 @@ class AclLog {
   size_t total_entries_allowed_;
 };
 
-}  // namespace dfly::acl
+}  // namespace acl
+}  // namespace dfly

src/server/acl/validator.h

@@ -6,17 +6,18 @@
 
 #include <utility>
 
+#include "facade/facade_types.h"
 #include "server/acl/acl_log.h"
 #include "server/command_registry.h"
-#include "server/conn_context.h"
 
 namespace dfly::acl {
 
+class AclKeys;
+
 std::pair<bool, AclLog::Reason> IsUserAllowedToInvokeCommandGeneric(
-    const std::vector<uint64_t>& acl_commands, const AclKeys& keys, CmdArgList tail_args,
+    const std::vector<uint64_t>& acl_commands, const AclKeys& keys, facade::CmdArgList tail_args,
     const CommandId& id);
 
 bool IsUserAllowedToInvokeCommand(const ConnectionContext& cntx, const CommandId& id,
-                                  CmdArgList tail_args);
-
+                                  facade::CmdArgList tail_args);
 }  // namespace dfly::acl

src/server/cluster/incoming_slot_migration.cc

@@ -4,8 +4,10 @@
 
 #include "server/cluster/incoming_slot_migration.h"
 
-#include "absl/cleanup/cleanup.h"
-#include "absl/strings/str_cat.h"
+#include <absl/cleanup/cleanup.h>
+#include <absl/strings/str_cat.h>
+
+#include "base/flags.h"
 #include "base/logging.h"
 #include "cluster_utility.h"
 #include "server/error.h"

src/server/db_slice.cc

@@ -479,6 +479,18 @@ OpResult<DbSlice::PrimeItAndExp> DbSlice::FindInternal(const Context& cntx, std:
     fetched_items_.insert(res.it->first.AsRef());
   }
 
+  // If the value has a pending stash, cancel it before any modification are applied.
+  // Rationale: we either look it up for reads - and then it's hot, or alternatively,
+  // we follow up with modifications during mutation operations, and in that case storing on disk
+  // does not make much sense.
+  if (res.it->second.HasStashPending()) {
+    owner_->tiered_storage()->CancelStash(cntx.db_index, key, &res.it->second);
+  }
+
+  // Mark this entry as being looked up. We use key (first) deliberately to preserve the hotness
+  // attribute of the entry in case of value overtides.
+  res.it->first.SetTouched(true);
+
   db.top_keys.Touch(key);
 
   std::move(update_stats_on_miss).Cancel();
@@ -491,6 +503,11 @@ OpResult<DbSlice::PrimeItAndExp> DbSlice::FindInternal(const Context& cntx, std:
       if (cluster::IsClusterEnabled()) {
         db.slots_stats[cluster::KeySlot(key)].total_reads++;
       }
+      if (res.it->second.IsExternal()) {
+        events_.ram_misses++;
+      } else {
+        events_.ram_hits++;
+      }
       break;
   }
   return res;
@@ -976,13 +993,6 @@ void DbSlice::PreUpdate(DbIndex db_ind, Iterator it, std::string_view key) {
   DVLOG(2) << "Running callbacks in dbid " << db_ind;
   CallChangeCallbacks(db_ind, ChangeReq{it.GetInnerIt()});
 
-  // If the value has a pending stash, cancel it before any modification are applied.
-  // Note: we don't delete offloaded values before updates, because a read-modify operation (like
-  // append) can be applied instead of a full overwrite. Deleting is reponsibility of the commands
-  if (it.IsOccupied() && it->second.HasIoPending()) {
-    owner_->tiered_storage()->CancelStash(db_ind, key, &it->second);
-  }
-
   it.GetInnerIt().SetVersion(NextVersion());
 }
 
@@ -1401,7 +1411,7 @@ void DbSlice::ClearOffloadedEntries(absl::Span<const DbIndex> indices, const DbT
       cursor = db_ptr->prime.Traverse(cursor, [&](PrimeIterator it) {
         if (it->second.IsExternal()) {
           tiered_storage->Delete(index, &it->second);
-        } else if (it->second.HasIoPending()) {
+        } else if (it->second.HasStashPending()) {
           tiered_storage->CancelStash(index, it->first.GetSlice(&scratch), &it->second);
         }
       });

src/server/string_family.cc

@@ -53,6 +53,8 @@ void CopyValueToBuffer(const PrimeValue& pv, char* dest) {
 
 string GetString(const PrimeValue& pv) {
   string res;
+  DCHECK_EQ(pv.ObjType(), OBJ_STRING);
+
   if (pv.ObjType() != OBJ_STRING)
     return res;
   res.resize(pv.Size());

src/server/test_utils.cc

@@ -30,7 +30,7 @@ using namespace std;
 ABSL_DECLARE_FLAG(string, dbfilename);
 ABSL_DECLARE_FLAG(uint32_t, num_shards);
 ABSL_FLAG(bool, force_epoll, false, "If true, uses epoll api instead iouring to run tests");
-
+ABSL_DECLARE_FLAG(size_t, acllog_max_len);
 namespace dfly {
 
 std::ostream& operator<<(std::ostream& os, const DbStats& stats) {

src/server/tiered_storage.cc

@@ -17,7 +17,6 @@
 #include "base/logging.h"
 #include "server/common.h"
 #include "server/db_slice.h"
-#include "server/engine_shard_set.h"
 #include "server/snapshot.h"
 #include "server/table.h"
 #include "server/tiering/common.h"
@@ -25,8 +24,11 @@
 #include "server/tiering/small_bins.h"
 #include "server/tx_base.h"
 
-ABSL_FLAG(bool, tiered_storage_cache_fetched, true,
-          "WIP: Load results of offloaded reads to memory");
+using namespace facade;
+
+ABSL_FLAG(uint32_t, tiered_storage_memory_margin, 1_MB,
+          "In bytes. If memory budget on a shard goes blow this limit, tiering stops "
+          "hot-loading values into ram.");
 
 ABSL_FLAG(unsigned, tiered_storage_write_depth, 50,
           "Maximum number of concurrent stash requests issued by background offload");
@@ -36,8 +38,6 @@ namespace dfly {
 using namespace std;
 using namespace util;
 
-using namespace tiering::literals;
-
 using KeyRef = tiering::OpManager::KeyRef;
 
 namespace {
@@ -80,14 +80,14 @@ class TieredStorage::ShardOpManager : public tiering::OpManager {
 
  public:
   ShardOpManager(TieredStorage* ts, DbSlice* db_slice, size_t max_size)
-      : tiering::OpManager{max_size}, ts_{ts}, db_slice_{db_slice} {
-    cache_fetched_ = absl::GetFlag(FLAGS_tiered_storage_cache_fetched);
+      : tiering::OpManager{max_size}, ts_{ts}, db_slice_{*db_slice} {
+    memory_margin_ = absl::GetFlag(FLAGS_tiered_storage_memory_margin);
   }
 
   // Clear IO pending flag for entry
   void ClearIoPending(OpManager::KeyRef key) {
     if (auto pv = Find(key); pv) {
-      pv->SetIoPending(false);
+      pv->SetStashPending(false);
       stats_.total_cancels++;
     }
   }
@@ -99,14 +99,14 @@ class TieredStorage::ShardOpManager : public tiering::OpManager {
   }
 
   DbTableStats* GetDbTableStats(DbIndex dbid) {
-    return db_slice_->MutableStats(dbid);
+    return db_slice_.MutableStats(dbid);
   }
 
  private:
   PrimeValue* Find(OpManager::KeyRef key) {
     // TODO: Get DbContext for transaction for correct dbid and time
     // Bypass all update and stat mechanisms
-    auto it = db_slice_->GetDBTable(key.first)->prime.Find(key.second);
+    auto it = db_slice_.GetDBTable(key.first)->prime.Find(key.second);
     return IsValid(it) ? &it->second : nullptr;
   }
 
@@ -141,7 +141,7 @@ class TieredStorage::ShardOpManager : public tiering::OpManager {
     if (auto pv = Find(key); pv) {
       RecordAdded(*pv, segment.length, GetDbTableStats(key.first));
 
-      pv->SetIoPending(false);
+      pv->SetStashPending(false);
       pv->SetExternal(segment.offset, segment.length);
 
       stats_.total_stashes++;
@@ -154,7 +154,11 @@ class TieredStorage::ShardOpManager : public tiering::OpManager {
       SetExternal({sub_dbid, sub_key}, sub_segment);
   }
 
-  bool cache_fetched_ = false;
+  bool HasEnoughMemoryMargin(int64_t value_len) {
+    return db_slice_.memory_budget() - memory_margin_ - value_len > 0;
+  }
+
+  int64_t memory_margin_ = 0;
 
   struct {
     size_t total_stashes = 0, total_cancels = 0, total_fetches = 0;
@@ -162,7 +166,7 @@ class TieredStorage::ShardOpManager : public tiering::OpManager {
   } stats_;
 
   TieredStorage* ts_;
-  DbSlice* db_slice_;
+  DbSlice& db_slice_;
 };
 
 void TieredStorage::ShardOpManager::Defragment(tiering::DiskSegment segment, string_view page) {
@@ -173,7 +177,7 @@ void TieredStorage::ShardOpManager::Defragment(tiering::DiskSegment segment, str
     auto predicate = [item_segment](const PrimeKey& key, const PrimeValue& probe) {
       return probe.IsExternal() && tiering::DiskSegment{probe.GetExternalSlice()} == item_segment;
     };
-    auto it = db_slice_->GetDBTable(dbid)->prime.FindFirst(hash, predicate);
+    auto it = db_slice_.GetDBTable(dbid)->prime.FindFirst(hash, predicate);
     if (!IsValid(it))
       continue;
 
@@ -200,7 +204,8 @@ bool TieredStorage::ShardOpManager::NotifyFetched(EntryId id, string_view value,
   //    the snapshotting.
   // TODO: to revisit this when we rewrite it with more efficient snapshotting algorithm.
 
-  bool should_upload = modified || (cache_fetched_ && !SliceSnapshot::IsSnaphotInProgress());
+  bool should_upload =
+      modified || (HasEnoughMemoryMargin(value.size()) && !SliceSnapshot::IsSnaphotInProgress());
 
   if (!should_upload)
     return false;
@@ -332,7 +337,7 @@ bool TieredStorage::TryStash(DbIndex dbid, string_view key, PrimeValue* value) {
   }
 
   StringOrView raw_string = value->GetRawString();
-  value->SetIoPending(true);
+  value->SetStashPending(true);
 
   tiering::OpManager::EntryId id;
   error_code ec;
@@ -364,13 +369,13 @@ void TieredStorage::Delete(DbIndex dbid, PrimeValue* value) {
 }
 
 void TieredStorage::CancelStash(DbIndex dbid, std::string_view key, PrimeValue* value) {
-  DCHECK(value->HasIoPending());
+  DCHECK(value->HasStashPending());
   if (OccupiesWholePages(value->Size())) {
     op_manager_->Delete(KeyRef(dbid, key));
   } else if (auto bin = bins_->Delete(dbid, key); bin) {
     op_manager_->Delete(*bin);
   }
-  value->SetIoPending(false);
+  value->SetStashPending(false);
 }
 
 float TieredStorage::WriteDepthUsage() const {
@@ -423,11 +428,18 @@ void TieredStorage::RunOffloading(DbIndex dbid) {
       disk_stats.max_file_size)
     return;
 
-  auto cb = [this, dbid, tmp = std::string{}](PrimeIterator it) mutable {
-    TryStash(dbid, it->first.GetSlice(&tmp), &it->second);
+  string tmp;
+  auto cb = [this, dbid, &tmp](PrimeIterator it) mutable {
+    if (ShouldStash(it->second)) {
+      if (it->first.WasTouched()) {
+        it->first.SetTouched(false);
+      } else {
+        TryStash(dbid, it->first.GetSlice(&tmp), &it->second);
+      }
+    }
   };
 
-  PrimeTable& table = op_manager_->db_slice_->GetDBTable(dbid)->prime;
+  PrimeTable& table = op_manager_->db_slice_.GetDBTable(dbid)->prime;
   PrimeTable::Cursor start_cursor{};
 
   // Loop while we haven't traversed all entries or reached our stash io device limit.
@@ -442,7 +454,7 @@ void TieredStorage::RunOffloading(DbIndex dbid) {
 
 bool TieredStorage::ShouldStash(const PrimeValue& pv) const {
   auto disk_stats = op_manager_->GetStats().disk_stats;
-  return !pv.IsExternal() && !pv.HasIoPending() && pv.ObjType() == OBJ_STRING &&
+  return !pv.IsExternal() && !pv.HasStashPending() && pv.ObjType() == OBJ_STRING &&
          pv.Size() >= kMinValueSize &&
          disk_stats.allocated_bytes + tiering::kPageSize + pv.Size() < disk_stats.max_file_size;
 }

src/server/tiered_storage_test.cc

@@ -23,8 +23,6 @@ using namespace testing;
 
 ABSL_DECLARE_FLAG(bool, force_epoll);
 ABSL_DECLARE_FLAG(string, tiered_prefix);
-ABSL_DECLARE_FLAG(bool, tiered_storage_cache_fetched);
-ABSL_DECLARE_FLAG(bool, backing_file_direct);
 ABSL_DECLARE_FLAG(float, tiered_offload_threshold);
 ABSL_DECLARE_FLAG(unsigned, tiered_storage_write_depth);
 
@@ -53,8 +51,6 @@ class TieredStorageTest : public BaseFamilyTest {
     if (GetFlag(FLAGS_tiered_prefix).empty()) {
       SetFlag(&FLAGS_tiered_prefix, "/tmp/tiered_storage_test");
     }
-    SetFlag(&FLAGS_tiered_storage_cache_fetched, true);
-    SetFlag(&FLAGS_backing_file_direct, true);
 
     BaseFamilyTest::SetUp();
   }

src/server/transaction.cc

@@ -6,6 +6,7 @@
 
 #include <absl/strings/match.h>
 
+#include "base/flags.h"
 #include "base/logging.h"
 #include "facade/op_status.h"
 #include "redis/redis_aux.h"