diff --git a/.bazelrc b/.bazelrc
index 3e3c3b6c4..b076a0799 100644
--- a/.bazelrc
+++ b/.bazelrc
@@ -2,5 +2,10 @@
 build --compilation_mode=opt
 build --action_env=PATH
 build --action_env=PYTHON_BIN_PATH
+# Warnings should be errors
+build --per_file_copt=-src/ray/thirdparty/hiredis/dict.c,-.*/arrow/util/logging.cc@-Werror
+# Ignore warnings for protobuf generated files and external projects.
+build --per_file_copt='\\.pb\\.cc$@-w'
+build --per_file_copt='external*@-w'
 # This workaround is needed due to https://github.com/bazelbuild/bazel/issues/4341
 build --per_file_copt="external/com_github_grpc_grpc/.*@-DGRPC_BAZEL_BUILD"
diff --git a/python/ray/_raylet.pyx b/python/ray/_raylet.pyx
index 494713e3e..d9edd13f2 100644
--- a/python/ray/_raylet.pyx
+++ b/python/ray/_raylet.pyx
@@ -86,7 +86,7 @@ cdef VectorToObjectIDs(c_vector[CObjectID] object_ids):
 
 
 def compute_put_id(TaskID task_id, int64_t put_index):
-    if put_index < 1 or put_index > CObjectID.MaxObjectIndex():
+    if put_index < 1 or put_index > <int64_t>CObjectID.MaxObjectIndex():
         raise ValueError("The range of 'put_index' should be [1, %d]"
                          % CObjectID.MaxObjectIndex())
     return ObjectID(CObjectID.ForPut(task_id.native(), put_index, 0).Binary())
diff --git a/src/ray/common/id.cc b/src/ray/common/id.cc
index e4e73fee6..0f1207dae 100644
--- a/src/ray/common/id.cc
+++ b/src/ray/common/id.cc
@@ -99,7 +99,7 @@ inline uint8_t GetTransportType(ObjectIDFlagsType flags) {
 template <typename T>
 void FillNil(T *data) {
   RAY_CHECK(data != nullptr);
-  for (int i = 0; i < data->size(); i++) {
+  for (size_t i = 0; i < data->size(); i++) {
     (*data)[i] = static_cast<uint8_t>(0xFF);
   }
 }
@@ -129,7 +129,7 @@ plasma::UniqueID ObjectID::ToPlasmaId() const {
 }
 
 ObjectID::ObjectID(const plasma::UniqueID &from) {
-  RAY_CHECK(from.size() <= ObjectID::Size()) << "Out of size.";
+  RAY_CHECK(from.size() <= static_cast<int64_t>(ObjectID::Size())) << "Out of size.";
   std::memcpy(this->MutableData(), from.data(), ObjectID::Size());
 }
 
diff --git a/src/ray/common/ray_config_def.h b/src/ray/common/ray_config_def.h
index b91b4c773..b3b81a906 100644
--- a/src/ray/common/ray_config_def.h
+++ b/src/ray/common/ray_config_def.h
@@ -151,7 +151,7 @@ RAY_CONFIG(int64_t, max_task_lease_timeout_ms, 60000)
 /// Note: this number should be set to at least 2. Because saving a application
 /// checkpoint isn't atomic with saving the backend checkpoint, and it will break
 /// if this number is set to 1 and users save application checkpoints in place.
-RAY_CONFIG(uint32_t, num_actor_checkpoints_to_keep, 20)
+RAY_CONFIG(int32_t, num_actor_checkpoints_to_keep, 20)
 
 /// Maximum number of ids in one batch to send to GCS to delete keys.
 RAY_CONFIG(uint32_t, maximum_gcs_deletion_batch_size, 1000)
diff --git a/src/ray/common/task/task.cc b/src/ray/common/task/task.cc
index 9c60e078a..61413d767 100644
--- a/src/ray/common/task/task.cc
+++ b/src/ray/common/task/task.cc
@@ -16,7 +16,7 @@ const std::vector<ObjectID> &Task::GetDependencies() const { return dependencies
 
 void Task::ComputeDependencies() {
   dependencies_.clear();
-  for (int i = 0; i < task_spec_.NumArgs(); ++i) {
+  for (size_t i = 0; i < task_spec_.NumArgs(); ++i) {
     int count = task_spec_.ArgIdCount(i);
     for (int j = 0; j < count; j++) {
       dependencies_.push_back(task_spec_.ArgId(i, j));
diff --git a/src/ray/common/task/task_spec.cc b/src/ray/common/task/task_spec.cc
index 06cba686a..1f04d3943 100644
--- a/src/ray/common/task/task_spec.cc
+++ b/src/ray/common/task/task_spec.cc
@@ -156,7 +156,7 @@ std::string TaskSpecification::DebugString() const {
   const auto list = VectorFromProtobuf(message_->function_descriptor());
   // The 4th is the code hash which is binary bits. No need to output it.
   const size_t size = std::min(static_cast<size_t>(3), list.size());
-  for (int i = 0; i < size; ++i) {
+  for (size_t i = 0; i < size; ++i) {
     if (i != 0) {
       stream << ",";
     }
diff --git a/src/ray/core_worker/object_interface.cc b/src/ray/core_worker/object_interface.cc
index 87b1f4ffc..e612314f9 100644
--- a/src/ray/core_worker/object_interface.cc
+++ b/src/ray/core_worker/object_interface.cc
@@ -58,16 +58,13 @@ std::unique_ptr<CoreWorkerStoreProvider> CoreWorkerObjectInterface::CreateStoreP
   case StoreProviderType::LOCAL_PLASMA:
     return std::unique_ptr<CoreWorkerStoreProvider>(
         new CoreWorkerLocalPlasmaStoreProvider(store_socket_));
-    break;
   case StoreProviderType::PLASMA:
     return std::unique_ptr<CoreWorkerStoreProvider>(
         new CoreWorkerPlasmaStoreProvider(store_socket_, raylet_client_));
-    break;
   default:
     RAY_LOG(FATAL) << "unknown store provider type " << static_cast<int>(type);
-    break;
+    return nullptr;
   }
-  return nullptr;
 }
 
 }  // namespace ray
diff --git a/src/ray/core_worker/store_provider/local_plasma_provider.cc b/src/ray/core_worker/store_provider/local_plasma_provider.cc
index 10ec0d07c..7441883c5 100644
--- a/src/ray/core_worker/store_provider/local_plasma_provider.cc
+++ b/src/ray/core_worker/store_provider/local_plasma_provider.cc
@@ -67,7 +67,7 @@ Status CoreWorkerLocalPlasmaStoreProvider::Wait(const std::vector<ObjectID> &obj
                                                 int num_objects, int64_t timeout_ms,
                                                 const TaskID &task_id,
                                                 std::vector<bool> *results) {
-  if (num_objects != object_ids.size()) {
+  if (num_objects != static_cast<int>(object_ids.size())) {
     return Status::Invalid("num_objects should equal to number of items in object_ids");
   }
 
diff --git a/src/ray/core_worker/store_provider/memory_store/memory_store.cc b/src/ray/core_worker/store_provider/memory_store/memory_store.cc
index d654eddc2..752ee7a8d 100644
--- a/src/ray/core_worker/store_provider/memory_store/memory_store.cc
+++ b/src/ray/core_worker/store_provider/memory_store/memory_store.cc
@@ -140,7 +140,7 @@ Status CoreWorkerMemoryStore::Get(const std::vector<ObjectID> &object_ids,
 
     std::unique_lock<std::mutex> lock(lock_);
     // Check for existing objects and see if this get request can be fullfilled.
-    for (int i = 0; i < object_ids.size(); i++) {
+    for (size_t i = 0; i < object_ids.size(); i++) {
       const auto &object_id = object_ids[i];
       auto iter = objects_.find(object_id);
       if (iter != objects_.end()) {
@@ -178,7 +178,7 @@ Status CoreWorkerMemoryStore::Get(const std::vector<ObjectID> &object_ids,
   {
     std::unique_lock<std::mutex> lock(lock_);
     // Populate results.
-    for (int i = 0; i < object_ids.size(); i++) {
+    for (size_t i = 0; i < object_ids.size(); i++) {
       const auto &object_id = object_ids[i];
       if ((*results)[i] == nullptr) {
         (*results)[i] = get_request->Get(object_id);
diff --git a/src/ray/core_worker/store_provider/memory_store_provider.cc b/src/ray/core_worker/store_provider/memory_store_provider.cc
index d1e07bb67..ef5c39e2c 100644
--- a/src/ray/core_worker/store_provider/memory_store_provider.cc
+++ b/src/ray/core_worker/store_provider/memory_store_provider.cc
@@ -31,7 +31,7 @@ Status CoreWorkerMemoryStoreProvider::Wait(const std::vector<ObjectID> &object_i
                                            int num_objects, int64_t timeout_ms,
                                            const TaskID &task_id,
                                            std::vector<bool> *results) {
-  if (num_objects != object_ids.size()) {
+  if (num_objects != static_cast<int>(object_ids.size())) {
     return Status::Invalid("num_objects should equal to number of items in object_ids");
   }
 
@@ -41,7 +41,7 @@ Status CoreWorkerMemoryStoreProvider::Wait(const std::vector<ObjectID> &object_i
   auto status = store_->Get(object_ids, timeout_ms, false, &result_objects);
   if (status.ok()) {
     RAY_CHECK(result_objects.size() == object_ids.size());
-    for (int i = 0; i < object_ids.size(); i++) {
+    for (size_t i = 0; i < object_ids.size(); i++) {
       (*results)[i] = (result_objects[i] != nullptr);
     }
   }
diff --git a/src/ray/core_worker/task_execution.cc b/src/ray/core_worker/task_execution.cc
index 20fe1df1b..c143c98a7 100644
--- a/src/ray/core_worker/task_execution.cc
+++ b/src/ray/core_worker/task_execution.cc
@@ -83,7 +83,7 @@ Status CoreWorkerTaskExecutionInterface::BuildArgsForExecutor(
   std::vector<ObjectID> object_ids_to_fetch;
   std::vector<int> indices;
 
-  for (int i = 0; i < task.NumArgs(); ++i) {
+  for (size_t i = 0; i < task.NumArgs(); ++i) {
     int count = task.ArgIdCount(i);
     if (count > 0) {
       // pass by reference.
diff --git a/src/ray/core_worker/task_interface.cc b/src/ray/core_worker/task_interface.cc
index fa35198fb..c6d47a39e 100644
--- a/src/ray/core_worker/task_interface.cc
+++ b/src/ray/core_worker/task_interface.cc
@@ -134,7 +134,7 @@ void CoreWorkerTaskInterface::BuildCommonTaskSpec(
 
   // Compute return IDs.
   (*return_ids).resize(num_returns);
-  for (int i = 0; i < num_returns; i++) {
+  for (size_t i = 0; i < num_returns; i++) {
     (*return_ids)[i] = ObjectID::ForTaskReturn(task_id, i + 1, /*transport_type=*/0);
   }
 }
diff --git a/src/ray/core_worker/test/core_worker_test.cc b/src/ray/core_worker/test/core_worker_test.cc
index 510b62983..206b0eeba 100644
--- a/src/ray/core_worker/test/core_worker_test.cc
+++ b/src/ray/core_worker/test/core_worker_test.cc
@@ -360,7 +360,7 @@ void CoreWorkerTest::TestActorReconstruction(
     for (int i = 0; i < num_tasks; i++) {
       if (i == task_index_to_kill_worker) {
         RAY_LOG(INFO) << "killing worker";
-        system("pkill mock_worker");
+        ASSERT_EQ(system("pkill mock_worker"), 0);
 
         // Wait for actor restruction event, and then for alive event.
         ASSERT_TRUE(WaitForDirectCallActorState(driver, actor_handle->ActorID(), false,
@@ -413,7 +413,7 @@ void CoreWorkerTest::TestActorFailure(
     for (int i = 0; i < num_tasks; i++) {
       if (i == task_index_to_kill_worker) {
         RAY_LOG(INFO) << "killing worker";
-        system("pkill mock_worker");
+        ASSERT_EQ(system("pkill mock_worker"), 0);
       }
 
       // wait for actor being reconstructed.
diff --git a/src/ray/core_worker/transport/direct_actor_transport.cc b/src/ray/core_worker/transport/direct_actor_transport.cc
index ecf4fed11..e7ec46f82 100644
--- a/src/ray/core_worker/transport/direct_actor_transport.cc
+++ b/src/ray/core_worker/transport/direct_actor_transport.cc
@@ -7,7 +7,7 @@ using ray::rpc::ActorTableData;
 namespace ray {
 
 bool HasByReferenceArgs(const TaskSpecification &spec) {
-  for (int i = 0; i < spec.NumArgs(); ++i) {
+  for (size_t i = 0; i < spec.NumArgs(); ++i) {
     if (spec.ArgIdCount(i) > 0) {
       return true;
     }
@@ -149,7 +149,8 @@ Status CoreWorkerDirectActorTaskSubmitter::PushTask(rpc::DirectActorClient &clie
                     reinterpret_cast<const uint8_t *>(return_object.metadata().data())),
                 return_object.metadata().size());
           }
-          store_provider_->Put(RayObject(data_buffer, metadata_buffer), object_id);
+          RAY_CHECK_OK(
+              store_provider_->Put(RayObject(data_buffer, metadata_buffer), object_id));
         }
       });
   return status;
@@ -163,7 +164,7 @@ void CoreWorkerDirectActorTaskSubmitter::TreatTaskAsFailed(
     std::string meta = std::to_string(static_cast<int>(error_type));
     auto metadata = const_cast<uint8_t *>(reinterpret_cast<const uint8_t *>(meta.data()));
     auto meta_buffer = std::make_shared<LocalMemoryBuffer>(metadata, meta.size());
-    store_provider_->Put(RayObject(nullptr, meta_buffer), object_id);
+    RAY_CHECK_OK(store_provider_->Put(RayObject(nullptr, meta_buffer), object_id));
   }
 }
 
@@ -203,7 +204,7 @@ void CoreWorkerDirectActorTaskReceiver::HandlePushTask(
   auto status = task_handler_(task_spec, &results);
   RAY_CHECK(results.size() == num_returns) << results.size() << "  " << num_returns;
 
-  for (int i = 0; i < results.size(); i++) {
+  for (size_t i = 0; i < results.size(); i++) {
     auto return_object = (*reply).add_return_objects();
     ObjectID id = ObjectID::ForTaskReturn(task_spec.TaskId(), /*index=*/i + 1,
                                           /*transport_type=*/0);
diff --git a/src/ray/core_worker/transport/raylet_transport.cc b/src/ray/core_worker/transport/raylet_transport.cc
index 776fb0a09..aa3e7e26a 100644
--- a/src/ray/core_worker/transport/raylet_transport.cc
+++ b/src/ray/core_worker/transport/raylet_transport.cc
@@ -40,10 +40,10 @@ void CoreWorkerRayletTaskReceiver::HandleAssignTask(
   }
 
   RAY_CHECK(results.size() == num_returns);
-  for (int i = 0; i < num_returns; i++) {
+  for (size_t i = 0; i < num_returns; i++) {
     ObjectID id = ObjectID::ForTaskReturn(task_spec.TaskId(), /*index=*/i + 1,
                                           /*transport_type=*/0);
-    object_interface_.Put(*results[i], id);
+    RAY_CHECK_OK(object_interface_.Put(*results[i], id));
   }
 
   // Notify raylet that current task is done via a `TaskDone` message. This is to
diff --git a/src/ray/gcs/actor_state_accessor_test.cc b/src/ray/gcs/actor_state_accessor_test.cc
index c7518e0e7..7f3bb8cf1 100644
--- a/src/ray/gcs/actor_state_accessor_test.cc
+++ b/src/ray/gcs/actor_state_accessor_test.cc
@@ -22,7 +22,7 @@ class ActorStateAccessorTest : public ::testing::Test {
     RAY_CHECK_OK(gcs_client_->Connect(io_service_));
 
     work_thread.reset(new std::thread([this] {
-      std::auto_ptr<boost::asio::io_service::work> work(
+      std::unique_ptr<boost::asio::io_service::work> work(
           new boost::asio::io_service::work(io_service_));
       io_service_.run();
     }));
@@ -91,10 +91,10 @@ TEST_F(ActorStateAccessorTest, RegisterAndGet) {
   for (const auto &elem : actor_datas_) {
     const auto &actor = elem.second;
     ++pending_count_;
-    actor_accessor.AsyncRegister(actor, [this](Status status) {
+    RAY_CHECK_OK(actor_accessor.AsyncRegister(actor, [this](Status status) {
       RAY_CHECK_OK(status);
       --pending_count_;
-    });
+    }));
   }
 
   std::chrono::milliseconds timeout(10000);
@@ -102,16 +102,15 @@ TEST_F(ActorStateAccessorTest, RegisterAndGet) {
 
   // get
   for (const auto &elem : actor_datas_) {
-    const auto &actor = elem.second;
     ++pending_count_;
-    actor_accessor.AsyncGet(elem.first,
-                            [this](Status status, std::vector<ActorTableData> datas) {
-                              ASSERT_EQ(datas.size(), 1U);
-                              ActorID actor_id = ActorID::FromBinary(datas[0].actor_id());
-                              auto it = actor_datas_.find(actor_id);
-                              ASSERT_TRUE(it != actor_datas_.end());
-                              --pending_count_;
-                            });
+    RAY_CHECK_OK(actor_accessor.AsyncGet(
+        elem.first, [this](Status status, std::vector<ActorTableData> datas) {
+          ASSERT_EQ(datas.size(), 1U);
+          ActorID actor_id = ActorID::FromBinary(datas[0].actor_id());
+          auto it = actor_datas_.find(actor_id);
+          ASSERT_TRUE(it != actor_datas_.end());
+          --pending_count_;
+        }));
   }
 
   WaitPendingDone(timeout);
@@ -135,7 +134,7 @@ TEST_F(ActorStateAccessorTest, Subscribe) {
   };
 
   ++do_sub_pending_count;
-  actor_accessor.AsyncSubscribe(subscribe, done);
+  RAY_CHECK_OK(actor_accessor.AsyncSubscribe(subscribe, done));
   // Wait until subscribe finishes.
   WaitPendingDone(do_sub_pending_count, timeout);
 
@@ -145,10 +144,11 @@ TEST_F(ActorStateAccessorTest, Subscribe) {
     const auto &actor = elem.second;
     ++sub_pending_count;
     ++register_pending_count;
-    actor_accessor.AsyncRegister(actor, [&register_pending_count](Status status) {
-      RAY_CHECK_OK(status);
-      --register_pending_count;
-    });
+    RAY_CHECK_OK(
+        actor_accessor.AsyncRegister(actor, [&register_pending_count](Status status) {
+          RAY_CHECK_OK(status);
+          --register_pending_count;
+        }));
   }
   // Wait until register finishes.
   WaitPendingDone(register_pending_count, timeout);
diff --git a/src/ray/gcs/redis_gcs_client_test.cc b/src/ray/gcs/redis_gcs_client_test.cc
index 34e7b359b..61ca14ee6 100644
--- a/src/ray/gcs/redis_gcs_client_test.cc
+++ b/src/ray/gcs/redis_gcs_client_test.cc
@@ -701,7 +701,7 @@ TEST_F(TestGcsWithAsio, TestSetSubscribeAll) {
 
 void TestTableSubscribeId(const JobID &job_id,
                           std::shared_ptr<gcs::RedisGcsClient> client) {
-  int num_modifications = 3;
+  size_t num_modifications = 3;
 
   // Add a table entry.
   TaskID task_id1 = RandomTaskId();
diff --git a/src/ray/gcs/redis_module/ray_redis_module.cc b/src/ray/gcs/redis_module/ray_redis_module.cc
index 197f36179..4d92c3b6c 100644
--- a/src/ray/gcs/redis_module/ray_redis_module.cc
+++ b/src/ray/gcs/redis_module/ray_redis_module.cc
@@ -585,7 +585,7 @@ int HashUpdate_DoWrite(RedisModuleCtx *ctx, RedisModuleString **argv, int argc,
     // This code path means they are updating command.
     size_t total_size = gcs_entry.entries_size();
     REPLY_AND_RETURN_IF_FALSE(total_size % 2 == 0, "Invalid Hash Update data vector.");
-    for (int i = 0; i < total_size; i += 2) {
+    for (size_t i = 0; i < total_size; i += 2) {
       // Reconstruct a key-value pair from a flattened list.
       RedisModuleString *entry_key = RedisModule_CreateString(
           ctx, gcs_entry.entries(i).data(), gcs_entry.entries(i).size());
@@ -603,7 +603,7 @@ int HashUpdate_DoWrite(RedisModuleCtx *ctx, RedisModuleString **argv, int argc,
     updated.set_change_mode(gcs_entry.change_mode());
 
     size_t total_size = gcs_entry.entries_size();
-    for (int i = 0; i < total_size; i++) {
+    for (size_t i = 0; i < total_size; i++) {
       RedisModuleString *entry_key = RedisModule_CreateString(
           ctx, gcs_entry.entries(i).data(), gcs_entry.entries(i).size());
       int deleted_num = RedisModule_HashSet(key, REDISMODULE_HASH_NONE, entry_key,
@@ -929,7 +929,7 @@ Status IsNil(bool *out, const std::string &data) {
     return Status::RedisError("Size of data doesn't match size of UniqueID");
   }
   const uint8_t *d = reinterpret_cast<const uint8_t *>(data.data());
-  for (int i = 0; i < kUniqueIDSize; ++i) {
+  for (size_t i = 0; i < kUniqueIDSize; ++i) {
     if (d[i] != 255) {
       *out = false;
     }
diff --git a/src/ray/gcs/tables.cc b/src/ray/gcs/tables.cc
index d60c1b0e2..5c99aaac8 100644
--- a/src/ray/gcs/tables.cc
+++ b/src/ray/gcs/tables.cc
@@ -92,7 +92,7 @@ Status Log<ID, Data>::Lookup(const JobID &job_id, const ID &id, const Callback &
         GcsEntry gcs_entry;
         gcs_entry.ParseFromString(reply.ReadAsString());
         RAY_CHECK(ID::FromBinary(gcs_entry.id()) == id);
-        for (size_t i = 0; i < gcs_entry.entries_size(); i++) {
+        for (int64_t i = 0; i < gcs_entry.entries_size(); i++) {
           Data data;
           data.ParseFromString(gcs_entry.entries(i));
           results.emplace_back(std::move(data));
@@ -142,7 +142,7 @@ Status Log<ID, Data>::Subscribe(const JobID &job_id, const ClientID &client_id,
         gcs_entry.ParseFromString(data);
         ID id = ID::FromBinary(gcs_entry.id());
         std::vector<Data> results;
-        for (size_t i = 0; i < gcs_entry.entries_size(); i++) {
+        for (int64_t i = 0; i < gcs_entry.entries_size(); i++) {
           Data result;
           result.ParseFromString(gcs_entry.entries(i));
           results.emplace_back(std::move(result));
diff --git a/src/ray/raylet/actor_registration.cc b/src/ray/raylet/actor_registration.cc
index afc3a7d89..7381d8d13 100644
--- a/src/ray/raylet/actor_registration.cc
+++ b/src/ray/raylet/actor_registration.cc
@@ -17,7 +17,7 @@ ActorRegistration::ActorRegistration(const ActorTableData &actor_table_data,
       execution_dependency_(
           ObjectID::FromBinary(checkpoint_data.execution_dependency())) {
   // Restore `frontier_`.
-  for (size_t i = 0; i < checkpoint_data.handle_ids_size(); i++) {
+  for (int64_t i = 0; i < checkpoint_data.handle_ids_size(); i++) {
     auto handle_id = ActorHandleID::FromBinary(checkpoint_data.handle_ids(i));
     auto &frontier_entry = frontier_[handle_id];
     frontier_entry.task_counter = checkpoint_data.task_counters(i);
@@ -25,7 +25,7 @@ ActorRegistration::ActorRegistration(const ActorTableData &actor_table_data,
         ObjectID::FromBinary(checkpoint_data.frontier_dependencies(i));
   }
   // Restore `dummy_objects_`.
-  for (size_t i = 0; i < checkpoint_data.unreleased_dummy_objects_size(); i++) {
+  for (int64_t i = 0; i < checkpoint_data.unreleased_dummy_objects_size(); i++) {
     auto dummy = ObjectID::FromBinary(checkpoint_data.unreleased_dummy_objects(i));
     dummy_objects_[dummy] = checkpoint_data.num_dummy_object_dependencies(i);
   }
diff --git a/src/ray/raylet/lineage_cache_test.cc b/src/ray/raylet/lineage_cache_test.cc
index d845e33ba..e9591f755 100644
--- a/src/ray/raylet/lineage_cache_test.cc
+++ b/src/ray/raylet/lineage_cache_test.cc
@@ -160,7 +160,7 @@ std::vector<ObjectID> InsertTaskChain(LineageCache &lineage_cache,
     lineage_cache.AddUncommittedLineage(task.GetTaskSpecification().TaskId(), lineage);
     inserted_tasks.push_back(task);
     arguments.clear();
-    for (int j = 0; j < task.GetTaskSpecification().NumReturns(); j++) {
+    for (size_t j = 0; j < task.GetTaskSpecification().NumReturns(); j++) {
       arguments.push_back(task.GetTaskSpecification().ReturnId(j));
     }
   }
diff --git a/src/ray/raylet/node_manager.cc b/src/ray/raylet/node_manager.cc
index 3770ff884..4b2e21f77 100644
--- a/src/ray/raylet/node_manager.cc
+++ b/src/ray/raylet/node_manager.cc
@@ -1021,7 +1021,7 @@ void NodeManager::HandleFetchOrReconstructRequest(
   WorkerID worker_id = WorkerID::FromBinary(request.worker_id());
   const auto &object_ids = request.object_ids();
   std::vector<ObjectID> required_object_ids;
-  for (size_t i = 0; i < object_ids.size(); ++i) {
+  for (int64_t i = 0; i < object_ids.size(); ++i) {
     ObjectID object_id = ObjectID::FromBinary(object_ids[i]);
     if (request.fetch_only()) {
       // If only a fetch is required, then do not subscribe to the
@@ -1513,7 +1513,7 @@ void NodeManager::SubmitTask(const Task &task, const Lineage &uncommitted_lineag
           // The actor is local.
           int64_t expected_task_counter = GetExpectedTaskCounter(
               actor_registry_, spec.ActorId(), spec.ActorHandleId());
-          if (spec.ActorCounter() < expected_task_counter) {
+          if (static_cast<int64_t>(spec.ActorCounter()) < expected_task_counter) {
             // A task that has already been executed before has been found. The
             // task will be treated as failed if at least one of the task's
             // return values have been evicted, to prevent the application from
@@ -1751,7 +1751,7 @@ bool NodeManager::AssignTask(const Task &task) {
     // expected task counter.
     int64_t expected_task_counter =
         GetExpectedTaskCounter(actor_registry_, spec.ActorId(), spec.ActorHandleId());
-    RAY_CHECK(spec.ActorCounter() == expected_task_counter)
+    RAY_CHECK(static_cast<int64_t>(spec.ActorCounter()) == expected_task_counter)
         << "Expected actor counter: " << expected_task_counter << ", task "
         << spec.TaskId() << " has: " << spec.ActorCounter();
   }
@@ -2341,7 +2341,7 @@ void NodeManager::ForwardTask(
         // Iterate through the object's arguments. NOTE(swang): We do not include
         // the execution dependencies here since those cannot be transferred
         // between nodes.
-        for (int i = 0; i < spec.NumArgs(); ++i) {
+        for (size_t i = 0; i < spec.NumArgs(); ++i) {
           int count = spec.ArgIdCount(i);
           for (int j = 0; j < count; j++) {
             ObjectID argument_id = spec.ArgId(i, j);
diff --git a/src/ray/raylet/task_dependency_manager_test.cc b/src/ray/raylet/task_dependency_manager_test.cc
index 8c4ed1dea..510bd0397 100644
--- a/src/ray/raylet/task_dependency_manager_test.cc
+++ b/src/ray/raylet/task_dependency_manager_test.cc
@@ -94,7 +94,7 @@ std::vector<Task> MakeTaskChain(int chain_size,
     auto task = ExampleTask(arguments, num_returns);
     task_chain.push_back(task);
     arguments.clear();
-    for (int j = 0; j < task.GetTaskSpecification().NumReturns(); j++) {
+    for (size_t j = 0; j < task.GetTaskSpecification().NumReturns(); j++) {
       arguments.push_back(task.GetTaskSpecification().ReturnId(j));
     }
   }
diff --git a/src/ray/rpc/raylet/raylet_client.cc b/src/ray/rpc/raylet/raylet_client.cc
index 7f43bf9fa..80636e858 100644
--- a/src/ray/rpc/raylet/raylet_client.cc
+++ b/src/ray/rpc/raylet/raylet_client.cc
@@ -109,7 +109,7 @@ ray::Status RayletClient::GetTask(std::unique_ptr<ray::TaskSpecification> *task_
   if (status.ok()) {
     resource_ids_.clear();
     // Parse resources that would be used by this assigned task.
-    for (size_t i = 0; i < reply.fractional_resource_ids().size(); ++i) {
+    for (int64_t i = 0; i < reply.fractional_resource_ids().size(); ++i) {
       auto const &fractional_resource_ids = reply.fractional_resource_ids()[i];
       auto &acquired_resources = resource_ids_[fractional_resource_ids.resource_name()];
 
diff --git a/src/ray/stats/metric.h b/src/ray/stats/metric.h
index 96f67bbd5..fb6c2d12a 100644
--- a/src/ray/stats/metric.h
+++ b/src/ray/stats/metric.h
@@ -41,10 +41,6 @@ class StatsConfig final {
   bool is_stats_disabled_ = true;
 };
 
-/// The helper function for registering a view.
-static void RegisterAsView(opencensus::stats::ViewDescriptor view_descriptor,
-                           const std::vector<opencensus::tags::TagKey> &keys);
-
 /// A thin wrapper that wraps the `opencensus::tag::measure` for using it simply.
 class Metric {
  public:
diff --git a/src/ray/stats/stats_test.cc b/src/ray/stats/stats_test.cc
index da8b7ffc3..bd6f0ceec 100644
--- a/src/ray/stats/stats_test.cc
+++ b/src/ray/stats/stats_test.cc
@@ -29,7 +29,7 @@ class MockExporter : public opencensus::stats::StatsExporter::Handler {
       ASSERT_EQ(opencensus::stats::ViewData::Type::kDouble, view_data.type());
 
       for (const auto row : view_data.double_data()) {
-        for (int i = 0; i < descriptor.columns().size(); ++i) {
+        for (size_t i = 0; i < descriptor.columns().size(); ++i) {
           if (descriptor.columns()[i].name() == "NodeAddress") {
             ASSERT_EQ("Localhost", row.first[i]);
           }
diff --git a/src/ray/util/logging.cc b/src/ray/util/logging.cc
index e57712d84..3c5ff7f4e 100644
--- a/src/ray/util/logging.cc
+++ b/src/ray/util/logging.cc
@@ -138,7 +138,6 @@ void RayLog::StartRayLog(const std::string &app_name, RayLogLevel severity_thres
   log_dir_ = log_dir;
 #ifdef RAY_USE_GLOG
   google::InitGoogleLogging(app_name_.c_str());
-  int mapped_severity_threshold = GetMappedSeverity(severity_threshold_);
   google::SetStderrLogging(GetMappedSeverity(RayLogLevel::ERROR));
   for (int i = static_cast<int>(severity_threshold_);
        i <= static_cast<int>(RayLogLevel::FATAL); ++i) {