delay destructiuon of rocksdb checkpoint handle

Summary:
X-link: facebookresearch/FBGEMM#1761

delay the destruction of rocksdb checkpoint handle, so that rdb checkpoint handle can be used multiple times.

this is a stopgap measure for the silvertorch publishing use case, until the rdb checkpoint lifecycle unification effort is ready.

Reviewed By: duduyi2013

Differential Revision: D80564473

Author: jiayulu

fbgemm_gpu/src/ssd_split_embeddings_cache/embedding_rocksdb_wrapper.h

@@ -218,14 +218,14 @@ class EmbeddingRocksDBWrapper : public torch::jit::CustomClassHolder {
     impl_->create_checkpoint(global_step);
   }
 
-  c10::intrusive_ptr<RocksdbCheckpointHandleWrapper> get_active_checkpoint_uuid(
-      int64_t global_step) {
+  std::optional<c10::intrusive_ptr<RocksdbCheckpointHandleWrapper>>
+  get_active_checkpoint_uuid(int64_t global_step) {
     auto uuid_opt = impl_->get_active_checkpoint_uuid(global_step);
     if (uuid_opt.has_value()) {
       return c10::make_intrusive<RocksdbCheckpointHandleWrapper>(
           uuid_opt.value(), impl_);
     } else {
-      return nullptr;
+      return std::nullopt;
     }
   }
 

fbgemm_gpu/src/ssd_split_embeddings_cache/kv_tensor_wrapper.h

@@ -47,7 +47,7 @@ struct RocksdbCheckpointHandleWrapper : public torch::jit::CustomClassHolder {
       const std::string& checkpoint_uuid,
       std::shared_ptr<EmbeddingRocksDB> db);
 
-  ~RocksdbCheckpointHandleWrapper();
+  //   ~RocksdbCheckpointHandleWrapper();
 
   std::string uuid;
   std::shared_ptr<EmbeddingRocksDB> db;

fbgemm_gpu/src/ssd_split_embeddings_cache/ssd_split_table_batched_embeddings.cpp

@@ -345,9 +345,12 @@ RocksdbCheckpointHandleWrapper::RocksdbCheckpointHandleWrapper(
     std::shared_ptr<EmbeddingRocksDB> db)
     : uuid(checkpoint_uuid), db(std::move(db)) {}
 
-RocksdbCheckpointHandleWrapper::~RocksdbCheckpointHandleWrapper() {
-  db->release_checkpoint(uuid);
-}
+// do not release uuid when RocksdbCheckpointHandleWrapper is destroyed
+// subsequent get_active_checkpoint_uuid() calls need to retrieve
+// the checkpoint uuid
+// RocksdbCheckpointHandleWrapper::~RocksdbCheckpointHandleWrapper() {
+//   db->release_checkpoint(uuid);
+// }
 
 KVTensorWrapper::KVTensorWrapper(
     std::vector<int64_t> shape,

fbgemm_gpu/src/ssd_split_embeddings_cache/ssd_table_batched_embeddings.h

@@ -593,7 +593,29 @@ class EmbeddingRocksDB : public kv_db::EmbeddingKVDB {
              "removing the prev rdb ckpt, please make sure it has fullfilled "
              "its use case, e.g. checkpoint and publish";
     }
+
+    // remove old checkoint handler as they are likely not needed anymore
+    // this can not be done the same way as rdb snapshot, because we only
+    // create 1 rdb checkpoint per use cases(ckpt/publish), however publish
+    // calls state_dict multiple times, most of them to just get the fqn, they
+    // throw away pmt immediately if such state_dict is called at the beginning,
+    // it will destroy the checkpoint handler and when the real use case needs
+    // rdb ckpt, it is removed already
+    if (global_step - 10000 > 0) {
+      std::vector<std::string> ckpt_uuids_to_purge;
+      for (const auto& [glb_step, ckpt_uuid] : global_step_to_ckpt_uuid_) {
+        if (glb_step < global_step - 10000) {
+          ckpt_uuids_to_purge.push_back(ckpt_uuid);
+        }
+      }
+      for (auto& ckpt_uuid : ckpt_uuids_to_purge) {
+        release_checkpoint(ckpt_uuid);
+      }
+    }
+
     auto ckpt_uuid = facebook::strings::generateUUID();
+
+    LOG(INFO) << "creating new rocksdb checkpoint, uuid:" << ckpt_uuid;
     auto handle = std::make_unique<CheckpointHandle>(
         this, tbe_uuid_, ckpt_uuid, path_, use_default_ssd_path_);
     checkpoints_[ckpt_uuid] = std::move(handle);

fbgemm_gpu/test/tbe/ssd/kv_backend_test.py

@@ -394,6 +394,32 @@ def test_rocksdb_get_discrete_ids(
 
         assert torch.equal(ids_in_range_ordered, id_tensor_ordered)
 
+    @given(**default_st)
+    @settings(**default_settings)
+    def test_ssd_rocksdb_checkpoint_handle(
+        self,
+        T: int,
+        D: int,
+        log_E: int,
+        mixed: bool,
+        weights_precision: SparseType,
+    ) -> None:
+        emb, _, _ = self.generate_fbgemm_kv_tbe(
+            T, D, log_E, weights_precision, mixed, False, 8
+        )
+
+        # expect no checkpoint handle
+        checkpoint_handle = emb._ssd_db.get_active_checkpoint_uuid(0)
+        assert checkpoint_handle is None, f"{checkpoint_handle=}"
+        # create a checkpoint
+        emb.create_rocksdb_hard_link_snapshot()
+        checkpoint_handle = emb._ssd_db.get_active_checkpoint_uuid(0)
+        assert checkpoint_handle is not None, f"{checkpoint_handle=}"
+        # delete checkpoint_handle, handle should still exist because emb holds a reference
+        del checkpoint_handle
+        checkpoint_handle = emb._ssd_db.get_active_checkpoint_uuid(0)
+        assert checkpoint_handle is not None, f"{checkpoint_handle=}"
+
     @given(
         E=st.integers(min_value=1000, max_value=10000),
         num_buckets=st.integers(min_value=10, max_value=15),