Add in_memory option to PersistentDataset for hybrid caching

monai/data/dataset.py

@@ -232,6 +232,7 @@ def __init__(
         reset_ops_id: bool = True,
         track_meta: bool = False,
         weights_only: bool = True,
+        in_memory: bool = False,
     ) -> None:
         """
         Args:
@@ -273,6 +274,10 @@ def __init__(
                 other safe objects. Setting this to `False` is required for loading `MetaTensor`
                 objects saved with `track_meta=True`, however this creates the possibility of remote
                 code execution through `torch.load` so be aware of the security implications of doing so.
+            in_memory: if `True`, keep the pre-processed data in an in-memory dictionary after first access.
+                This combines the benefits of persistent storage (data survives restarts) with faster RAM access.
+                When data is accessed, it is first loaded from disk cache and then stored in memory.
+                Default to `False`.
 
         Raises:
             ValueError: When both `track_meta=True` and `weights_only=True`, since this combination
@@ -299,6 +304,13 @@ def __init__(
             )
         self.track_meta = track_meta
         self.weights_only = weights_only
+        self.in_memory = in_memory
+        self._memory_cache: dict[str, Any] = {}
+
+    @property
+    def memory_cache_size(self) -> int:
+        """Return the number of items currently stored in the in-memory cache."""
+        return len(self._memory_cache)
 
     def set_transform_hash(self, hash_xform_func: Callable[..., bytes]):
         """Get hashable transforms, and then hash them. Hashable transforms
@@ -326,6 +338,7 @@ def set_data(self, data: Sequence):
 
         """
         self.data = data
+        self._memory_cache = {}
         if self.cache_dir is not None and self.cache_dir.exists():
             shutil.rmtree(self.cache_dir, ignore_errors=True)
             self.cache_dir.mkdir(parents=True, exist_ok=True)
@@ -389,14 +402,24 @@ def _cachecheck(self, item_transformed):
 
         """
         hashfile = None
+        # compute cache key once for both disk and memory caching
+        data_item_md5 = self.hash_func(item_transformed).decode("utf-8")
+        data_item_md5 += self.transform_hash
+        cache_key = f"{data_item_md5}.pt"
+
         if self.cache_dir is not None:
-            data_item_md5 = self.hash_func(item_transformed).decode("utf-8")
-            data_item_md5 += self.transform_hash
-            hashfile = self.cache_dir / f"{data_item_md5}.pt"
+            hashfile = self.cache_dir / cache_key
+
+        # check in-memory cache first
+        if self.in_memory and cache_key in self._memory_cache:
+            return self._memory_cache[cache_key]
 
         if hashfile is not None and hashfile.is_file():  # cache hit
             try:
-                return torch.load(hashfile, weights_only=self.weights_only)
+                _item_transformed = torch.load(hashfile, weights_only=self.weights_only)
+                if self.in_memory:
+                    self._memory_cache[cache_key] = _item_transformed
+                return _item_transformed
             except PermissionError as e:
                 if sys.platform != "win32":
                     raise e
@@ -409,6 +432,8 @@ def _cachecheck(self, item_transformed):
 
         _item_transformed = self._pre_transform(deepcopy(item_transformed))  # keep the original hashed
         if hashfile is None:
+            if self.in_memory:
+                self._memory_cache[cache_key] = _item_transformed
             return _item_transformed
         try:
             # NOTE: Writing to a temporary directory and then using a nearly atomic rename operation
@@ -431,6 +456,8 @@ def _cachecheck(self, item_transformed):
                         pass
         except PermissionError:  # project-monai/monai issue #3613
             pass
+        if self.in_memory:
+            self._memory_cache[cache_key] = _item_transformed
         return _item_transformed
 
     def _transform(self, index: int):

tests/data/test_persistentdataset.py

@@ -15,6 +15,7 @@
 import os
 import tempfile
 import unittest
+from pathlib import Path
 
 import nibabel as nib
 import numpy as np
@@ -200,6 +201,111 @@ def test_track_meta_and_weights_only(self, track_meta, weights_only, expected_er
                 im = test_dataset[0]["image"]
                 self.assertIsInstance(im, expected_type)
 
+    def test_in_memory_cache(self):
+        """Test in_memory caching feature that combines persistent storage with RAM caching."""
+        items = [[list(range(i))] for i in range(5)]
+
+        with tempfile.TemporaryDirectory() as tempdir:
+            # First, create the persistent cache
+            ds1 = PersistentDataset(data=items, transform=_InplaceXform(), cache_dir=tempdir, in_memory=False)
+            # Access all items to populate disk cache
+            _ = list(ds1)
+
+            # Now create a new dataset with in_memory=True
+            ds2 = PersistentDataset(data=items, transform=_InplaceXform(), cache_dir=tempdir, in_memory=True)
+
+            # Memory cache should be empty initially
+            self.assertEqual(ds2.memory_cache_size, 0)
+
+            # Access items - they should be loaded from disk and cached in memory
+            _ = ds2[0]
+            self.assertEqual(ds2.memory_cache_size, 1)
+
+            _ = ds2[1]
+            self.assertEqual(ds2.memory_cache_size, 2)
+
+            # Access all items
+            _ = list(ds2)
+            self.assertEqual(ds2.memory_cache_size, 5)
+
+            # Accessing same item again should use memory cache (same result)
+            result1 = ds2[0]
+            result2 = ds2[0]
+            self.assertEqual(result1, result2)
+
+            # Test set_data clears in-memory cache
+            ds2.set_data(items[:3])
+            self.assertEqual(ds2.memory_cache_size, 0)
+
+    def test_in_memory_without_cache_dir(self):
+        """Test in_memory caching works even without a cache_dir (pure RAM cache)."""
+        items = [[list(range(i))] for i in range(3)]
+
+        ds = PersistentDataset(data=items, transform=_InplaceXform(), cache_dir=None, in_memory=True)
+
+        # Memory cache should be empty initially
+        self.assertEqual(ds.memory_cache_size, 0)
+
+        # Access items - they should be cached in memory
+        _ = ds[0]
+        self.assertEqual(ds.memory_cache_size, 1)
+
+        _ = list(ds)
+        self.assertEqual(ds.memory_cache_size, 3)
+
+    def test_automatic_hybrid_caching(self):
+        """
+        Test that in_memory=True provides automatic hybrid caching:
+        - ALL samples automatically persist to disk
+        - ALL samples automatically cache to RAM after first access
+        - No manual specification of which samples go where (unlike torchdatasets)
+        - Simulates restart scenario: disk cache survives, RAM cache rebuilds automatically
+        """
+        items = [[list(range(i))] for i in range(5)]
+
+        with tempfile.TemporaryDirectory() as tempdir:
+            # === First "session": populate both disk and RAM cache ===
+            ds1 = PersistentDataset(data=items, transform=_InplaceXform(), cache_dir=tempdir, in_memory=True)
+
+            # Access all items - should automatically cache to BOTH disk AND RAM
+            for i in range(len(items)):
+                _ = ds1[i]
+
+            # Verify: ALL samples are in RAM (automatic, no manual specification)
+            self.assertEqual(ds1.memory_cache_size, 5)
+
+            # Verify: ALL samples are on disk (count .pt files)
+            cache_files = list(Path(tempdir).glob("*.pt"))
+            self.assertEqual(len(cache_files), 5)
+
+            # === Simulate "restart": new dataset instance, same cache_dir ===
+            # This is the key benefit over CacheDataset - disk cache survives restart
+            ds2 = PersistentDataset(data=items, transform=_InplaceXform(), cache_dir=tempdir, in_memory=True)
+
+            # RAM cache starts empty (simulating fresh process)
+            self.assertEqual(ds2.memory_cache_size, 0)
+
+            # Access all items - should load from disk and automatically cache to RAM
+            results = [ds2[i] for i in range(len(items))]
+
+            # Verify: ALL samples now in RAM again (automatic rebuild from disk)
+            self.assertEqual(ds2.memory_cache_size, 5)
+
+            # Verify: Results are correct
+            for i, result in enumerate(results):
+                self.assertEqual(result, [list(range(i))])
+
+            # === Verify RAM cache provides fast repeated access ===
+            # Accessing same items again should hit RAM cache (same objects)
+            for i in range(len(items)):
+                result1 = ds2[i]
+                result2 = ds2[i]
+                # Should return equivalent results
+                self.assertEqual(result1, result2)
+
+            # RAM cache size unchanged (no duplicate entries)
+            self.assertEqual(ds2.memory_cache_size, 5)
+
 
 if __name__ == "__main__":
     unittest.main()