add skip_storage flag to HNSW (#3487)

Summary:
Pull Request resolved: https://github.com/facebookresearch/faiss/pull/3487

Sometimes it is not useful to serialize the storage index along with a HNSW index. This diff adds a flag that supports skipping the storage of the index.

Searchign and adding to the index is not possible until a storage index is added back in.

Reviewed By: junjieqi

Differential Revision: D57911060

fbshipit-source-id: 5a4ceee4a8f53f6f746df59af3942b813a99c14f

faiss/IndexHNSW.cpp

@@ -5,8 +5,6 @@
  * LICENSE file in the root directory of this source tree.
  */
 
-// -*- c++ -*-
-
 #include <faiss/IndexHNSW.h>
 
 #include <omp.h>
@@ -251,7 +249,8 @@ void hnsw_search(
         const SearchParameters* params_in) {
     FAISS_THROW_IF_NOT_MSG(
             index->storage,
-            "Please use IndexHNSWFlat (or variants) instead of IndexHNSW directly");
+            "No storage index, please use IndexHNSWFlat (or variants) "
+            "instead of IndexHNSW directly");
     const SearchParametersHNSW* params = nullptr;
     const HNSW& hnsw = index->hnsw;
 

faiss/impl/index_read.cpp

@@ -5,8 +5,6 @@
  * LICENSE file in the root directory of this source tree.
  */
 
-// -*- c++ -*-
-
 #include <faiss/index_io.h>
 
 #include <faiss/impl/io_macros.h>
@@ -531,7 +529,11 @@ Index* read_index(IOReader* f, int io_flags) {
     Index* idx = nullptr;
     uint32_t h;
     READ1(h);
-    if (h == fourcc("IxFI") || h == fourcc("IxF2") || h == fourcc("IxFl")) {
+    if (h == fourcc("null")) {
+        // denotes a missing index, useful for some cases
+        return nullptr;
+    } else if (
+            h == fourcc("IxFI") || h == fourcc("IxF2") || h == fourcc("IxFl")) {
         IndexFlat* idxf;
         if (h == fourcc("IxFI")) {
             idxf = new IndexFlatIP();
@@ -961,7 +963,7 @@ Index* read_index(IOReader* f, int io_flags) {
         read_index_header(idxhnsw, f);
         read_HNSW(&idxhnsw->hnsw, f);
         idxhnsw->storage = read_index(f, io_flags);
-        idxhnsw->own_fields = true;
+        idxhnsw->own_fields = idxhnsw->storage != nullptr;
         if (h == fourcc("IHNp") && !(io_flags & IO_FLAG_PQ_SKIP_SDC_TABLE)) {
             dynamic_cast<IndexPQ*>(idxhnsw->storage)->pq.compute_sdc_table();
         }

faiss/impl/index_write.cpp

@@ -5,8 +5,6 @@
  * LICENSE file in the root directory of this source tree.
  */
 
-// -*- c++ -*-
-
 #include <faiss/index_io.h>
 
 #include <faiss/impl/io.h>
@@ -390,8 +388,12 @@ static void write_ivf_header(const IndexIVF* ivf, IOWriter* f) {
     write_direct_map(&ivf->direct_map, f);
 }
 
-void write_index(const Index* idx, IOWriter* f) {
-    if (const IndexFlat* idxf = dynamic_cast<const IndexFlat*>(idx)) {
+void write_index(const Index* idx, IOWriter* f, int io_flags) {
+    if (idx == nullptr) {
+        // eg. for a storage component of HNSW that is set to nullptr
+        uint32_t h = fourcc("null");
+        WRITE1(h);
+    } else if (const IndexFlat* idxf = dynamic_cast<const IndexFlat*>(idx)) {
         uint32_t h =
                 fourcc(idxf->metric_type == METRIC_INNER_PRODUCT ? "IxFI"
                                : idxf->metric_type == METRIC_L2  ? "IxF2"
@@ -765,7 +767,12 @@ void write_index(const Index* idx, IOWriter* f) {
         WRITE1(h);
         write_index_header(idxhnsw, f);
         write_HNSW(&idxhnsw->hnsw, f);
-        write_index(idxhnsw->storage, f);
+        if (io_flags & IO_FLAG_SKIP_STORAGE) {
+            uint32_t n4 = fourcc("null");
+            WRITE1(n4);
+        } else {
+            write_index(idxhnsw->storage, f);
+        }
     } else if (const IndexNSG* idxnsg = dynamic_cast<const IndexNSG*>(idx)) {
         uint32_t h = dynamic_cast<const IndexNSGFlat*>(idx) ? fourcc("INSf")
                 : dynamic_cast<const IndexNSGPQ*>(idx)      ? fourcc("INSp")
@@ -841,14 +848,14 @@ void write_index(const Index* idx, IOWriter* f) {
     }
 }
 
-void write_index(const Index* idx, FILE* f) {
+void write_index(const Index* idx, FILE* f, int io_flags) {
     FileIOWriter writer(f);
-    write_index(idx, &writer);
+    write_index(idx, &writer, io_flags);
 }
 
-void write_index(const Index* idx, const char* fname) {
+void write_index(const Index* idx, const char* fname, int io_flags) {
     FileIOWriter writer(fname);
-    write_index(idx, &writer);
+    write_index(idx, &writer, io_flags);
 }
 
 void write_VectorTransform(const VectorTransform* vt, const char* fname) {

faiss/index_io.h

@@ -5,8 +5,6 @@
  * LICENSE file in the root directory of this source tree.
  */
 
-// -*- c++ -*-
-
 // I/O code for indexes
 
 #ifndef FAISS_INDEX_IO_H
@@ -35,9 +33,12 @@ struct IOReader;
 struct IOWriter;
 struct InvertedLists;
 
-void write_index(const Index* idx, const char* fname);
-void write_index(const Index* idx, FILE* f);
-void write_index(const Index* idx, IOWriter* writer);
+/// skip the storage for graph-based indexes
+const int IO_FLAG_SKIP_STORAGE = 1;
+
+void write_index(const Index* idx, const char* fname, int io_flags = 0);
+void write_index(const Index* idx, FILE* f, int io_flags = 0);
+void write_index(const Index* idx, IOWriter* writer, int io_flags = 0);
 
 void write_index_binary(const IndexBinary* idx, const char* fname);
 void write_index_binary(const IndexBinary* idx, FILE* f);

faiss/python/__init__.py

@@ -292,10 +292,10 @@ IVFSearchParameters = SearchParametersIVF
 ###########################################
 
 
-def serialize_index(index):
+def serialize_index(index, io_flags=0):
     """ convert an index to a numpy uint8 array  """
     writer = VectorIOWriter()
-    write_index(index, writer)
+    write_index(index, writer, io_flags)
     return vector_to_array(writer.data)
 
 

tests/test_graph_based.py

@@ -133,6 +133,42 @@ class TestHNSW(unittest.TestCase):
         Dhnsw, Ihnsw = index.search(self.xq, 1)
         self.assertGreater(stats.ndis, len(self.xq) * index.hnsw.efSearch)
 
+    def test_io_no_storage(self):
+        d = self.xq.shape[1]
+        index = faiss.IndexHNSWFlat(d, 16)
+        index.add(self.xb)
+
+        Dref, Iref = index.search(self.xq, 5)
+
+        # test writing without storage
+        index2 = faiss.deserialize_index(
+            faiss.serialize_index(index, faiss.IO_FLAG_SKIP_STORAGE)
+        )
+        self.assertEquals(index2.storage, None)
+        self.assertRaises(
+            RuntimeError,
+            index2.search, self.xb, 1)
+
+        # make sure we can store an index with empty storage
+        index4 = faiss.deserialize_index(
+            faiss.serialize_index(index2))
+
+        # add storage afterwards
+        index.storage = faiss.clone_index(index.storage)
+        index.own_fields = True
+
+        Dnew, Inew = index.search(self.xq, 5)
+        np.testing.assert_array_equal(Dnew, Dref)
+        np.testing.assert_array_equal(Inew, Iref)
+
+        if False:
+            # test reading without storage
+            # not implemented because it is hard to skip over an index
+            index3 = faiss.deserialize_index(
+                faiss.serialize_index(index), faiss.IO_FLAG_SKIP_STORAGE
+            )
+            self.assertEquals(index3.storage, None)
+
 
 class TestNSG(unittest.TestCase):