[MOD-12983] [SVS] Add WriteInPlace mode support to TieredSVSIndex::runGC() (#861)

* Add `runGCInPlaceMode` test

* Add WriteInPlace support

* Spelling fixes in comments

* Code review s1e1

* Update tests to run GC on empty index

Author: rfsaliev

src/VecSim/algorithms/svs/svs_tiered.h

@@ -579,6 +579,10 @@ class TieredSVSIndex : public VecSimTieredIndex<DataType, float> {
         index->backendIndex->log(VecSimCommonStrings::LOG_VERBOSE_STRING,
                                  "running asynchronous GC for tiered SVS index");
         auto svs_index = index->GetSVSIndex();
+        if (index->backendIndex->indexSize() == 0) {
+            // No need to run GC on an empty index.
+            return;
+        }
         svs_index->setNumThreads(std::min(availableThreads, index->backendIndex->indexSize()));
         // VecSimIndexAbstract::runGC() is protected
         static_cast<VecSimIndexInterface *>(index->backendIndex)->runGC();
@@ -1009,6 +1013,21 @@ class TieredSVSIndex : public VecSimTieredIndex<DataType, float> {
     }
 
     void runGC() override {
+        if (this->getWriteMode() == VecSim_WriteInPlace) {
+            TIERED_LOG(VecSimCommonStrings::LOG_VERBOSE_STRING,
+                       "running synchronous GC for tiered SVS index in write-in-place mode");
+            // In write-in-place mode, we run GC synchronously.
+            std::lock_guard lock{this->mainIndexGuard};
+            if (this->backendIndex->indexSize() == 0) {
+                // No need to run GC on an empty index.
+                return;
+            }
+            // Force single thread for write-in-place mode.
+            this->GetSVSIndex()->setNumThreads(1);
+            // VecSimIndexAbstract::runGC() is protected
+            static_cast<VecSimIndexInterface *>(this->backendIndex)->runGC();
+            return;
+        }
         TIERED_LOG(VecSimCommonStrings::LOG_VERBOSE_STRING,
                    "scheduling asynchronous GC for tiered SVS index");
         scheduleSVSIndexGC();

tests/unit/test_svs_tiered.cpp

@@ -3163,6 +3163,13 @@ TYPED_TEST(SVSTieredIndexTestBasic, runGCParallel) {
     ASSERT_INDEX(tiered_index);
     auto allocator = tiered_index->getAllocator();
 
+    // Run GC on empty index
+    VecSimTieredIndex_GC(tiered_index);
+    mock_thread_pool.init_threads();
+    mock_thread_pool.thread_pool_join();
+    ASSERT_EQ(tiered_index->getNumMarkedDeleted(), 0);
+    ASSERT_EQ(mock_thread_pool.jobQ.size(), 0);
+
     // Insert n vectors directly to SVS.
     std::srand(10); // create pseudo random generator with any arbitrary seed.
     for (size_t i = 0; i < n; i++) {
@@ -3211,6 +3218,75 @@ TYPED_TEST(SVSTieredIndexTestBasic, runGCParallel) {
     EXPECT_EQ(tiered_index->statisticInfo().numberOfMarkedDeleted, 0);
 }
 
+TYPED_TEST(SVSTieredIndexTestBasic, runGCInPlaceMode) {
+    // Create TieredSVS index instance with a mock queue.
+    size_t dim = 4;
+    size_t threshold = 1024;
+    const size_t n = threshold * 4;
+    SVSParams params = {.type = TypeParam::get_index_type(), .dim = dim, .metric = VecSimMetric_L2};
+    VecSimParams svs_params = CreateParams(params);
+    auto mock_thread_pool = tieredIndexMock();
+
+    // Force trigger the first update job for 64 first vectors.
+    auto *tiered_index = this->CreateTieredSVSIndex(svs_params, mock_thread_pool, 64);
+    ASSERT_INDEX(tiered_index);
+    auto allocator = tiered_index->getAllocator();
+
+    VecSim_SetWriteMode(VecSim_WriteInPlace);
+
+    // Run GC on empty index
+    VecSimTieredIndex_GC(tiered_index);
+    ASSERT_EQ(tiered_index->getNumMarkedDeleted(), 0);
+    ASSERT_EQ(mock_thread_pool.jobQ.size(), 0);
+
+    // Insert n vectors directly to SVS.
+    std::srand(10); // create pseudo random generator with any arbitrary seed.
+    for (size_t i = 0; i < n; i++) {
+        TEST_DATA_T vector[dim];
+        for (size_t j = 0; j < dim; j++) {
+            vector[j] = std::rand() / (TEST_DATA_T)RAND_MAX;
+        }
+        VecSimIndex_AddVector(tiered_index->GetBackendIndex(), vector, i);
+    }
+
+    ASSERT_EQ(tiered_index->indexSize(), n);
+    ASSERT_EQ(tiered_index->GetBackendIndex()->indexSize(), n);
+
+    // Run the mess of add, delete, GC
+    for (size_t i = 0; i < threshold; i++) {
+        // Run GC for every 64 iterations.
+        if (i % 64 == 0) {
+            VecSimTieredIndex_GC(tiered_index);
+            ASSERT_EQ(tiered_index->getNumMarkedDeleted(), 0);
+            ASSERT_EQ(mock_thread_pool.jobQ.size(), 0);
+        }
+        // Add a new vector
+        TEST_DATA_T vector[dim];
+        for (size_t j = 0; j < dim; j++) {
+            vector[j] = std::rand() / (TEST_DATA_T)RAND_MAX;
+        }
+        VecSimIndex_AddVector(tiered_index, vector, n + i);
+        // Delete an existing vector
+        tiered_index->deleteVector(i + threshold);
+    }
+    // Final GC after all operations.
+    VecSimTieredIndex_GC(tiered_index);
+
+    // Validate if no jobs scheduled to the queue.
+    ASSERT_EQ(mock_thread_pool.jobQ.size(), 0);
+
+    // Validate sizes after GC.
+    auto tiered_size = tiered_index->indexSize();
+    auto flat_size = tiered_index->GetFlatIndex()->indexSize();
+    auto backend_size = tiered_index->GetBackendIndex()->indexSize();
+    ASSERT_EQ(tiered_size, n);
+    ASSERT_EQ(tiered_size, backend_size + flat_size);
+    // Expect that GC cleaned all the deleted vectors.
+    ASSERT_EQ(tiered_index->GetSVSIndex()->indexStorageSize(), backend_size);
+    ASSERT_EQ(tiered_index->GetSVSIndex()->getNumMarkedDeleted(), 0);
+    EXPECT_EQ(tiered_index->statisticInfo().numberOfMarkedDeleted, 0);
+}
+
 TYPED_TEST(SVSTieredIndexTestBasic, switchDeleteModes) {
     // Create TieredSVS index instance with a mock queue.
     size_t dim = 16;