CANN: implement LRU cache for ACL graphs (#15814)

* CANN: implement LRU cache for ACL graphs in CANN backend

- Introduce ggml_cann_graph_lru_cache to store multiple ggml_cann_graph objects.
- Graphs are loaded on demand and evicted using LRU policy when capacity is exceeded.
- Updated push, move_to_front, and clear methods to manage cached graphs efficiently.
- Ensures reuse of graphs, reducing graph reconstruction overhead in CANN backend.

* fix typo

* The LRU cache capacity can be configured via an env variable

Signed-off-by: noemotiovon <757486878@qq.com>

* refactory acl graph

* refactory && fix review comments

Signed-off-by: noemotiovon <757486878@qq.com>

---------

Signed-off-by: noemotiovon <757486878@qq.com>

docs/backend/CANN.md

@@ -314,3 +314,7 @@ Converting the matmul weight format from ND to NZ to improve performance. Enable
 ### GGML_CANN_ACL_GRAPH
 
 Operators are executed using ACL graph execution, rather than in op-by-op (eager) mode. Enabled by default.
+
+### GGML_CANN_GRAPH_CACHE_CAPACITY
+
+Maximum number of compiled CANN graphs kept in the LRU cache, default is 12. When the number of cached graphs exceeds this capacity, the least recently used graph will be evicted.

ggml/src/ggml-cann/common.h

@@ -38,6 +38,7 @@
 #include <unistd.h>
 #include <functional>
 #include <optional>
+#include <list>
 
 #include "../include/ggml-cann.h"
 #include "../include/ggml.h"
@@ -106,6 +107,7 @@ int32_t ggml_cann_get_device();
 
 std::optional<std::string> get_env(const std::string& name);
 bool parse_bool(const std::string& value);
+int parse_integer(const std::string& value);
 
 /**
  * @brief Abstract base class for memory pools used by CANN.
@@ -350,7 +352,7 @@ struct ggml_graph_node_properties {
 struct ggml_cann_graph {
     ~ggml_cann_graph() {
         if (graph != nullptr) {
-            aclmdlRIDestroy(graph);
+            ACL_CHECK(aclmdlRIDestroy(graph));
         }
     }
 
@@ -358,6 +360,64 @@ struct ggml_cann_graph {
 
     std::vector<ggml_graph_node_properties> ggml_graph_properties;
 };
+
+/**
+ * @brief LRU cache for managing ggml_cann_graph objects.
+ *
+ * This class maintains a list of shared_ptr to ggml_cann_graph objects
+ * and enforces a maximum capacity. It provides methods to push new graphs,
+ * move existing graphs to the front (most recently used), and clear the cache.
+ */
+struct ggml_cann_graph_lru_cache {
+    size_t capacity;  /**< Maximum number of graphs in the cache. */
+
+    std::list<ggml_cann_graph*> cache_list; /**< List storing cached graphs as raw pointers. */
+
+    ggml_cann_graph_lru_cache() {
+        capacity = parse_integer(get_env("GGML_CANN_GRAPH_CACHE_CAPACITY").value_or("12"));
+    }
+
+    /**
+     * @brief Push a new graph to the front of the cache.
+     * If the cache exceeds capacity, the least recently used graph is deleted.
+     * @param new_node Pointer to the new ggml_cann_graph to cache.
+     *        Ownership is transferred to the cache (cache will delete it).
+     */
+    void push(ggml_cann_graph* new_node) {
+        if (cache_list.size() >= capacity) {
+            ggml_cann_graph* old = cache_list.back();
+            cache_list.pop_back();
+            delete old; // free the old graph
+        }
+        cache_list.push_front(new_node);
+    }
+
+    /**
+     * @brief Move an existing graph to the front of the cache.
+     * @param node Pointer to the ggml_cann_graph to move.
+     */
+    void move_to_front(ggml_cann_graph* node) {
+        cache_list.remove(node);
+        cache_list.push_front(node);
+    }
+
+    /**
+     * @brief Clear all graphs from the cache (also frees memory).
+     */
+    void clear() {
+        for (auto ptr : cache_list) {
+            delete ptr;
+        }
+        cache_list.clear();
+    }
+
+    /**
+     * @brief Destructor that clears the cache and frees all cached graphs.
+     */
+    ~ggml_cann_graph_lru_cache() {
+        clear();
+    }
+};
 #endif  // USE_ACL_GRAPH
 
 struct ggml_cann_rope_cache {
@@ -394,7 +454,7 @@ struct ggml_backend_cann_context {
     aclrtEvent copy_event = nullptr; /**< Event for managing copy operations. */
 #ifdef USE_ACL_GRAPH
     /// Cached CANN ACL graph used for executing the current ggml computation graph.
-    std::unique_ptr<ggml_cann_graph> cann_graph;
+    ggml_cann_graph_lru_cache graph_lru_cache;
     bool acl_graph_mode = true;
 #endif
     cann_task_queue task_queue;

ggml/src/ggml-cann/ggml-cann.cpp

@@ -116,6 +116,24 @@ bool parse_bool(const std::string& value) {
     return valid_values.find(value) != valid_values.end();
 }
 
+/**
+ * @brief Parse a string as an integer, returning 0 if invalid.
+ *
+ * This function attempts to convert the input string `value` to an `int`.
+ * If the string is not a valid integer or is out of the `int` range,
+ * it returns 0.
+ *
+ * @param value The string to parse.
+ * @return The parsed integer, or 0 if conversion fails.
+ */
+int parse_integer(const std::string& value) {
+    try {
+        return std::stoi(value);
+    } catch (...) {
+        return 0;
+    }
+}
+
 /**
  * @brief Initialize the CANN device information.
  *
@@ -2131,30 +2149,52 @@ static void ggml_backend_cann_synchronize(ggml_backend_t backend) {
 
 #ifdef USE_ACL_GRAPH
 /**
- * @brief Populate the internal CANN graph node properties from the ggml computation graph.
+ * @brief Add a new CANN graph to the LRU cache by populating node properties from the ggml graph.
+ *
+ * This function creates a new ggml_cann_graph object and fills its node properties
+ * (operation type, dimensions, strides, input sources, and operation parameters)
+ * based on the current ggml computation graph.
  *
- * This function copies all node attributes (operation type, dimensions, strides, input sources,
- * and operation parameters) into the cached CANN graph structure for later reuse or comparison.
+ * Each node in the ggml graph is mapped to a property entry in the new CANN graph:
+ * - node address
+ * - operation type
+ * - shape (ne) and strides (nb)
+ * - source tensor addresses
+ * - operation parameters
  *
- * @param cann_ctx  The CANN backend context.
- * @param cgraph    The ggml computational graph.
+ * After initialization, the new graph is pushed into the LRU cache owned by the
+ * CANN backend context. The cache takes ownership of the graph and manages its
+ * lifetime (including deletion upon eviction).
+ *
+ * @param cann_ctx  The CANN backend context containing the graph cache.
+ * @param cgraph    The current ggml computation graph.
  */
-static void set_ggml_graph_node_properties(ggml_backend_cann_context * cann_ctx, ggml_cgraph * cgraph) {
-    for (int node_idx = 0; node_idx < cgraph->n_nodes; node_idx++) {
+static void add_lru_matched_graph_node_properties(
+        ggml_backend_cann_context * cann_ctx,
+        ggml_cgraph * cgraph) {
+    // Create a new ggml_cann_graph object on the heap (its lifetime is managed by the cache).
+    ggml_cann_graph * new_graph = new ggml_cann_graph();
+    new_graph->ggml_graph_properties.resize(cgraph->n_nodes);
+
+    for (int node_idx = 0; node_idx < cgraph->n_nodes; ++node_idx) {
         ggml_tensor * node = cgraph->nodes[node_idx];
-        cann_ctx->cann_graph->ggml_graph_properties[node_idx].node_address = node->data;
-        cann_ctx->cann_graph->ggml_graph_properties[node_idx].node_op = node->op;
+        auto & prop = new_graph->ggml_graph_properties[node_idx];
 
-        for (int dim = 0; dim < GGML_MAX_DIMS; dim++) {
-            cann_ctx->cann_graph->ggml_graph_properties[node_idx].ne[dim] = node->ne[dim];
-            cann_ctx->cann_graph->ggml_graph_properties[node_idx].nb[dim] = node->nb[dim];
-        }
-        for (int src = 0; src < GGML_MAX_SRC; src++) {
-            cann_ctx->cann_graph->ggml_graph_properties[node_idx].src_address[src] =
-                node->src[src] ? node->src[src]->data : nullptr;
+        prop.node_address = node->data;
+        prop.node_op      = node->op;
+
+        std::copy_n(node->ne, GGML_MAX_DIMS, prop.ne);
+        std::copy_n(node->nb, GGML_MAX_DIMS, prop.nb);
+
+        for (int src = 0; src < GGML_MAX_SRC; ++src) {
+            prop.src_address[src] = node->src[src] ? node->src[src]->data : nullptr;
         }
-        memcpy(cann_ctx->cann_graph->ggml_graph_properties[node_idx].op_params, node->op_params, GGML_MAX_OP_PARAMS);
+
+        memcpy(prop.op_params, node->op_params, GGML_MAX_OP_PARAMS);
     }
+
+    // Insert into the LRU cache (cache takes ownership and will delete it when evicted).
+    cann_ctx->graph_lru_cache.push(new_graph);
 }
 
 /**
@@ -2199,30 +2239,45 @@ static bool ggml_graph_node_has_matching_properties(ggml_tensor * node, ggml_gra
 }
 
 /**
- * @brief Determine if the CANN graph needs to be rebuilt due to graph changes.
+ * @brief Check whether there is a cached CANN graph that matches the current ggml graph.
+ *
+ * This function iterates through the cached CANN graphs stored in the LRU cache and
+ * compares them against the given ggml computation graph. A match requires that the
+ * number of nodes is the same and that each node’s properties (operation type,
+ * dimensions, strides, inputs, and operation parameters) are identical.
  *
- * This checks whether the number or properties of ggml graph nodes have changed
- * compared to the last captured CANN graph. If so, the CANN graph must be re-captured.
+ * If a matching graph is found, it is promoted to the front of the LRU cache and the
+ * function returns true. Otherwise, the function returns false, indicating that a new
+ * CANN graph needs to be captured.
  *
- * @param cann_ctx  The CANN backend context.
+ * @param cann_ctx  The CANN backend context containing the graph cache.
  * @param cgraph    The current ggml computation graph.
- * @return true if an update is required; false otherwise.
- */
-static bool is_cann_graph_update_required(ggml_backend_cann_context * cann_ctx, ggml_cgraph * cgraph) {
-    // The number of nodes is different, so the graph needs to be reconstructed.
-    if (cann_ctx->cann_graph->ggml_graph_properties.size() != (size_t)cgraph->n_nodes) {
-        cann_ctx->cann_graph->ggml_graph_properties.resize(cgraph->n_nodes);
-        return true;
-    }
+ * @return true if a matching cached graph exists; false otherwise.
+ */
+static bool is_matched_graph(ggml_backend_cann_context * cann_ctx, ggml_cgraph * cgraph) {
+    ggml_cann_graph_lru_cache &lru_cache = cann_ctx->graph_lru_cache;
+    for (auto &graph_ptr : lru_cache.cache_list) {
+        // Skip graphs with a different number of nodes.
+        if (graph_ptr->ggml_graph_properties.size() != static_cast<size_t>(cgraph->n_nodes)) {
+            continue;
+        }
 
-    // The number of nodes is the same; iterate over each node to check whether they match.
-    for (int i = 0; i < cgraph->n_nodes; i++) {
-        bool has_matching_properties = ggml_graph_node_has_matching_properties(
-            cgraph->nodes[i], &cann_ctx->cann_graph->ggml_graph_properties[i]);
-        if(!has_matching_properties) {
+        // Check if all nodes match.
+        bool all_match = true;
+        for (int i = 0; i < cgraph->n_nodes; ++i) {
+            if (!ggml_graph_node_has_matching_properties(cgraph->nodes[i], &graph_ptr->ggml_graph_properties[i])) {
+                all_match = false;
+                break;
+            }
+        }
+
+        if (all_match) {
+            // update cache_list && renturn graph_ptr
+            lru_cache.move_to_front(graph_ptr);
             return true;
         }
     }
+
     return false;
 }
 #endif  // USE_ACL_GRAPH
@@ -2241,17 +2296,13 @@ static bool is_cann_graph_update_required(ggml_backend_cann_context * cann_ctx,
  * @param cann_graph_update_required Whether graph capture is needed due to graph changes.
  */
 static void evaluate_and_capture_cann_graph(ggml_backend_cann_context * cann_ctx, ggml_cgraph * cgraph,
-    bool & use_cann_graph, bool & cann_graph_update_required) {
+    bool & use_cann_graph,  bool & cann_graph_update_required) {
 #ifdef USE_ACL_GRAPH
+    ggml_cann_graph* matched_graph = cann_ctx->graph_lru_cache.cache_list.front();
     if (use_cann_graph && cann_graph_update_required) {
-        if (cann_ctx->cann_graph->graph != nullptr) {
-            ACL_CHECK(aclmdlRIDestroy(cann_ctx->cann_graph->graph));
-            cann_ctx->cann_graph->graph = nullptr;
-        }
         ACL_CHECK(aclmdlRICaptureBegin(cann_ctx->stream(), ACL_MODEL_RI_CAPTURE_MODE_GLOBAL));
     }
 #endif // USE_ACL_GRAPH
-
     // Only perform the graph execution if CANN graphs are not enabled, or we are capturing the graph.
     // With the use of CANN graphs, the execution will be performed by the graph launch.
     if (!use_cann_graph || cann_graph_update_required) {
@@ -2272,12 +2323,12 @@ static void evaluate_and_capture_cann_graph(ggml_backend_cann_context * cann_ctx
 
 #ifdef USE_ACL_GRAPH
     if (use_cann_graph && cann_graph_update_required) { // End CANN graph capture
-        ACL_CHECK(aclmdlRICaptureEnd(cann_ctx->stream(), &cann_ctx->cann_graph->graph));
+        ACL_CHECK(aclmdlRICaptureEnd(cann_ctx->stream(), &matched_graph->graph));
     }
 
     if (use_cann_graph) {
         // Execute graph
-        ACL_CHECK(aclmdlRIExecuteAsync(cann_ctx->cann_graph->graph, cann_ctx->stream()));
+        ACL_CHECK(aclmdlRIExecuteAsync(matched_graph->graph, cann_ctx->stream()));
     }
 #endif // USE_ACL_GRAPH
 }
@@ -2311,19 +2362,17 @@ static enum ggml_status ggml_backend_cann_graph_compute(
     }
 
     if (use_cann_graph) {
-        if (cann_ctx->cann_graph == nullptr) {
-            cann_ctx->cann_graph.reset(new ggml_cann_graph());
-            cann_graph_update_required = true;
+        // If no matching graph is found, the graph needs to be recaptured.
+        cann_graph_update_required = !is_matched_graph(cann_ctx, cgraph);
+        if (cann_graph_update_required) {
+            // If no matching graph is found, add a new ACL graph.
+            add_lru_matched_graph_node_properties(cann_ctx, cgraph);
         }
-
-        cann_graph_update_required = is_cann_graph_update_required(cann_ctx, cgraph);
-        set_ggml_graph_node_properties(cann_ctx, cgraph);
     }
 #else
     bool use_cann_graph = false;
     bool cann_graph_update_required = false;
 #endif  // USE_ACL_GRAPH
-
     evaluate_and_capture_cann_graph(
         cann_ctx,
         cgraph,