SYCL: reduce allocation overhead during flash attention (#22732)

* SYCL: reduce allocation overhead during flash attention

* tidy up whitespace

* add a note about the flag

* move ggml_sycl_fattn_* into fattn-buffers.hpp

* refactor implementation into fattn-buffers.cpp

* move new_fattn_kv_buffers back into ggml-sycl.cpp

ggml/src/ggml-sycl/ggml-sycl.cpp

@@ -1286,6 +1286,23 @@ struct ggml_sycl_pool_leg : public ggml_sycl_pool {
     explicit ggml_sycl_pool_leg(queue_ptr qptr_, int device_) : device(device_), qptr(qptr_) {}
 
     ~ggml_sycl_pool_leg() {
+#ifdef DEBUG_SYCL_POOL
+        int    n_cached    = 0;
+        size_t bytes_cached = 0;
+        for (int i = 0; i < MAX_SYCL_BUFFERS; ++i) {
+            if (buffer_pool[i].ptr != nullptr) {
+                ++n_cached;
+                bytes_cached += buffer_pool[i].size;
+            }
+        }
+        GGML_LOG_INFO("%s: %d buffers, cached = %.2f MiB\n", __func__,
+                      n_cached, bytes_cached / 1024.0 / 1024.0);
+        const auto slots = format_slots_in_alloc_order();
+        if (!slots.empty()) {
+            GGML_LOG_INFO("%s: slots MiB: %s\n", __func__, slots.c_str());
+        }
+#endif
+
         for (int i = 0; i < MAX_SYCL_BUFFERS; ++i) {
             ggml_sycl_buffer & b = buffer_pool[i];
             if (b.ptr != nullptr) {
@@ -1296,6 +1313,26 @@ struct ggml_sycl_pool_leg : public ggml_sycl_pool {
         GGML_ASSERT(pool_size == 0);
     }
 
+#ifdef DEBUG_SYCL_POOL
+    std::string format_slots_in_alloc_order() const {
+        std::string line;
+        char buf[32];
+        bool first = true;
+        for (int i = 0; i < MAX_SYCL_BUFFERS; ++i) {
+            if (buffer_pool[i].ptr == nullptr) {
+                continue;
+            }
+            if (!first) {
+                line += '/';
+            }
+            first = false;
+            snprintf(buf, sizeof(buf), "%.2f", buffer_pool[i].size / 1024.0 / 1024.0);
+            line += buf;
+        }
+        return line;
+    }
+#endif
+
     void * alloc(size_t size, size_t * actual_size) override {
 #ifdef DEBUG_sycl_MALLOC
         int nnz = 0;
@@ -1459,6 +1496,10 @@ std::unique_ptr<ggml_sycl_pool> ggml_backend_sycl_context::new_pool_for_device(q
    return std::unique_ptr<ggml_sycl_pool>(new ggml_sycl_pool_leg(qptr, device));
 }
 
+std::unique_ptr<ggml_sycl_fattn_kv_buffers> ggml_backend_sycl_context::new_fattn_kv_buffers(queue_ptr qptr, int device) {
+    return std::unique_ptr<ggml_sycl_fattn_kv_buffers>(new ggml_sycl_fattn_kv_buffers(qptr, device));
+}
+
 // TBD pool with virtual memory management
 // struct ggml_sycl_pool_vmm : public ggml_sycl_pool