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

docs/backend/SYCL.md

@@ -737,6 +737,14 @@ use 1 SYCL GPUs: [0] with Max compute units:512
 | ZES_ENABLE_SYSMAN | 0 (default) or 1 | Support to get free memory of GPU by sycl::aspect::ext_intel_free_memory.<br>Recommended to use when --split-mode = layer |
 | UR_L0_ENABLE_RELAXED_ALLOCATION_LIMITS | 0 (default) or 1 | Support malloc device memory more than 4GB.|
 
+## Compile-time Flags
+
+Pass these via `CXXFLAGS` or add a one-off `#define` to enable a flag on the spot.
+
+| Name            | Function                                                                         |
+|-----------------|----------------------------------------------------------------------------------|
+| DEBUG_SYCL_POOL | Enable device memory pool logging on teardown. Useful for profiling allocations. |
+
 ## Design Rule
 
 - Open to all contributors.

ggml/src/ggml-sycl/common.hpp

@@ -25,6 +25,7 @@
 #include "presets.hpp"
 #include "type.hpp"
 #include "sycl_hw.hpp"
+#include "fattn-buffers.hpp"
 
 namespace syclexp = sycl::ext::oneapi::experimental;
 
@@ -404,12 +405,16 @@ struct ggml_backend_sycl_context {
     std::unique_ptr<ggml_sycl_pool> pools[GGML_SYCL_MAX_DEVICES];
     std::unordered_map<sycl::queue *, std::unique_ptr<ggml_sycl_pool_alloc<uint8_t>>> scratchpad_map;
 
+    std::unique_ptr<ggml_sycl_fattn_kv_buffers> fattn_bufs[GGML_SYCL_MAX_DEVICES];
+
     std::unique_ptr<ggml_sycl_pool> host_pools[GGML_SYCL_MAX_DEVICES];
 
     static std::unique_ptr<ggml_sycl_pool> new_pool_for_device(queue_ptr qptr, int device);
 
     static std::unique_ptr<ggml_sycl_pool> new_pool_for_host(queue_ptr qptr, int device);
 
+    static std::unique_ptr<ggml_sycl_fattn_kv_buffers> new_fattn_kv_buffers(queue_ptr qptr, int device);
+
     ggml_sycl_pool & pool(int device) {
         if (pools[device] == nullptr) {
             pools[device] = new_pool_for_device(stream(device,0), device);
@@ -421,6 +426,17 @@ struct ggml_backend_sycl_context {
         return pool(device);
     }
 
+    ggml_sycl_fattn_kv_buffers & fattn_buffers(int device) {
+        if (fattn_bufs[device] == nullptr) {
+            fattn_bufs[device] = new_fattn_kv_buffers(stream(device, 0), device);
+        }
+        return *fattn_bufs[device];
+    }
+
+    ggml_sycl_fattn_kv_buffers & fattn_buffers() {
+        return fattn_buffers(device);
+    }
+
 #ifdef GGML_SYCL_GRAPH
     std::unique_ptr<sycl_ex::command_graph<sycl_ex::graph_state::executable>> exec_graph = nullptr;
 #endif

ggml/src/ggml-sycl/fattn-buffers.cpp

@@ -0,0 +1,56 @@
+//
+// MIT license
+// Copyright (C) 2025 Intel Corporation
+// SPDX-License-Identifier: MIT
+//
+
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+
+#include "common.hpp"
+
+sycl::half * ggml_sycl_fattn_kv_buffers::kv_buffer::ensure_half(size_t n_elems) {
+    const size_t need_bytes = n_elems * sizeof(sycl::half);
+
+    if (capacity >= need_bytes) {
+        return ptr;
+    }
+
+    if (ptr) {
+        SYCL_CHECK(CHECK_TRY_ERROR(qptr->wait()));
+        SYCL_CHECK(CHECK_TRY_ERROR(sycl::free(ptr, *qptr)));
+        ptr = nullptr;
+        capacity = 0;
+    }
+
+    size_t cap = 0;
+    while (cap < need_bytes) {
+        cap += CHUNK_SIZE;
+    }
+
+    void * dev_ptr;
+    SYCL_CHECK(
+        CHECK_TRY_ERROR(dev_ptr = sycl::malloc_device(
+                        cap, *qptr)));
+
+    if (!dev_ptr) {
+        GGML_LOG_ERROR("%s: can't allocate %lu Bytes of memory on device\n", __func__, cap);
+        GGML_ABORT("fattn buffer alloc failed");
+    }
+
+    ptr = static_cast<sycl::half *>(dev_ptr);
+    capacity = cap;
+    return ptr;
+}
+
+ggml_sycl_fattn_kv_buffers::kv_buffer::~kv_buffer() {
+#ifdef DEBUG_SYCL_POOL
+    GGML_LOG_INFO("ggml_sycl_fattn_kv_buffer[%d]: %.2f MiB\n", device, capacity / 1024.0 / 1024.0);
+#endif
+    if (ptr) {
+        SYCL_CHECK(CHECK_TRY_ERROR(sycl::free(ptr, *qptr)));
+    }
+}

ggml/src/ggml-sycl/fattn-buffers.hpp

@@ -0,0 +1,63 @@
+//
+// MIT license
+// Copyright (C) 2025 Intel Corporation
+// SPDX-License-Identifier: MIT
+//
+
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+
+#ifndef GGML_SYCL_FATTN_BUFFERS_HPP
+#define GGML_SYCL_FATTN_BUFFERS_HPP
+
+#include <sycl/sycl.hpp>
+
+typedef sycl::queue *queue_ptr;
+
+struct ggml_sycl_fattn_kv_buffers {
+    // buffers grow in chunks of this size
+    static constexpr size_t CHUNK_SIZE = 16ull << 20; // 16 MiB
+
+    struct kv_buffer {
+        kv_buffer(queue_ptr qptr_, int device_) : qptr(qptr_), device(device_) {}
+        ~kv_buffer();
+
+        kv_buffer(const kv_buffer &) = delete;
+        kv_buffer & operator=(const kv_buffer &) = delete;
+
+        sycl::half * ensure_half(size_t n_elems);
+
+    private:
+        sycl::half * ptr      = nullptr;
+        size_t       capacity = 0;
+        queue_ptr    qptr     = nullptr;
+        [[maybe_unused]] int device = 0;
+    };
+
+    kv_buffer K;
+    kv_buffer V;
+
+    ggml_sycl_fattn_kv_buffers(queue_ptr qptr, int device) : K(qptr, device), V(qptr, device) {}
+
+    ggml_sycl_fattn_kv_buffers(const ggml_sycl_fattn_kv_buffers &) = delete;
+    ggml_sycl_fattn_kv_buffers & operator=(const ggml_sycl_fattn_kv_buffers &) = delete;
+};
+
+/**
+ * Imitates `ggml_sycl_pool_alloc` to keep the code calling alloc unchanged.
+ */
+struct ggml_sycl_fattn_alloc {
+    ggml_sycl_fattn_kv_buffers::kv_buffer & buf;
+    sycl::half *                         ptr = nullptr;
+
+    explicit ggml_sycl_fattn_alloc(ggml_sycl_fattn_kv_buffers::kv_buffer & buf_) : buf(buf_) {}
+
+    sycl::half * alloc(size_t n_elems) {
+        ptr = buf.ensure_half(n_elems);
+        return ptr;
+    }
+};
+#endif

ggml/src/ggml-sycl/fattn-common.hpp

@@ -5,6 +5,7 @@
 #include "common.hpp"
 #include "convert.hpp"
 #include "vecdotq.hpp"
+#include "fattn-buffers.hpp"
 
 #include "ggml.h"
 
@@ -918,12 +919,13 @@ void launch_fattn(
     GGML_ASSERT(!mask || mask->type == GGML_TYPE_F16);
 
     ggml_sycl_pool & pool = ctx.pool();
+    ggml_sycl_fattn_kv_buffers & fbuf = ctx.fattn_buffers();
     dpct::queue_ptr  main_stream = ctx.stream();
     const int id  = ggml_sycl_get_device();
     const int nsm = ggml_sycl_info().devices[id].nsm;
 
-    ggml_sycl_pool_alloc<sycl::half>   K_f16(pool);
-    ggml_sycl_pool_alloc<sycl::half>   V_f16(pool);
+    ggml_sycl_fattn_alloc        K_f16(fbuf.K);
+    ggml_sycl_fattn_alloc        V_f16(fbuf.V);
     ggml_sycl_pool_alloc<int>    KV_max(pool);
     ggml_sycl_pool_alloc<float>  dst_tmp(pool);
     ggml_sycl_pool_alloc<sycl::float2> dst_tmp_meta(pool);

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