""" High-performance CPU-GPU KV cache transfer engine. Key design principles for CUDA Graph compatibility: 1. All tensor addresses are fixed at initialization 2. Only index tensor contents change between graph replays 3. Supports both async transfer (for prefill) and graph-based transfer (for decode) """ import torch import torch.cuda.nvtx from torch import Tensor from typing import Dict, List, Tuple, Optional from dataclasses import dataclass from nanovllm.kvcache.kernels import gathered_copy_kv from nanovllm.comm import memcpy_2d_async from nanovllm.utils.logger import get_logger logger = get_logger("offload_engine") @dataclass class TransferEvent: """Tracks a pending async transfer.""" event: torch.cuda.Event layer_id: int src_block_id: int dst_block_id: int direction: str # "h2d" or "d2h" class OffloadEngine: """ High-performance CPU-GPU async transfer engine for KV cache offloading. Memory layout: - GPU cache: [num_layers, num_gpu_blocks, block_size, kv_heads, head_dim] - CPU cache: [num_layers, num_cpu_blocks, block_size, kv_heads, head_dim] (pinned) - Gather indices: [num_layers, num_gpu_blocks] (fixed address, variable content) CUDA Graph compatibility: - gathered_h2d_layer() can be captured into CUDA graphs - update_gather_indices() is called outside graphs to prepare indices - All tensor addresses remain fixed across graph replays """ def __init__( self, num_layers: int, num_gpu_blocks: int, num_cpu_blocks: int, block_size: int, num_kv_heads: int, head_dim: int, dtype: torch.dtype = torch.float16, num_streams: int = 4, ): self.num_layers = num_layers self.num_gpu_blocks = num_gpu_blocks self.num_cpu_blocks = num_cpu_blocks self.block_size = block_size self.num_kv_heads = num_kv_heads self.head_dim = head_dim self.dtype = dtype self.kv_dim = num_kv_heads * head_dim self.block_numel = block_size * self.kv_dim # ========== sgDMA pitch parameters for strided transfers ========== self.dtype_size = dtype.itemsize self.cpu_pitch = num_cpu_blocks * self.block_numel * self.dtype_size self.gpu_pitch = num_gpu_blocks * self.block_numel * self.dtype_size self.width = self.block_numel * self.dtype_size self.height = num_layers logger.info(f"sgDMA parameters: cpu_pitch={self.cpu_pitch}, gpu_pitch={self.gpu_pitch}, " f"width={self.width}, height={self.height}") # ========== Unified Ring Buffer configuration ========== # Constraint checks assert num_gpu_blocks >= 2, \ f"Need at least 2 GPU blocks for ring buffer, got {num_gpu_blocks}" # Unified Ring Buffer: all slots cycle for prefill # Prefill: use ALL slots as ring buffer (slot[chunk_idx % N]) # Decode: slot[0] as decode_slot, slots[1:] for loading previous chunks self.num_ring_slots = num_gpu_blocks self.ring_slots = list(range(num_gpu_blocks)) # Decode phase uses slot[0] for writing new token's KV self.decode_slot = 0 # Decode phase uses slots[1:] for loading previous chunks from CPU self.decode_load_slots = list(range(1, num_gpu_blocks)) self.num_decode_load_slots = len(self.decode_load_slots) self.num_gpu_slots = num_gpu_blocks # alias logger.info(f"Unified Ring Buffer: {self.num_ring_slots} slots total") logger.info(f" Prefill: all slots as ring buffer [0..{num_gpu_blocks-1}]") logger.info(f" Decode: slot[0] as decode_slot, slots[1..{num_gpu_blocks-1}] for loading") # ========== Fixed-address GPU KV cache ========== # Shape: [num_layers, num_gpu_blocks, block_size, kv_heads, head_dim] # Use zeros initialization to avoid uninitialized memory issues self.k_cache_gpu = torch.zeros( num_layers, num_gpu_blocks, block_size, num_kv_heads, head_dim, dtype=dtype, device="cuda" ) self.v_cache_gpu = torch.zeros( num_layers, num_gpu_blocks, block_size, num_kv_heads, head_dim, dtype=dtype, device="cuda" ) # ========== Fixed-address CPU KV cache (pinned memory) ========== self.k_cache_cpu = torch.zeros( num_layers, num_cpu_blocks, block_size, num_kv_heads, head_dim, dtype=dtype, device="cpu", pin_memory=True ) self.v_cache_cpu = torch.zeros( num_layers, num_cpu_blocks, block_size, num_kv_heads, head_dim, dtype=dtype, device="cpu", pin_memory=True ) # ========== Fixed-address gather indices (content is variable) ========== # gather_indices[layer][i] = CPU block id to copy to GPU slot i # -1 means no-op (skip this slot) self.gather_indices_cpu = torch.empty( num_layers, num_gpu_blocks, dtype=torch.int64, device="cpu", pin_memory=True ) self.gather_indices_cpu.fill_(-1) self.gather_indices_gpu = torch.full( (num_layers, num_gpu_blocks), -1, dtype=torch.int64, device="cuda" ) # Log memory allocation gpu_mem_mb = self.gpu_memory_bytes() / (1024 * 1024) cpu_mem_mb = self.cpu_memory_bytes() / (1024 * 1024) logger.info(f" GPU memory: {gpu_mem_mb:.1f} MB, CPU memory: {cpu_mem_mb:.1f} MB") # ========== Transfer streams for async operations ========== self.transfer_streams = [torch.cuda.Stream() for _ in range(num_streams)] # IMPORTANT: Create a dedicated compute stream (not default stream!) # Default stream has implicit synchronization with other streams, # which prevents overlap between transfer and compute. self.compute_stream = torch.cuda.Stream() self._stream_idx = 0 # ========== Per-slot transfer streams for parallel H2D ========== # Each slot has its own stream to enable parallel transfers # This allows multiple slots to load simultaneously self.slot_transfer_streams = [torch.cuda.Stream() for _ in range(self.num_ring_slots)] logger.info(f" Created {self.num_ring_slots} per-slot transfer streams") # ========== Ring Buffer dedicated stream and events ========== self.transfer_stream_main = torch.cuda.Stream() # Main transfer stream (for legacy/batch ops) # Decode offload event self.decode_offload_done = torch.cuda.Event() # ========== Per-slot Per-layer events for ring buffer ========== # ring_slot_ready[slot_idx][layer_id] = CUDA Event for H2D completion # ring_slot_offload_done[slot_idx][layer_id] = CUDA Event for D2H completion self.ring_slot_ready = [ [torch.cuda.Event() for _ in range(num_layers)] for _ in range(self.num_ring_slots) ] self.ring_slot_offload_done = [ [torch.cuda.Event() for _ in range(num_layers)] for _ in range(self.num_ring_slots) ] # Per-slot events for all-layer operations (used in some legacy paths) self.ring_slot_all_layers_ready = [torch.cuda.Event() for _ in range(self.num_ring_slots)] self.ring_slot_all_layers_offload_done = [torch.cuda.Event() for _ in range(self.num_ring_slots)] # ========== Per-slot Per-layer compute_done events for async pipeline ========== # ring_slot_compute_done[slot_idx][layer_id] = CUDA Event for compute completion # This is used to ensure we don't overwrite data before it's been read by attention self.ring_slot_compute_done = [ [torch.cuda.Event() for _ in range(num_layers)] for _ in range(self.num_ring_slots) ] # Initialize all compute_done events (record them once) # This prevents undefined behavior on first load_to_slot_layer call for slot_idx in range(self.num_ring_slots): for layer_id in range(num_layers): self.ring_slot_compute_done[slot_idx][layer_id].record() torch.cuda.synchronize() # Ensure all events are recorded # ========== Event tracking for async transfers ========== self.pending_events: Dict[Tuple[int, int], torch.cuda.Event] = {} # ========== Debug hook mode ========== self._debug_mode = False self._debug_hooks: List = [] # External hooks for debug events def _get_next_stream(self) -> torch.cuda.Stream: """Round-robin stream selection for parallel transfers.""" stream = self.transfer_streams[self._stream_idx] self._stream_idx = (self._stream_idx + 1) % len(self.transfer_streams) return stream # ========== CUDA Graph compatible methods ========== def gathered_h2d_layer(self, layer_id: int) -> None: """ Execute gathered H2D copy for a single layer. This method is CUDA Graph compatible - can be captured into a graph. Before calling, update_gather_indices() must be called to set up which CPU blocks to copy to which GPU slots. Args: layer_id: Layer index to transfer """ gathered_copy_kv( k_src=self.k_cache_cpu[layer_id], v_src=self.v_cache_cpu[layer_id], k_dst=self.k_cache_gpu[layer_id], v_dst=self.v_cache_gpu[layer_id], indices=self.gather_indices_gpu[layer_id], ) def gathered_h2d_all_layers(self) -> None: """ Execute gathered H2D copy for all layers. CUDA Graph compatible - can be captured into a single graph. """ for layer_id in range(self.num_layers): self.gathered_h2d_layer(layer_id) def update_gather_indices( self, layer_id: int, mappings: List[Tuple[int, int]], ) -> None: """ Update gather indices for a layer (call OUTSIDE CUDA graph). Args: layer_id: Layer index mappings: List of (cpu_block_id, gpu_slot) tuples Only these slots will be updated; others keep their values """ for cpu_block_id, gpu_slot in mappings: self.gather_indices_cpu[layer_id, gpu_slot] = cpu_block_id # Async copy to GPU self.gather_indices_gpu[layer_id].copy_( self.gather_indices_cpu[layer_id], non_blocking=True ) def update_gather_indices_all_layers( self, mappings_per_layer: List[List[Tuple[int, int]]], ) -> None: """ Update gather indices for all layers. Args: mappings_per_layer: mappings_per_layer[layer_id] = [(cpu_block_id, gpu_slot), ...] """ for layer_id, mappings in enumerate(mappings_per_layer): for cpu_block_id, gpu_slot in mappings: self.gather_indices_cpu[layer_id, gpu_slot] = cpu_block_id # Batch copy all layers self.gather_indices_gpu.copy_(self.gather_indices_cpu, non_blocking=True) def clear_gather_indices(self, layer_id: Optional[int] = None) -> None: """ Clear gather indices (set all to -1, meaning no-op). Args: layer_id: If provided, clear only this layer; otherwise clear all """ if layer_id is not None: self.gather_indices_cpu[layer_id].fill_(-1) self.gather_indices_gpu[layer_id].fill_(-1) else: self.gather_indices_cpu.fill_(-1) self.gather_indices_gpu.fill_(-1) # ========== Async transfer methods (for prefill, outside CUDA graph) ========== def prefetch_block_async( self, layer_id: int, cpu_block_id: int, gpu_block_id: int, ) -> torch.cuda.Event: """ Async prefetch a single block from CPU to GPU. For use in prefill phase where CUDA graphs are not used. Args: layer_id: Layer index cpu_block_id: Source block in CPU cache gpu_block_id: Destination slot in GPU cache Returns: CUDA event that signals completion """ stream = self._get_next_stream() event = torch.cuda.Event() logger.debug(f"H2D prefetch: layer={layer_id}, CPU[{cpu_block_id}] -> GPU[{gpu_block_id}]") with torch.cuda.stream(stream): # K cache self.k_cache_gpu[layer_id, gpu_block_id].copy_( self.k_cache_cpu[layer_id, cpu_block_id], non_blocking=True ) # V cache self.v_cache_gpu[layer_id, gpu_block_id].copy_( self.v_cache_cpu[layer_id, cpu_block_id], non_blocking=True ) event.record() self.pending_events[(layer_id, gpu_block_id)] = event return event def prefetch_blocks_batch_async( self, transfers: List[Tuple[int, int, int]], # [(layer_id, cpu_block_id, gpu_block_id), ...] ) -> List[torch.cuda.Event]: """ Batch async prefetch multiple blocks. Args: transfers: List of (layer_id, cpu_block_id, gpu_block_id) tuples Returns: List of CUDA events for each transfer """ events = [] for layer_id, cpu_block_id, gpu_block_id in transfers: event = self.prefetch_block_async(layer_id, cpu_block_id, gpu_block_id) events.append(event) return events def offload_block_async( self, layer_id: int, gpu_block_id: int, cpu_block_id: int, ) -> torch.cuda.Event: """ Async offload a block from GPU to CPU. Args: layer_id: Layer index gpu_block_id: Source slot in GPU cache cpu_block_id: Destination block in CPU cache Returns: CUDA event that signals completion """ stream = self._get_next_stream() event = torch.cuda.Event() logger.debug(f"D2H offload: layer={layer_id}, GPU[{gpu_block_id}] -> CPU[{cpu_block_id}]") with torch.cuda.stream(stream): # Wait for any compute using this block stream.wait_stream(self.compute_stream) # K cache self.k_cache_cpu[layer_id, cpu_block_id].copy_( self.k_cache_gpu[layer_id, gpu_block_id], non_blocking=True ) # V cache self.v_cache_cpu[layer_id, cpu_block_id].copy_( self.v_cache_gpu[layer_id, gpu_block_id], non_blocking=True ) event.record() return event def offload_blocks_batch_async( self, transfers: List[Tuple[int, int, int]], # [(layer_id, gpu_block_id, cpu_block_id), ...] ) -> List[torch.cuda.Event]: """ Batch async offload multiple blocks. Args: transfers: List of (layer_id, gpu_block_id, cpu_block_id) tuples Returns: List of CUDA events """ events = [] for layer_id, gpu_block_id, cpu_block_id in transfers: event = self.offload_block_async(layer_id, gpu_block_id, cpu_block_id) events.append(event) return events # ========== Chunked Decode: Load CPU blocks to GPU slots ========== def load_cpu_blocks_to_gpu_slots( self, layer_id: int, cpu_block_ids: List[int], gpu_slot_ids: List[int], ) -> None: """ Load CPU blocks to specific GPU slots for chunked decode. Uses the main GPU KV cache slots, not a separate temp buffer. This is the same mechanism as chunked prefill uses. Args: layer_id: Layer index cpu_block_ids: List of CPU block IDs to load gpu_slot_ids: List of GPU slot IDs to load into (must be same length) """ assert len(cpu_block_ids) == len(gpu_slot_ids) if cpu_block_ids: logger.debug(f"H2D chunked load: layer={layer_id}, CPU{cpu_block_ids} -> GPU{gpu_slot_ids}") stream = self._get_next_stream() with torch.cuda.stream(stream): for cpu_block_id, gpu_slot in zip(cpu_block_ids, gpu_slot_ids): # Copy from pinned CPU memory to GPU KV cache slot self.k_cache_gpu[layer_id, gpu_slot].copy_( self.k_cache_cpu[layer_id, cpu_block_id], non_blocking=True ) self.v_cache_gpu[layer_id, gpu_slot].copy_( self.v_cache_cpu[layer_id, cpu_block_id], non_blocking=True ) # Wait for transfer to complete stream.synchronize() def load_cpu_blocks_to_gpu_slots_async( self, layer_id: int, cpu_block_ids: List[int], gpu_slot_ids: List[int], ) -> torch.cuda.Event: """ Async version: Load CPU blocks to GPU slots. Args: layer_id: Layer index cpu_block_ids: List of CPU block IDs to load gpu_slot_ids: List of GPU slot IDs to load into Returns: CUDA event to wait on """ assert len(cpu_block_ids) == len(gpu_slot_ids) if cpu_block_ids: logger.debug(f"H2D chunked load async: layer={layer_id}, CPU{cpu_block_ids} -> GPU{gpu_slot_ids}") stream = self._get_next_stream() event = torch.cuda.Event() with torch.cuda.stream(stream): for cpu_block_id, gpu_slot in zip(cpu_block_ids, gpu_slot_ids): self.k_cache_gpu[layer_id, gpu_slot].copy_( self.k_cache_cpu[layer_id, cpu_block_id], non_blocking=True ) self.v_cache_gpu[layer_id, gpu_slot].copy_( self.v_cache_cpu[layer_id, cpu_block_id], non_blocking=True ) event.record() return event def load_cpu_blocks_to_gpu_slots_all_layers( self, cpu_block_ids: List[int], gpu_slot_ids: List[int], ) -> None: """ Load CPU blocks to GPU slots for ALL layers at once. More efficient than per-layer loading when we know the mapping upfront. Args: cpu_block_ids: List of CPU block IDs to load gpu_slot_ids: List of GPU slot IDs to load into """ assert len(cpu_block_ids) == len(gpu_slot_ids) if cpu_block_ids: logger.debug(f"H2D all layers: CPU{cpu_block_ids} -> GPU{gpu_slot_ids}") stream = self._get_next_stream() with torch.cuda.stream(stream): for cpu_block_id, gpu_slot in zip(cpu_block_ids, gpu_slot_ids): # Copy all layers at once using sgDMA memcpy_2d_async( self.k_cache_gpu[:, gpu_slot], self.k_cache_cpu[:, cpu_block_id], self.gpu_pitch, self.cpu_pitch, self.width, self.height, "h2d", stream=stream ) memcpy_2d_async( self.v_cache_gpu[:, gpu_slot], self.v_cache_cpu[:, cpu_block_id], self.gpu_pitch, self.cpu_pitch, self.width, self.height, "h2d", stream=stream ) stream.synchronize() # ========== Synchronization methods ========== def wait_for_block(self, layer_id: int, gpu_block_id: int) -> None: """Wait for a specific block's transfer to complete.""" key = (layer_id, gpu_block_id) if key in self.pending_events: self.pending_events[key].synchronize() del self.pending_events[key] def wait_all_transfers(self) -> None: """Wait for all pending transfers to complete.""" for stream in self.transfer_streams: stream.synchronize() self.pending_events.clear() def sync_indices(self) -> None: """Synchronize to ensure all index updates are complete.""" torch.cuda.default_stream().synchronize() # ========== Cache access methods ========== def get_layer_cache(self, layer_id: int) -> Tuple[Tensor, Tensor]: """ Get GPU K/V cache tensors for a specific layer. Returns: (k_cache, v_cache) tensors for the layer Shape: [num_gpu_blocks, block_size, kv_heads, head_dim] """ return self.k_cache_gpu[layer_id], self.v_cache_gpu[layer_id] def get_all_gpu_cache(self) -> Tuple[Tensor, Tensor]: """ Get full GPU K/V cache tensors. Returns: (k_cache, v_cache) tensors Shape: [num_layers, num_gpu_blocks, block_size, kv_heads, head_dim] """ return self.k_cache_gpu, self.v_cache_gpu def get_cpu_block( self, layer_id: int, cpu_block_id: int, ) -> Tuple[Tensor, Tensor]: """ Get a specific CPU block's K/V cache. Returns: (k_cache, v_cache) for the block Shape: [block_size, kv_heads, head_dim] """ return ( self.k_cache_cpu[layer_id, cpu_block_id], self.v_cache_cpu[layer_id, cpu_block_id], ) # ========== Memory info ========== def gpu_memory_bytes(self) -> int: """Total GPU memory used by KV caches.""" return ( self.k_cache_gpu.numel() * self.k_cache_gpu.element_size() + self.v_cache_gpu.numel() * self.v_cache_gpu.element_size() + self.gather_indices_gpu.numel() * self.gather_indices_gpu.element_size() ) def cpu_memory_bytes(self) -> int: """Total CPU memory used by KV caches.""" return ( self.k_cache_cpu.numel() * self.k_cache_cpu.element_size() + self.v_cache_cpu.numel() * self.v_cache_cpu.element_size() + self.gather_indices_cpu.numel() * self.gather_indices_cpu.element_size() ) def __repr__(self) -> str: return ( f"OffloadEngine(\n" f" num_layers={self.num_layers},\n" f" num_gpu_blocks={self.num_gpu_blocks},\n" f" num_cpu_blocks={self.num_cpu_blocks},\n" f" block_size={self.block_size},\n" f" kv_heads={self.num_kv_heads},\n" f" head_dim={self.head_dim},\n" f" dtype={self.dtype},\n" f" ring_buffer: {self.num_ring_slots} slots, decode_slot={self.decode_slot}, decode_load_slots={self.decode_load_slots},\n" f" gpu_memory={self.gpu_memory_bytes() / 1024**2:.1f}MB,\n" f" cpu_memory={self.cpu_memory_bytes() / 1024**2:.1f}MB\n" f")" ) def wait_all_offload_done(self) -> None: """Wait for all offload operations to complete.""" self.transfer_stream_main.synchronize() # ========== Unified Ring Buffer methods ========== # ----- Prefill: Ring Buffer slot management ----- def get_write_slot_for_prefill(self, chunk_idx: int) -> int: """ Get ring buffer slot for writing prefill chunk. For prefill, ALL slots are used as ring buffer, cycling through. Args: chunk_idx: Current chunk index (0, 1, 2, ...) Returns: GPU slot index for writing """ return chunk_idx % self.num_ring_slots def get_load_slots_for_prefill(self, write_slot_idx: int) -> List[int]: """ Get available slots for loading previous chunks during prefill. Excludes the current write slot to avoid conflict. Args: write_slot_idx: Current write slot index Returns: List of slot indices available for loading (N-1 slots) """ return [i for i in range(self.num_ring_slots) if i != write_slot_idx] # ----- Decode: slot management ----- def get_load_slots_for_decode(self) -> List[int]: """ Get slots available for loading during decode. Excludes decode_slot (slot[0]) since it's used for writing new token's KV. Returns: List of slot indices for loading (slots[1:]) """ return self.decode_load_slots # ----- Per-slot Per-layer loading methods ----- def record_slot_compute_done(self, slot_idx: int, layer_id: int) -> None: """ Record that computation using this slot's data is done. This event is used by load_to_slot_layer to ensure we don't overwrite data before it's been read by attention computation. Args: slot_idx: GPU slot index that was just used for computation layer_id: Layer index """ self.ring_slot_compute_done[slot_idx][layer_id].record() def load_to_slot_layer(self, slot_idx: int, layer_id: int, cpu_block_id: int) -> None: """ Async load a single CPU block to a ring buffer slot for one layer. This is the core building block for ring buffer pipelining. Before starting the transfer, waits for: 1. Any previous compute on this slot to complete 2. Any pending offload of this slot to complete Args: slot_idx: Target GPU slot index layer_id: Layer index to load cpu_block_id: Source CPU block ID """ logger.debug(f"Ring load: layer={layer_id}, CPU[{cpu_block_id}] -> GPU slot[{slot_idx}]") # Use per-slot stream for parallel transfers across different slots stream = self.slot_transfer_streams[slot_idx] torch.cuda.nvtx.range_push(f"H2D: L{layer_id} CPU[{cpu_block_id}]->Slot[{slot_idx}]") with torch.cuda.stream(stream): # Wait for previous compute on this slot to complete before overwriting # This prevents data race: transfer must not start until attention finishes reading stream.wait_event(self.ring_slot_compute_done[slot_idx][layer_id]) # Also wait for any pending offload of this slot to complete # This prevents race: load must not write GPU slot while offload is reading from it stream.wait_event(self.ring_slot_all_layers_offload_done[slot_idx]) self.k_cache_gpu[layer_id, slot_idx].copy_( self.k_cache_cpu[layer_id, cpu_block_id], non_blocking=True ) self.v_cache_gpu[layer_id, slot_idx].copy_( self.v_cache_cpu[layer_id, cpu_block_id], non_blocking=True ) self.ring_slot_ready[slot_idx][layer_id].record(stream) torch.cuda.nvtx.range_pop() def wait_slot_layer(self, slot_idx: int, layer_id: int) -> None: """ Wait for a slot's loading to complete for a specific layer. Args: slot_idx: GPU slot index to wait for layer_id: Layer index to wait for """ self.compute_stream.wait_event(self.ring_slot_ready[slot_idx][layer_id]) def load_to_slot_all_layers(self, slot_idx: int, cpu_block_id: int) -> None: """ Async load a CPU block to a ring buffer slot for ALL layers. Args: slot_idx: Target GPU slot index cpu_block_id: Source CPU block ID """ logger.debug(f"Ring load all layers: CPU[{cpu_block_id}] -> GPU slot[{slot_idx}]") with torch.cuda.stream(self.transfer_stream_main): memcpy_2d_async( self.k_cache_gpu[:, slot_idx], self.k_cache_cpu[:, cpu_block_id], self.gpu_pitch, self.cpu_pitch, self.width, self.height, "h2d", stream=self.transfer_stream_main ) memcpy_2d_async( self.v_cache_gpu[:, slot_idx], self.v_cache_cpu[:, cpu_block_id], self.gpu_pitch, self.cpu_pitch, self.width, self.height, "h2d", stream=self.transfer_stream_main ) self.ring_slot_all_layers_ready[slot_idx].record(self.transfer_stream_main) def wait_slot_all_layers(self, slot_idx: int) -> None: """Wait for a slot's loading to complete for ALL layers.""" self.compute_stream.wait_event(self.ring_slot_all_layers_ready[slot_idx]) # ----- Slot offload methods ----- def offload_slot_to_cpu(self, slot_idx: int, cpu_block_id: int) -> None: """ Async offload a ring buffer slot to CPU (all layers). Args: slot_idx: Source GPU slot index cpu_block_id: Target CPU block ID """ logger.debug(f"Ring offload: GPU slot[{slot_idx}] -> CPU[{cpu_block_id}]") torch.cuda.nvtx.range_push(f"D2H: Slot[{slot_idx}]->CPU[{cpu_block_id}]") with torch.cuda.stream(self.transfer_stream_main): # Wait for both compute_stream and default stream # - compute_stream: for flash attention operations # - default_stream: for store_kvcache which runs on default stream self.transfer_stream_main.wait_stream(self.compute_stream) self.transfer_stream_main.wait_stream(torch.cuda.default_stream()) memcpy_2d_async( self.k_cache_cpu[:, cpu_block_id], self.k_cache_gpu[:, slot_idx], self.cpu_pitch, self.gpu_pitch, self.width, self.height, "d2h", stream=self.transfer_stream_main ) memcpy_2d_async( self.v_cache_cpu[:, cpu_block_id], self.v_cache_gpu[:, slot_idx], self.cpu_pitch, self.gpu_pitch, self.width, self.height, "d2h", stream=self.transfer_stream_main ) self.ring_slot_all_layers_offload_done[slot_idx].record(self.transfer_stream_main) torch.cuda.nvtx.range_pop() def wait_slot_offload(self, slot_idx: int) -> None: """Wait for slot offload to complete.""" self.compute_stream.wait_event(self.ring_slot_all_layers_offload_done[slot_idx]) def offload_slot_layer_to_cpu(self, slot_idx: int, layer_id: int, cpu_block_id: int) -> None: """ Async offload a ring buffer slot to CPU for one layer. Args: slot_idx: Source GPU slot index layer_id: Layer index to offload cpu_block_id: Target CPU block ID """ with torch.cuda.stream(self.transfer_stream_main): # Wait for both compute_stream and default stream self.transfer_stream_main.wait_stream(self.compute_stream) self.transfer_stream_main.wait_stream(torch.cuda.default_stream()) self.k_cache_cpu[layer_id, cpu_block_id].copy_( self.k_cache_gpu[layer_id, slot_idx], non_blocking=True ) self.v_cache_cpu[layer_id, cpu_block_id].copy_( self.v_cache_gpu[layer_id, slot_idx], non_blocking=True ) self.ring_slot_offload_done[slot_idx][layer_id].record(self.transfer_stream_main) def wait_slot_offload_layer(self, slot_idx: int, layer_id: int) -> None: """Wait for slot offload to complete for a specific layer.""" self.compute_stream.wait_event(self.ring_slot_offload_done[slot_idx][layer_id]) # ----- KV access methods for ring buffer ----- def get_kv_for_slot(self, slot_idx: int, layer_id: int) -> Tuple[Tensor, Tensor]: """ Get KV for a single ring buffer slot. Args: slot_idx: GPU slot index layer_id: Layer ID Returns: (k_cache, v_cache), shape: [1, block_size, kv_heads, head_dim] """ k = self.k_cache_gpu[layer_id, slot_idx].unsqueeze(0) # [1, block_size, heads, dim] v = self.v_cache_gpu[layer_id, slot_idx].unsqueeze(0) return k, v def get_kv_for_slots( self, layer_id: int, slot_indices: List[int], ) -> Tuple[Tensor, Tensor]: """ Get KV for multiple ring buffer slots. Args: layer_id: Layer ID slot_indices: List of GPU slot indices Returns: (k_cache, v_cache), shape: [1, len(slots) * block_size, kv_heads, head_dim] """ if not slot_indices: return None, None k = self.k_cache_gpu[layer_id, slot_indices] v = self.v_cache_gpu[layer_id, slot_indices] k = k.reshape(1, -1, self.num_kv_heads, self.head_dim) v = v.reshape(1, -1, self.num_kv_heads, self.head_dim) return k, v # ----- Decode slot methods (kept for decode phase) ----- def offload_decode_slot(self, cpu_block_id: int) -> None: """ Offload KV from decode slot (slot[0]) to CPU. Args: cpu_block_id: Target CPU block ID """ logger.debug(f"Decode offload: GPU slot[{self.decode_slot}] -> CPU[{cpu_block_id}]") with torch.cuda.stream(self.transfer_stream_main): self.transfer_stream_main.wait_stream(self.compute_stream) memcpy_2d_async( self.k_cache_cpu[:, cpu_block_id], self.k_cache_gpu[:, self.decode_slot], self.cpu_pitch, self.gpu_pitch, self.width, self.height, "d2h", stream=self.transfer_stream_main ) memcpy_2d_async( self.v_cache_cpu[:, cpu_block_id], self.v_cache_gpu[:, self.decode_slot], self.cpu_pitch, self.gpu_pitch, self.width, self.height, "d2h", stream=self.transfer_stream_main ) self.decode_offload_done.record(self.transfer_stream_main) def wait_decode_offload(self) -> None: """Wait for decode slot offload to complete.""" self.compute_stream.wait_event(self.decode_offload_done) def get_kv_for_decode_slot( self, layer_id: int, pos_in_block: int, ) -> Tuple[Tensor, Tensor]: """ Get KV at specified position in decode slot. Args: layer_id: Layer ID pos_in_block: Token position within block (0 to block_size-1) Returns: (k_cache, v_cache), shape: [1, 1, kv_heads, head_dim] """ k = self.k_cache_gpu[layer_id, self.decode_slot, pos_in_block:pos_in_block+1] v = self.v_cache_gpu[layer_id, self.decode_slot, pos_in_block:pos_in_block+1] k = k.unsqueeze(0) v = v.unsqueeze(0) return k, v def get_kv_for_decode_slot_accumulated( self, layer_id: int, num_tokens: int, ) -> Tuple[Tensor, Tensor]: """ Get accumulated KV in decode slot (positions 0 to num_tokens-1). Args: layer_id: Layer ID num_tokens: Number of accumulated tokens (1 to block_size) Returns: (k_cache, v_cache), shape: [1, num_tokens, kv_heads, head_dim] """ k = self.k_cache_gpu[layer_id, self.decode_slot, :num_tokens] v = self.v_cache_gpu[layer_id, self.decode_slot, :num_tokens] k = k.unsqueeze(0) v = v.unsqueeze(0) return k, v # ----- Legacy compatibility methods (for decode double-buffering) ----- def load_to_compute_layer(self, layer_id: int, cpu_block_ids: List[int]) -> None: """ Legacy: Load CPU blocks to decode_load_slots for decode double-buffering. Uses first half of decode_load_slots as 'compute' region. """ if not cpu_block_ids: return half = max(1, len(self.decode_load_slots) // 2) slots = self.decode_load_slots[:half] num_to_load = min(len(cpu_block_ids), len(slots)) with torch.cuda.stream(self.transfer_stream_main): for i in range(num_to_load): cpu_id = cpu_block_ids[i] gpu_slot = slots[i] self.k_cache_gpu[layer_id, gpu_slot].copy_( self.k_cache_cpu[layer_id, cpu_id], non_blocking=True ) self.v_cache_gpu[layer_id, gpu_slot].copy_( self.v_cache_cpu[layer_id, cpu_id], non_blocking=True ) if num_to_load > 0: self.ring_slot_ready[slots[0]][layer_id].record(self.transfer_stream_main) def wait_compute_layer(self, layer_id: int) -> None: """Legacy: Wait for 'compute' region loading.""" half = max(1, len(self.decode_load_slots) // 2) if self.decode_load_slots: self.wait_slot_layer(self.decode_load_slots[0], layer_id) def load_to_prefetch_layer(self, layer_id: int, cpu_block_ids: List[int]) -> None: """ Legacy: Load CPU blocks to decode_load_slots for decode double-buffering. Uses second half of decode_load_slots as 'prefetch' region. """ if not cpu_block_ids: return half = max(1, len(self.decode_load_slots) // 2) slots = self.decode_load_slots[half:] if not slots: slots = self.decode_load_slots # Fallback if only 1-2 slots num_to_load = min(len(cpu_block_ids), len(slots)) with torch.cuda.stream(self.transfer_stream_main): for i in range(num_to_load): cpu_id = cpu_block_ids[i] gpu_slot = slots[i] self.k_cache_gpu[layer_id, gpu_slot].copy_( self.k_cache_cpu[layer_id, cpu_id], non_blocking=True ) self.v_cache_gpu[layer_id, gpu_slot].copy_( self.v_cache_cpu[layer_id, cpu_id], non_blocking=True ) if num_to_load > 0: self.ring_slot_ready[slots[0]][layer_id].record(self.transfer_stream_main) def wait_prefetch_layer(self, layer_id: int) -> None: """Legacy: Wait for 'prefetch' region loading.""" half = max(1, len(self.decode_load_slots) // 2) slots = self.decode_load_slots[half:] if slots: self.wait_slot_layer(slots[0], layer_id) elif self.decode_load_slots: self.wait_slot_layer(self.decode_load_slots[0], layer_id) def get_kv_for_compute( self, layer_id: int, num_blocks: int, ) -> Tuple[Tensor, Tensor]: """Legacy: Get KV from 'compute' region (first half of decode_load_slots).""" half = max(1, len(self.decode_load_slots) // 2) slots = self.decode_load_slots[:half][:num_blocks] return self.get_kv_for_slots(layer_id, slots) def get_kv_for_prefetch( self, layer_id: int, num_blocks: int, ) -> Tuple[Tensor, Tensor]: """Legacy: Get KV from 'prefetch' region (second half of decode_load_slots).""" half = max(1, len(self.decode_load_slots) // 2) slots = self.decode_load_slots[half:] if not slots: slots = self.decode_load_slots slots = slots[:num_blocks] return self.get_kv_for_slots(layer_id, slots) # ========== Debug Hook Interface ========== # # Minimal generic hook system for debugging. # Framework only provides hook registration and tensor access. # All verification logic is external. def enable_debug_mode(self) -> None: """Enable debug mode.""" self._debug_mode = True logger.info("OffloadEngine debug mode ENABLED") def disable_debug_mode(self) -> None: """Disable debug mode and clear all hooks.""" self._debug_mode = False self._debug_hooks.clear() logger.info("OffloadEngine debug mode DISABLED") @property def debug_mode(self) -> bool: """Check if debug mode is enabled.""" return self._debug_mode def register_debug_hook(self, hook_fn) -> None: """ Register a debug hook. The hook is called after H2D load completes (after wait_slot_layer), receiving the loaded tensor for inspection. Args: hook_fn: Callable with signature: (slot_idx: int, layer_id: int, cpu_block_id: int, k: Tensor, v: Tensor) -> None - k, v: GPU tensor views for the loaded slot Example: def my_hook(slot_idx, layer_id, cpu_block_id, k, v): if layer_id == 0: k_val = k.float().mean().item() print(f"Loaded block {cpu_block_id}, K mean = {k_val}") offload_engine.register_debug_hook(my_hook) """ self._debug_hooks.append(hook_fn) def remove_debug_hook(self, hook_fn) -> None: """Remove a registered debug hook.""" if hook_fn in self._debug_hooks: self._debug_hooks.remove(hook_fn) def _call_debug_hooks(self, slot_idx: int, layer_id: int, cpu_block_id: int) -> None: """ Call all registered debug hooks with loaded tensor (internal use). Called by attention.py after wait_slot_layer completes. """ if not self._debug_mode or not self._debug_hooks: return k = self.k_cache_gpu[layer_id, slot_idx] v = self.v_cache_gpu[layer_id, slot_idx] for hook in self._debug_hooks: try: hook(slot_idx, layer_id, cpu_block_id, k, v) except Exception as e: logger.warning(f"Debug hook error: {e}")