[PD] Add doc and simplify sender.send (#6019)

docs/backend/pd_disaggregation.md

+# PD Disaggregation
+
+## Why and What is PD Disaggregation?
+
+Large Language Model (LLM) inference comprises two distinct phases: **Prefill** and **Decode**. The Prefill phase is computation-intensive, processing the entire input sequence, while the Decode phase is memory-intensive, managing the Key-Value (KV) cache for token generation. Traditionally, these phases are handled within a unified engine, where combined scheduling of prefill and decode batches introduces inefficiencies. To address these challenges, we introduce **Prefill and Decoding (PD) Disaggregation** in SGLang.
+
+### Issues with Unified Scheduling
+
+The conventional unified engine, which processes prefill and decode batches together, results in two significant problems:
+
+1. **Prefill Interruption**: Incoming prefill batches frequently interrupt ongoing decode batches, causing substantial delays in token generation.
+2. **DP Attention Imbalance**: In data-parallel (DP) attention, one DP worker may process a prefill batch while another handles a decode batch simultaneously, leading to increased decode latency.
+
+PD Disaggregation resolves these by separating the two stages, enabling tailored optimizations for each.
+
+For the design details, please refer to [link](https://docs.google.com/document/d/1rQXJwKd5b9b1aOzLh98mnyMhBMhlxXA5ATZTHoQrwvc/edit?tab=t.0).
+
+Currently, we support Mooncake and NIXL as the transfer engine.
+
+
+## Mooncake
+### Requirements
+
+```bash
+uv pip install mooncake-transfer-engine
+```
+
+### Usage
+
+### Llama Single Node
+
+```bash
+$ python -m sglang.launch_server --model-path meta-llama/Llama-3.1-8B-Instruct --disaggregation-mode prefill --disaggregation-ib-device mlx5_roce0
+$ python -m sglang.launch_server --model-path meta-llama/Llama-3.1-8B-Instruct --disaggregation-mode decode --port 30001 --base-gpu-id 1 --disaggregation-ib-device mlx5_roce0
+$ python -m sglang.srt.disaggregation.mini_lb --prefill http://127.0.0.1:30000 --decode http://127.0.0.1:30001 --host 0.0.0.0 --port 8000
+```
+
+### DeepSeek Multi-Node
+
+```bash
+# prefill 0
+$ python -m sglang.launch_server --model-path deepseek-ai/DeepSeek-V3-0324 --disaggregation-ib-device ${device_name} --disaggregation-mode prefill --host ${local_ip} --port 30000 --trust-remote-code --dist-init-addr ${prefill_master_ip}:5000 --nnodes 2 --node-rank 0 --tp-size 16 --dp-size 8 --enable-dp-attention --enable-deepep-moe --deepep-mode normal --mem-fraction-static 0.8
+# prefill 1
+$ python -m sglang.launch_server --model-path deepseek-ai/DeepSeek-V3-0324 --disaggregation-ib-device ${device_name} --disaggregation-mode prefill --host ${local_ip} --port 30000 --trust-remote-code --dist-init-addr ${prefill_master_ip}:5000 --nnodes 2 --node-rank 1 --tp-size 16 --dp-size 8 --enable-dp-attention --enable-deepep-moe --deepep-mode normal --mem-fraction-static 0.8
+# decode 0
+$ python -m sglang.launch_server --model-path deepseek-ai/DeepSeek-V3-0324 --disaggregation-ib-device ${device_name} --disaggregation-mode decode --host ${local_ip} --port 30001 --trust-remote-code --dist-init-addr ${decode_master_ip}:5000 --nnodes 2 --node-rank 0 --tp-size 16 --dp-size 8 --enable-dp-attention --enable-deepep-moe --deepep-mode low_latency --mem-fraction-static 0.8 --max-running-requests 128
+# decode 1
+$ python -m sglang.launch_server --model-path deepseek-ai/DeepSeek-V3-0324 --disaggregation-ib-device ${device_name} --disaggregation-mode decode --host ${local_ip} --port 30001 --trust-remote-code --dist-init-addr ${decode_master_ip}:5000 --nnodes 2 --node-rank 1 --tp-size 16 --dp-size 8 --enable-dp-attention --enable-deepep-moe --deepep-mode low_latency --mem-fraction-static 0.8 --max-running-requests 128
+```

docs/index.rst

@@ -55,6 +55,7 @@ The core features include:
    backend/custom_chat_template.md
    backend/quantization.md
    backend/lora.ipynb
+   backend/pd_disaggregation.md
 
 .. toctree::
    :maxdepth: 1

python/sglang/srt/disaggregation/fake/conn.py

@@ -33,28 +33,18 @@ class FakeKVSender(BaseKVSender):
         self,
         kv_indices: list[int],
         aux_index: Optional[int] = None,
-        dest_ranks: Optional[list[int]] = None,
     ):
         logger.info(
-            f"FakeKVSender init with kv_indices: {kv_indices}, aux_index: {aux_index}, dest_ranks: {dest_ranks}"
+            f"FakeKVSender init with kv_indices: {kv_indices}, aux_index: {aux_index}"
         )
         pass
 
     def send(
         self,
         kv_indices: npt.NDArray[np.int64],
-        index_slice: slice,
-        is_last: bool,
     ):
-        logger.info(
-            f"FakeKVSender send with kv_indices: {kv_indices}, index_slice: {index_slice}, is_last: {is_last}"
-        )
-        if is_last:
-            self.has_sent = True
-            logger.info(f"FakeKVSender send success")
-        else:
-            self.has_sent = False
-            logger.info(f"FakeKVSender send fake transferring")
+        self.has_sent = True
+        logger.info(f"FakeKVSender send with kv_indices: {kv_indices}")
 
     def failure_exception(self):
         raise Exception("Fake KVSender Exception")

python/sglang/srt/disaggregation/mooncake/conn.py

@@ -464,6 +464,8 @@ class MooncakeKVSender(BaseKVSender):
         self.aux_index = None
         self.bootstrap_server_url = bootstrap_addr
         self.session_id = self.kv_mgr.get_session_id()
+        # inner state
+        self.curr_idx = 0
 
     def init(self, num_kv_indices: int, aux_index: Optional[int] = None):
         self.num_kv_indices = num_kv_indices
@@ -472,9 +474,11 @@ class MooncakeKVSender(BaseKVSender):
     def send(
         self,
         kv_indices: npt.NDArray[np.int64],
-        index_slice: slice,
-        is_last: bool,
     ):
+        index_slice = slice(self.curr_idx, self.curr_idx + len(kv_indices))
+        self.curr_idx += len(kv_indices)
+        is_last = self.curr_idx == self.num_kv_indices
+
         if not is_last:
             self.kv_mgr.add_transfer_request(
                 self.bootstrap_room, kv_indices, index_slice, False

python/sglang/srt/disaggregation/prefill.py

@@ -384,11 +384,10 @@ class SchedulerDisaggregationPrefillMixin:
             if end_idx is not None
             else min(len(req.fill_ids), len(req.origin_input_ids))
         )
+
         last_chunk = token_id is not None
 
-        if (not last_chunk) and (
-            end_idx % page_size != 0
-        ):  # todo: remove the second condition
+        if not last_chunk:
             # if not the last chunk and the last page is partial, delay the last partial page to the next send
             end_idx = end_idx - end_idx % page_size
 
@@ -405,16 +404,10 @@ class SchedulerDisaggregationPrefillMixin:
                 req.metadata_buffer_index, token_id
             )
         page_indices = kv_to_page_indices(kv_indices, page_size)
-
-        page_start_idx = start_idx // page_size
-        page_end_idx = page_start_idx + len(page_indices)
-
         if len(page_indices) == 0:
             logger.info(
                 f"Skip sending kv chunk for request {req.rid=} {req.bootstrap_room=} because page_indices is empty"
             )
             return
 
-        req.disagg_kv_sender.send(
-            page_indices, slice(page_start_idx, page_end_idx), last_chunk
-        )
+        req.disagg_kv_sender.send(page_indices)

python/sglang/srt/managers/io_struct.py

@@ -407,6 +407,7 @@ class GenerateReqInput:
                 else None
             ),
             return_hidden_states=self.return_hidden_states,
+            # if `__getitem__` is called, the bootstrap_host, bootstrap_port, bootstrap_room must be a list
             bootstrap_host=(
                 self.bootstrap_host[i] if self.bootstrap_host is not None else None
             ),