MORI-IO Introduction

September 18, 2026 · View on GitHub

MORI-IO is AMD's point-to-point communication library that leverages GDR (GPU Direct RDMA) to achieve low-latency and high-bandwidth. Its current main use case is KVCache transfer in LLM inference.

Design & Concepts
Workflow
Architecture
Python API Quick Reference
Example: Basic Read/Write
Environment & Configuration
Profiling MORI-IO with ROCTX Markers
Source Files

Design & Concepts

IOEngine: The primary interface for interacting with MORI-IO. It abstracts low-level details of P2P communications and provides high-level APIs for engine registration, memory registration, P2P transfer, etc.
Backend: A backend represents and manages a specific transfer medium (e.g., RDMA, XGMI, TCP). It must be created before any data transfer can occur over that medium.
Engine Registration: Before two engines can communicate, the remote engine must be registered with the local engine. This establishes the necessary context for initiating data transfers between them.
Memory Registration: Application memory must be registered with a local engine before it can participate in data transfer. This ensures the engine can access and manage the memory efficiently during communication.
Read/Write: One-sided transfer operations initiated by the initiator engine without active involvement from the target engine. These operations can move data directly between registered memory regions.
Batch Read/Write: A batched form of one-sided operations, where multiple transfers are grouped and launched together. Batching reduces per-operation launch overhead and improves bandwidth utilization.
Session: A pre-established transfer context between a pair of MemoryDesc objects. Sessions eliminate repetitive overheads such as connection setup, metadata exchange, and resource management, providing a lightweight and efficient path for repeated transfers.

Workflow

The image below shows a typical workflow of using MORI-IO.

Architecture

From the application's perspective, MORI-IO provides 3 kinds of functionalities: engine registration, memory registration, and transfers.

The application is responsible for passing and registering engine descriptors among engines where transfers are expected to happen. Once engines are registered with each other, the application registers memory buffers on both initiator-side and target-side. Before initiating transfers from the initiator, the application needs to first pass the memory descriptors from the target side to the initiator side. After that, the application is ready to initiate transfers from the initiator engine. The transfer APIs return a TransferStatus that the application can use to query the state of corresponding transfers. Transfers are initiated asynchronously.

Inside MORI-IO, there are 5 components:

Component	Description
Slow Control Plane	Exchanges metadata such as memory descriptors, RDMA QP numbers, and custom notification messages. Uses a built-in TCP server with a lightweight protocol (no 3rd-party libraries).
Fast Control Plane	Exchanges metadata on the critical path of transfers (e.g., remote engine completion notifications). Uses RDMA network to minimize performance penalty.
Data Plane	Transfers bulk data. Supported transports: RDMA, XGMI, TCP.
Metadata Store	Manages per-engine metadata. Each engine manages its own metadata (no centralized store).
Transport Store	Manages multiple transfer backends (RDMA, TCP, XGMI). Provides failover if one backend is in a failure state.

Python API Quick Reference

Imports:

from mori.io import (
    IOEngine, IOEngineSession, IOEngineConfig,
    BackendType, MemoryLocationType, StatusCode, PollCqMode,
    RdmaBackendConfig, XgmiBackendConfig,
    EngineDesc, MemoryDesc,
    set_log_level,
)

Core classes:

Class	Description
`IOEngine`	Primary engine for P2P transfers — create backends, register engines/memory, issue transfers
`IOEngineSession`	Lightweight reusable session between two memory regions, lower per-transfer overhead
`IOEngineConfig`	Engine configuration: `host` (str), `port` (int)
`RdmaBackendConfig`	RDMA backend config: `qp_per_transfer`, `post_batch_size`, `num_worker_threads`, `poll_cq_mode`, `enable_notification`, `enable_transfer_chunking`, `chunk_bytes`, `max_chunks_per_transfer`, `num_nics_per_transfer`
`XgmiBackendConfig`	XGMI backend config: `num_streams` (default 64), `num_events` (default 64)

Enums:

Enum	Values
`BackendType`	`Unknown`, `XGMI`, `RDMA`, `TCP`
`MemoryLocationType`	`Unknown`, `CPU`, `GPU`
`StatusCode`	`SUCCESS`, `INIT`, `IN_PROGRESS`, `ERR_INVALID_ARGS`, `ERR_NOT_FOUND`, `ERR_RDMA_OP`, `ERR_BAD_STATE`, `ERR_GPU_OP`
`PollCqMode`	`POLLING`, `EVENT`

IOEngine methods:

Method	Description
`get_engine_desc()`	Get engine descriptor for remote registration
`create_backend(type, config)`	Create RDMA/XGMI/TCP backend
`remove_backend(type)`	Remove a backend
`register_remote_engine(desc)`	Register a remote engine
`deregister_remote_engine(desc)`	Deregister a remote engine
`register_memory(ptr, size, device_id, mem_loc)`	Register memory by raw pointer
`register_torch_tensor(tensor)`	Register a PyTorch tensor
`deregister_memory(mem_desc)`	Deregister memory
`allocate_transfer_uid()`	Allocate unique transfer ID
`read(local_mem, l_offset, remote_mem, r_offset, size, uid)`	One-sided read (remote → local)
`write(local_mem, l_offset, remote_mem, r_offset, size, uid)`	One-sided write (local → remote)
`batch_read(...)` / `batch_write(...)`	Batched transfers
`create_session(local_mem, remote_mem)`	Create reusable session
`pop_inbound_transfer_status(key, uid)`	Check inbound transfer status

TransferStatus methods:

Method	Description
`InProgress()`	Transfer still in progress
`Succeeded()`	Transfer completed successfully
`Failed()`	Transfer failed
`Code()`	Get status code
`Message()`	Get status message
`Wait()`	Block until transfer completes

Example: Basic Read/Write

import torch
from mori.io import (
    IOEngine, IOEngineConfig, BackendType,
    RdmaBackendConfig, EngineDesc,
)

# Create engines on initiator and target
config = IOEngineConfig(host="127.0.0.1", port=8080)
initiator = IOEngine(key="initiator", config=config)
config.port = 8081
target = IOEngine(key="target", config=config)

# Create RDMA backends
rdma_config = RdmaBackendConfig(qp_per_transfer=1)
initiator.create_backend(BackendType.RDMA, rdma_config)
target.create_backend(BackendType.RDMA, rdma_config)

# Exchange and register engine descriptors
initiator_desc = initiator.get_engine_desc()
target_desc = target.get_engine_desc()
initiator.register_remote_engine(target_desc)
target.register_remote_engine(initiator_desc)

# Register GPU tensors
size = 1024 * 1024  # 1 MB
src_tensor = torch.randint(0, 256, (size,), device="cuda:0", dtype=torch.uint8)
dst_tensor = torch.zeros(size, device="cuda:1", dtype=torch.uint8)
initiator_mem = initiator.register_torch_tensor(dst_tensor)
target_mem = target.register_torch_tensor(src_tensor)

# Read: copy from target (remote) to initiator (local)
uid = initiator.allocate_transfer_uid()
status = initiator.read(initiator_mem, 0, target_mem, 0, size, uid)

# Wait for completion
while status.InProgress():
    pass
assert status.Succeeded()

# With sessions (lower overhead for repeated transfers)
sess = initiator.create_session(initiator_mem, target_mem)
uid = sess.allocate_transfer_uid()
status = sess.read(0, 0, size, uid)
while status.InProgress():
    pass

See examples/io/example.py for more complete examples including batch transfers.

Environment & Configuration

Setting	Description
`set_log_level(level)`	Set MORI-IO log verbosity level
`RdmaBackendConfig.qp_per_transfer`	Queue pairs per transfer (default 1, increase for higher bandwidth; with multi-NIC these QPs are spread across NICs, so aim for ≥2 QP per NIC). Env: `MORI_IO_QP_PER_TRANSFER`
`RdmaBackendConfig.poll_cq_mode`	CQ polling mode: `POLLING` (busy-wait, lower latency) or `EVENT` (interrupt-driven). Env: `MORI_IO_POLL_CQ_MODE` (`0`/`polling`, `1`/`event`)
`RdmaBackendConfig.post_batch_size`	WRs per `ibv_post_send` chain (default `-1` = auto). Env: `MORI_IO_POST_BATCH_SIZE`
`RdmaBackendConfig.num_worker_threads`	Worker threads for batch posting (default `1`; ignored when chunking / multi-NIC forces inline posting). Env: `MORI_IO_NUM_WORKER_THREADS`
`RdmaBackendConfig.enable_notification`	Enable target-side completion notifications (default `True`). Env: `MORI_IO_ENABLE_NOTIFICATION`
`RdmaBackendConfig.enable_transfer_chunking`	Split a large single transfer into `chunk_bytes` chunks pipelined across QPs (default `False`). Lifts single-transfer bandwidth from single-outstanding (~28 GB/s) to NIC line rate. Forces single-thread inline posting (ignores `num_worker_threads`). Env: `MORI_IO_ENABLE_CHUNKING`
`RdmaBackendConfig.chunk_bytes`	Chunk size when chunking is on (default `65536` = 64 KB). Messages ≤ this are unchanged. Env: `MORI_IO_CHUNK_BYTES`
`RdmaBackendConfig.max_chunks_per_transfer`	Cap on chunks per transfer to bound WR/SQ usage (default `64`). Env: `MORI_IO_MAX_CHUNKS`
`RdmaBackendConfig.num_nics_per_transfer`	Stripe a transfer across this many NICs (default `1`). Adaptive by memory type: GPU memory stays single-NIC (PCIe-bound); host memory stripes across NUMA-local NICs. Env: `MORI_IO_NUM_NICS_PER_TRANSFER`
`IOEngineConfig.port = 0`	Auto-bind to a free port
`LD_LIBRARY_PATH`	MORI-IO loads libibverbs dynamically at runtime (`dlopen` of `libibverbs.so` / `libibverbs.so.1`) rather than linking it. To use an out-of-tree libibverbs, put its directory on `LD_LIBRARY_PATH`.

UMBP (the upper-layer cache pool) exposes a separate set of runtime-tunable env vars for distributed master / pool client / SPDK proxy timing. Those are out of scope for MORI-IO; see src/umbp/doc/runtime-env-vars.md.

Profiling MORI-IO with ROCTX Markers

MORI-IO provides optional, runtime-gated ROCTX timeline markers for use with rocprofv3 --marker-trace. The following variables are disabled by default and operate independently:

MORI_ROCTX enables same-thread host submission ranges around MORI-IO batch-write dispatch and RDMA posting paths. They measure host-side submission and post work only, not completion latency.
MORI_ROCTX_TRANSFER enables process-wide ranges that begin immediately before posting a signaled RDMA work request and end on the CQ polling thread after its CQE is observed. They measure post-to-completion latency.

Marker fields bytes= and id= identify application payload bytes (not physical RoCE wire bytes) and MORI's logical transfer ID, respectively. Key host marker families are mori.io.* and mori.rdma.batch_post.*; transfer markers use mori.rdma.io_transfer[.read].

Although generic to MORI-IO, these markers can help observe KV-cache post-to-completion latency in prefill/decode-disaggregated vLLM or SGLang deployments configured to use MORI-IO.

export MORI_ROCTX=1
export MORI_ROCTX_TRANSFER=1
rocprofv3 --marker-trace -- <your MORI command>

Either variable can be enabled alone. Enabled values include 1, on, and true; disable a variable by unsetting it or setting it to 0, off, or false.

Source Files

File	Description
`python/mori/io/__init__.py`	Public exports
`python/mori/io/engine.py`	Python IOEngine and IOEngineSession classes
`src/pybind/pybind.cpp`	Python module entry point
`src/pybind/mori.cpp`	IO binding registration (`RegisterMoriIo`)
`examples/io/example.py`	Complete usage examples (read, write, batch, session)
`tests/python/io/test_engine.py`	Comprehensive test suite
`tests/cpp/io/bench_engine.cpp`	Performance benchmark (default, nixlbench-matching)
`tests/python/io/benchmark.py`	Performance benchmark (Python, kept for parity)
`docs/MORI-IO-BENCHMARK.md`	Benchmark commands and results