Configuration
UCX/UCX-Py can be configured with a wide variety of options and optimizations including: transport, caching, etc. Users can configure
UCX/UCX-Py either with environment variables or programmatically during initialization. Below we demonstrate setting UCX_MEMTYPE_CACHE
to
n
and checking the configuration:
import ucp
options = {"MEMTYPE_CACHE": "n"}
ucp.init(options)
assert ucp.get_config()['MEMTYPE_CACHE'] is 'n'
Note
When programmatically configuring UCX-Py, the UCX
prefix is not used.
For novice users we recommend using UCX-Py defaults, see the next section for details.
UCX-Py vs UCX Defaults
UCX-Py redefines some of the UCX defaults for a variety of reasons, including better performance for the more common Python use cases, or to work around known limitations or bugs of UCX. To verify UCX default configurations, for the currently installed UCX version please run the command-line tool ucx_info -f
.
Below is a list of the UCX-Py redefined default values, and what conditions are required for them to apply.
Apply to all UCX versions:
UCX_RNDV_THRESH=8192
UCX_RNDV_SCHEME=get_zcopy
Apply to UCX >= 1.12.0, older UCX versions rely on UCX defaults:
UCX_CUDA_COPY_MAX_REG_RATIO=1.0
UCX_MAX_RNDV_RAILS=1
Please note that UCX_CUDA_COPY_MAX_REG_RATIO=1.0
is only set provided at least one GPU is present with a BAR1 size smaller than its total memory (e.g., NVIDIA T4).
UCX Environment Variables in UCX-Py
In this section we go over a brief overview of some of the more relevant variables for current UCX-Py usage, along with some comments on their uses and limitations. To see a complete list of UCX environment variables, their descriptions and default values, please run the command-line tool ucx_info -f
.
DEBUG
Debug variables for both UCX and UCX-Py can be set
UCXPY_LOG_LEVEL/UCX_LOG_LEVEL
Values: DEBUG, TRACE
If UCX has been built with debug mode enabled
MEMORY
UCX_MEMTYPE_CACHE
This is a UCX Memory optimization which toggles whether UCX library intercepts cu*alloc* calls. UCX-Py defaults this value to n
. There known issues when using this feature.
Values: n
/y
UCX_CUDA_IPC_CACHE
This is a UCX CUDA Memory optimization which enables/disables a remote endpoint IPC memhandle mapping cache. UCX/UCX-Py defaults this value to y
Values: n
/y
UCX_MEMTYPE_REG_WHOLE_ALLOC_TYPES
By defining UCX_MEMTYPE_REG_WHOLE_ALLOC_TYPES=cuda
(default in UCX >= 1.12.0), UCX enables registration cache based on a buffer’s base address, thus preventing multiple time-consuming registrations for the same buffer. This is particularly useful when using a CUDA memory pool, thus requiring a single registration between two ends for the entire pool, providing considerable performance gains, especially when using InfiniBand.
TRANSPORTS
UCX_MAX_RNDV_RAILS
Limiting the number of rails (network devices) to 1
allows UCX to use only the closest device according to NUMA locality and system topology. Particularly useful with InfiniBand and CUDA GPUs, ensuring all transfers from/to the GPU will use the closest InfiniBand device and thus implicitly enable GPUDirectRDMA.
Note
On CPU-only systems, better network bandwidth performance with infiniband transports may be achieved by letting UCX use more than a single network device. This can be achieved by explicitly setting UCX_MAX_RNDV_RAILS
to 2
or higher.
Values: Int (UCX-Py default: 1
)
UCX_RNDV_THRESH
This is a configurable parameter used by UCX to help determine which transport method should be used. For example, on machines with multiple GPUs, and with NVLink enabled, UCX can deliver messages either through TCP or NVLink. Sending GPU buffers over TCP is costly as it triggers a device-to-host on the sender side, and then host-to-device transfer on the receiver side – we want to avoid these kinds of transfers when NVLink is available. If a buffer is below the threshold, Rendezvous-Protocol is triggered and for UCX-Py users, this will typically mean messages will be delivered through TCP. Depending on the application, messages can be quite small, therefore, we recommend setting a small value if the application uses NVLink or InfiniBand: UCX_RNDV_THRESH=8192
Values: Int (UCX-Py default: 8192
)
UCX_RNDV_SCHEME
Communication scheme in RNDV protocol
Values:
put_zcopy
get_zcopy
auto
(default)
UCX_TCP_RX_SEG_SIZE
Size of send copy-out buffer when receiving. This environment variable controls the size of the buffer on the host when receiving data over TCP.
UCX_TCP_TX_SEG_SIZE
Size of send copy-out buffer when transmitting. This environment variable controls the size of the buffer on the host when sending data over TCP.
Note
Users should take care to properly tune UCX_TCP_{RX/TX}_SEG_SIZE
parameters when mixing TCP with other transports methods as well as when
using TCP over UCX in isolation. These variables will impact CUDA transfers when no NVLink or InfiniBand is available between UCX-Py processes.
These parameters will cause the HostToDevice and DeviceToHost copies of buffers to be broken down in several
chunks when the size of a buffer exceeds the size defined by these two variables. If an application is expected to transfer very
large buffers, increasing such values may improve overall performance.
UCX_TLS
Transport Methods (Simplified):
all
-> use all the available transportsrc
-> InfiniBand (ibv_post_send, ibv_post_recv, ibv_poll_cq) uses rc_v and rc_x (preferably if available)cuda_copy
-> cuMemHostRegister, cuMemcpyAsynccuda_ipc
-> CUDA Interprocess Communication (cuIpcCloseMemHandle, cuIpcOpenMemHandle, cuMemcpyAsync)sm/shm
-> all shared memory transports (mm, cma, knem)mm
-> shared memory transports - only memory mappersugni
-> ugni_smsg and ugni_rdma (uses ugni_udt for bootstrap)ib
-> all infiniband transports (rc/rc_mlx5, ud/ud_mlx5, dc_mlx5)rc_v
-> rc verbs (uses ud for bootstrap)rc_x
-> rc with accelerated verbs (uses ud_mlx5 for bootstrap)ud_v
-> ud verbsud_x
-> ud with accelerated verbsud
-> ud_v and ud_x (preferably if available)dc/dc_x
-> dc with accelerated verbstcp
-> sockets over TCP/IPcuda
-> CUDA (NVIDIA GPU) memory supportrocm
-> ROCm (AMD GPU) memory support
SOCKADDR_TLS_PRIORITY
Priority of sockaddr transports
InfiniBand Device
Select InfiniBand Device
UCX_NET_DEVICES
Typically these will be the InfiniBand device corresponding to a particular set of GPUs. Values:
mlx5_0:1
To find more information on the topology of InfiniBand-GPU pairing run the following:
nvidia-smi topo -m
Example Configs
InfiniBand – No NVLink
UCX_RNDV_SCHEME=get_zcopy UCX_MEMTYPE_CACHE=n UCX_TLS=rc,tcp,cuda_copy <SCRIPT>
InfiniBand – With NVLink
UCX_RNDV_SCHEME=get_zcopy UCX_MEMTYPE_CACHE=n UCX_TLS=rc,tcp,cuda_copy,cuda_ipc <SCRIPT>
TLS/Socket – No NVLink
UCX_RNDV_SCHEME=get_zcopy UCX_MEMTYPE_CACHE=n UCX_TLS=tcp,cuda_copy <SCRIPT>
TLS/Socket – With NVLink
UCX_RNDV_SCHEME=get_zcopy UCX_MEMTYPE_CACHE=n UCX_TLS=tcp,cuda_copy,cuda_ipc <SCRIPT>