Environment Variables¶
NCCL has an extensive set of environment variables to tune for specific usage.
They can also be set statically in /etc/nccl.conf (for an administrator to set system-wide values) or in ~/.nccl.conf (for users). For example, those files could contain :
NCCL_DEBUG=WARN
NCCL_SOCKET_IFNAME==ens1f0
NCCL_P2P_DISABLE¶
The NCCL_P2P_DISABLE variable disables the peer to peer (P2P) transport, which uses CUDA direct access between GPUs, using NVLink or PCI.
Values accepted¶
Define and set to 1 to disable direct GPU-to-GPU (P2P) communication.
NCCL_P2P_LEVEL¶
(since 2.3.4)
The NCCL_P2P_LEVEL variable allows the user to finely control when to use the peer to peer (P2P) transport between GPUs.
The level defines the maximum distance between GPUs where NCCL will use the P2P transport. A short string representing
the path type should be used to specify the topographical cutoff for using the P2P transport.
If this isn’t specified, NCCL will attempt to optimally select a value based on the architecture and environment it’s run in.
Values accepted¶
- LOC : Never use P2P (always disabled)
- NVL : Use P2P when GPUs are connected through NVLink
- PIX : Use P2P when GPUs are on the same PCI switch.
- PXB : Use P2P when GPUs are connected through PCI switches (potentially multiple hops).
- PHB : Use P2P when GPUs are on the same NUMA node. Traffic will go through the CPU.
- SYS : Use P2P between NUMA nodes, potentially crossing the SMP interconnect (e.g. QPI/UPI).
Integer Values (Legacy)¶
There is also the option to declare NCCL_P2P_LEVEL as an integer corresponding to the path type. These numerical values were kept for retro-compatibility, for those who used numerical values before strings were allowed.
Integer values are discouraged due to breaking changes in path types - the literal values can change over time. To avoid headaches debugging your configuration, use string identifiers.
- LOC : 0
- PIX : 1
- PXB : 2
- PHB : 3
- SYS : 4
Values greater than 4 will be interpreted as SYS. NVL is not supported using the legacy integer values.
NCCL_P2P_DIRECT_DISABLE¶
The NCCL_P2P_DIRECT_DISABLE variable forbids NCCL to directly access user buffers through P2P between GPUs of the same process. This is useful when user buffers are allocated with APIs which do not automatically make them accessible to other GPUs managed by the same process and with P2P access.
Values accepted¶
Define and set to 1 to disable direct user buffer access across GPUs.
NCCL_SHM_DISABLE¶
The NCCL_SHM_DISABLE variable disables the Shared Memory (SHM) transports. SHM is used between devices when peer-to-peer cannot happen, therefore, host memory is used. NCCL will use the network (i.e. InfiniBand or IP sockets) to communicate between the CPU sockets when SHM is disabled.
Values accepted¶
Define and set to 1 to disable communication through shared memory (SHM).
NCCL_SOCKET_IFNAME¶
The NCCL_SOCKET_IFNAME variable specifies which IP interfaces to use for communication.
Values accepted¶
Define to a list of prefixes to filter interfaces to be used by NCCL.
Multiple prefixes can be provided, separated by the , symbol.
Using the ^ symbol, NCCL will exclude interfaces starting with any prefix in that list.
To match (or not) an exact interface name, begin the prefix string with the = character.
Examples:
eth : Use all interfaces starting with eth, e.g. eth0, eth1, …
=eth0 : Use only interface eth0
=eth0,eth1 : Use only interfaces eth0 and eth1
^docker : Do not use any interface starting with docker
^=docker0 : Do not use interface docker0.
Note: By default, the loopback interface (lo) and docker interfaces (docker*) would not be selected unless there are no other interfaces available. If you prefer to use lo or docker* over other interfaces, you would need to explicitly select them using NCCL_SOCKET_IFNAME. The default algorithm will also favor interfaces starting with ib over others. Setting NCCL_SOCKET_IFNAME will bypass the automatic interface selection algorithm and may use all interfaces matching the manual selection.
NCCL_SOCKET_FAMILY¶
The NCCL_SOCKET_FAMILY variable allows users to force NCCL to use only IPv4 or IPv6 interface.
Values accepted¶
Set to AF_INET to force the use of IPv4, or AF_INET6 to force IPv6 usage.
NCCL_SOCKET_NTHREADS¶
(since 2.4.8)
The NCCL_SOCKET_NTHREADS variable specifies the number of CPU helper threads used per network connection for socket transport. Increasing this value may increase the socket transport performance, at the cost of a higher CPU usage.
Values accepted¶
1 to 16. On AWS, the default value is 2; on Google Cloud instances with the gVNIC network interface, the default value is 4 (since 2.5.6); in other cases, the default value is 1.
For generic 100G networks, this value can be manually set to 4. However, the product of NCCL_SOCKET_NTHREADS and NCCL_NSOCKS_PERTHREAD cannot exceed 64. See also NCCL_NSOCKS_PERTHREAD.
NCCL_NSOCKS_PERTHREAD¶
(since 2.4.8)
The NCCL_NSOCKS_PERTHREAD variable specifies the number of sockets opened by each helper thread of the socket transport. In environments where per-socket speed is limited, setting this variable larger than 1 may improve the network performance.
Values accepted¶
On AWS, the default value is 8; in other cases, the default value is 1.
For generic 100G networks, this value can be manually set to 4. However, the product of NCCL_SOCKET_NTHREADS and NCCL_NSOCKS_PERTHREAD cannot exceed 64. See also NCCL_SOCKET_NTHREADS.
NCCL_DEBUG¶
The NCCL_DEBUG variable controls the debug information that is displayed from NCCL. This variable is commonly used for debugging.
Values accepted¶
VERSION - Prints the NCCL version at the start of the program.
WARN - Prints an explicit error message whenever any NCCL call errors out.
INFO - Prints debug information
TRACE - Prints replayable trace information on every call.
NCCL_BUFFSIZE¶
The NCCL_BUFFSIZE variable controls the size of the buffer used by NCCL when communicating data between pairs of GPUs.
Use this variable if you encounter memory constraint issues when using NCCL or you think that a different buffer size would improve performance.
Values accepted¶
The default is 4194304 (4 MB).
Values are integers, in bytes. The recommendation is to use powers of 2. For example, 1024 will give a 1K buffer.
NCCL_NTHREADS¶
The NCCL_NTHREADS variable sets the number of CUDA threads per CUDA block. NCCL will launch one CUDA block per communication channel.
Use this variable if you think your GPU clocks are low and you want to increase the number of threads.
You can also use this variable to reduce the number of threads to decrease the GPU workload.
Values accepted¶
The default is 512 for recent generation GPUs, and 256 for some older generations.
The values allowed are 64, 128, 256 and 512.
NCCL_MAX_NCHANNELS¶
(NCCL_MAX_NRINGS since 2.0.5, NCCL_MAX_NCHANNELS since 2.5.0)
The NCCL_MAX_NCHANNELS variable limits the number of channels NCCL can use. Reducing the number of channels also reduces the
number of CUDA blocks used for communication, hence the impact on GPU computing resources.
The old NCCL_MAX_NRINGS variable (used until 2.4) still works as an alias in newer versions but is ignored if NCCL_MAX_NCHANNELS is set.
Values accepted¶
Any value above or equal to 1.
NCCL_MIN_NCHANNELS¶
(NCCL_MIN_NRINGS since 2.2.0, NCCL_MIN_NCHANNELS since 2.5.0)
The NCCL_MIN_NCHANNELS variable controls the minimum number of channels you want NCCL to use.
Increasing the number of channels also increases the number of
CUDA blocks NCCL uses, which may be useful to improve performance; however, it uses more CUDA compute resources.
This is especially useful when using aggregated collectives on platforms where NCCL would usually only create one channel.
The old NCCL_MIN_NRINGS variable (used until 2.4) still works as an alias in newer versions, but is ignored if NCCL_MIN_NCHANNELS is set.
Values accepted¶
The default is platform dependent. Set to an integer value, up to 12 (up to 2.2), 16 (2.3 and 2.4) or 32 (2.5 and later).
NCCL_CROSS_NIC¶
The NCCL_CROSS_NIC variable controls whether NCCL should allow rings/trees to use different NICs,
causing inter-node communication to use different NICs on different nodes.
To maximize inter-node communication performance when using multiple NICs, NCCL tries to use the
same NICs when communicating
between nodes, to allow for a network design where each NIC on a node connects to
a different network switch (network rail), and avoid any risk of traffic flow interference.
The NCCL_CROSS_NIC setting is therefore dependent on the network topology, and in particular
on whether the network fabric is rail-optimized or not.
This has no effect on systems with only one NIC.
Values accepted¶
0: Always use the same NIC for the same ring/tree, to avoid crossing network rails. Suited for networks with per NIC switches (rails), with a slow inter-rail connection. Note that if the communicator does not contain the same GPUs on each node, NCCL may still need to communicate across NICs.
1: Allow the use of different NICs for the same ring/tree. This is suited for networks where all NICs from a node are connected to the same switch, hence trying to communicate across the same NICs does not help avoiding flow collisions.
2: (Default) Try to use the same NIC for the same ring/tree, but still allow for the use of different NICs if it would result in a better performance.
NCCL_CHECKS_DISABLE¶
(since 2.0.5, deprecated in 2.2.12)
The NCCL_CHECKS_DISABLE variable can be used to disable argument checks on each collective call.
Checks are useful during development but can increase the latency. They can be disabled to
improve performance in production.
Values accepted¶
The default is 0, set to 1 to disable checks.
NCCL_CHECK_POINTERS¶
(since 2.2.12)
The NCCL_CHECK_POINTERS variable enables checking of the CUDA memory pointers on each collective call.
Checks are useful during development but can increase the latency.
Values accepted¶
The default is 0, set to 1 to enable checking.
Setting to 1 restores the original behavior of NCCL prior to 2.2.12.
NCCL_LAUNCH_MODE¶
(since 2.1.0)
The NCCL_LAUNCH_MODE variable controls how NCCL launches CUDA kernels.
Values accepted¶
The default value is PARALLEL.
Setting is to GROUP will use cooperative groups (CUDA 9.0 and later) for processes managing more than one GPU. This is deprecated in 2.9 and may be removed in future versions.
NCCL_IB_DISABLE¶
The NCCL_IB_DISABLE variable prevents the IB/RoCE transport from being used by NCCL. Instead, NCCL will fall back to
using IP sockets.
Values accepted¶
Define and set to 1 to disable the use of InfiniBand Verbs for communication (and force another method, e.g. IP sockets).
NCCL_IB_HCA¶
The NCCL_IB_HCA variable specifies which RDMA interfaces to use for communication.
Values accepted¶
Define to filter IB Verbs interfaces to be used by NCCL. The list is comma-separated; port numbers can be specified using
the : symbol. An optional prefix ^ indicates the list is an exclude list. A second optional prefix = indicates
that the tokens are exact names, otherwise by default NCCL would treat each token as a prefix.
Examples:
mlx5 : Use all ports of all cards starting with mlx5
=mlx5_0:1,mlx5_1:1 : Use ports 1 of cards mlx5_0 and mlx5_1.
^=mlx5_1,mlx5_4 : Do not use cards mlx5_1 and mlx5_4.
Note: using mlx5_1 without a preceding = will select mlx5_1 as well as mlx5_10 to mlx5_19, if they exist.
It is therefore always recommended to add the = prefix to ensure an exact match.
NCCL_IB_TIMEOUT¶
The NCCL_IB_TIMEOUT variable controls the InfiniBand Verbs Timeout.
The timeout is computed as 4.096 µs * 2 ^ timeout, and the correct value is dependent on the size of the network. Increasing that value can help on very large networks, for example, if NCCL is failing on a call to ibv_poll_cq with error 12.
For more information, see section 12.7.34 of the InfiniBand specification Volume 1 (Local Ack Timeout).
Values accepted¶
The default value used by NCCL is 18, i.e. just over 1 second (since 2.14; it was 14 in earlier versions).
Values can be 1-22.
NCCL_IB_RETRY_CNT¶
(since 2.1.15)
The NCCL_IB_RETRY_CNT variable controls the InfiniBand retry count.
For more information, see section 12.7.38 of the InfiniBand specification Volume 1.
Values accepted¶
The default value is 7.
NCCL_IB_GID_INDEX¶
(since 2.1.4)
The NCCL_IB_GID_INDEX variable defines the Global ID index used in RoCE mode.
See the InfiniBand show_gids command in order to set this value.
For more information, see the InfiniBand specification Volume 1 or vendor documentation.
Values accepted¶
The default value is -1.
NCCL_IB_ADDR_FAMILY¶
(since 2.21)
The NCCL_IB_ADDR_FAMILY variable defines the IP address family associated to
the infiniband GID dynamically selected by NCCL when NCCL_IB_GID_INDEX is left
unset.
Values accepted¶
The default value is “AF_INET”.
NCCL_IB_ADDR_RANGE¶
(since 2.21)
The NCCL_IB_ADDR_RANGE variable defines the range of valid GIDs dynamically
selected by NCCL when NCCL_IB_GID_INDEX is left unset.
Values accepted¶
By default, ignored if unset.
GID ranges can be defined using the Classless Inter-Domain Routing (CIDR) format for IPv4 and IPv6 families.
NCCL_IB_ROCE_VERSION_NUM¶
(since 2.21)
The NCCL_IB_ROCE_VERSION_NUM variable defines the RoCE version associated to
the infiniband GID dynamically selected by NCCL when NCCL_IB_GID_INDEX is left
unset.
Values accepted¶
The default value is 2.
NCCL_IB_SL¶
(since 2.1.4)
Defines the InfiniBand Service Level.
For more information, see the InfiniBand specification Volume 1 or vendor documentation.
Values accepted¶
The default value is 0.
NCCL_IB_TC¶
(since 2.1.15)
Defines the InfiniBand traffic class field.
For more information, see the InfiniBand specification Volume 1 or vendor documentation.
Values accepted¶
The default value is 0.
NCCL_IB_AR_THRESHOLD¶
(since 2.6)
Threshold above which we send InfiniBand data in a separate message which can leverage adaptive routing.
Values accepted¶
Size in bytes, the default value is 8192.
Setting it above NCCL_BUFFSIZE will disable the use of adaptive routing completely.
NCCL_IB_CUDA_SUPPORT¶
(removed in 2.4.0, see NCCL_NET_GDR_LEVEL)
The NCCL_IB_CUDA_SUPPORT variable is used to force or disable the usage of GPU Direct RDMA.
By default, NCCL enables GPU Direct RDMA if the topology permits it. This variable can disable this behavior or force
the usage of GPU Direct RDMA in all cases.
Values accepted¶
Define and set to 0 to disable GPU Direct RDMA.
Define and set to 1 to force the usage of GPU Direct RDMA.
NCCL_IB_QPS_PER_CONNECTION¶
(since 2.10)
Number of IB queue pairs to use for each connection between two ranks. This can be useful on multi-level fabrics which need multiple queue pairs to have good routing entropy.
See NCCL_IB_SPLIT_DATA_ON_QPS for different ways to split data on multiple QPs, as it can affect performance.
Values accepted¶
Number between 1 and 128, default is 1.
NCCL_IB_SPLIT_DATA_ON_QPS¶
(since 2.18)
This parameter controls how we use the queue pairs when we create more than one. Set to 1 (split mode, default), each message will be split evenly on each queue pair. This may cause a visible latency degradation if we use many QPs. Set to 0 (round-robin mode), queue pairs will be used in round-robin mode for each message we send. Operations which do not send multiple messages will not use all QPs.
Values accepted¶
0 or 1. Default is 1. Setting it to 0 will switch to round-robin mode.
NCCL_IB_PCI_RELAXED_ORDERING¶
(since 2.12)
Enable the use of Relaxed Ordering for the IB Verbs transport. Relaxed Ordering can greatly help the performance of InfiniBand networks in virtualized environments.
Values accepted¶
Set to 2 to automatically use Relaxed Ordering if available. Set to 1 to force the use of Relaxed Ordering and fail if not available. Set to 0 to disable the use of Relaxed Ordering. Default is 2.
NCCL_IB_ADAPTIVE_ROUTING¶
(since 2.16)
Enable the use of Adaptive Routing capable data transfers for the IB Verbs transport. Adaptive routing can improve the performance of communications at scale. A system defined Adaptive Routing enabled SL has to be selected accordingly (cf. NCCL_IB_SL).
Values accepted¶
Enabled (1) by default on IB networks. Disabled (0) by default on RoCE networks. Set to 1 to force use of Adaptive Routing capable data transmission.
NCCL_MEM_SYNC_DOMAIN¶
(since 2.16)
Sets the default Memory Sync Domain for NCCL kernels (CUDA 12.0 & sm90 and later). Memory Sync Domains can help eliminate interference between the NCCL kernels and the application compute kernels, when they use different domains.
Values accepted¶
Default value is cudaLaunchMemSyncDomainRemote (1). Currently supported values are 0 and 1.
NCCL_CUMEM_ENABLE¶
(since 2.18)
Use CUDA cuMem* functions to allocate memory in NCCL.
Values accepted¶
0 or 1. Default is 0 in 2.18 (disabled); since 2.19 this feature is auto-enabled by default if the system supports it (NCCL_CUMEM_ENABLE can still be used to override the autodetection).
NCCL_NET¶
(since 2.10)
Forces NCCL to use a specific network, for example to make sure NCCL uses an external plugin and doesn’t automatically fall back on the internal IB or Socket implementation. Setting this environment variable will override the netName configuration in all communicators (see ncclConfig_t); if not set (undefined), the network module will be determined by the configuration; if not passing configuration, NCCL will automatically choose the best network module.
Values accepted¶
The value of NCCL_NET has to match exactly the name of the NCCL network used (case-insensitive). Internal network names are “IB” (generic IB verbs) and “Socket” (TCP/IP sockets). External network plugins define their own names. Default value is undefined.
NCCL_NET_PLUGIN¶
(since 2.11)
Set it to a suffix string to choose among multiple NCCL net plugins. This setting will cause NCCL to look for the file “libnccl-net-<suffix>.so” instead of the default “libnccl-net.so”.
For example, setting NCCL_NET_PLUGIN=aws will cause NCCL to use libnccl-net-aws.so (provided that it exists on the system). Setting NCCL_NET_PLUGIN=none will cause NCCL not to use any plugin.
Values accepted¶
Suffix string of the plugin file name, or “none”.
NCCL_NET_GDR_LEVEL (formerly NCCL_IB_GDR_LEVEL)¶
(since 2.3.4. In 2.4.0, NCCL_IB_GDR_LEVEL was renamed to NCCL_NET_GDR_LEVEL)
The NCCL_NET_GDR_LEVEL variable allows the user to finely control when to use GPU Direct RDMA between a NIC and a GPU.
The level defines the maximum distance between the NIC and the GPU. A string representing the path type should be used to specify the topographical cutoff for GpuDirect.
If this isn’t specified, NCCL will attempt to optimally select a value based on the architecture and environment it’s run in.
Values accepted¶
- LOC : Never use GPU Direct RDMA (always disabled).
- PIX : Use GPU Direct RDMA when GPU and NIC are on the same PCI switch.
- PXB : Use GPU Direct RDMA when GPU and NIC are connected through PCI switches (potentially multiple hops).
- PHB : Use GPU Direct RDMA when GPU and NIC are on the same NUMA node. Traffic will go through the CPU.
- SYS : Use GPU Direct RDMA even across the SMP interconnect between NUMA nodes (e.g., QPI/UPI) (always enabled).
Integer Values (Legacy)¶
There is also the option to declare NCCL_NET_GDR_LEVEL as an integer corresponding to the path type. These numerical values were kept for retro-compatibility, for those who used numerical values before strings were allowed.
Integer values are discouraged due to breaking changes in path types - the literal values can change over time. To avoid headaches debugging your configuration, use string identifiers.
- LOC : 0
- PIX : 1
- PXB : 2
- PHB : 3
- SYS : 4
Values greater than 4 will be interpreted as SYS.
NCCL_NET_GDR_READ¶
The NCCL_NET_GDR_READ variable enables GPU Direct RDMA when sending data as long as the GPU-NIC distance is within the distance specified by NCCL_NET_GDR_LEVEL. Before 2.4.2, GDR read is disabled by default, i.e. when sending data, the data is first stored in CPU memory, then goes to the InfiniBand card. Since 2.4.2, GDR read is enabled by default for NVLink-based platforms.
Note: Reading directly from GPU memory when sending data is known to be slightly slower than reading from CPU memory on some platforms, such as PCI-E.
Values accepted¶
0 or 1. Define and set to 1 to use GPU Direct RDMA to send data to the NIC directly (bypassing CPU).
Before 2.4.2, the default value is 0 for all platforms. Since 2.4.2, the default value is 1 for NVLink-based platforms and 0 otherwise.
NCCL_SINGLE_RING_THRESHOLD¶
(since 2.1, removed in 2.3)
The NCCL_SINGLE_RING_THRESHOLD variable sets the limit under which NCCL will only use one ring.
This will limit bandwidth but improve latency.
Values accepted¶
The default value is 262144 (256kB) on GPUs with compute capability 7 and above. Otherwise, the default value is 131072 (128kB).
Values are integers, in bytes.
NCCL_LL_THRESHOLD¶
(since 2.1, removed in 2.5)
The NCCL_LL_THRESHOLD variable sets the size limit under which NCCL uses low-latency algorithms.
Values accepted¶
The default is 16384 (up to 2.2) or is dependent on the number of ranks (2.3 and later).
Values are integers, in bytes.
NCCL_TREE_THRESHOLD¶
(since 2.4, removed in 2.5)
The NCCL_TREE_THRESHOLD variable sets the size limit under which NCCL uses tree algorithms instead of rings.
NCCL_ALGO¶
(since 2.5)
The NCCL_ALGO variable defines which algorithms NCCL will use.
Values accepted¶
Comma-separated list of algorithms (not case sensitive) among: Tree, Ring, Collnet (up to 2.13), CollnetDirect (2.14+) and CollnetChain (2.14+). NVLS (2.17+) is the algorithm used to enable NVLink SHARP offload. To specify algorithms to exclude (instead of include), start the list with ^.
The default is Tree,Ring,CollnetDirect,CollnetChain,NVLS,NVLSTree.
NCCL_PROTO¶
(since 2.5)
The NCCL_PROTO variable defines which protocol NCCL will use.
Values accepted¶
Comma-separated list of protocols (not case sensitive) among: LL, LL128, Simple. To specify protocols to exclude (instead of include), start the list with ^.
The default is LL,LL128,Simple on platforms which support LL128, LL,Simple otherwise.
Users are discouraged from setting this variable, with the exception of disabling a specific protocol in case a bug in NCCL is suspected. In particular, enabling LL128 on platforms that don’t support it can lead to data corruption.
NCCL_IGNORE_CPU_AFFINITY¶
(since 2.4.6)
The NCCL_IGNORE_CPU_AFFINITY variable can be used to cause NCCL to ignore the job’s supplied CPU affinity and instead use the GPU affinity only.
Values accepted¶
The default is 0, set to 1 to cause NCCL to ignore the job’s supplied CPU affinity.
NCCL_DEBUG_FILE¶
(since 2.2.12)
The NCCL_DEBUG_FILE variable directs the NCCL debug logging output to a file.
The filename format can be set to filename.%h.%p where %h is replaced with the
hostname and %p is replaced with the process PID. This does not accept the ~ character as part of the path, please convert to a relative or absolute path first.
Values accepted¶
The default output file is stdout unless this environment variable is set.
Setting NCCL_DEBUG_FILE will cause NCCL to create and overwrite any previous files of that name.
Note: If the filename is not unique across all the job processes, then the output may be lost or corrupted.
NCCL_DEBUG_SUBSYS¶
(since 2.3.4)
The NCCL_DEBUG_SUBSYS variable allows the user to filter the NCCL_DEBUG=INFO output based on subsystems.
The value should be a comma separated list of the subsystems to include in the NCCL debug log traces.
Prefixing the subsystem name with ‘^’ will disable the logging for that subsystem.
Values accepted¶
The default value is INIT.
Supported subsystem names are INIT (stands for initialization), COLL (stands for collectives), P2P (stands for peer-to-peer), SHM (stands for shared memory), NET (stands for network), GRAPH (stands for topology detection and graph search), TUNING (stands for algorithm/protocol tuning), ENV (stands for environment settings), ALLOC (stands for memory allocations), and ALL (includes every subsystem).
NCCL_COLLNET_ENABLE¶
(since 2.6)
Enable the use of the CollNet plugin.
Value accepted¶
Default is 0, define and set to 1 to use the CollNet plugin.
NCCL_COLLNET_NODE_THRESHOLD¶
(since 2.9.9)
A threshold for the number of nodes below which CollNet will not be enabled.
Value accepted¶
Default is 2, define and set to an integer.
NCCL_TOPO_FILE¶
(since 2.6)
Path to an XML file to load before detecting the topology. By default, NCCL will load /var/run/nvidia-topologyd/virtualTopology.xml if present.
Value accepted¶
A path to an accessible file describing part or all of the topology.
NCCL_TOPO_DUMP_FILE¶
(since 2.6)
Path to a file to dump the XML topology to after detection.
Value accepted¶
A path to a file which will be created or overwritten.
NCCL_NVB_DISABLE¶
(since 2.11)
Disable intra-node communication through NVLink via an intermediate GPU.
Value accepted¶
Default is 0, set to 1 to disable this mechanism.
NCCL_PXN_DISABLE¶
(since 2.12)
Disable inter-node communication using a non-local NIC, using NVLink and an intermediate GPU.
Value accepted¶
Default is 0, set to 1 to disable this mechanism.
NCCL_P2P_PXN_LEVEL¶
(since 2.12)
Control in which cases PXN is used for send/receive operations.
Value accepted¶
A value of 0 will disable the use of PXN for send/receive. A value of 1 will enable the use of PXN when the NIC preferred by the destination is not directly accessible. A value of 2 (default) will cause PXN to always be used, even if the NIC is directly accessible, storing data from all GPUs within the node on an intermediate GPU to maximize aggregation.
NCCL_GRAPH_REGISTER¶
(since 2.11)
Enable user buffer registration when NCCL calls are captured by CUDA Graphs.
Effective only when: (i) the CollNet algorithm is being used; (ii) all GPUs within a node have P2P access to each other; (iii) there is at most one GPU per process.
User buffer registration may reduce the number of data copies between user buffers and the internal buffers of NCCL. The user buffers will be automatically de-registered when the CUDA Graphs are destroyed.
Value accepted¶
0 or 1. Default value is 1 (enabled).
NCCL_LOCAL_REGISTER¶
(since 2.19)
Enable user local buffer registration when users explicitly call ncclCommRegister.
Value accepted¶
0 or 1. Default value is 1 (enabled).
NCCL_SET_STACK_SIZE¶
(since 2.9)
Set CUDA kernel stack size to the maximum stack size amongst all NCCL kernels.
It may avoid a CUDA memory reconfiguration on load. Set to 1 if you experience hang due to CUDA memory reconfiguration.
Value accepted¶
0 or 1. Default value is 0 (disabled).
NCCL_SET_THREAD_NAME¶
(since 2.12)
Give more meaningful names to NCCL CPU threads to ease debugging and analysis.
Value accepted¶
0 or 1. Default is 0 (disabled).
NCCL_GRAPH_MIXING_SUPPORT¶
(since 2.13)
Enable/disable support for multiple outstanding NCCL calls from parallel CUDA graphs or a CUDA graph and non-captured NCCL calls. NCCL calls are considered outstanding starting from their host-side launch (e.g., a call to ncclAllreduce() for non-captured calls or cudaGraphLaunch() for captured calls) and ending when the device kernel execution completes. With graph mixing support disabled, the following use cases are NOT supported:
- Using a NCCL communicator (or split-shared communicators) from parallel graph launches, where parallel means on different streams without dependencies that would serialize their execution.
- Launching a non-captured NCCL collective during an outstanding graph launch that uses the same communicator (or split-shared communicators), regardless of stream ordering.
The ability to disable support is motivated by observed hangs in the CUDA launches when support is enabled and multiple ranks have work launched via cudaGraphLaunch from the same thread.
Value accepted¶
0 or 1. Default is 1 (enabled).
NCCL_DMABUF_ENABLE¶
(since 2.13)
Enable GPU Direct RDMA buffer registration using the Linux dma-buf subsystem.
The Linux dma-buf subsystem allows GPU Direct RDMA capable NICs to read and write CUDA buffers directly without CPU involvement.
Value accepted¶
0 or 1. Default value is 1 (enabled), but the feature is automatically disabled if the Linux kernel or the CUDA/NIC driver do not support it.
NCCL_P2P_NET_CHUNKSIZE¶
(since 2.14)
The NCCL_P2P_NET_CHUNKSIZE controls the size of messages sent through the network for ncclSend/ncclRecv operations.
Values accepted¶
The default is 131072 (128 K).
Values are integers, in bytes. The recommendation is to use powers of 2, hence 262144 would be the next value.
NCCL_P2P_LL_THRESHOLD¶
(since 2.14)
The NCCL_P2P_LL_THRESHOLD is the maximum message size that NCCL will use the LL protocol for P2P operations.
Values accepted¶
Decimal number. Default is 16384.
NCCL_ALLOC_P2P_NET_LL_BUFFERS¶
(since 2.14)
NCCL_ALLOC_P2P_NET_LL_BUFFERS instructs communicators to allocate dedicated LL buffers for all P2P network connections. This enables all ranks to use the LL protocol for latency-bound send and receive operations below NCCL_P2P_LL_THRESHOLD sizes.
Intranode P2P transfers always have dedicated LL buffers allocated. If running all-to-all workloads with high numbers of ranks, this will result in a high scaling memory overhead.
Values accepted¶
0 or 1. Default value is 0 (disabled).
NCCL_COMM_BLOCKING¶
(since 2.14)
The NCCL_COMM_BLOCKING variable controls whether NCCL calls are allowed to block or not. This includes all calls to NCCL, including init/finalize functions, as well as communication functions which may also block due to the lazy initialization of connections for send/receive calls. Setting this environment variable will override the blocking configuration in all communicators (see ncclConfig_t); if not set (undefined), communicator behavior will be determined by the configuration; if not passing configuration, communicators are blocking.
Values accepted¶
0 or 1. 1 indicates blocking communicators, and 0 indicates nonblocking communicators. The default value is undefined.
NCCL_CGA_CLUSTER_SIZE¶
(since 2.16)
Set CUDA Cooperative Group Array (CGA) cluster size. On sm90 and later we have an extra level of hierarchy where we
can group together several blocks within the Grid, called Thread Block Clusters. Setting this to non-zero will cause
NCCL to launch the communication kernels with the Cluster Dimension attribute set accordingly. Setting this environment
variable will override the cgaClusterSize configuration in all communicators (see ncclConfig_t); if not set
(undefined), CGA cluster size will be determined by the configuration; if not passing configuration, NCCL will
automatically choose the best value.
Values accepted¶
0 to 8. Default value is undefined.
NCCL_MAX_CTAS¶
(since 2.17)
Set the maximal number of CTAs the NCCL should use. Setting this environment variable will override the maxCTAs configuration in all communicators (see ncclConfig_t); if not set (undefined), maximal CTAs will be determined by the configuration; if not passing configuration, NCCL will automatically choose the best value.
Values accepted¶
Set to a positive integer value up to 32. Default value is undefined.
NCCL_MIN_CTAS¶
(since 2.17)
Set the minimal number of CTAs the NCCL should use. Setting this environment variable will override the minCTAs configuration in all communicators (see ncclConfig_t); if not set (undefined), minimal CTAs will be determined by the configuration; if not passing configuration, NCCL will automatically choose the best value.
Values accepted¶
Set to a positive integer value up to 32. Default value is undefined.
NCCL_NVLS_ENABLE¶
(since 2.17)
Enable the use of NVLink SHARP (NVLS). NVLink SHARP is available in third-generation NVSwitch systems (NVLink4) with Hopper and later GPU architectures, allowing collectives such as ncclAllReduce to be offloaded to the NVSwitch domain.
NVLS will be disabled automatically on systems which do not support the feature.
Values accepted¶
Default is automatic detection, define and set to 0 to disable use of NVLink SHARP.
NCCL_IB_MERGE_NICS¶
(since 2.20)
Enable NCCL to combine dual-port IB NICs into a single logical network device. This allows NCCL to more easily aggregate dual-port NIC bandwidth.
Values accepted¶
Default is 1 (enabled), define and set to 0 to disable NIC merging
NCCL_MNNVL_ENABLE¶
(since 2.21)
Enable NCCL to use Multi-Node NVLink (MNNVL) when available. If the system or driver are not Multi-Node NVLink capable then MNNVL will automatically be disabled. This feature also requires NCCL CUMEM support (NCCL_CUMEM_ENABLE) to be enabled.
MNNVL requires a fully configured and operational IMEX domain for all the nodes that form the NVLink domain. See the CUDA documentation for more details on IMEX domains.
Values accepted¶
Default is automatic detection, define and set to 0 to disable MNNVL support.