# 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_SOCKET_IFNAME=eth0
NCCL_DEBUG=WARN


## 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.

### Values accepted¶

LOC or 0 : Never use P2P (always disabled)

NVL : Use P2P when GPUs are connected through NVLink

PIX or 1 : Use P2P when GPUs are on the same PCI switch.

PXB or 2 : Use P2P when GPUs are connected through PCI switches (potentially multiple hops).

PHB or 3, or 4 : Use P2P when GPUs are on the same NUMA node. Traffic will go through the CPU.

SYS or 5 : Use P2P betweem NUMA nodes, potentially crossing the SMP interconnect (e.g. QPI/UPI).

## 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 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 interface to use for communication.

### Values accepted¶

Define to a list of prefixes to filter interfaces to be used by NCCL. For example, eth,ib would only select interfaces starting with eth or ib. Using the ^ symbol, NCCL will exclude interfaces starting with any prefix in that list. For example, ^eth,ib would select interfaces not starting with eth or ib.

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.

## 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 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

## 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 communicate between same NICs between nodes, to allow for network design where each NIC from each 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 depending 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 there are corner cases for which NCCL may still cause cross-rail communication, so rails still need to be connected at the top.

1: Do not attempt to use the same NIC 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 it if it would result in 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 disables the IB/RoCE transport that is to be used by NCCL. Instead, NCCL will fallback 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 be a list of prefixes to filter interfaces to be used by NCCL.

Using the ^ symbol, NCCL will exclude interfaces starting with any prefix in that list of prefix. Specific ports can also be specified using “:”.

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_1:2 : Do not use port 2 of card mlx5_1.

## 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 (https://www.infinibandta.org/ibta-specifications-download) (Local Ack Timeout).

### Values accepted¶

The default value used by NCCL is 14.

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 (https://www.infinibandta.org/ibta-specifications-download).

### 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 (https://www.infinibandta.org/ibta-specifications-download) or vendor documentation.

### Values accepted¶

The default value is 0.

## NCCL_IB_SL¶

(since 2.1.4)

Defines the InfiniBand Service Level.

For more information, see the InfiniBand specification Volume 1 (https://www.infinibandta.org/ibta-specifications-download) 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 (https://www.infinibandta.org/ibta-specifications-download) or vendor documentation.

### Values accepted¶

The default value is 0.

## NCCL_IB_AR_THRESHOLD¶

(since 2.6)

Threshold after 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. Each message, regardless of its size, will be split in N parts and sent on each queue pair. Therefore, increasing this number can cause a latency increase as well as a bandwidth reduction.

### Values accepted¶

Number between 1 and 128, default is 1. Values beyond 8 usually cause degraded bandwidth.

## NCCL_IB_PCI_RELAXED_ORDERING¶

(since 2.12)

Enable 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 use of Relaxed Ordering and fail if not available. Set to 0 to disable use of Relaxed Ordering. Default is 2.

## 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.

### Values accepted¶

The value of NCCL_NET has to match exactly the name of the NCCL network used (case-sensitive). Internal network names are “IB” (generic IB verbs) and “Socket” (TCP/IP sockets). External network plugins define their own names.

## 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 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 is renamed 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.

### Values accepted¶

0 : Never use GPU Direct RDMA. (always disabled)

1 : Use GPU Direct RDMA when GPU and NIC are on the same PCI switch.

2 : Use GPU Direct RDMA when GPU and NIC are connected through PCI switches (potentially multiple hops).

3 : Use GPU Direct RDMA when GPU and NIC are on the same PCI root complex, potentially going through the CPU.

4 : (Since 2.4.7) Use GPU Direct RDMA even across PCI root complexes, as long as GPU and NIC are within the same NUMA node. (Before 2.4.7) Use GPU Direct RDMA even across PCI root complexes, regardless of whether GPU and NIC are within the same NUMA node (always enabled).

5 : Use GPU Direct RDMA even across the SMP interconnect between NUMA nodes (e.g., QPI/UPI). (always enabled)

The default value is 2.

## 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.

### Values accepted¶

The default is dependent on the number of ranks.

Values are integers, in bytes.

## NCCL_ALGO¶

(since 2.5)

The NCCL_ALGO variable defines which algorithms NCCL will use.

### Values accepted¶

Coma-separated list of algorithms (not case sensitive) among: Tree, Ring, Collnet. To specify algorithms to exclude (instead of include), start the list with ^.

The default is Tree,Ring,Collnet.

## NCCL_PROTO¶

(since 2.5)

The NCCL_PROTO variable defines which protocol NCCL will use.

### Values accepted¶

Coma-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.

## 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.

### 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. 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 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 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 an XML file to dump the topology after detection.

### Value accepted¶

A path to a file which will be created or overwritten.

## 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 0.

## 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.