11.2.2. TCP


TODO This section needs to be converted from FAQ Q&A style to regular documentation style. How do I specify to use the IP network for MPI messages?

Open MPI will generally automatically use the tcp BTL when:

  1. The tcp BTL is available at run time (which it should be on most POSIX-like systems), and

  2. A higher-performance network is not available

When the tcp BTL is used, it is typically also (automatically) used with the self and sm BTLs for process-loopback and node-loopback communication, respectively.

If you want to guarantee that the tcp, sm, and self BTLs are used, you can explicitly specify them on the mpirun command line:

shell$ mpirun --mca pml ob1 --mca btl tcp,sm,self ...


Failure to specify the sm BTL will likely result in lower performance when Open MPI uses the TCP network stack to send to peers on the same host.


Failure to specify the self BTL may result in Open MPI being unable to complete send-to-self scenarios (meaning that your program will run fine until a process tries to send to itself). But wait — I’m using a high-speed network. Do I have to disable the TCP BTL?

No. Following the so-called “Law of Least Astonishment”, Open MPI assumes that if you have both an IP network and at least one high-speed network (such InfiniBand), you will likely only want to use the high-speed network(s) for MPI message passing. Hence, the tcp BTL component will sense this and automatically deactivate itself.

That being said, Open MPI may still use TCP for setup and teardown information — so you’ll see traffic across your IP network during startup and shutdown of your MPI job. This is normal and does not affect the MPI message passing channels. How do I know what MCA parameters are available for tuning MPI performance?

The ompi_info command can display all the parameters available for the tcp BTL component (i.e., the component that uses TCP for MPI communications):

shell$ ompi_info --param btl tcp --level 9 Does Open MPI use the IP loopback interface?

Usually not.

In general message passing usage, there are two scenarios where using the operating system IP loopback interface could be used:

  1. Sending a message from one process to itself

  2. Sending a message from one process to another process on the same machine

The TCP BTL does not handle “send-to-self” scenarios in Open MPI; indeed, it is not even capable of doing so. Instead, the self BTL component is used for all send-to-self MPI communications. Not only does this allow all Open MPI BTL components to avoid special case code for send-to-self scenarios, it also allows avoiding using inefficient loopback network stacks (such as the IP loopback device).

Specifically: the self component uses its own mechanisms for send-to-self scenarios; it does not use operating system network interfaces such as the IP loopback interface.

When sending to other processes on the same machine, Open MPI will default to using a shared memory BTL (sm). If the user has deactivated these BTLs, depending on what other BTL components are available, it is possible that the TCP BTL will be chosen for message passing to processes on the same node, in which case the IP loopback device will likely be used. But this is not the default; either shared memory has to fail to startup properly or the user must specifically request not to use the shared memory BTL. I have multiple IP networks on some/all of my cluster nodes. Which ones will Open MPI use?

In general, Open MPI will greedily use all IP networks that it finds per its reachability computations.

To change this behavior, you can either specifically include certain networks or specifically exclude certain networks. See this FAQ entry for more details.

I’m getting TCP-related errors. What do they mean?

TCP-related errors are usually reported by Open MPI in a message similar to these:

btl_tcp_endpoint.c:572:mca_btl_tcp_endpoint_complete_connect: connect() failed with errno=113
mca_btl_tcp_frag_send: writev failed with errno=104

If an errno number is displayed with no explanation string, you can see what that specific error number means on your operating system. On Linux, you can use the perror command:

# See what errno 113 is
shell$ perror 113
OS error code 113:  No route to host

# See what errno 104 is
shell$ perror 104
OS error code 104:  Connection reset by peer

Two types of errors are commonly reported to the Open MPI user’s mailing list:

  1. No route to host: These types of errors usually mean that there are multiple IP interfaces available and they do not obey Open MPI’s assumptions about routability. See the TCP routability assumptions FAQ entry and the TCP selection FAQ entry for more information.

  2. Connection reset by peer: These types of errors usually occur after MPI_INIT has completed, and typically indicate that an MPI process has died unexpectedly (e.g., due to a catastrophic error such as a segmentation fault). The specific error message indicates that a peer MPI process tried to write to the now-dead MPI process and failed. How do I tell Open MPI which IP interfaces / networks to use?

In some HPC environments, it is not uncommon to have multiple IP interfaces on each node — for example, one IP network may be “slow” and used for control information such as a batch scheduler, a networked filesystem, and/or interactive logins. Another IP network (or networks) may be “fast” and be intended for parallel applications to use during their runs. As another example, some operating systems may also have virtual interfaces for communicating with virtual machines.

Unless otherwise specified, Open MPI will greedily use all “up” IP networks that it can find and try to connect to all peers upon demand (i.e., Open MPI does not open sockets to all of its MPI peers during MPI_INIT — see this FAQ entry for more details). Hence, if you want MPI jobs to not use specific IP networks — or not use any IP networks at all — then you need to tell Open MPI.


Aggressively using all “up” interfaces can cause problems in some cases. For example, if you have a machine with a local-only interface (e.g., the loopback device, or a virtual-machine bridge device that can only be used on that machine, and cannot be used to communicate with MPI processes on other machines), you will likely need to tell Open MPI to ignore these networks.

Open MPI usually ignores loopback devices by default, but other local-only devices must be manually ignored. Users have reported cases where RHEL6 automatically installed a virbr0 device for Xen virtualization. This interface was automatically given an IP address in the subnet and marked as “up”. Since Open MPI saw this “up” interface in all MPI processes on all nodes, it assumed that that network was usable for MPI communications. This is obviously incorrect, and it led to MPI applications hanging when they tried to send or receive MPI messages.

  1. To disable Open MPI from using TCP for MPI communications, the tcp MCA parameter should be set accordingly. You can either exclude the TCP component or include all other components. Specifically:

    # This says to exclude the TCP BTL component
    # (implicitly including all others)
    shell$ mpirun --mca btl ^tcp...
    # This says to include only the listed BTL components
    # (tcp is not listed, and therefore will not be used)
    shell$ mpirun --mca btl self,vader,openib ...
  2. If you want to use TCP for MPI communications, but want to restrict it from certain networks, use the btl_tcp_if_include or btl_tcp_if_exclude MCA parameters (only one of the two should be set). The values of these parameters can be a comma-delimited list of network interfaces. For example:

    # This says to not use the eth0 and lo interfaces.
    # (and implicitly use all the rest).  Per the description
    # above, IP loopback and all local-only devices *must*
    # be included if the exclude list is specified.
    shell$ mpirun --mca btl_tcp_if_exclude lo,eth0 ...
    # This says to only use the eth1 and eth2 interfaces
    # (and implicitly ignore the rest)
    shell$ mpirun --mca btl_tcp_if_include eth1,eth2 ...
  3. You can also specify subnets in the include or exclude lists in CIDR notation. For example:

    # Only use the and subnets for MPI
    # communications:
    shell$ mpirun --mca btl_tcp_if_include, ...


    You must specify the CIDR notation for a given network precisely. For example, if you have two IP networks and, Open MPI will not recognize either of them if you specify “”.


If you use the btl_tcp_if_include and btl_tcp_if_exclude MCA parameters to shape the behavior of the TCP BTL for MPI communications, you may also need/want to investigate the corresponding MCA parameters oob_tcp_if_include and oob_tcp_if_exclude, which are used to shape non-MPI TCP-based communication (e.g., communications setup and coordination during MPI_INIT and MPI_FINALIZE).


TODO do corresponding OOB TCP params still exist in PMIx?

Note that Open MPI will still use TCP for control messages, such as data between mpirun and the MPI processes, rendezvous information during MPI_INIT, etc. To disable TCP altogether, you also need to disable the tcp component from the OOB framework.


TODO Is this possible in PMIx? I doubt it…? Does Open MPI open a bunch of sockets during MPI_INIT?

Although Open MPI is likely to open multiple TCP sockets during MPI_INIT, the tcp BTL component does not open one socket per MPI peer process during MPI_INIT. Open MPI opens sockets as they are required — so the first time a process sends a message to a peer and there is no TCP connection between the two, Open MPI will automatically open a new socket.

Hence, you should not have scalability issues with running large numbers of processes (e.g., running out of per-process file descriptors) if your parallel application is sparse in its communication with peers. Are there any Linux kernel TCP parameters that I should set?

Everyone has different opinions on this, and it also depends on your exact hardware and environment. Below are general guidelines that some users have found helpful.

  1. net.ipv4.tcp_syn_retries: Some Linux systems have very large initial connection timeouts — they retry sending SYN packets many times before determining that a connection cannot be made. If MPI is going to fail to make socket connections, it would be better for them to fail somewhat quickly (minutes vs. hours). You might want to reduce this value to a smaller value; YMMV.

  2. net.ipv4.tcp_keepalive_time: Some MPI applications send an initial burst of MPI messages (over TCP) and then send nothing for long periods of time (e.g., embarrassingly parallel applications). Linux may decide that these dormant TCP sockets are dead because it has seen no traffic on them for long periods of time. You might therefore need to lengthen the TCP inactivity timeout. Many Linux systems default to 7,200 seconds; increase it if necessary.

  3. Increase TCP buffering for 10G or 40G Ethernet. Many Linux distributions come with good buffering presets for 1G Ethernet. In a datacenter/HPC cluster with 10G or 40G Ethernet NICs, this amount of kernel buffering is typically insufficient. Here’s a set of parameters that some have used for good 10G/40G TCP bandwidth:

    • net.core.rmem_max: 16777216

    • net.core.wmem_max: 16777216

    • net.ipv4.tcp_rmem: 4096 87380 16777216

    • net.ipv4.tcp_wmem: 4096 65536 16777216

    • net.core.netdev_max_backlog: 30000

    • net.core.rmem_default: 16777216

    • net.core.wmem_default: 16777216

    • net.ipv4.tcp_mem: ‘16777216 16777216 16777216’

    • net.ipv4.route.flush: 1

    Each of the above items is a Linux kernel parameter that can be set in multiple different ways.

    1. You can change the running kernel via the /proc filesystem:

      shell# cat /proc/sys/net/ipv4/tcp_syn_retries
      shell# echo 6 > /proc/sys/net/ipv4/tcp_syn_retries
    2. You can also use the sysctl command:

      shell# sysctl net.ipv4.tcp_syn_retries
      net.ipv4.tcp_syn_retries = 5
      shell# sysctl -w net.ipv4.tcp_syn_retries=6
      net.ipv4.tcp_syn_retries = 6
    3. Or you can set them by adding entries in /etc/sysctl.conf, which are persistent across reboots:

      shell$ grep tcp_syn_retries /etc/sysctl.conf
      net.ipv4.tcp_syn_retries = 6
    4. Your Linux distro may also support putting individual files in /etc/sysctl.d (even if that directory does not yet exist), which is actually better practice than putting them in /etc/sysctl.conf. For example:

      shell$ cat /etc/sysctl.d/my-tcp-settings
      net.ipv4.tcp_syn_retries = 6 How does Open MPI know which IP addresses are routable to each other?

Open MPI assumes that all interfaces are routable as long as they have the same address family, IPv4 or IPv6. We use graph theory and give each possible connection a weight depending on the quality of the connection. This allows the library to select the best connections between nodes. This method also supports striping but prevents more than one connection to any interface.

The quality of the connection is defined as follows, with a higher number meaning better connection. Note that when giving a weight to a connection consisting of a private address and a public address, it will give it the weight of PRIVATE_DIFFERENT_NETWORK.

            NO_CONNECTION = 0

An example will best illustrate how two processes on two different nodes would connect up. Here we have two nodes with a variety of interfaces:

       Node A                Node B
 ----------------       ----------------
|      lo0       |     |      lo0       |
|    |     |    |
|                |     |                |
|      eth0      |     |      eth0      |
|   |     |   |
|                |     |                |
|      eth1      |     |      eth1      |
| |     | |
|                |     |                |
|      eth2      |     |                |
| |     |                |
 ----------------      ------------------

From these two nodes, the software builds up a bipartite graph that shows all the possible connections with all the possible weights. The lo0 interfaces are excluded as the btl_tcp_if_exclude MCA parameter is set to lo by default. Here is what all the possible connections with their weights look like.

      Node A       Node B
eth0 --------- 2 -------- eth0
       ------- 1 -------- eth1

eth1 --------- 1 -------- eth0
       ------- 2 -------- eth1

eth2 --------- 1 -------- eth0
       ------- 1 -------- eth1

The library then examines all the connections and picks the optimal ones. This leaves us with two connections being established between the two nodes.

If you are curious about the actual connect() calls being made by the processes, then you can run with --mca btl_base_verbose 30. This can be useful if you notice your job hanging and believe it may be the library trying to make connections to unreachable hosts.

# Here is an example with some of the output deleted for clarity.
# One can see the connections that are attempted.
shell$ mpirun --mca btl self,sm,tcp --mca btl_base_verbose 30 -n 2 -host NodeA,NodeB a.out
[NodeA:18003] btl: tcp: attempting to connect() to address on port 59822
[NodeA:18003] btl: tcp: attempting to connect() to address on port 59822
[NodeB:16842] btl: tcp: attempting to connect() to address on port 44500

In case you want more details about the theory behind the connection code, you can find the background story in this IEEE paper. Does Open MPI ever close TCP sockets?

In general, no.

Although TCP sockets are opened “lazily” (meaning that MPI connections / TCP sockets are only opened upon demand — as opposed to opening all possible sockets between MPI peer processes during MPI_INIT), they are never closed. Does Open MPI support IP interfaces that have more than one IP address?

In general, no.

For example, if the output from your ifconfig has a single IP device with multiple IP addresses like this:

0: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP qlen 1000
   link/ether 00:18:ae:f4:d2:29 brd ff:ff:ff:ff:ff:ff
   inet brd scope global eth0:1
   inet brf scope global eth0
   inet6 fe80::218:aef2:29b4:2c4/64 scope link
      valid_lft forever preferred_lft forever

(note the two inet lines in there)

Then Open MPI will be unable to use this device. Does Open MPI support virtual IP interfaces?


For example, if the output of your ifconfig has both eth0 and eth0:0, Open MPI will get confused if you use the TCP BTL, and may hang or otherwise act unpredictably.

Note that using btl_tcp_if_include or btl_tcp_if_exclude to avoid using the virtual interface will not solve the issue. Can I use multiple TCP connections to improve network performance?

Open MPI can use multiple TCP connections between any pair of MPI processes, striping large messages across the connections. The btl_tcp_links parameter can be used to set how many TCP connections should be established between MPI processes.

Note that this may not improve application performance for common use cases of nearest-neighbor exchanges when there many MPI processes on each host. In these cases, there are already many TCP connections between any two hosts (because of the many processes all communicating), so the extra TCP connections are likely just consuming extra resources and adding work to the MPI implementation.

However, for highly multi-threaded applications, where there are only one or two MPI processes per host, the btl_tcp_links option may improve TCP throughput considerably.