The ExaNIC Sockets acceleration library allows applications to benefit from the low latency of direct access to the ExaNIC without requiring modifications to the application. This is achieved by intercepting calls to the Linux socket APIs.
While ExaNIC Sockets should be compatible with most applications using Linux socket APIs, there are some cases where programs may not work as expected. Feedback and bug reports would be greatly appreciated (contact us at https://exablaze.com/support).
Build the ExaNIC driver and libraries as per the ExaNIC Installation and Configuration Guide. ExaNIC Sockets is built and installed as a standard component, and the exasock kernel module is loaded automatically when an ExaNIC interface is brought up.
First ensure that the application works without ExaNIC Sockets. All IP addresses should be configured as if you were running the application through the normal Linux network interface corresponding to the ExaNIC.
Then, to accelerate the application, simply prefix it with the
command. For example, to run the UNIX netcat (
nc) utility to listen
for UDP datagrams on port 1234:
$ exasock nc -u -l 1234
Another simple example application that receives and sends UDP multicast
datagrams is located in the ExaNIC source code distribution
examples/exasock/multicast-echo.c). Note that this is a normal Linux
sockets application that can be run either with or without the ExaNIC
Sockets acceleration library.
Sometimes it can be difficult to determine if the kernel bypass is
functioning correctly. Setting the
EXASOCK_DEBUG environment variable
prints extra debugging information that can help. For example:
$ EXASOCK_DEBUG=1 exasock nc -u -l 1234 exasock: enabled bypass on fd 4
In this case, the message
exasock: enabled bypass on fd 4 indicates
that kernel bypass has been enabled for the socket associated with file
The exasock command itself can take several arguments:
--help: display a message summarising these arguments
--debug: use the debug version of
--trace: record system calls, their arguments and their return values to stdout
--no-warn: turn off warning messages
--no-auto: disable acceleration on new sockets by default. It is still possible to opt-in to acceleration by calling
SO_EXA_NO_ACCELand supplying zero as the argument.
Displaying Exanic sockets accelerated connections
To provide insight into the current ExaNIC accelerated socket connections the utility
exasock-stat is provided. By default running
exasock-stat will display all accelerated UDP and TCP, listening and connected sockets.
exasock-stat application was introduced with the v2.0.0 ExaNIC driver and software package and can be found within the
This application will build as part of the utils build however the
libnl3-devel package is not present by default in which case building
exasock-stat will be skipped.
To ensure it can be built first run the appropriate command to install the missing libnl-3-dev package e.g.
sudo apt-get install libnl-3-dev or
sudo yum install libnl3-devel.x86_64 then re-run
make && make install.
Then when running:
you should see a table similar to the following:
Active ExaNIC Sockets accelerated connections (servers and established): Proto | Recv-Q | Send-Q | Local Address | Foreign Address | State UDP | 0 | 0 | 192.168.10.10:12345 | *:* | -
The coloumns shown are:
Proto: The protocol used by the socket (TCP or UDP) Recv-Q: Connected: The count of bytes not copied by the user program connected to this socket Listening: The count of connections waiting to be accepted by the user program Send-Q: Connected: The count of bytes not acknowledged by the remote host Listening: N/A Local Address: Address and port number of the local end of the socket Foreign Address: Address and port number of the remote end of the socket State: The state of the socket Extended Output not shown (-e/--extend enabled): User: The username or the user id (UID) of the owner of the socket PID:FD: PID of the process that owns the socket and value of the socket's file descriptor Program: Process name of the process that owns the socket
Exactly what the application displays can be controlled by providing arguements from the command line. To see the arguements available run
Disabling acceleration per-socket
If only the ExaNIC Sockets acceleration library is used, then each socket
bound to either an ExaNIC interface or to a wildcard address
INADDR_ANY) gets automatically accelerated (ie kernel is bypassed to
allow direct access to the ExaNIC).
As of exasock version 2.0.0 it is possible to disable default acceleration on a given socket, even if bound to an ExaNIC interface (or bound to a wildcard address or joined a multicast group with an ExaNIC interface).
In order to use this feature the application is required to
<exasock/socket.h> header file and to disable the acceleration
as needs be for each socket. This is done by either setting the exasock private
SO_EXA_NO_ACCEL socket option, or alternatively by calling the
exasock_disable_acceleration() helper function.
Disabling acceleration on a socket is not allowed if the socket has already been accelerated (either by binding it to an ExaNIC interface or joining a multicast group with an ExaNIC interface).
Once acceleration has been disabled on a socket, it can no longer be re-enabled.
Documentation for both the exasock private socket option and the helper function can
be found in the header file
Versions of exasock older than 2.0.0 automatically accelerate each socket bound
to a multicast address. Newer versions (2.0.0 and beyond) accelerate a multicast
socket only if joined a multicast group (via
IP_ADD_MEMBERSHIP socket option)
with an ExaNIC interface. For any accelerated socket exasock version 2.0.0 (or
later) receives multicast packets only from the interface with which the socket
has joined the multicast group.
exasock 2.0.0 and later:
If a socket bound to a wildcard address (
INADDR_ANY) is to be used for
receiving multicast traffic, it is worth to keep in mind that it will always be
accelerated. Multicast packets are going to be discarded on this socket unless
it has been set with
IP_ADD_MEMBERSHIP option to join given multicast group
and multicast packets are arriving through the ExaNIC interface specified in the
exasock 2.0.0 and later:
If a socket bound to a multicast address but not associated with an ExaNIC
IP_ADD_MEMBERSHIP option is to be used, then it will not get
accelerated. It will receive multicast packets through the native kernel
networking stack instead.
exasock extension API
exasock 1.7.0 and later include a library called
allows user applications to detect that they are running under exasock
and access functionality beyond the standard Linux socket calls. In
particular, the current version provides a TCP acceleration feature that
allows programmers to achieve even lower TCP latencies than possible
with the normal send/sendto/sendmsg APIs.
Using this TCP acceleration feature, an application can construct
partial or complete TCP packets ahead of time. These pre-built packets
can then be transmitted through the lower level libexanic library, or
can even be pushed to a user FPGA application on the ExaNIC card for
ultra-low latency responses to triggers. Documentation for the extension
functions is in the header file
<exasock/extensions.h>, and an
example program is located in the ExaNIC source distribution
If you are using the exasock TCP extension then you need to ensure that you do not modify the packet between
If you are using the exasock TCP extension then you need to ensure that if you have prepared a TCP frame for transmission, that it is the next frame transmitted. If, for example, another TCP frame is sent via exasock using
send(), then the prepared TCP frame will have the incorrect sequence number etc., and so must be discarded.
Known issues and limitations
- Each thread that calls a blocking I/O call - e.g.
accept()- will spin waiting on data. This normally provides optimal latency but can induce performance problems if there are more threads than available CPUs. Other blocking modes will be provided in the future.
- If a socket is bound to a wildcard address (
INADDR_ANY), it will only receive packets that arrive on ExaNIC interfaces when run with the acceleration library.
- If exasock version older than 2.0.0 is used and a socket is bound to
a multicast address, or exasock version 2.0.0 or newer is used and
a socket has joined a multicast group (
IP_ADD_MEMBERSHIPsocket option) with an ExaNIC interface, it will only receive packets that arrive on ExaNIC interfaces when run with the acceleration library.
- Connecting to an accelerated socket from the same host is not supported (for example, if a socket is bound to 192.168.1.1:80, then it is not possible to connect to 192.168.1.1:80 from the local host).
- Transmitted multicast datagrams are not looped back to local sockets.
recv()is not currently supported (to be resolved).
- No support for recursive addition of epoll file descriptors to epoll sets.
- No support for IP fragmentation.
- Sockets may not be correctly maintained across
- Sockets cannot be transferred to other processes with
recvmmsg()duplicates the Linux behavior of only checking the timeout after the receipt of each datagram, so that if up to vlen-1 datagrams received before the timeout expires, but then no further datagrams are received, the call will block forever.
Tips for best performance
- Wherever possible, do not mix accelerated sockets with
non-accelerated sockets and other file descriptors in
- For the best possible performance, pin threads to CPU cores in the CPU socket directly connected to the ExaNIC.
This page was last updated on Oct-12-2018.