OnixS C++ FIX Engine  4.10.1
API Documentation
Solarflare Specific Features

Solarflare Onload

Solarflare Onload is a sockets acceleration technology for the transparent acceleration of sockets-based applications.

On Linux, the FIX Engine uses Solarflare Onload Extensions API to support additional features of Solarflare network cards. If you use such network cards, in your system, you can get an additional benefit from advanced features of such network cards.

FIX Engine automatically detects if the corresponding Onload library installed in your system and if so, then loads it in run-time and provides an ability to use additional features. In this case, there should be the record, in the FIX Engine log, that the corresponding version of the Onload extension library is used.

If you want to test Onload features on the loopback network interface, then EF_TCP_CLIENT_LOOPBACK and EF_TCP_SERVER_LOOPBACK parameters should be used, e.g. :

Please see the "Onload User Guide".

Currently, the FIX Engine supports the following Onload features:

When you use the OnixS::FIX::Session::warmUp method, the FIX Engine automatically enables this feature if possible. As a result, you can get the maximal warmup effect including the complete warm-up of the sending path of the TCP stack.

The onload_stackdump utility could be used to monitor the usage of this feature.

For example:

onload_stackdump lots | grep warm

In order to use this feature you need to use the overloaded OnixS::FIX::Session::send(FlatMessage * msg, SessionSendMode::Enum mode) method with the OnixS::FIX::SessionSendMode::OnloadZeroCopy mode. As a result, you can avoid the additional data copying from an application allocated buffer to the network adapter buffer and can decrease the sending latency.

This feature also is supported when one is sending messages in a batch. In this case, you need to use the overloaded OnixS::FIX::Session::send(FlatMessageBatch & msgs, SessionSendMode::Enum mode, size_t maxPacketSize) method.

If a Solarflare network card is not present in the system and you use this feature then the usual socket send call will be performed.
The Zero-Copy feature requires the allocation of specific buffers from NIC. After each successful send, the corresponding buffers become invalid, so it needs to reallocate them. This reallocation is performed right after the send method is completed. This adds additional latency to the OnixS::FIX::Session::send overall latency.

Solarflare TCPDirect

Solarflare's TCPDirect is a highly accelerated network middleware. It is similar to Onload but provides lower latency.


Before a TCPDirect-enabled session is created, the application must create an OnixS::FIX::TCPDirect::Stack instance.

TCPDirect::Attributes attributes;
// The default interface name could be provided by environment variable
// ZF_ATTR="interface=<interface-name>", see TCPDirect specs,
// or could be set manually:
// attr.set("interface", "interface-name");
TCPDirect::Stack stack(attributes);

Attributes (OnixS::FIX::TCPDirect::Attributes) control the configuration of the stack and its behavior. Please see the "Solarflare TCPDirect User Guide" for a complete list of available attributes and their description.

The application should create TCPDirect-enabled sessions in the same thread where it has created the corresponding OnixS::FIX::TCPDirect::Stack instance.

The pointer to the stack is provided in Session's constructor, for example:

SessionListener listener;
Session session(&stack, SenderCompId, TargetCompId, FixProtocolVersion, &listener);

By default, each TCPDirect stack can handle up to 64 TCP endpoints, so this is the maximum number of sessions that could be created using the same stack instance.

The max_tcp_endpoints attribute can set the maximum number of TCP endpoints. Please refer to Solarflare TCPDirect documentation.

See also max_tcp_listen_endpoints, max_tcp_syn_backlog, max_udp_rx_endpoints, max_udp_tx_endpoints attributes.

These attributes could be set to a minimum value to save hardware and software resources.

The TCPDirect does not support loopback connections, so it needs to test it between different machines or NICs.
The TCPDirect requires the allocation of huge pages. Please see the "Solarflare TCPDirect User Guide" for details. Huge pages are generally enabled by default. One can use, e.g., the following command to check the current state of huge pages:
cat /sys/kernel/mm/transparent_hugepage/enabled

Asynchronous Logon and Logout

Applications must call asynchronous Session's methods to logon (OnixS::FIX::Session::logonAsInitiatorAsync) and logout (OnixS::FIX::Session::logoutAsync).

session.logonAsInitiatorAsync(counterpartyHost, counterpartyPort, heartBtInt, ONIXS_FIXENGINE_NULLPTR, true);
// Dispatching events..
session.logoutAsync("End of connection");

Dispatching Network Events

Applications must call OnixS::FIX::TCPDirect::Stack::dispatchEvents frequently for each stack that is in use.

bool finished = false;
while (!finished)

Thread Safety

Access to the stack instance at each moment is possible from only one thread. The TCPDirect middleware does not use inter-threaded synchronization, does not serialize calls, and does not check the correctness of access to its API by the user. Therefore, the event dispatching and message sending should be performed from the same thread.


The TCPDirect stack requires finishing all outstanding work and handling all outstanding events. Before destroying the OnixS::FIX::TCPDirect::Stack instance, the following code should be executed:

while (!stack.isQuiescent())

Method OnixS::FIX::TCPDirect::Stack::isQuiescent returns a boolean value indicating whether a stack is quiescent.

This can be used to ensure that all connections have been closed gracefully before destroying a stack (or exiting the application). Destroying a stack while it is not quiescent is permitted by the API, but when doing so there is no guarantee that sent data has been acknowledged by the peer or even transmitted, and there is the possibility that peers' connections will be reset.

TCPDirect for Acceptor Sessions

There is an ability to use the TCPDirect mode for acceptor sessions too. However, to dispatch listening events and accept incoming TCP connections, one must pass the pointer to the OnixS::FIX::TCPDirect::Stack to the OnixS::FIX::Engine::init() method. The pointer to the same stack should also be passed to the acceptor session's constructor. In this case, the FIX Engine does not create an additional thread to listen for incoming connections and uses the corresponding stack to dispatch these events:

EngineSettings engineSettings;
// Set listen local network interface, which corresponds to Solarflare's NIC.
// Set the listen port.
TCPDirect::Attributes attributes;
// Set the interface name, which corresponds to Solarflare's NIC.
TCPDirect::Stack stack(attributes);
// Invoke the Engine::init with the given stack class instance.
Engine::init(engineSettings, &stack);
SessionListener listener;
Session acceptor(&stack, SenderCompId, TargetCompId, FixProtocolVersion, &listener);
bool finished = false;
// Dispatching events..
while (!finished)
while (!stack.isQuiescent())
It needs explicitly configure the listen local network interface, which corresponds to Solarflare's NIC. This can be done by the "OnixS::FIX::EngineSettings::localNetworkInterface() setting.

See Also